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Versions: 00 01 02 03

Network Working Group                                        A. Fuldseth
Internet-Draft                                            G. Bjontegaard
Intended status: Standards Track                           S. Midtskogen
Expires: May 4, 2017                                           T. Davies
                                                               M. Zanaty
                                                                   Cisco
                                                        October 31, 2016


                            Thor Video Codec
                      draft-fuldseth-netvc-thor-03

Abstract

   This document provides a high-level description of the Thor video
   codec.  Thor is designed to achieve high compression efficiency with
   moderate complexity, using the well-known hybrid video coding
   approach of motion-compensated prediction and transform coding.

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 May 4, 2017.

Copyright Notice

   Copyright (c) 2016 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  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Definitions . . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.1.  Requirements Language . . . . . . . . . . . . . . . . . .   5
     2.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   6
   3.  Block Structure . . . . . . . . . . . . . . . . . . . . . . .   6
     3.1.  Super Blocks and Coding Blocks  . . . . . . . . . . . . .   6
     3.2.  Special Processing at Frame Boundaries  . . . . . . . . .   7
     3.3.  Transform Blocks  . . . . . . . . . . . . . . . . . . . .   8
     3.4.  Prediction Blocks . . . . . . . . . . . . . . . . . . . .   8
   4.  Intra Prediction  . . . . . . . . . . . . . . . . . . . . . .   8
   5.  Inter Prediction  . . . . . . . . . . . . . . . . . . . . . .   9
     5.1.  Multiple Reference Frames . . . . . . . . . . . . . . . .   9
     5.2.  Bi-Prediction . . . . . . . . . . . . . . . . . . . . . .  10
     5.3.  Improved chroma prediction  . . . . . . . . . . . . . . .  10
     5.4.  Reordered Frames  . . . . . . . . . . . . . . . . . . . .  10
     5.5.  Interpolated Reference Frames . . . . . . . . . . . . . .  10
     5.6.  Sub-Pixel Interpolation . . . . . . . . . . . . . . . . .  10
       5.6.1.  Luma Poly-phase Filter  . . . . . . . . . . . . . . .  10
       5.6.2.  Luma Special Filter Position  . . . . . . . . . . . .  12
       5.6.3.  Chroma Poly-phase Filter  . . . . . . . . . . . . . .  13
     5.7.  Motion Vector Coding  . . . . . . . . . . . . . . . . . .  13
       5.7.1.  Inter0 and Inter1 Modes . . . . . . . . . . . . . . .  13
       5.7.2.  Inter2 and Bi-Prediction Modes  . . . . . . . . . . .  15
       5.7.3.  Motion Vector Direction . . . . . . . . . . . . . . .  16
   6.  Transforms  . . . . . . . . . . . . . . . . . . . . . . . . .  16
   7.  Quantization  . . . . . . . . . . . . . . . . . . . . . . . .  16
     7.1.  Quantization matrices . . . . . . . . . . . . . . . . . .  17
       7.1.1.  Quantization matrix selection . . . . . . . . . . . .  17
       7.1.2.  Quantization matrix design  . . . . . . . . . . . . .  18
   8.  Loop Filtering  . . . . . . . . . . . . . . . . . . . . . . .  18
     8.1.  Deblocking  . . . . . . . . . . . . . . . . . . . . . . .  18
       8.1.1.  Luma deblocking . . . . . . . . . . . . . . . . . . .  18
       8.1.2.  Chroma Deblocking . . . . . . . . . . . . . . . . . .  19
     8.2.  Constrained Low Pass Filter (CLPF)  . . . . . . . . . . .  20
   9.  Entropy coding  . . . . . . . . . . . . . . . . . . . . . . .  20
     9.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .  20
     9.2.  Low Level Syntax  . . . . . . . . . . . . . . . . . . . .  21
       9.2.1.  CB Level  . . . . . . . . . . . . . . . . . . . . . .  21
       9.2.2.  PB Level  . . . . . . . . . . . . . . . . . . . . . .  21
       9.2.3.  TB Level  . . . . . . . . . . . . . . . . . . . . . .  22
       9.2.4.  Super Mode  . . . . . . . . . . . . . . . . . . . . .  22
       9.2.5.  CBP . . . . . . . . . . . . . . . . . . . . . . . . .  23
       9.2.6.  Transform Coefficients  . . . . . . . . . . . . . . .  23



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   10. High Level Syntax . . . . . . . . . . . . . . . . . . . . . .  25
     10.1.  Sequence Header  . . . . . . . . . . . . . . . . . . . .  25
     10.2.  Frame Header . . . . . . . . . . . . . . . . . . . . . .  26
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  27
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  27
   13. Normative References  . . . . . . . . . . . . . . . . . . . .  27
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  27

1.  Introduction

   This document provides a high-level description of the Thor video
   codec.  Thor is designed to achieve high compression efficiency with
   moderate complexity, using the well-known hybrid video coding
   approach of motion-compensated prediction and transform coding.

   The Thor video codec is a block-based hybrid video codec similar in
   structure to widespread standards.  The high level encoder and
   decoder structures are illustrated in Figure 1 and Figure 2
   respectively.
































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                  +---+   +-----------+   +-----------+   +--------+
       Input--+-->| + |-->| Transform |-->| Quantizer |-->| Entropy|
       Video  |   +---+   +-----------+   +-----------+   | Coding |
              |     ^ -                         |         +--------+
              |     |                           v              |
              |     |                     +-----------+        v
              |     |                     |  Inverse  |     Output
              |     |                     | Transform |    Bitstream
              |     |                     +-----------+
              |     |                           |
              |     |                           v
              |     |                         +---+
              |     +------------------------>| + |
              |     |      +-------------+    +---+
              |     |   ___| Intra Frame |      |
              |     |  /   | Prediction  |<-----+
              |     | /    +-------------+      |
              |     |/                          v
              |      \     +-------------+  +---------+
              |       \    | Inter Frame |  |  Loop   |
              |        \___| Prediction  |  | Filters |
              |            +-------------+  +---------+
              |                   ^             |
              |                   |             v
              |            +------------+   +---------------+
              |            |   Motion   |   | Reconstructed |
              +----------->| Estimation |<--| Frame Memory  |
                           +------------+   +---------------+


                        Figure 1: Encoder Structure




















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                       +----------+      +-----------+
        Input  ------->| Entropy  |----->|  Inverse  |
      Bitstream        | Decoding |      | Transform |
                       +----------+      +-----------+
                                               |
                                               v
                                             +---+
                   +------------------------>| + |
                   |      +-------------+    +---+
                   |   ___| Intra Frame |      |
                   |  /   | Prediction  |<-----+
                   | /    +-------------+      |
                   |/                          v
                    \     +-------------+  +---------+
                     \    | Inter Frame |  |  Loop   |
                      \___| Prediction  |  | Filters |
                          +-------------+  +---------+
                                 ^             |-------------> Output
                                 |             v               Video
                        +--------------+   +---------------+
                        |     Motion   |   | Reconstructed |
                        | Compensation |<--| Frame Memory  |
                        +--------------+   +---------------+


                        Figure 2: Decoder Structure

   The remainder of this document is organized as follows.  First, some
   requirements language and terms are defined.  Block structures are
   described in detail, followed by intra-frame prediction techniques,
   inter-frame prediction techniques, transforms, quantization, loop
   filters, entropy coding, and finally high level syntax.

   An open source reference implementation is available at
   github.com/cisco/thor.

2.  Definitions

2.1.  Requirements Language

   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 RFC 2119 [RFC2119].








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2.2.  Terminology

   This document frequently uses the following terms.

      SB: Super Block - 64x64 or 128x128 block (luma pixels) which can
      be divided into CBs.

      CB: Coding Block - Subdivision of a SB, down to 8x8 (luma pixels).

      PB: Prediction Block - Subdivision of a CB, into 1, 2 or 4 equal
      blocks.

      TB: Transform Block - Subdivision of a CB, into 1 or 4 equal
      blocks.

3.  Block Structure

3.1.  Super Blocks and Coding Blocks

   Input frames with bitdepths of 8, 10 or 12 are supported.  The
   internal bitdepth can be 8, 10 or 12 regardless if input bitdepth.
   The bitdepth of the output frames always follows the input frames.
   Chroma can be subsampled in both directions (4:2:0) or have full
   resolution (4:4:4).

   Each frame is divided into 64x64 or 128x128 Super Blocks (SB) which
   are processed in raster-scan order.  The SB size is signaled in the
   sequence header.  Each SB can be divided into Coding Blocks (CB)
   using a quad-tree structure.  The smallest allowed CB size is 8x8
   luma pixels.  The four CBs of a larger block are coded/signaled in
   the following order; upleft, downleft, upright, and downright.

   The following modes are signaled at the CB level:

   o  Intra

   o  Inter0 (skip): MV index, no residual information

   o  Inter1 (merge): MV index, residual information

   o  Inter2 (uni-pred): explicit motion information, residual
      information

   o  Inter3 (ni-pred): explicit motion information, residual
      information






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3.2.  Special Processing at Frame Boundaries

   At frame boundaries some square blocks might not be complete.  For
   example, for 1920x1080 resolutions, the bottom row would consist of
   rectangular blocks of size 64x56.  Rectangular blocks at frame
   boundaries are handled as follows.  For each rectangular block, send
   one bit to choose between:

   o  A rectangular inter0 block and

   o  Further split.

   For the bottom part of a 1920x1080 frame, this implies the following:

   o  For each 64x56 block, transmit one bit to signal a 64x56 inter0
      block or a split into two 32x32 blocks and two 32x24 blocks.

   o  For each 32x24 block, transmit one bit to signal a 32x24 inter0
      block or a split into two 16x16 blocks and two 16x8 blocks.

   o  For each 16x8 block, transmit one bit to signal a 16x8 inter0
      block or a split into two 8x8 blocks.

   Two examples of handling 64x56 blocks at the bottom row of a
   1920x1080 frame are shown in Figure 3 and Figure 4 respectively.


                                          64
                           +-------------------------------+
                           |                               |
                           |                               |
                           |                               |
                           |                               |
                           |                               |
                           |                               |
                           |                               |
                       64  | 56           64x56            |
                           |              SKIP             |
                           |                               |
                           |                               |
                           |                               |
                           |                               |
         - - - - - - - - - + - - - - - - - - - - - - - - - + - - -
         Frame boundary    | 8                             |
                           +-------------------------------+


                  Figure 3: Super block at frame boundary



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                                          64
                           +---------------+---------------+
                           |               |               |
                           |               |               |
                           |               |               |
                           |               |               |
                           |               |               |
                           |               |               |
                           |               |               |
                       64  +---------------+-------+-------+
                           |               |       |       |
                           |               |       |       |
                           |     32x24     |       |       |
                           |     SKIP      +---+---+-------+
                           |               |   |   | 16x8  |
         - - - - - - - - - + - - - - - - - +---+---+ - - - + - - -
         Frame boundary    | 8             |   |   | SKIP  |
                           +---------------+---+---+-------+


                 Figure 4: Coding block at frame boundary

3.3.  Transform Blocks

   A coding block (CB) can be divided into four smaller transform blocks
   (TBs).

3.4.  Prediction Blocks

   A coding block (CB) can also be divided into smaller prediction
   blocks (PBs) for the purpose of motion-compensated prediction.
   Horizontal, vertical and quad split are used.

4.  Intra Prediction

   8 intra prediction modes are used:

   1.  DC

   2.  Vertical (V)

   3.  Horizontal (H)

   4.  Upupright (north-northeast)

   5.  Upupleft (north-northwest)

   6.  Upleft (northwest)



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   7.  Upleftleft (west-northwest)

   8.  Downleftleft (west-southwest)

   The definition of DC, vertical, and horizontal modes are
   straightforward.

   The upleft direction is exactly 45 degrees.

   The upupright, upupleft, and upleftleft directions are equal to
   arctan(1/2) from the horizontal or vertical direction, since they are
   defined by going one pixel horizontally and two pixels vertically (or
   vice versa).

   For the 5 angular intra modes (i.e. angle different from 90 degrees),
   the pixels of the neighbor blocks are filtered before they are used
   for prediction:

   y(n) = (x(n-1) + 2*x(n) + x(n+1) + 2)/4

   For the angular intra modes that are not 45 degrees, the prediction
   sometimes requires sample values at a half-pixel position.  These
   sample values are determined by an additional filter:

   z(n + 1/2) = (y(n) + y(n+1))/2

5.  Inter Prediction

5.1.  Multiple Reference Frames

   Multiple reference frames are currently implemented as follows.

   o  Use a sliding-window process to keep the N most recent
      reconstructed frames in memory.  The value of N is signaled in the
      sequence header.

   o  In the frame header, signal which of these frames shall be active
      for the current frame.

   o  For each CB, signal which of the active frames to be used for MC.

   Combined with re-ordering, this allows for MPEG-1 style B frames.

   A desirable future extension is to allow long-term reference frames
   in addition to the short-term reference frames defined by the
   sliding-window process.





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5.2.  Bi-Prediction

   In case of bi-prediction, two reference indices and two motion
   vectors are signaled per CB.  In the current version, PB-split is not
   allowed in bi-prediction mode.  Sub-pixel interpolation is performed
   for each motion vector/reference index separately before doing an
   average between the two predicted blocks:

   p(x,y) = (p0(x,y) + p1(x,y))/2

5.3.  Improved chroma prediction

   If specified in the sequence header, the chroma prediction, both
   intra and inter, or either, is improved by using the luma
   reconstruction if certain criteria are met.  The process is described
   in the separate CLPF draft [I-D.midtskogen-netvc-chromapred].

5.4.  Reordered Frames

   Frames may be transmitted out of order.  Reference frames are
   selected from the sliding window buffer as normal.

5.5.  Interpolated Reference Frames

   A flag is sent in the sequence header indicating that interpolated
   reference frames may be used.

   If a frame is using an interpolated reference frame, it will be the
   first reference in the reference list, and will be interpolated from
   the second and third reference in the list.  It is indicated by a
   reference index of -1 and has a frame number equal to that of the
   current frame.

   The interpolated reference is created by a deterministic process
   common to the encoder and decoder, and described in the separate
   IRFVC draft [I-D.davies-netvc-irfvc].

5.6.  Sub-Pixel Interpolation

5.6.1.  Luma Poly-phase Filter

   Inter prediction uses traditional block-based motion compensated
   prediction with quarter pixel resolution.  A separable 6-tap poly-
   phase filter is the basis method for doing MC with sub-pixel
   accuracy.  The luma filter coefficients are as follows:

   When bi-prediction is enabled in the sequence header:




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   1/4 phase: [2,-10,59,17,-5,1]/64

   2/4 phase: [1,-8,39,39,-8,1]/64

   3/4 phase: [1,-5,17,59,-10,2]/64

   When bi-prediction is disabled in the sequence header:

   1/4 phase: [1,-7,55,19,-5,1]/64

   2/4 phase: [1,-7,38,38,-7,1]/64

   3/4 phase: [1,-5,19,55,-7,1]/64

   With reference to Figure 5, a fractional sample value, e.g. i0,0
   which has a phase of 1/4 in the horizontal dimension and a phase of
   1/2 in the vertical dimension is calculated as follows:

   a0,j = 2*A-2,i - 10*A-1,i + 59*A0,i + 17*A1,i - 5*A2,i + 1*A3,i

   where j = -2,...,3

   i0,0 = (1*a0,-2 - 8*a0,-1 + 39*a0,0 + 39*a0,1 - 8*a0,2 + 1*a0,3 +
   2048)/4096

   The minimum sub-block size is 8x8.

























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          +-----+-----+-----+-----+-----+-----+-----+-----+-----+
          |A    |     |     |     |A    |a    |b    |c    |A    |
          |-1,-1|     |     |     | 0,-1| 0,-1| 0,-1| 0,-1| 1,-1|
          +-----+-----+-----+-----+-----+-----+-----+-----+-----+
          |     |     |     |     |     |     |     |     |     |
          |     |     |     |     |     |     |     |     |     |
          +-----+-----+-----+-----+-----+-----+-----+-----+-----+
          |     |     |     |     |     |     |     |     |     |
          |     |     |     |     |     |     |     |     |     |
          +-----+-----+-----+-----+-----+-----+-----+-----+-----+
          |     |     |     |     |     |     |     |     |     |
          |     |     |     |     |     |     |     |     |     |
          +-----+-----+-----+-----+-----+-----+-----+-----+-----+
          |A    |     |     |     |A    |a    |b    |c    |A    |
          |-1,0 |     |     |     | 0,0 | 0,0 | 0,0 | 0,0 | 1,0 |
          +-----+-----+-----+-----+-----+-----+-----+-----+-----+
          |d    |     |     |     |d    |e    |f    |g    |d    |
          |-1,0 |     |     |     | 0,0 | 0,0 | 0,0 | 0,0 | 1,0 |
          +-----+-----+-----+-----+-----+-----+-----+-----+-----+
          |h    |     |     |     |h    |i    |j    |k    |h    |
          |-1,0 |     |     |     | 0,0 | 0,0 | 0,0 | 0,0 | 1,0 |
          +-----+-----+-----+-----+-----+-----+-----+-----+-----+
          |l    |     |     |     |l    |m    |n    |o    |l    |
          |-1,0 |     |     |     | 0,0 | 0,0 | 0,0 | 0,0 | 1,0 |
          +-----+-----+-----+-----+-----+-----+-----+-----+-----+
          |A    |     |     |     |A    |a    |b    |c    |A    |
          |-1,1 |     |     |     | 0,1 | 0,1 | 0,1 | 0,1 | 1,1 |
          +-----+-----+-----+-----+-----+-----+-----+-----+-----+



                       Figure 5: Sub-pixel positions

5.6.2.  Luma Special Filter Position

   For the fractional pixel position having exactly 2 quarter pixel
   offsets in each dimension, a non-separable filter is used to
   calculate the interpolated value.  With reference to Figure 5, the
   center position j0,0 is calculated as follows:

   j0,0 =

   [0*A-1,-1 + 1*A0,-1 + 1*A1,-1 + 0*A2,-1 +

   1*A-1,0 + 2*A0,0 + 2*A1,0 + 1*A2,0 +

   1*A-1,1 + 2*A0,1 + 2*A1,1 + 1*A2,1 +




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   0*A-1,2 + 1*A0,2 + 1*A1,2 + 0*A2,2 + 8]/16

5.6.3.  Chroma Poly-phase Filter

   Chroma interpolation is performed with 1/8 pixel resolution using the
   following poly-phase filter.

   1/8 phase: [-2, 58, 10, -2]/64

   2/8 phase: [-4, 54, 16, -2]/64

   3/8 phase: [-4, 44, 28, -4]/64

   4/8 phase: [-4, 36, 36, -4]/64

   5/8 phase: [-4, 28, 44, -4]/64

   6/8 phase: [-2, 16, 54, -4]/64

   7/8 phase: [-2, 10, 58, -2]/64

5.7.  Motion Vector Coding

5.7.1.  Inter0 and Inter1 Modes

   Inter0 and inter1 modes imply signaling of a motion vector index to
   choose a motion vector from a list of candidate motion vectors with
   associated reference frame index.  A list of motion vector candidates
   are derived from at most two different neighbor blocks, each having a
   unique motion vector/reference frame index.  Signaling of the motion
   vector index uses 0 or 1 bit, dependent on the number of unique
   motion vector candidates.  If the chosen neighbor block is coded in
   bi-prediction mode, the inter0 or inter1 block inherits both motion
   vectors, both reference indices and the bi-prediction property of the
   neighbor block.

   For block sizes less than 64x64, inter0 has only one motion vector
   candidate, and its value is always zero.

   Which neighbor blocks to use for motion vector candidates depends on
   the availability of the neighbor blocks (i.e. whether the neighbor
   blocks have already been coded, belong to the same slice and are not
   outside the frame boundaries).  Four different availabilities, U, UR,
   L, and LL, are defined as illustrated in Figure 6.  If the neighbor
   block is intra it is considered to be available but with a zero
   motion vector.





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                               |           |
                               |     U     |    UR
                    -----------+-----------+-----------
                               |           |
                               |  current  |
                         L     |   block   |
                               |           |
                               |           |
                    -----------+-----------+
                               |
                               |
                         LL    |
                               |


                 Figure 6: Availability of neighbor blocks

   Based on the four availabilities defined above, each of the motion
   vector candidates is derived from one of the possible neighbor blocks
   defined in Figure 7.


                  +----+----+      +----+    +----+----+
                  | UL | U0 |      | U1 |    | U2 | UR |
                  +----+----+------+----+----+----+----+
                  | L0 |                          |
                  +----+                          |
                       |                          |
                       |                          |
                  +----+        current           |
                  | L1 |         block            |
                  +----+                          |
                       |                          |
                  +----+                          |
                  | L2 |                          |
                  +----+--------------------------+
                  | LL |
                  +----+


                    Figure 7: Motion vector candidates

   The choice of motion vector candidates depends on the availability of
   neighbor blocks as shown in Table 1.







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            +----+-----+----+-----+---------------------------+
            | U  | UR  | L  | LL  | Motion vector candidates  |
            +----+-----+----+-----+---------------------------+
            | 0  | 0   | 0  | 0   | zero vector               |
            | 1  | 0   | 0  | 0   | U2, zero vector           |
            | 0  | 1   | 0  | 0   | NA                        |
            | 1  | 1   | 0  | 0   | U2,zero vector            |
            | 0  | 0   | 1  | 0   | L2, zero vector           |
            | 1  | 0   | 1  | 0   | U2,L2                     |
            | 0  | 1   | 1  | 0   | NA                        |
            | 1  | 1   | 1  | 0   | U2,L2                     |
            | 0  | 0   | 0  | 1   | NA                        |
            | 1  | 0   | 0  | 1   | NA                        |
            | 0  | 1   | 0  | 1   | NA                        |
            | 1  | 1   | 0  | 1   | NA                        |
            | 0  | 0   | 1  | 1   | L2, zero vector           |
            | 1  | 0   | 1  | 1   | U2,L2                     |
            | 0  | 1   | 1  | 1   | NA                        |
            | 1  | 1   | 1  | 1   | U2,L2                     |
            +----+-----+----+-----+---------------------------+

      Table 1: Motion vector candidates for different availability of
                              neighbor blocks

5.7.2.  Inter2 and Bi-Prediction Modes

   Motion vectors are coded using motion vector prediction.  The motion
   vector predictor is defined as the median of the motion vectors from
   three neighbor blocks.  Definition of the motion vector predictor
   uses the same definition of availability and neighbors as in Figure 6
   and Figure 7 respectively.  The three vectors used for median
   filtering depends on the availability of neighbor blocks as shown in
   Table 2.  If the neighbor block is coded in bi-prediction mode, only
   the first motion vector (in transmission order), MV0, is used as
   input to the median operator.
















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      +----+-----+----+-----+--------------------------------------+
      | U  | UR  | L  | LL  | Motion vectors for median filtering  |
      +----+-----+----+-----+--------------------------------------+
      | 0  | 0   | 0  | 0   | 3 x zero vector                      |
      | 1  | 0   | 0  | 0   | U0,U1,U2                             |
      | 0  | 1   | 0  | 0   | NA                                   |
      | 1  | 1   | 0  | 0   | U0,U2,UR                             |
      | 0  | 0   | 1  | 0   | L0,L1,L2                             |
      | 1  | 0   | 1  | 0   | UL,U2,L2                             |
      | 0  | 1   | 1  | 0   | NA                                   |
      | 1  | 1   | 1  | 0   | U0,UR,L2,L0                          |
      | 0  | 0   | 0  | 1   | NA                                   |
      | 1  | 0   | 0  | 1   | NA                                   |
      | 0  | 1   | 0  | 1   | NA                                   |
      | 1  | 1   | 0  | 1   | NA                                   |
      | 0  | 0   | 1  | 1   | L0,L2,LL                             |
      | 1  | 0   | 1  | 1   | U2,L0,LL                             |
      | 0  | 1   | 1  | 1   | NA                                   |
      | 1  | 1   | 1  | 1   | U0,UR,L0                             |
      +----+-----+----+-----+--------------------------------------+

      Table 2: Neighbor blocks used to define motion vector predictor
                         through median filtering

5.7.3.  Motion Vector Direction

   Motion vectors referring to reference frames later in time than the
   current frame are stored with their sign reversed, and these reversed
   values are used for coding and motion vector prediction.

6.  Transforms

   Transforms are applied at the TB or CB level, implying that transform
   sizes range from 4x4 to 128x128.  The transforms form an embedded
   structure meaning the transform matrix elements of the smaller
   transforms can be extracted from the larger transforms.

7.  Quantization

   For the 32x32, 64x64 and 128x128 transform sizes, only the 16x16 low
   frequency coefficients are quantized and transmitted.

   The 64x64 inverse transform is defined as a 32x32 transform followed
   by duplicating each output sample into a 2x2 block.  The 128x128
   inverse transform is defined as a 32x32 transform followed by
   duplicating each output sample into a 4x4 block.





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7.1.  Quantization matrices

   A flag is transmitted in the sequence header to indicate whether
   quantization matrices are used.  If this flag is true, a 6 bit value
   qmtx_offset is transmitted in the sequence header to indicate matrix
   strength.

   If used, then in dequantization a separate scaling factor is applied
   to each coefficient, so that the dequantized value of a coefficient
   ci at position i is:


            (ci * d(q) * IW(i,c,s,t,q) + 2^(k + 5)) >> (k + 6)


                           Figure 8: Equation 1

   where IW is the scale factor for coefficient position i with size s,
   frame type (inter/inter) t, component (Y, Cb or Cr) c and quantizer
   q; and k=k(s,q) is the dequantization shift.  IW has scale 64, that
   is, a weight value of 64 is no different to unweighted
   dequantization.

7.1.1.  Quantization matrix selection

   The current luma qp value qpY and the offset value qmtx_offset
   determine a quantisation matrix set by the formula:


         qmlevel = max(0,min(11,((qpY + qmtx_offset) * 12) / 44))


                           Figure 9: Equation 2

   This selects one of the 12 different sets of default quantization
   matrix, with increasing qmlevel indicating increasing flatness.

   For a given value of qmlevel, different weighting matrices are
   provided for all combinations of transform block size, type (intra/
   inter), and component (Y, Cb, Cr).  Matrices at low qmlevel are flat
   (constant value 64).  Matrices for inter frames have unity DC gain
   (i.e. value 64 at position 0), whereas those for intra frames are
   designed such that the inverse weighting matrix has unity energy gain
   (i.e. normalized sum-squared of the scaling factors is 1).







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7.1.2.  Quantization matrix design

   Further details on the quantization matrix and implementation can be
   found in the separate QMTX draft [I-D.davies-netvc-qmtx].

8.  Loop Filtering

8.1.  Deblocking

8.1.1.  Luma deblocking

   Luma deblocking is performed on an 8x8 grid as follows:

   1.  For each vertical edge between two 8x8 blocks, calculate the
       following for each of line 2 and line 5 respectively:

       d = abs(a-b) + abs(c-d),

       where a and b, are on the left hand side of the block edge and c
       and d are on the right hand side of the block edge:

       a b | c d

   2.  For each line crossing the vertical edge, perform deblocking if
       and only if all of the following conditions are true:

       *  d2+d5 < beta(QP)

       *  The edge is also a transform block edge

       *  abs(mvx(left)) > 2, or abs(mvx(right)) > 2, or

          abs(mvy(left)) > 2, or abs(mvy(right)) > 2, or

          One of the transform blocks on each side of the edge has non-
          zero coefficients, or

          One of the transform blocks on each side of the edge is coded
          using intra mode.

   3.  If deblocking is performed, calculate a delta value as follows:

       delta = clip((18*(c-b) - 6*(d-a) + 16)/32,tc,-tc),

       where tc is a QP-dependent value.

   4.  Next, modify two pixels on each side of the block edge as
       follows:



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       a' = a + delta/2

       b' = b + delta

       c' = c + delta

       d' = d + delta/2

   5.  The same procedure is followed for horizontal block edges.

   The relative positions of the samples, a, b, c, d and the motion
   vectors, MV, are illustrated in Figure 10.


                                    |
                                    | block edge
                                    |
                            +---+---+---+---+
                            | a | b | c | d |
                            +---+---+---+---+
                                    |
                           mv       | mv
                             x,left |   x,right
                                    |
                           mv         mv
                             y,left     y,right


               Figure 10: Deblocking filter pixel positions

8.1.2.  Chroma Deblocking

   Chroma deblocking is performed on a 4x4 grid as follows:

   1.  Delocking of the edge between two 4x4 blocks is performed if and
       only if:

       *  The pixels on either side of the block edge belongs to an
          intra block.

       *  The block edge is also an edge between two transform blocks.

   2.  If deblocking is performed, calculate a delta value as follows:

       delta = clip((4*(c-b) + (d-a) + 4)/8,tc,-tc),

       where tc is a QP-dependent value.




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   3.  Next, modify one pixel on each side of the block edge as follows:

       b' = b + delta

       c' = c + delta

8.2.  Constrained Low Pass Filter (CLPF)

   A low-pass filter is applied after the deblocking filter if signaled
   in the sequence header.  It can still be switched off for individual
   frames in the frame header.  Also signaled in the frame header is
   whether to apply the filter for all qualified 128x128 blocks or to
   transmit a flag for each such block.  A super block does not qualify
   if it only contains Inter0 (skip) coding block and no signal is
   transmitted for these blocks.

   The filter is described in the separate CLPF draft
   [I-D.midtskogen-netvc-clpf].

9.  Entropy coding

9.1.  Overview

   The following information is signaled at the sequence level:

   o  Sequence header

   The following information is signaled at the frame level:

   o  Frame header

   The following information is signaled at the CB level:

   o  Super-mode (mode, split, reference index for uni-prediction)

   o  Intra prediction mode

   o  PB-split (none, hor, ver, quad)

   o  TB-split (none or quad)

   o  Reference frame indices for bi-prediction

   o  Motion vector candidate index

   o  Transform coefficients if TB-split=0

   The following information is signaled at the TB level:



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   o  CBP (8 combinations of CBPY, CBPU, and CBPV)

   o  Transform coefficients

   The following information is signaled at the PB level:

   o  Motion vector differences

9.2.  Low Level Syntax

9.2.1.  CB Level

    super-mode          (inter0/split/inter1/inter2-ref0/intra/inter2-ref1/inter2-ref2/inter2-ref3,..)

    if (mode == inter0 || mode == inter1)

      mv_idx                    (one of up to 2 motion vector candidates)

    else if (mode == INTRA)

      intra_mode                (one of up to 8 intra modes)

      tb_split                  (NONE or QUAD, coded jointly with CBP for tb_split=NONE)

    else if (mode == INTER)

      pb_split          (NONE,VER,HOR,QUAD)

      tb_split_and_cbp  (NONE or QUAD and CBP)

    else if (mode == BIPRED)

      mvd_x0, mvd_y0    (motion vector difference for first vector)

      mvd_x1, mvd_y1    (motion vector difference for second vector)

      ref_idx0, ref_idx1        (two reference indices)

9.2.2.  PB Level

       if (mode == INTER2 || mode == BIPRED)

         mvd_x, mvd_y              (motion vector differences)








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9.2.3.  TB Level

       if (mode != INTER0 and tb_split == 1)

         cbp                       (8 possibilities for CBPY/CBPU/CBPV)

       if (mode != INTER0)

         transform coefficients

9.2.4.  Super Mode

   For each block of size NxN (64>=N>8), the following mutually
   exclusive events are jointly encoded using a single VLC code as
   follows (example using 4 reference frames):

   If there is no interpolated reference frame:

   INTER0      1
   SPLIT       01
   INTER1      001
   INTER2-REF0 0001
   BIPRED      00001
   INTRA       000001
   INTER2-REF1     0000001
   INTER2-REF2     00000001
   INTER2-REF3     00000000

   If there is an interpolated reference frame:

   INTER0      1
   SPLIT       01
   INTER1      001
   BIPRED      0001
   INTRA       00001
   INTER2-REF1     000001
   INTER2-REF2     0000001
   INTER2-REF3     00000001
   INTER2-REF0 00000000

   If less than 4 reference frames is used, a shorter VLC table is used.
   If bi-pred is not possible, or split is not possible, they are
   omitted from the table and shorter codes are used for subsequent
   elements.

   Additionally, depending on information from the blocks to the left
   and above (meta data and CBP), a different sorting of the events can
   be used, e.g.:



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     SPLIT       1
     INTER1      01
     INTER2-REF0 001
     INTER0      0001
     INTRA       00001
     INTER2-REF1 000001
     INTER2-REF2 0000001
     INTER2-REF3 00000001
     BIPRED      00000000

9.2.5.  CBP

   Calculate code as follows:

       if (tb-split == 0)

         N = 4*CBPV + 2*CBPU + CBPY

       else

         N = 8

   Map the value of N to code through a table lookup:

   code = table[N]

   where the purpose of the table lookup is the sort the different
   values of code according to decreasing probability (typically CBPY=1,
   CBPU=0, CBPV=0 having the highest probability).

   Use a different table depending on the values of CBPY in neighbor
   blocks (left and above).

   Encode the value of code using a systematic VLC code.

9.2.6.  Transform Coefficients

   Transform coefficient coding uses a traditional zig-zag scan pattern
   to convert a 2D array of quantized transform coefficients, coeff, to
   a 1D array of samples.  VLC coding of quantized transform
   coefficients starts from the low frequency end of the 1D array using
   two different modes; level-mode and run-mode, starting in level-mode:

   o  Level-mode

      *  Encode each coefficient, coeff, separately

      *  Each coefficient is encoded by:



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         +  The absolute value, level=abs(coeff), using a VLC code and

         +  If level > 0, the sign bit (sign=0 or sign=1 for coeff>0 and
            coeff<0 respectively).

      *  If coefficient N is zero, switch to run-mode, starting from
         coefficient N+1.

   o  Run-mode

      *  For each non-zero coefficient, encode the combined event of:

         1.  Length of the zero-run, i.e. the number of zeros since the
             last non-zero coefficient.

         2.  Whether or not level=abs(coeff) is greater than 1.

         3.  End of block (EOB) indicating that there are no more non-
             zero coefficients.

      *  Additionally, if level = 1, code the sign bit.

      *  Additionally, if level > 1 define code = 2*(level-2)+sign,

      *  If the absolute value of coefficient N is larger than 1, switch
         to level-mode, starting from coefficient N+1.

   Example

   Figure 11 illustrates an example where 16 quantized transform
   coefficients are encoded.


                    4
                                         3
              2     |                       2
                 1  |  1        1        |           1
              |     |     0  0     0  0  |  |  0  0     0  0
              |__|__|__|________|________|__|________|_______



                     Figure 11: Coefficients to encode

   Table 3 shows the mode, VLC number and symbols to be coded for each
   coefficient.





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   +--------+-------------+-------------+------------------------------+
   | Index  | abs(coeff)  | Mode        | Encoded symbols              |
   +--------+-------------+-------------+------------------------------+
   | 0      | 2           | level-mode  | level=2,sign                 |
   | 1      | 1           | level-mode  | level=1,sign                 |
   | 2      | 4           | level-mode  | level=4,sign                 |
   | 3      | 1           | level-mode  | level=1,sign                 |
   | 4      | 0           | level-mode  | level=0                      |
   | 5      | 0           | run-mode    |                              |
   | 6      | 1           | run-mode    | (run=1,level=1)              |
   | 7      | 0           | run-mode    |                              |
   | 8      | 0           | run-mode    |                              |
   | 9      | 3           | run-mode    | (run=1,level>1),             |
   |        |             |             | 2*(3-2)+sign                 |
   | 10     | 2           | level-mode  | level=2, sign                |
   | 11     | 0           | level-mode  | level=0                      |
   | 12     | 0           | run-mode    |                              |
   | 13     | 1           | run-mode    | (run=1,level=1)              |
   | 14     | 0           | run-mode    | EOB                          |
   | 15     | 0           | run-mode    |                              |
   +--------+-------------+-------------+------------------------------+

      Table 3: Transform coefficient encoding for the example above.

10.  High Level Syntax

   High level syntax is currently very simple and rudimentary as the
   primary focus so far has been on compression performance.  It is
   expected to evolve as functionality is added.

10.1.  Sequence Header

   o  Width - 16 bits

   o  Height - 16 bits

   o  Enable/disable PB-split - 1 bit

   o  SB size - 3 bits

   o  Enable/disable TB-split - 1 bit

   o  Number of active reference frames (may go into frame header) - 2
      bits (max 4)

   o  Enable/disable interpolated reference frames - 1 bit

   o  Enable/disable delta qp - 1 bit



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   o  Enable/disable deblocking - 1 bit

   o  Constrained low-pass filter (CLPF) enable/disable - 1 bit

   o  Enable/disable block context coding - 1 bit

   o  Enable/disable bi-prediction - 1 bit

   o  Enable/disable quantization matrices - 1 bit

   o  If quantization matrices enabled: quantization matrix offset - 6
      bits

   o  Select 420 or 444 input - 1 bit

   o  Number of reordered frames - 4 bits

   o  Enable/disable chroma intra prediction from luma - 1 bit

   o  Enable/disable chroma inter prediction from luma - 1 bit

   o  Internal frame bitdepth (8, 10 or 12 bits) - 2 bits

   o  Input video bitdepth (8, 10 or 12 bits) - 2 bits

10.2.  Frame Header

   o  Frame type - 1 bit

   o  QP - 8 bits

   o  Identification of active reference frames - num_ref*4 bits

   o  Number of intra modes - 4 bits

   o  Number of active reference frames - 2 bits

   o  Active reference frames - number of active reference frames * 6
      bits

   o  Frame number - 16 bits

   o  If CLPF is enabled in the sequence header: Constrained low-pass
      filter (CLPF) strength - 2 bits (00 = off, 01 = strength 1, 10 =
      strength 2, 11 = strength 4)

   o  IF CLPF is enabled in the sequence header: Enable/disable CLPF
      signal for each qualified filter block



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11.  IANA Considerations

   This document has no IANA considerations yet.  TBD

12.  Security Considerations

   This document has no security considerations yet.  TBD

13.  Normative References

   [I-D.davies-netvc-irfvc]
              Davies, T., "Interpolated reference frames for video
              coding", draft-davies-netvc-irfvc-00 (work in progress),
              October 2015.

   [I-D.davies-netvc-qmtx]
              Davies, T., "Quantisation matrices for Thor video coding",
              draft-davies-netvc-qmtx-00 (work in progress), March 2016.

   [I-D.midtskogen-netvc-chromapred]
              Midtskogen, S., "Improved chroma prediction", draft-
              midtskogen-netvc-chromapred-02 (work in progress), October
              2016.

   [I-D.midtskogen-netvc-clpf]
              Midtskogen, S., Fuldseth, A., and M. Zanaty, "Constrained
              Low Pass Filter", draft-midtskogen-netvc-clpf-02 (work in
              progress), April 2016.

   [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>.

Authors' Addresses

   Arild Fuldseth
   Cisco
   Lysaker
   Norway

   Email: arilfuld@cisco.com









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   Gisle Bjontegaard
   Cisco
   Lysaker
   Norway

   Email: gbjonteg@cisco.com


   Steinar Midtskogen
   Cisco
   Lysaker
   Norway

   Email: stemidts@cisco.com


   Thomas Davies
   Cisco
   London
   UK

   Email: thdavies@cisco.com


   Mo Zanaty
   Cisco
   RTP,NC
   USA

   Email: mzanaty@cisco.com





















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