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Versions: (draft-niedermayer-cellar-ffv1) 00

cellar                                                    M. Niedermayer
Internet-Draft
Intended status: Standards Track                                 D. Rice
Expires: January 3, 2018
                                                             J. Martinez
                                                            July 2, 2017


                            FF Video Codec 1
                       draft-ietf-cellar-ffv1-00

Abstract

   This document defines FFV1, a lossless intra-frame video encoding
   format.  FFV1 is designed to efficiently compress video data in a
   variety of pixel formats.  Compared to uncompressed video, FFV1
   offers storage compression, frame fixity, and self-description, which
   makes FFV1 useful as a preservation or intermediate video format.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 3, 2018.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of



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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Notation and Conventions  . . . . . . . . . . . . . . . . . .   4
     2.1.  Definitions . . . . . . . . . . . . . . . . . . . . . . .   4
     2.2.  Conventions . . . . . . . . . . . . . . . . . . . . . . .   5
       2.2.1.  Arithmetic operators  . . . . . . . . . . . . . . . .   5
       2.2.2.  Assignment operators  . . . . . . . . . . . . . . . .   6
       2.2.3.  Comparison operators  . . . . . . . . . . . . . . . .   6
       2.2.4.  Mathematical functions  . . . . . . . . . . . . . . .   6
       2.2.5.  Order of operation precedence . . . . . . . . . . . .   7
       2.2.6.  Pseudo-code . . . . . . . . . . . . . . . . . . . . .   7
       2.2.7.  Range . . . . . . . . . . . . . . . . . . . . . . . .   7
       2.2.8.  NumBytes  . . . . . . . . . . . . . . . . . . . . . .   8
       2.2.9.  Bitstream functions . . . . . . . . . . . . . . . . .   8
   3.  General Description . . . . . . . . . . . . . . . . . . . . .   8
     3.1.  Border  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     3.2.  Samples . . . . . . . . . . . . . . . . . . . . . . . . .   9
     3.3.  Median predictor  . . . . . . . . . . . . . . . . . . . .   9
     3.4.  Context . . . . . . . . . . . . . . . . . . . . . . . . .  10
     3.5.  Quantization Table Sets . . . . . . . . . . . . . . . . .  10
     3.6.  Quantization Table Set indexes  . . . . . . . . . . . . .  11
     3.7.  Color space . . . . . . . . . . . . . . . . . . . . . . .  11
       3.7.1.  YCbCr . . . . . . . . . . . . . . . . . . . . . . . .  11
       3.7.2.  JPEG2000-RCT  . . . . . . . . . . . . . . . . . . . .  12
     3.8.  Coding of the Sample Difference . . . . . . . . . . . . .  13
       3.8.1.  Range coding mode . . . . . . . . . . . . . . . . . .  13
       3.8.2.  Huffman coding mode . . . . . . . . . . . . . . . . .  17
   4.  Bitstream . . . . . . . . . . . . . . . . . . . . . . . . . .  19
     4.1.  Configuration Record  . . . . . . . . . . . . . . . . . .  20
       4.1.1.  reserved_for_future_use . . . . . . . . . . . . . . .  21
       4.1.2.  configuration_record_crc_parity . . . . . . . . . . .  21
       4.1.3.  Mapping FFV1 into Containers  . . . . . . . . . . . .  21
     4.2.  Frame . . . . . . . . . . . . . . . . . . . . . . . . . .  22
     4.3.  Slice . . . . . . . . . . . . . . . . . . . . . . . . . .  22
     4.4.  Slice Header  . . . . . . . . . . . . . . . . . . . . . .  23
       4.4.1.  slice_x . . . . . . . . . . . . . . . . . . . . . . .  23
       4.4.2.  slice_y . . . . . . . . . . . . . . . . . . . . . . .  23
       4.4.3.  slice_width . . . . . . . . . . . . . . . . . . . . .  23
       4.4.4.  slice_height  . . . . . . . . . . . . . . . . . . . .  23
       4.4.5.  quant_table_set_index_count . . . . . . . . . . . . .  23
       4.4.6.  quant_table_set_index . . . . . . . . . . . . . . . .  24
       4.4.7.  picture_structure . . . . . . . . . . . . . . . . . .  24
       4.4.8.  sar_num . . . . . . . . . . . . . . . . . . . . . . .  24
       4.4.9.  sar_den . . . . . . . . . . . . . . . . . . . . . . .  24



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       4.4.10. reset_contexts  . . . . . . . . . . . . . . . . . . .  24
       4.4.11. slice_coding_mode . . . . . . . . . . . . . . . . . .  24
     4.5.  Slice Content . . . . . . . . . . . . . . . . . . . . . .  25
       4.5.1.  primary_color_count . . . . . . . . . . . . . . . . .  25
       4.5.2.  plane_pixel_height  . . . . . . . . . . . . . . . . .  25
       4.5.3.  slice_pixel_height  . . . . . . . . . . . . . . . . .  25
       4.5.4.  slice_pixel_y . . . . . . . . . . . . . . . . . . . .  25
     4.6.  Line  . . . . . . . . . . . . . . . . . . . . . . . . . .  25
       4.6.1.  plane_pixel_width . . . . . . . . . . . . . . . . . .  26
       4.6.2.  slice_pixel_width . . . . . . . . . . . . . . . . . .  26
       4.6.3.  slice_pixel_x . . . . . . . . . . . . . . . . . . . .  26
     4.7.  Slice Footer  . . . . . . . . . . . . . . . . . . . . . .  26
       4.7.1.  slice_size  . . . . . . . . . . . . . . . . . . . . .  27
       4.7.2.  error_status  . . . . . . . . . . . . . . . . . . . .  27
       4.7.3.  slice_crc_parity  . . . . . . . . . . . . . . . . . .  27
     4.8.  Parameters  . . . . . . . . . . . . . . . . . . . . . . .  27
       4.8.1.  version . . . . . . . . . . . . . . . . . . . . . . .  28
       4.8.2.  micro_version . . . . . . . . . . . . . . . . . . . .  29
       4.8.3.  coder_type  . . . . . . . . . . . . . . . . . . . . .  30
       4.8.4.  state_transition_delta  . . . . . . . . . . . . . . .  30
       4.8.5.  colorspace_type . . . . . . . . . . . . . . . . . . .  30
       4.8.6.  chroma_planes . . . . . . . . . . . . . . . . . . . .  30
       4.8.7.  bits_per_raw_sample . . . . . . . . . . . . . . . . .  31
       4.8.8.  h_chroma_subsample  . . . . . . . . . . . . . . . . .  31
       4.8.9.  v_chroma_subsample  . . . . . . . . . . . . . . . . .  31
       4.8.10. alpha_plane . . . . . . . . . . . . . . . . . . . . .  31
       4.8.11. num_h_slices  . . . . . . . . . . . . . . . . . . . .  31
       4.8.12. num_v_slices  . . . . . . . . . . . . . . . . . . . .  32
       4.8.13. quant_table_set_count . . . . . . . . . . . . . . . .  32
       4.8.14. states_coded  . . . . . . . . . . . . . . . . . . . .  32
       4.8.15. initial_state_delta . . . . . . . . . . . . . . . . .  32
       4.8.16. ec  . . . . . . . . . . . . . . . . . . . . . . . . .  32
       4.8.17. intra . . . . . . . . . . . . . . . . . . . . . . . .  32
     4.9.  Quantization Table Set  . . . . . . . . . . . . . . . . .  33
       4.9.1.  quant_tables  . . . . . . . . . . . . . . . . . . . .  34
       4.9.2.  context_count . . . . . . . . . . . . . . . . . . . .  34
   5.  Restrictions  . . . . . . . . . . . . . . . . . . . . . . . .  34
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  35
   7.  Appendixes  . . . . . . . . . . . . . . . . . . . . . . . . .  35
     7.1.  Decoder implementation suggestions  . . . . . . . . . . .  36
       7.1.1.  Multi-threading support and independence of slices  .  36
   8.  Changelog . . . . . . . . . . . . . . . . . . . . . . . . . .  36
   9.  ToDo  . . . . . . . . . . . . . . . . . . . . . . . . . . . .  36
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  37
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  37
     10.2.  Informative References . . . . . . . . . . . . . . . . .  37
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  39




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1.  Introduction

   This document describes FFV1, a lossless video encoding format.  The
   design of FFV1 considers the storage of image characteristics, data
   fixity, and the optimized use of encoding time and storage
   requirements.  FFV1 is designed to support a wide range of lossless
   video applications such as long-term audiovisual preservation,
   scientific imaging, screen recording, and other video encoding
   scenarios that seek to avoid the generational loss of lossy video
   encodings.

   This document defines a version 0, 1, and 3 of FFV1.  The
   distinctions of the versions are provided throughout the document,
   but in summary:

   o  Version 0 of FFV1 was the original implementation of FFV1 and has
      been in non-experimental use since April 14, 2006 [FFV1_V0].

   o  Version 1 of FFV1 adds support of more video bit depths and has
      been in use since April 24, 2009 [FFV1_V1].

   o  Version 2 of FFV1 only existed in experimental form and is not
      described by this document.

   o  Version 3 of FFV1 adds several features such as increased
      description of the characteristics of the encoding images and
      embedded CRC data to support fixity verification of the encoding.
      Version 3 has been in non-experimental use since August 17, 2013
      [FFV1_V3].

   The latest version of this document is available at
   <https://raw.github.com/FFmpeg/FFV1/master/ffv1.md>

   This document assumes familiarity with mathematical and coding
   concepts such as Range coding [range-coding] and YCbCr color spaces
   [YCbCr].

2.  Notation and Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

2.1.  Definitions

   "ESC": An ESCape symbol to indicate that the symbol to be stored is
   too large for normal storage and that an alternate storage method.




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   "MSB": Most Significant Bit, the bit that can cause the largest
   change in magnitude of the symbol.

   "RCT": Reversible Color Transform, a near linear, exactly reversible
   integer transform that converts between RGB and YCbCr representations
   of a sample.

   "VLC": Variable Length Code, a code which maps source symbols to a
   variable number of bits.

   "RGB": A reference to the method of storing the value of a sample by
   using three numeric values that represent Red, Green, and Blue.

   "YCbCr": A reference to the method of storing the value of a sample
   by using three numeric values that represent the luminance of the
   sample (Y) and the chrominance of the sample (Cb and Cr).

   "TBA": To Be Announced.  Used in reference to the development of
   future iterations of the FFV1 specification.

2.2.  Conventions

   Note: the operators and the order of precedence are the same as used
   in the C programming language [ISO.9899.1990].

2.2.1.  Arithmetic operators

   "a + b" means a plus b.

   "a - b" means a minus b.

   "-a" means negation of a.

   "a * b" means a multiplied by b.

   "a / b" means a divided by b.

   "a & b" means bit-wise "and" of a and b.

   "a | b" means bit-wise "or" of a and b.

   "a >> b" means arithmetic right shift of two's complement integer
   representation of a by b binary digits.

   "a << b" means arithmetic left shift of two's complement integer
   representation of a by b binary digits.





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2.2.2.  Assignment operators

   "a = b" means a is assigned b.

   "a++" is equivalent to a is assigned a + 1.

   "a--" is equivalent to a is assigned a - 1.

   "a += b" is equivalent to a is assigned a + b.

   "a -= b" is equivalent to a is assigned a - b.

   "a *= b" is equivalent to a is assigned a * b.

2.2.3.  Comparison operators

   "a > b" means a is greater than b.

   "a >= b" means a is greater than or equal to b.

   "a < b" means a is less than b.

   "a <= b" means a is less than or equal b.

   "a == b" means a is equal to b.

   "a != b" means a is not equal to b.

   "a && b" means Boolean logical "and" of a and b.

   "a || b" means Boolean logical "or" of a and b.

   "!a" means Boolean logical "not" of a.

   "a ? b : c" if a is true, then b, otherwise c.

2.2.4.  Mathematical functions

   floor(a) the largest integer less than or equal to a

   ceil(a) the largest integer less than or equal to a

   sign(a) extracts the sign of a number, i.e. if a < 0 then -1, else if
   a > 0 then 1, else 0

   abs(a) the absolute value of a, i.e. abs(a) = sign(a)*a

   log2(a) the base-two logarithm of a



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   min(a,b) the smallest of two values a and b

   max(a,b) the largest of two values a and b

   median(a,b,c) the numerical middle value in a data set of a, b, and
   c, i.e. a+b+c-min(a,b,c)-max(a,b,c)

   a_{b} the b-th value of a sequence of a

   a_{b,c} the 'b,c'-th value of a sequence of a

2.2.5.  Order of operation precedence

   When order of precedence is not indicated explicitly by use of
   parentheses, operations are evaluated in the following order (from
   top to bottom, operations of same precedence being evaluated from
   left to right).  This order of operations is based on the order of
   operations used in Standard C.

                       a++, a--
                       !a, -a
                       a * b, a / b, a % b
                       a + b, a - b
                       a << b, a >> b
                       a < b, a <= b, a > b, a >= b
                       a == b, a != b
                       a & b
                       a | b
                       a && b
                       a || b
                       a ? b : c
                       a = b, a += b, a -= b, a *= b

2.2.6.  Pseudo-code

   Several components of FFV1 are described in this document using
   pseudo-code.  Note that the pseudo-code is used for clarity in order
   to illustrate the structure of FFV1 and not intended to specify any
   particular implementation.  The pseudo-code used is based upon the C
   programming language [ISO.9899.1990] as uses its "if/else", "while"
   and "for" functions as well as functions defined within this
   document.

2.2.7.  Range

   "a...b" means any value starting from a to b, inclusive.





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2.2.8.  NumBytes

   NumBytes is a non-negative integer that expresses the size in 8-bit
   octets of particular FFV1 components such as the Configuration Record
   and Frame.  FFV1 relies on its container to store the NumBytes
   values, see Section 4.1.3.

2.2.9.  Bitstream functions

2.2.9.1.  remaining_bits_in_bitstream

   "remaining_bits_in_bitstream( )" means the count of remaining bits
   after the current position in that bitstream component.  It is
   computed from the NumBytes value multiplied by 8 minus the count of
   bits of that component already read by the bitstream parser.

2.2.9.2.  byte_aligned

   "byte_aligned( )" is true if "remaining_bits_in_bitstream( NumBytes
   )" is a multiple of 8, otherwise false.

3.  General Description

   Samples within a plane are coded in raster scan order (left->right,
   top->bottom).  Each sample is predicted by the median predictor from
   samples in the same plane and the difference is stored see
   Section 3.8.

3.1.  Border

   A border is assumed for each coded slice for the purpose of the
   predictor and context according to the following rules:

   o  one column of samples to the left of the coded slice is assumed as
      identical to the samples of the leftmost column of the coded slice
      shifted down by one row

   o  one column of samples to the right of the coded slice is assumed
      as identical to the samples of the rightmost column of the coded
      slice

   o  an additional column of samples to the left of the coded slice and
      two rows of samples above the coded slice are assumed to be "0"

   The following table depicts a slice of samples "a,b,c,d,e,f,g,h,i"
   along with its assumed border.





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                     +---+---+---+---+---+---+---+---+
                     | 0 | 0 |   | 0 | 0 | 0 |   | 0 |
                     +---+---+---+---+---+---+---+---+
                     | 0 | 0 |   | 0 | 0 | 0 |   | 0 |
                     +---+---+---+---+---+---+---+---+
                     |   |   |   |   |   |   |   |   |
                     +---+---+---+---+---+---+---+---+
                     | 0 | 0 |   | a | b | c |   | c |
                     +---+---+---+---+---+---+---+---+
                     | 0 | a |   | d | e | f |   | f |
                     +---+---+---+---+---+---+---+---+
                     | 0 | d |   | g | h | i |   | i |
                     +---+---+---+---+---+---+---+---+

3.2.  Samples

   Positions used for context and median predictor are:

                             +---+---+---+---+
                             |   |   | T |   |
                             +---+---+---+---+
                             |   |tl | t |tr |
                             +---+---+---+---+
                             | L | l | X |   |
                             +---+---+---+---+

   "X" is the current processed Sample.
   The identifers are made of the first letters of the words Top, Left
   and Right.

3.3.  Median predictor

   The prediction for any sample value at position "X" may be computed
   based upon the relative neighboring values of "l", "t", and "tl" via
   this equation:

   "median(l, t, l + t - tl)".

   Note, this prediction template is also used in [ISO.14495-1.1999] and
   [HuffYUV].

   Exception for the media predictor: if colorspace_type == 0 &&
   bits_per_raw_sample == 16 && ( coder_type == 1 || coder_type == 2 ),
   the following media predictor MUST be used:

   "median(left16s, top16s, left16s + top16s - diag16s)"

   where:



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                left16s = l  >= 32768 ? ( l  - 65536 ) : l
                top16s  = t  >= 32768 ? ( t  - 65536 ) : t
                diag16s = tl >= 32768 ? ( tl - 65536 ) : tl

   Background: a two's complement signed 16-bit signed integer was used
   for storing pixel values in all known implementations of FFV1
   bitstream.  So in some circumstances, the most significant bit was
   wrongly interpreted (used as a sign bit instead of the 16th bit of an
   unsigned integer).  Note that when the issue is discovered, the only
   configuration of all known implementations being impacted is 16-bit
   YCbCr color space with Range Coder coder, as other potentially
   impacted configurations (e.g. 15/16-bit JPEG2000-RCT color space with
   Range Coder coder, or 16-bit any color space with Golomb Rice coder)
   were implemented nowhere.  In the meanwhile, 16-bit JPEG2000-RCT
   color space with Range Coder coder was implemented without this issue
   in one implementation and validated by one conformance checker.  It
   is expected (to be confirmed) to remove this exception for the media
   predictor in the next version of the bitstream.

3.4.  Context

   Relative to any sample "X", the Quantized Sample Differences "L-l",
   "l-tl", "tl-t", "T-t", and "t-tr" are used as context:

                         context = Q_{0}[l - tl] +
                                   Q_{1}[tl - t] +
                                   Q_{2}[t - tr] +
                                   Q_{3}[L - l]  +
                                   Q_{4}[T - t]

   If "context >= 0" then "context" is used and the difference between
   the sample and its predicted value is encoded as is, else "-context"
   is used and the difference between the sample and its predicted value
   is encoded with a flipped sign.

3.5.  Quantization Table Sets

   The bitstream contains 1 or more Quantization Table Sets.
   Each Quantization Table Set contains exactly 5 Quantization Tables,
   each Quantization Table corresponding to 1 of the 5 Quantized Sample
   Differences.
   For each Quantization Table, both the number of quantization steps
   and their distribution are stored in the bitstream; each Quantization
   Table has exactly 256 entries, and the 8 least significant bits of
   the Quantized Sample Difference are used as index:

                   Q_{j}[k] = quant_tables[i][j][k&255]




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   In this formula, "i" is the Quantization Table Set index, "j" is the
   Quantized Table index, "k" the Quantized Sample Difference.

3.6.  Quantization Table Set indexes

   For each plane of each slice, a Quantization Table Set is selected
   from an index:

   o  For Y plane, "quant_table_set_index [ 0 ]" index is used

   o  For Cb and Cr planes, "quant_table_set_index [ 1 ]" index is used

   o  For Alpha plane, "quant_table_set_index [ (version <= 3 ||
      chroma_planes) ? 2 : 1 ]" index is used

   Background: in first implementations of FFV1 bitstream, the index for
   Cb and Cr planes was stored even if it is not used (chroma_planes set
   to 0), this index is kept for version <= 3 in order to keep
   compatibility with bitstreams in the wild.

3.7.  Color space

   FFV1 supports two color spaces: YCbCr and JPEG2000-RCT.  Both color
   spaces allow an optional Alpha plane that can be used to code
   transparency data.

3.7.1.  YCbCr

   In YCbCr color space, the Cb and Cr planes are optional, but if used
   then MUST be used together.  Omitting the Cb and Cr planes codes the
   frames in grayscale without color data.  An FFV1 frame using YCbCr
   MUST use one of the following arrangements:

   o  Y

   o  Y, Alpha

   o  Y, Cb, Cr

   o  Y, Cb, Cr, Alpha

   When FFV1 uses the YCbCr color space, the Y plane MUST be coded
   first.  If the Cb and Cr planes are used then they MUST be coded
   after the Y plane.  If an Alpha (transparency) plane is used, then it
   MUST be coded last.






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3.7.2.  JPEG2000-RCT

   JPEG2000-RCT is a Reversible Color Transform that codes RGB (red,
   green, blue) planes losslessly in a modified YCbCr color space.
   Reversible conversions between YCbCr and RGB use the following
   formulae.

                                  Cb=b-g

                                  Cr=r-g

                              Y=g+(Cb+Cr)>>2

                              g=Y-(Cb+Cr)>>2

                                  r=Cr+g

                                  b=Cb+g

   Exception for the reversible conversions between YCbCr and RGB: if
   bits_per_raw_sample is between 9 and 15 inclusive, the following
   formulae for reversible conversions between YCbCr and RGB MUST be
   used instead of the ones above:

                                  Cb=g-b

                                  Cr=r-b

                              Y=b+(Cb+Cr)>>2

                              b=Y-(Cb+Cr)>>2

                                  r=Cr+b

                                  g=Cb+b

   Background: At the time of this writing, in all known implementations
   of FFV1 bitstream, when bits_per_raw_sample was between 9 and 15
   inclusive, GBR planes were used as BGR planes during both encoding
   and decoding.  In the meanwhile, 16-bit JPEG2000-RCT color space was
   implemented without this issue in one implementation and validated by
   one conformance checker.  Methods to address this exception for the
   transform are under consideration for the next version of the
   bitstream.

   [ISO.15444-1.2016]





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   An FFV1 frame using JPEG2000-RCT MUST use one of the following
   arrangements:

   o  Y, Cb, Cr

   o  Y, Cb, Cr, Alpha

   When FFV1 uses the JPEG2000-RCT color space, the horizontal lines are
   interleaved to improve caching efficiency since it is most likely
   that the RCT will immediately be converted to RGB during decoding.
   The interleaved coding order is also Y, then Cb, then Cr, and then if
   used Alpha.

   As an example, a frame that is two pixels wide and two pixels high,
   could be comprised of the following structure:

            +------------------------+------------------------+
            | Pixel[1,1]             | Pixel[2,1]             |
            | Y[1,1] Cb[1,1] Cr[1,1] | Y[2,1] Cb[2,1] Cr[2,1] |
            +------------------------+------------------------+
            | Pixel[1,2]             | Pixel[2,2]             |
            | Y[1,2] Cb[1,2] Cr[1,2] | Y[2,2] Cb[2,2] Cr[2,2] |
            +------------------------+------------------------+

   In JPEG2000-RCT color space, the coding order would be left to right
   and then top to bottom, with values interleaved by lines and stored
   in this order:

   Y[1,1] Y[2,1] Cb[1,1] Cb[2,1] Cr[1,1] Cr[2,1] Y[1,2] Y[2,2] Cb[1,2]
   Cb[2,2] Cr[1,2] Cr[2,2]

3.8.  Coding of the Sample Difference

   Instead of coding the n+1 bits of the Sample Difference with Huffman
   or Range coding (or n+2 bits, in the case of RCT), only the n (or
   n+1) least significant bits are used, since this is sufficient to
   recover the original sample.  In the equation below, the term "bits"
   represents bits_per_raw_sample+1 for RCT or bits_per_raw_sample
   otherwise:

    coder_input =
        [(sample_difference + 2^(bits-1)) & (2^bits - 1)] - 2^(bits-1)

3.8.1.  Range coding mode

   Early experimental versions of FFV1 used the CABAC Arithmetic coder
   from H.264 as defined in [ISO.14496-10.2014] but due to the uncertain
   patent/royalty situation, as well as its slightly worse performance,



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   CABAC was replaced by a Range coder based on an algorithm defined by
   _G.  Nigel_ and _N.  Martin_ in 1979 [range-coding].

3.8.1.1.  Range binary values

   To encode binary digits efficiently a Range coder is used.  "C_{i}"
   is the i-th Context.  "B_{i}" is the i-th byte of the bytestream.
   "b_{i}" is the i-th Range coded binary value, "S_{0,i}" is the i-th
   initial state, which is 128.  The length of the bytestream encoding n
   binary symbols is "j_{n}" bytes.

              r_{i} = floor( ( R_{i} * S_{i,C_{i}} ) / 2^8 )

               S_{i+1,C_{i}} =  zero_state_{S_{i,C_{i}}} XOR
                         l_i =  L_i                     XOR
                         t_i =  R_i - r_i               <==
                         b_i =  0                       <==>
                         L_i <  R_i - r_i

               S_{i+1,C_{i}} =  one_state_{S_{i,C_{i}}}  XOR
                         l_i =  L_i - R_i + r_i         XOR
                         t_i =  r_i                     <==
                         b_i =  1                       <==>
                         L_i >= R_i - r_i

                     S_{i+1,k} = S_{i,k} <== C_i != k

               R_{i+1} =  2^8 * t_{i}                   XOR
               L_{i+1} =  2^8 * l_{i} + B_{j_{i}}       XOR
               j_{i+1} =  j_{i} + 1                     <==
               t_{i}   <  2^8

               R_{i+1} =  t_{i}                         XOR
               L_{i+1} =  l_{i}                         XOR
               j_{i+1} =  j_{i}                         <==
               t_{i}   >= 2^8

                               R_{0} = 65280

                        L_{0} = 2^8 * B_{0} + B_{1}

                                 j_{0} = 2

3.8.1.2.  Range non binary values

   To encode scalar integers, it would be possible to encode each bit
   separately and use the past bits as context.  However that would mean
   255 contexts per 8-bit symbol which is not only a waste of memory but



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   also requires more past data to reach a reasonably good estimate of
   the probabilities.  Alternatively assuming a Laplacian distribution
   and only dealing with its variance and mean (as in Huffman coding)
   would also be possible, however, for maximum flexibility and
   simplicity, the chosen method uses a single symbol to encode if a
   number is 0 and if not encodes the number using its exponent,
   mantissa and sign.  The exact contexts used are best described by the
   following code, followed by some comments.

   pseudo-code                                                   | type
   --------------------------------------------------------------|-----
   void put_symbol(RangeCoder *c, uint8_t *state, int v, int \   |
   is_signed) {                                                  |
       int i;                                                    |
       put_rac(c, state+0, !v);                                  |
       if (v) {                                                  |
           int a= abs(v);                                        |
           int e= log2(a);                                       |
                                                                 |
           for (i=0; i<e; i++)                                   |
               put_rac(c, state+1+min(i,9), 1);  //1..10         |
                                                                 |
           put_rac(c, state+1+min(i,9), 0);                      |
           for (i=e-1; i>=0; i--)                                |
               put_rac(c, state+22+min(i,9), (a>>i)&1); //22..31 |
                                                                 |
           if (is_signed)                                        |
               put_rac(c, state+11 + min(e, 10), v < 0); //11..21|
       }                                                         |
   }                                                             |

3.8.1.3.  Initial values for the context model

   At keyframes all Range coder state variables are set to their initial
   state.

3.8.1.4.  State transition table

     one_state_{i} =
            default_state_transition_{i} + state_transition_delta_{i}

                 zero_state_{i} = 256 - one_state_{256-i}

3.8.1.5.  default_state_transition







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       0,  0,  0,  0,  0,  0,  0,  0, 20, 21, 22, 23, 24, 25, 26, 27,

      28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 37, 38, 39, 40, 41, 42,

      43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 56, 57,

      58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73,

      74, 75, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88,

      89, 90, 91, 92, 93, 94, 94, 95, 96, 97, 98, 99,100,101,102,103,

     104,105,106,107,108,109,110,111,112,113,114,114,115,116,117,118,

     119,120,121,122,123,124,125,126,127,128,129,130,131,132,133,133,

     134,135,136,137,138,139,140,141,142,143,144,145,146,147,148,149,

     150,151,152,152,153,154,155,156,157,158,159,160,161,162,163,164,

     165,166,167,168,169,170,171,171,172,173,174,175,176,177,178,179,

     180,181,182,183,184,185,186,187,188,189,190,190,191,192,194,194,

     195,196,197,198,199,200,201,202,202,204,205,206,207,208,209,209,

     210,211,212,213,215,215,216,217,218,219,220,220,222,223,224,225,

     226,227,227,229,229,230,231,232,234,234,235,236,237,238,239,240,

     241,242,243,244,245,246,247,248,248,  0,  0,  0,  0,  0,  0,  0,

3.8.1.6.  alternative state transition table

   The alternative state transition table has been built using iterative
   minimization of frame sizes and generally performs better than the
   default.  To use it, the coder_type MUST be set to 2 and the
   difference to the default MUST be stored in the parameters.  The
   reference implementation of FFV1 in FFmpeg uses this table by default
   at the time of this writing when Range coding is used.











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       0, 10, 10, 10, 10, 16, 16, 16, 28, 16, 16, 29, 42, 49, 20, 49,

      59, 25, 26, 26, 27, 31, 33, 33, 33, 34, 34, 37, 67, 38, 39, 39,

      40, 40, 41, 79, 43, 44, 45, 45, 48, 48, 64, 50, 51, 52, 88, 52,

      53, 74, 55, 57, 58, 58, 74, 60,101, 61, 62, 84, 66, 66, 68, 69,

      87, 82, 71, 97, 73, 73, 82, 75,111, 77, 94, 78, 87, 81, 83, 97,

      85, 83, 94, 86, 99, 89, 90, 99,111, 92, 93,134, 95, 98,105, 98,

     105,110,102,108,102,118,103,106,106,113,109,112,114,112,116,125,

     115,116,117,117,126,119,125,121,121,123,145,124,126,131,127,129,

     165,130,132,138,133,135,145,136,137,139,146,141,143,142,144,148,

     147,155,151,149,151,150,152,157,153,154,156,168,158,162,161,160,

     172,163,169,164,166,184,167,170,177,174,171,173,182,176,180,178,

     175,189,179,181,186,183,192,185,200,187,191,188,190,197,193,196,

     197,194,195,196,198,202,199,201,210,203,207,204,205,206,208,214,

     209,211,221,212,213,215,224,216,217,218,219,220,222,228,223,225,

     226,224,227,229,240,230,231,232,233,234,235,236,238,239,237,242,

     241,243,242,244,245,246,247,248,249,250,251,252,252,253,254,255,

3.8.2.  Huffman coding mode

   This coding mode uses Golomb Rice codes.  The VLC code is split into
   2 parts, the prefix stores the most significant bits, the suffix
   stores the k least significant bits or stores the whole number in the
   ESC case.  The end of the bitstream (of the frame) is filled with
   0-bits until that the bitstream contains a multiple of 8 bits.

3.8.2.1.  Prefix










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                        +----------------+-------+
                        | bits           | value |
                        +----------------+-------+
                        | 1              | 0     |
                        | 01             | 1     |
                        | ...            | ...   |
                        | 0000 0000 0001 | 11    |
                        | 0000 0000 0000 | ESC   |
                        +----------------+-------+

3.8.2.2.  Suffix

   +-------+-----------------------------------------------------------+
   | non   | the k least significant bits MSB first                    |
   | ESC   |                                                           |
   | ESC   | the value - 11, in MSB first order, ESC may only be used  |
   |       | if the value cannot be coded as non ESC                   |
   +-------+-----------------------------------------------------------+

3.8.2.3.  Examples

                 +-----+-------------------------+-------+
                 |  k  | bits                    | value |
                 +-----+-------------------------+-------+
                 |  0  | "1"                     |     0 |
                 |  0  | "001"                   |     2 |
                 |  2  | "1 00"                  |     0 |
                 |  2  | "1 10"                  |     2 |
                 |  2  | "01 01"                 |     5 |
                 | any | "000000000000 10000000" |   139 |
                 +-----+-------------------------+-------+

3.8.2.4.  Run mode

   Run mode is entered when the context is 0 and left as soon as a non-0
   difference is found.  The level is identical to the predicted one.
   The run and the first different level is coded.

3.8.2.5.  Run length coding

   The run value is encoded in 2 parts, the prefix part stores the more
   significant part of the run as well as adjusting the run_index which
   determines the number of bits in the less significant part of the
   run.  The 2nd part of the value stores the less significant part of
   the run as it is.  The run_index is reset for each plane and slice to
   0.





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   pseudo-code                                                   | type
   --------------------------------------------------------------|-----
   log2_run[41]={                                                |
    0, 0, 0, 0, 1, 1, 1, 1,                                      |
    2, 2, 2, 2, 3, 3, 3, 3,                                      |
    4, 4, 5, 5, 6, 6, 7, 7,                                      |
    8, 9,10,11,12,13,14,15,                                      |
   16,17,18,19,20,21,22,23,                                      |
   24,                                                           |
   };                                                            |
                                                                 |
   if (run_count == 0 && run_mode == 1) {                        |
       if (get_bits1()) {                                        |
           run_count = 1 << log2_run[run_index];                 |
           if (x + run_count <= w)                               |
               run_index++;                                      |
       } else {                                                  |
           if (log2_run[run_index])                              |
               run_count = get_bits(log2_run[run_index]);        |
           else                                                  |
               run_count = 0;                                    |
           if (run_index)                                        |
               run_index--;                                      |
           run_mode = 2;                                         |
       }                                                         |
   }                                                             |

   The log2_run function is also used within [ISO.14495-1.1999].

3.8.2.6.  Level coding

   Level coding is identical to the normal difference coding with the
   exception that the 0 value is removed as it cannot occur:

                              if (diff>0) diff--;
                              encode(diff);

   Note, this is different from JPEG-LS, which doesn't use prediction in
   run mode and uses a different encoding and context model for the last
   difference On a small set of test samples the use of prediction
   slightly improved the compression rate.

4.  Bitstream








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   +--------+----------------------------------------------------------+
   | Symbol |                        Definition                        |
   +--------+----------------------------------------------------------+
   |  u(n)  |         unsigned big endian integer using n bits         |
   |   sg   |  Golomb Rice coded signed scalar symbol coded with the   |
   |        |            method described in Section 3.8.2             |
   |   br   |    Range coded Boolean (1-bit) symbol with the method    |
   |        |               described in Section 3.8.1.1               |
   |   ur   | Range coded unsigned scalar symbol coded with the method |
   |        |               described in Section 3.8.1.2               |
   |   sr   |  Range coded signed scalar symbol coded with the method  |
   |        |               described in Section 3.8.1.2               |
   +--------+----------------------------------------------------------+

   The same context which is initialized to 128 is used for all fields
   in the header.

   The following MUST be provided by external means during
   initialization of the decoder:

   "frame_pixel_width" is defined as frame width in pixels.

   "frame_pixel_height" is defined as frame height in pixels.

   Default values at the decoder initialization phase:

   "ConfigurationRecordIsPresent" is set to 0.

4.1.  Configuration Record

   In the case of a bitstream with "version >= 3", a Configuration
   Record is stored in the underlying container, at the track header
   level.  It contains the parameters used for all frames.  The size of
   the Configuration Record, NumBytes, is supplied by the underlying
   container.

   pseudo-code                                                   | type
   --------------------------------------------------------------|-----
   ConfigurationRecord( NumBytes ) {                             |
       ConfigurationRecordIsPresent = 1                          |
       Parameters( )                                             |
       while( remaining_bits_in_bitstream( NumBytes ) > 32 )     |
           reserved_for_future_use                               | u(1)
       configuration_record_crc_parity                           | u(32)
   }                                                             |






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4.1.1.  reserved_for_future_use

   "reserved_for_future_use" has semantics that are reserved for future
   use.  Encoders conforming to this version of this specification
   SHALL NOT write this value.  Decoders conforming to this version of
   this specification SHALL ignore its value.

4.1.2.  configuration_record_crc_parity

   "configuration_record_crc_parity" 32 bits that are chosen so that the
   Configuration Record as a whole has a crc remainder of 0.  This is
   equivalent to storing the crc remainder in the 32-bit parity.  The
   CRC generator polynomial used is the standard IEEE CRC polynomial
   (0x104C11DB7) with initial value 0.

4.1.3.  Mapping FFV1 into Containers

   This Configuration Record can be placed in any file format supporting
   Configuration Records, fitting as much as possible with how the file
   format uses to store Configuration Records.  The Configuration Record
   storage place and NumBytes are currently defined and supported by
   this version of this specification for the following container
   formats:

4.1.3.1.  In AVI File Format

   The Configuration Record extends the stream format chunk ("AVI ",
   "hdlr", "strl", "strf") with the ConfigurationRecord bitstream.  See
   [AVI] for more information about chunks.

   "NumBytes" is defined as the size, in bytes, of the strf chunk
   indicated in the chunk header minus the size of the stream format
   structure.

4.1.3.2.  In ISO/IEC 14496-12 (MP4 File Format)

   The Configuration Record extends the sample description box ("moov",
   "trak", "mdia", "minf", "stbl", "stsd") with a "glbl" box which
   contains the ConfigurationRecord bitstream.  See [ISO.14496-12.2015]
   for more information about boxes.

   "NumBytes" is defined as the size, in bytes, of the "glbl" box
   indicated in the box header minus the size of the box header.








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4.1.3.3.  In NUT File Format

   The codec_specific_data element (in "stream_header" packet) contains
   the ConfigurationRecord bitstream.  See [NUT] for more information
   about elements.

   "NumBytes" is defined as the size, in bytes, of the
   codec_specific_data element as indicated in the "length" field of
   codec_specific_data

4.1.3.4.  In Matroska File Format

   FFV1 SHOULD use "V_FFV1" as the Matroska "Codec ID".  For FFV1
   versions 2 or less, the Matroska "CodecPrivate" Element SHOULD NOT be
   used.  For FFV1 versions 3 or greater, the Matroska "CodecPrivate"
   Element MUST contain the FFV1 Configuration Record structure and no
   other data.  See [Matroska] for more information about elements.

4.2.  Frame

   A frame consists of the keyframe field, parameters (if version <=1),
   and a sequence of independent slices.

   pseudo-code                                                   | type
   --------------------------------------------------------------|-----
   Frame( NumBytes ) {                                           |
       keyframe                                                  | br
       if (keyframe && !ConfigurationRecordIsPresent             |
           Parameters( )                                         |
       while ( remaining_bits_in_bitstream( NumBytes ) )         |
           Slice( )                                              |
   }                                                             |

4.3.  Slice

   pseudo-code                                                   | type
   --------------------------------------------------------------|-----
   Slice( ) {                                                    |
       if (version >= 3)                                         |
           SliceHeader( )                                        |
       SliceContent( )                                           |
       if (coder_type == 0)                                      |
           while (!byte_aligned())                               |
               padding                                           | u(1)
       if (version >= 3)                                         |
           SliceFooter( )                                        |
   }                                                             |




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   "padding" specifies a bit without any significance and used only for
   byte alignment.  MUST be 0.

4.4.  Slice Header

   pseudo-code                                                   | type
   --------------------------------------------------------------|-----
   SliceHeader( ) {                                              |
       slice_x                                                   | ur
       slice_y                                                   | ur
       slice_width - 1                                           | ur
       slice_height - 1                                          | ur
       for( i = 0; i < quant_table_set_index_count; i++ )        |
           quant_table_set_index [ i ]                           | ur
       picture_structure                                         | ur
       sar_num                                                   | ur
       sar_den                                                   | ur
       if (version >= 4) {                                       |
           reset_contexts                                        | br
           slice_coding_mode                                     | ur
      }                                                          |
   }                                                             |

4.4.1.  slice_x

   "slice_x" indicates the x position on the slice raster formed by
   num_h_slices.  Inferred to be 0 if not present.

4.4.2.  slice_y

   "slice_y" indicates the y position on the slice raster formed by
   num_v_slices.  Inferred to be 0 if not present.

4.4.3.  slice_width

   "slice_width" indicates the width on the slice raster formed by
   num_h_slices.  Inferred to be 1 if not present.

4.4.4.  slice_height

   "slice_height" indicates the height on the slice raster formed by
   num_v_slices.  Inferred to be 1 if not present.

4.4.5.  quant_table_set_index_count

   "quant_table_set_index_count" is defined as 1 + ( ( chroma_planes ||
   version <= 3 ) ? 1 : 0 ) + ( alpha_plane ? 1 : 0 ).




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4.4.6.  quant_table_set_index

   "quant_table_set_index" indicates the Quantization Table Set index to
   select the Quantization Table Set and the initial states for the
   slice.  Inferred to be 0 if not present.

4.4.7.  picture_structure

   "picture_structure" specifies the picture structure.  Inferred to be
   0 if not present.

                    +-------+-------------------------+
                    | value | picture structure used  |
                    +-------+-------------------------+
                    |   0   | unknown                 |
                    |   1   | top field first         |
                    |   2   | bottom field first      |
                    |   3   | progressive             |
                    | Other | reserved for future use |
                    +-------+-------------------------+

4.4.8.  sar_num

   "sar_num" specifies the sample aspect ratio numerator.  Inferred to
   be 0 if not present.  MUST be 0 if sample aspect ratio is unknown.

4.4.9.  sar_den

   "sar_den" specifies the sample aspect ratio numerator.  Inferred to
   be 0 if not present.  MUST be 0 if sample aspect ratio is unknown.

4.4.10.  reset_contexts

   "reset_contexts" indicates if slice contexts must be reset.  Inferred
   to be 0 if not present.

4.4.11.  slice_coding_mode

   "slice_coding_mode" indicates the slice coding mode.  Inferred to be
   0 if not present.

                  +-------+----------------------------+
                  | value | slice coding mode          |
                  +-------+----------------------------+
                  |   0   | normal Range Coding or VLC |
                  |   1   | raw PCM                    |
                  | Other | reserved for future use    |
                  +-------+----------------------------+



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4.5.  Slice Content

   pseudo-code                                                   | type
   --------------------------------------------------------------|-----
   SliceContent( ) {                                             |
       if (colorspace_type == 0) {                               |
           for( p = 0; p < primary_color_count; p++ ) {          |
               for( y = 0; y < plane_pixel_height[ p ]; y++ )    |
                   Line( p, y )                                  |
       } else if (colorspace_type == 1) {                        |
           for( y = 0; y < slice_pixel_height; y++ )             |
               for( p = 0; p < primary_color_count; p++ ) {      |
                   Line( p, y )                                  |
       }                                                         |
   }                                                             |

4.5.1.  primary_color_count

   "primary_color_count" is defined as 1 + ( chroma_planes ? 2 : 0 ) + (
   alpha_plane ? 1 : 0 ).

4.5.2.  plane_pixel_height

   "plane_pixel_height[ p ]" is the height in pixels of plane p of the
   slice.  "plane_pixel_height[ 0 ]" and "plane_pixel_height[ 1 + (
   chroma_planes ? 2 : 0 ) ]" value is "slice_pixel_height".  If
   "chroma_planes" is set to 1, "plane_pixel_height[ 1 ]" and
   "plane_pixel_height[ 2 ]" value is "ceil(slice_pixel_height /
   v_chroma_subsample)".

4.5.3.  slice_pixel_height

   "slice_pixel_height" is the height in pixels of the slice.  Its value
   is "floor(( slice_y + slice_height ) * slice_pixel_height /
   num_v_slices) - slice_pixel_y".

4.5.4.  slice_pixel_y

   "slice_pixel_y" is the slice vertical position in pixels.  Its value
   is "floor(slice_y * frame_pixel_height / num_v_slices)".

4.6.  Line









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   pseudo-code                                                   | type
   --------------------------------------------------------------|-----
   Line( p, y ) {                                                |
       if (colorspace_type == 0) {                               |
           for( x = 0; x < plane_pixel_width[ p ]; x++ )         |
               Pixel( p, y, x )                                  |
       } else if (colorspace_type == 1) {                        |
           for( x = 0; x < slice_pixel_width; x++ )              |
               Pixel( p, y, x )                                  |
       }                                                         |
   }                                                             |

4.6.1.  plane_pixel_width

   "plane_pixel_width[ p ]" is the width in pixels of plane p of the
   slice.  "plane_pixel_width[ 0 ]" and "plane_pixel_width[ 1 + (
   chroma_planes ? 2 : 0 ) ]" value is "slice_pixel_width".  If
   "chroma_planes" is set to 1, "plane_pixel_width[ 1 ]" and
   "plane_pixel_width[ 2 ]" value is "ceil(slice_pixel_width /
   v_chroma_subsample)".

4.6.2.  slice_pixel_width

   "slice_pixel_width" is the width in pixels of the slice.  Its value
   is "floor(( slice_x + slice_width ) * slice_pixel_width /
   num_h_slices) - slice_pixel_x".

4.6.3.  slice_pixel_x

   "slice_pixel_x" is the slice horizontal position in pixels.  Its
   value is "floor(slice_x * frame_pixel_width / num_h_slices)".

4.7.  Slice Footer

   Note: slice footer is always byte aligned.

   pseudo-code                                                   | type
   --------------------------------------------------------------|-----
   SliceFooter( ) {                                              |
       slice_size                                                | u(24)
       if (ec) {                                                 |
           error_status                                          | u(8)
           slice_crc_parity                                      | u(32)
       }                                                         |
   }                                                             |






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4.7.1.  slice_size

   "slice_size" indicates the size of the slice in bytes.  Note: this
   allows finding the start of slices before previous slices have been
   fully decoded.  And allows this way parallel decoding as well as
   error resilience.

4.7.2.  error_status

   "error_status" specifies the error status.

             +-------+--------------------------------------+
             | value | error status                         |
             +-------+--------------------------------------+
             |   0   | no error                             |
             |   1   | slice contains a correctable error   |
             |   2   | slice contains a uncorrectable error |
             | Other | reserved for future use              |
             +-------+--------------------------------------+

4.7.3.  slice_crc_parity

   "slice_crc_parity" 32 bits that are chosen so that the slice as a
   whole has a crc remainder of 0.  This is equivalent to storing the
   crc remainder in the 32-bit parity.  The CRC generator polynomial
   used is the standard IEEE CRC polynomial (0x104C11DB7) with initial
   value 0.

4.8.  Parameters






















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   pseudo-code                                                   | type
   --------------------------------------------------------------|-----
   Parameters( ) {                                               |
       version                                                   | ur
       if (version >= 3)                                         |
           micro_version                                         | ur
       coder_type                                                | ur
       if (coder_type > 1)                                       |
           for (i = 1; i < 256; i++)                             |
               state_transition_delta[ i ]                       | sr
       colorspace_type                                           | ur
       if (version >= 1)                                         |
           bits_per_raw_sample                                   | ur
       chroma_planes                                             | br
       log2( h_chroma_subsample )                                | ur
       log2( v_chroma_subsample )                                | ur
       alpha_plane                                               | br
       if (version >= 3) {                                       |
           num_h_slices - 1                                      | ur
           num_v_slices - 1                                      | ur
           quant_table_set_count                                 | ur
       }                                                         |
       for( i = 0; i < quant_table_set_count; i++ )              |
           QuantizationTableSet( i )                             |
       if (version >= 3) {                                       |
           for( i = 0; i < quant_table_set_count; i++ ) {        |
               states_coded                                      | br
               if (states_coded)                                 |
                   for( j = 0; j < context_count[ i ]; j++ )     |
                       for( k = 0; k < CONTEXT_SIZE; k++ )       |
                           initial_state_delta[ i ][ j ][ k ]    | sr
           }                                                     |
           ec                                                    | ur
           intra                                                 | ur
       }                                                         |
   }                                                             |

4.8.1.  version

   "version" specifies the version of the bitstream.  Each version is
   incompatible with others versions: decoders SHOULD reject a file due
   to unknown version.  Decoders SHOULD reject a file with version <= 1
   && ConfigurationRecordIsPresent == 1.  Decoders SHOULD reject a file
   with version >= 3 && ConfigurationRecordIsPresent == 0.







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                    +-------+-------------------------+
                    | value | version                 |
                    +-------+-------------------------+
                    | 0     | FFV1 version 0          |
                    | 1     | FFV1 version 1          |
                    | 2     | reserved*               |
                    | 3     | FFV1 version 3          |
                    | Other | reserved for future use |
                    +-------+-------------------------+

   * Version 2 was never enabled in the encoder thus version 2 files
   SHOULD NOT exist, and this document does not describe them to keep
   the text simpler.

4.8.2.  micro_version

   "micro_version" specifies the micro-version of the bitstream.  After
   a version is considered stable (a micro-version value is assigned to
   be the first stable variant of a specific version), each new micro-
   version after this first stable variant is compatible with the
   previous micro-version: decoders SHOULD NOT reject a file due to an
   unknown micro-version equal or above the micro-version considered as
   stable.

   Meaning of micro_version for version 3:

                    +-------+-------------------------+
                    | value | micro_version           |
                    +-------+-------------------------+
                    | 0...3 | reserved*               |
                    |   4   | first stable variant    |
                    | Other | reserved for future use |
                    +-------+-------------------------+

   * were development versions which may be incompatible with the stable
   variants.

   Meaning of micro_version for version 4 (note: at the time of writing
   of this specification, version 4 is not considered stable so the
   first stable version value is to be announced in the future):

                   +---------+-------------------------+
                   |  value  | micro_version           |
                   +---------+-------------------------+
                   | 0...TBA | reserved*               |
                   |   TBA   | first stable variant    |
                   |  Other  | reserved for future use |
                   +---------+-------------------------+



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   * were development versions which may be incompatible with the stable
   variants.

4.8.3.  coder_type

   "coder_type" specifies the coder used

        +-------+-------------------------------------------------+
        | value | coder used                                      |
        +-------+-------------------------------------------------+
        |   0   | Golomb Rice                                     |
        |   1   | Range Coder with default state transition table |
        |   2   | Range Coder with custom state transition table  |
        | Other | reserved for future use                         |
        +-------+-------------------------------------------------+

4.8.4.  state_transition_delta

   "state_transition_delta" specifies the Range coder custom state
   transition table.  If state_transition_delta is not present in the
   bitstream, all Range coder custom state transition table elements are
   assumed to be 0.

4.8.5.  colorspace_type

   "colorspace_type" specifies the color space.

                    +-------+-------------------------+
                    | value | color space used        |
                    +-------+-------------------------+
                    |   0   | YCbCr                   |
                    |   1   | JPEG2000-RCT            |
                    | Other | reserved for future use |
                    +-------+-------------------------+

4.8.6.  chroma_planes

   "chroma_planes" indicates if chroma (color) planes are present.

                 +-------+-------------------------------+
                 | value | color space used              |
                 +-------+-------------------------------+
                 |   0   | chroma planes are not present |
                 |   1   | chroma planes are present     |
                 +-------+-------------------------------+






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4.8.7.  bits_per_raw_sample

   "bits_per_raw_sample" indicates the number of bits for each luma and
   chroma sample.  Inferred to be 8 if not present.

        +-------+-------------------------------------------------+
        | value | bits for each luma and chroma sample            |
        +-------+-------------------------------------------------+
        |   0   | reserved*                                       |
        | Other | the actual bits for each luma and chroma sample |
        +-------+-------------------------------------------------+

   * Encoders MUST NOT store bits_per_raw_sample = 0 Decoders SHOULD
   accept and interpret bits_per_raw_sample = 0 as 8.

4.8.8.  h_chroma_subsample

   "h_chroma_subsample" indicates the subsample factor between luma and
   chroma width ("chroma_width = 2^(-log2_h_chroma_subsample) *
   luma_width").

4.8.9.  v_chroma_subsample

   "v_chroma_subsample" indicates the subsample factor between luma and
   chroma height ("chroma_height=2^(-log2_v_chroma_subsample) *
   luma_height").

4.8.10.  alpha_plane

   alpha_plane
      indicates if a transparency plane is present.

               +-------+-----------------------------------+
               | value | color space used                  |
               +-------+-----------------------------------+
               |   0   | transparency plane is not present |
               |   1   | transparency plane is present     |
               +-------+-----------------------------------+

4.8.11.  num_h_slices

   "num_h_slices" indicates the number of horizontal elements of the
   slice raster.  Inferred to be 1 if not present.








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4.8.12.  num_v_slices

   "num_v_slices" indicates the number of vertical elements of the slice
   raster.  Inferred to be 1 if not present.

4.8.13.  quant_table_set_count

   "quant_table_set_count" indicates the number of Quantization
   Table Sets.  Inferred to be 1 if not present.  MUST NOT be 0.

4.8.14.  states_coded

   "states_coded" indicates if the respective Quantization Table Set has
   the initial states coded.  Inferred to be 0 if not present.

   +-------+-----------------------------------------------------------+
   | value | initial states                                            |
   +-------+-----------------------------------------------------------+
   |   0   | initial states are not present and are assumed to be all  |
   |       | 128                                                       |
   |   1   | initial states are present                                |
   +-------+-----------------------------------------------------------+

4.8.15.  initial_state_delta

   "initial_state_delta" [ i ][ j ][ k ] indicates the initial Range
   coder state, it is encoded using k as context index and pred = j ?
   initial_states[ i ][j - 1][ k ] : 128 initial_state[ i ][ j ][ k ] =
   ( pred + initial_state_delta[ i ][ j ][ k ] ) & 255

4.8.16.  ec

   "ec" indicates the error detection/correction type.

          +-------+--------------------------------------------+
          | value | error detection/correction type            |
          +-------+--------------------------------------------+
          |   0   | 32-bit CRC on the global header            |
          |   1   | 32-bit CRC per slice and the global header |
          | Other | reserved for future use                    |
          +-------+--------------------------------------------+

4.8.17.  intra

   "intra" indicates the relationship between frames.  Inferred to be 0
   if not present.





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   +-------+-----------------------------------------------------------+
   | value | relationship                                              |
   +-------+-----------------------------------------------------------+
   |   0   | frames are independent or dependent (keyframes and non    |
   |       | keyframes)                                                |
   |   1   | frames are independent (keyframes only)                   |
   | Other | reserved for future use                                   |
   +-------+-----------------------------------------------------------+

4.9.  Quantization Table Set

   The Quantization Table Sets are stored by storing the number of equal
   entries -1 of the first half of the table (represented as "len - 1"
   in the pseudo-code below) using the method described in
   Section 3.8.1.2.  The second half doesn't need to be stored as it is
   identical to the first with flipped sign.

   example:

   Table: 0 0 1 1 1 1 2 2 -2 -2 -2 -1 -1 -1 -1 0

   Stored values: 1, 3, 1

   pseudo-code                                                   | type
   --------------------------------------------------------------|-----
   QuantizationTableSet( i ) {                                   |
       scale = 1                                                 |
       for( j = 0; j < MAX_CONTEXT_INPUTS; j++ ) {               |
           QuantizationTable( i, j, scale )                      |
           scale *= 2 * len_count[ i ][ j ] - 1                  |
       }                                                         |
       context_count[ i ] = ( scale + 1 ) / 2                    |
   }                                                             |

   MAX_CONTEXT_INPUTS is 5.
















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   pseudo-code                                                   | type
   --------------------------------------------------------------|-----
   QuantizationTable(i, j, scale) {                              |
       v = 0                                                     |
       for( k = 0; k < 128; ) {                                  |
           len - 1                                               | ur
           for( a = 0; a < len; a++ ) {                          |
               quant_tables[ i ][ j ][ k ] = scale* v            |
               k++                                               |
           }                                                     |
           v++                                                   |
       }                                                         |
       for( k = 1; k < 128; k++ ) {                              |
           quant_tables[ i ][ j ][ 256 - k ] = \                 |
           -quant_tables[ i ][ j ][ k ]                          |
       }                                                         |
       quant_tables[ i ][ j ][ 128 ] = \                         |
       -quant_tables[ i ][ j ][ 127 ]                            |
       len_count[ i ][ j ] = v                                   |
   }                                                             |

4.9.1.  quant_tables

   "quant_tables[ i ][ j ][ k ]" indicates the quantification table
   value of the Quantized Sample Difference "k" of the Quantization
   Table "j" of the Set Quantization Table Set "i".

4.9.2.  context_count

   "context_count[ i ]" indicates the count of contexts for Quantization
   Table Set "i".

5.  Restrictions

   To ensure that fast multithreaded decoding is possible, starting
   version 3 and if frame_pixel_width * frame_pixel_height is more than
   101376, slice_width * slice_height MUST be less or equal to
   num_h_slices * num_v_slices / 4.  Note: 101376 is the frame size in
   pixels of a 352x288 frame also known as CIF ("Common Intermediate
   Format") frame size format.

   For each frame, each position in the slice raster MUST be filled by
   one and only one slice of the frame (no missing slice position, no
   slice overlapping).

   For each Frame with keyframe value of 0, each slice MUST have the
   same value of slice_x, slice_y, slice_width, slice_height as a slice
   in the previous frame, except if reset_contexts is 1.



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6.  Security Considerations

   Like any other codec, (such as [RFC6716]), FFV1 should not be used
   with insecure ciphers or cipher-modes that are vulnerable to known
   plaintext attacks.  Some of the header bits as well as the padding
   are easily predictable.

   Implementations of the FFV1 codec need to take appropriate security
   considerations into account, as outlined in [RFC4732].  It is
   extremely important for the decoder to be robust against malicious
   payloads.  Malicious payloads must not cause the decoder to overrun
   its allocated memory or to take an excessive amount of resources to
   decode.  Although problems in encoders are typically rarer, the same
   applies to the encoder.  Malicious video streams must not cause the
   encoder to misbehave because this would allow an attacker to attack
   transcoding gateways.  A frequent security problem in image and video
   codecs is also to not check for integer overflows in pixel count
   computations, that is to allocate width * height without considering
   that the multiplication result may have overflowed the arithmetic
   types range.

   The reference implementation [REFIMPL] contains no known buffer
   overflow or cases where a specially crafted packet or video segment
   could cause a significant increase in CPU load.

   The reference implementation [REFIMPL] was validated in the following
   conditions:

   o  Sending the decoder valid packets generated by the reference
      encoder and verifying that the decoder's output matches the
      encoders input.

   o  Sending the decoder packets generated by the reference encoder and
      then subjected to random corruption.

   o  Sending the decoder random packets that are not FFV1.

   In all of the conditions above, the decoder and encoder was run
   inside the [VALGRIND] memory debugger as well as clangs address
   sanitizer [Address-Sanitizer], which track reads and writes to
   invalid memory regions as well as the use of uninitialized memory.
   There were no errors reported on any of the tested conditions.

7.  Appendixes







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7.1.  Decoder implementation suggestions

7.1.1.  Multi-threading support and independence of slices

   The bitstream is parsable in two ways: in sequential order as
   described in this document or with the pre-analysis of the footer of
   each slice.  Each slice footer contains a slice_size field so the
   boundary of each slice is computable without having to parse the
   slice content.  That allows multi-threading as well as independence
   of slice content (a bitstream error in a slice header or slice
   content has no impact on the decoding of the other slices).

   After having checked keyframe field, a decoder SHOULD parse
   slice_size fields, from slice_size of the last slice at the end of
   the frame up to slice_size of the first slice at the beginning of the
   frame, before parsing slices, in order to have slices boundaries.  A
   decoder MAY fallback on sequential order e.g. in case of corrupted
   frame (frame size unknown, slice_size of slices not coherent...) or
   if there is no possibility of seek into the stream.

   Architecture overview of slices in a frame:

    +-----------------------------------------------------------------+
    | first slice header                                              |
    | first slice content                                             |
    | first slice footer                                              |
    | --------------------------------------------------------------- |
    | second slice header                                             |
    | second slice content                                            |
    | second slice footer                                             |
    | --------------------------------------------------------------- |
    | ...                                                             |
    | --------------------------------------------------------------- |
    | last slice header                                               |
    | last slice content                                              |
    | last slice footer                                               |
    +-----------------------------------------------------------------+

8.  Changelog

   See <https://github.com/FFmpeg/FFV1/commits/master>

9.  ToDo

   o  mean,k estimation for the Golomb Rice codes






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10.  References

10.1.  Normative References

   [ISO.15444-1.2016]
              International Organization for Standardization,
              "Information technology -- JPEG 2000 image coding system:
              Core coding system", October 2016.

   [ISO.9899.1990]
              International Organization for Standardization,
              "Programming languages - C", ISO Standard 9899, 1990.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

10.2.  Informative References

   [Address-Sanitizer]
              The Clang Team, "ASAN AddressSanitizer website", undated,
              <https://clang.llvm.org/docs/AddressSanitizer.html>.

   [AVI]      Microsoft, "AVI RIFF File Reference", undated,
              <https://msdn.microsoft.com/en-us/library/windows/desktop/
              dd318189%28v=vs.85%29.aspx>.

   [FFV1_V0]  Niedermayer, M., "Commit to mark FFV1 version 0 as non-
              experimental", April 2006, <https://git.videolan.org/?p=ff
              mpeg.git;a=commit;h=b548f2b91b701e1235608ac882ea6df915167c
              7e>.

   [FFV1_V1]  Niedermayer, M., "Commit to release FFV1 version 1", April
              2009, <https://git.videolan.org/?p=ffmpeg.git;a=commit;h=6
              8f8d33becbd73b4d0aa277f472a6e8e72ea6849>.

   [FFV1_V3]  Niedermayer, M., "Commit to mark FFV1 version 3 as non-
              experimental", August 2013, <https://git.videolan.org/?p=f
              fmpeg.git;a=commit;h=abe76b851c05eea8743f6c899cbe5f7409b0f
              301>.

   [HuffYUV]  Rudiak-Gould, B., "HuffYUV", December 2003,
              <https://web.archive.org/web/20040402121343/
              http://cultact-server.novi.dk/kpo/huffyuv/huffyuv.html>.






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   [ISO.14495-1.1999]
              International Organization for Standardization,
              "Information technology -- Lossless and near-lossless
              compression of continuous-tone still images: Baseline",
              December 1999.

   [ISO.14496-10.2014]
              International Organization for Standardization,
              "Information technology -- Coding of audio-visual objects
              -- Part 10: Advanced Video Coding", September 2014.

   [ISO.14496-12.2015]
              International Organization for Standardization,
              "Information technology -- Coding of audio-visual objects
              -- Part 12: ISO base media file format", December 2015.

   [Matroska]
              IETF, "Matroska", 2016, <https://datatracker.ietf.org/doc/
              draft-lhomme-cellar-matroska/>.

   [NUT]      Niedermayer, M., "NUT Open Container Format", December
              2013, <https://ffmpeg.org/~michael/nut.txt>.

   [range-coding]
              Nigel, G. and N. Martin, "Range encoding: an algorithm for
              removing redundancy from a digitised message.", Proc.
              Institution of Electronic and Radio Engineers
              International Conference on Video and Data Recording ,
              July 1979.

   [REFIMPL]  Niedermayer, M., "The reference FFV1 implementation / the
              FFV1 codec in FFmpeg", undated, <https://ffmpeg.org>.

   [RFC4732]  Handley, M., Ed., Rescorla, E., Ed., and IAB, "Internet
              Denial-of-Service Considerations", RFC 4732,
              DOI 10.17487/RFC4732, December 2006,
              <http://www.rfc-editor.org/info/rfc4732>.

   [RFC6716]  Valin, JM., Vos, K., and T. Terriberry, "Definition of the
              Opus Audio Codec", RFC 6716, DOI 10.17487/RFC6716,
              September 2012, <http://www.rfc-editor.org/info/rfc6716>.

   [VALGRIND]
              Valgrind Developers, "Valgrind website", undated,
              <https://valgrind.org/>.

   [YCbCr]    Wikipedia, "YCbCr", undated, <https://en.wikipedia.org/w/
              index.php?title=YCbCr>.



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Authors' Addresses

   Michael Niedermayer

   Email: michael@niedermayer.cc


   Dave Rice

   Email: dave@dericed.com


   Jerome Martinez

   Email: jerome@mediaarea.net




































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