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Internet Engineering Task Force          Audio-Video Transport Working Group
INTERNET-DRAFT                                                       L. Berc
draft-ietf-avt-jpeg-03.txt                     Digital Equipment Corporation
                                                                   W. Fenner
                                                                  Xerox PARC
                                                                R. Frederick
                                                                  Xerox PARC
                                                                  S. McCanne
                                                Lawrence Berkeley Laboratory
                                                                July 7, 1996
                                                             Expires: 1/1/97


              RTP Payload Format for JPEG-compressed Video

Status of this Memo

This document is an Internet Draft.   Internet Drafts are working  docu-
ments of the Internet Engineering  Task Force (IETF), its  Areas, and
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Distribution of this document is unlimited.

                                Abstract

     This draft describes the RTP payload format for JPEG video streams.
     The packet format is optimized for real-time video streams where
     codec parameters change rarely from frame to frame.

This document is a product of the Audio-Video Transport working group
within the Internet Engineering Task Force.  Comments are solicited and
should be addressed to the working group's mailing list at rem-
conf@es.net and/or the author(s).







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1.  Introduction

The Joint Photographic Experts Group (JPEG) standard [1,2,3] defines a
family of compression algorithms for continuous-tone, still images.
This still image compression standard can be applied to video by
compressing each frame of video as an independent still image and
transmitting them in series.  Video coded in this fashion is often
called Motion-JPEG.

We first give an overview of JPEG and then describe the specific subset
of JPEG that is supported in RTP and the mechanism by which JPEG frames
are carried as RTP payloads.

The JPEG standard defines four modes of operation: the sequential DCT
mode, the progressive DCT mode, the lossless mode, and the hierarchical
mode.  Depending on the mode, the image is represented in one or more
passes.  Each pass (called a frame in the JPEG standard) is further bro-
ken down into one or more scans.  Within each scan, there are one to
four components,which represent the three components of a color signal
(e.g., ``red, green, and blue'', or a luminance signal and two chroman-
ince signals).  These components can be encoded as separate scans or
interleaved into a single scan.

Each frame and scan is preceded with a header containing optional defin-
itions for compression parameters like quantization tables and Huffman
coding tables.  The headers and optional parameters are identified with
``markers'' and comprise a marker segment; each scan appears as an
entropy-coded bit stream within two marker segments.  Markers are
aligned to byte boundaries and (in general) cannot appear in the
entropy-coded segment, allowing scan boundaries to be determined without
parsing the bit stream.

Compressed data is represented in one of three formats: the interchange
format, the abbreviated format, or the table-specification format.  The
interchange format contains definitions for all the table used in the by
the entropy-coded segments, while the abbreviated format might omit some
assuming they were defined out-of-band or by a ``previous'' image.

The JPEG standard does not define the meaning or format of the com-
ponents that comprise the image.  Attributes like the color space and
pixel aspect ratio must be specified out-of-band with respect to the
JPEG bit stream.  The JPEG File Interchange Format (JFIF) [4] is a
defacto standard that provides this extra information using an applica-
tion marker segment (APP0).  Note that a JFIF file is simply a JPEG
interchange format image along with the APP0 segment.  In the case of
video, additional parameters must be defined out-of-band (e.g., frame
rate, interlaced vs. non-interlaced, etc.).




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While the JPEG standard provides a rich set of algorithms for flexible
compression, cost-effective hardware implementations of the full stan-
dard have not appeared.  Instead, most hardware JPEG video codecs imple-
ment only a subset of the sequential DCT mode of operation.  Typically,
marker segments are interpreted in software (which ``re-programs'' the
hardware) and the hardware is presented with a single, interleaved
entropy-coded scan represented in the YUV color space.

2.  JPEG Over RTP

To maximize interoperability among hardware-based codecs, we assume the
sequential DCT operating mode [1,Annex F] and restrict the set of pre-
defined RTP/JPEG ``type codes'' (defined below) to single-scan, inter-
leaved images.  While this is more restrictive than even baseline JPEG,
many hardware implementation fall short of the baseline specification
(e.g., most hardware cannot decode non-interleaved scans).

In practice, most of the table-specification data rarely changes from
frame to frame within a single video stream.  Therefore, RTP/JPEG data
is represented in abbreviated format, with all of the tables omitted
from the bit stream.  Each image begins immediately with the (single)
entropy-coded scan.  The information that would otherwise be in both the
frame and scan headers is represented entirely within a 64-bit RTP/JPEG
header (defined below) that lies between the RTP header and the JPEG
scan and is present in every packet.

While parameters like Huffman tables and color space are likely to
remain fixed for the lifetime of the video stream, other parameters
should be allowed to vary, notably the quantization tables and image
size (e.g., to implement rate-adaptive transmission or allow a user to
adjust the ``quality level'' or resolution manually).  Thus explicit
fields in the RTP/JPEG header are allocated to represent this informa-
tion.  Since only a small set of quantization tables are typically used,
we encode the entire set of quantization tables in a small integer
field.   The image width and height are encoded explicitly.

Because JPEG frames are typically larger than the underlying network's
maximum packet size, frames must often be fragmented into several pack-
ets.  One approach is to allow the network layer below RTP (e.g., IP) to
perform the fragmentation.  However, this precludes rate-controlling the
resulting packet stream or partial delivery in the presence of loss.
For example, IP will not deliver a fragmented datagram to the applica-
tion if one or more fragments is lost, or IP might fragment an 8000 byte
frame into a burst of 8 back-to-back packets.  Instead, RTP/JPEG defines
a simple fragmentation and reassembly scheme at the RTP level.






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3.  RTP/JPEG Packet Format

The RTP timestamp is in units of 90000Hz.  The same timestamp must
appear across all fragments of a single frame.  The RTP marker bit is
set in the last packet of a frame.

3.1.  JPEG header

A special header is added to each packet that immediately follows the
RTP header:

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type specific |              Fragment Offset                  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|      Type     |       Q       |     Width     |     Height    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


3.1.1.  Type specific: 8 bits

     Interpretation depends on the value of the type field.

3.1.2.  Fragment Offset: 24 bits

     The Fragment Offset is the data offset in bytes of the current
     packet in the JPEG scan.

3.1.3.  Type: 8 bits

     The type field specifies the information that would otherwise be
     present in a JPEG abbreviated table-specification as well as the
     additional JFIF-style parameters not defined by JPEG.  Types 0-127
     are reserved as fixed, well-known mappings to be defined by this
     document and future revisions of this document.  Types 128-255 are
     free to be dynamically defined by a session setup protocol (which
     is beyond the scope of this document).

3.1.4.  Q: 8 bits

     The Q field defines the quantization tables for this frame using an
     algorithm that determined by the Type field (see below).

3.1.5.  Width: 8 bits

     This field encodes the width of the image in 8-pixel multiples
     (e.g., a width of 40 denotes an image 320 pixels wide).



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3.1.6.  Height: 8 bits

     This field encodes the height of the image in 8-pixel multiples
     (e.g., a height of 30 denotes an image 240 pixels tall).

3.1.7.  Data

     The data following the RTP/JPEG header is an entropy-coded segment
     consisting of a single scan.  The scan header is not present and is
     inferred from the RTP/JPEG header.  The scan is terminated either
     implicitly (i.e., the point at which the image is fully parsed), or
     explicitly with an EOI marker.  The scan may be padded to arbitrary
     length with undefined bytes.  (Existing hardware codecs generate
     extra lines at the bottom of a video frame and removal of these
     lines would require a Huffman-decoding pass over the data.)

     As defined by JPEG, restart markers are the only type of marker
     that may appear embedded in the entropy-coded segment.  The ``type
     code'' determines whether a restart interval is defined, and there-
     fore whether restart markers may be present. It also determines if
     the restart intervals will be aligned with RTP packets, allowing for
     partial decode of frames, thus increasing resiliance to packet
     drop. If restart markers are present, the 6-byte DRI segment (define
     restart interval marker [1, Sec. B.2.4.4] precedes the scan).

     JPEG markers appear explicitly on byte aligned boundaries beginning
     with an 0xFF.  A ``stuffed'' 0x00 byte follows any 0xFF byte gen-
     erated by the entropy coder [1, Sec. B.1.1.5].

4.  Discussion

4.1.  The Type Field

The Type field defines the abbreviated table-specification and addi-
tional JFIF-style parameters not defined by JPEG, since they are not
present in the body of the transmitted JPEG data.  The Type field must
remain constant for the duration of a session.

Six type codes are currently defined.  They correspond to an abbre-
viated table-specification indicating the ``Baseline DCT sequential''
mode, 8-bit samples, square pixels, three components in the YUV color
space, standard Huffman tables as defined in [1, Annex K.3], and a
single interleaved scan with a scan component selector indicating
components 0, 1, and 2 in that order.  The Y, U, and V color planes
correspond to component numbers 0, 1, and 2, respectively.  Component
0 (i.e., the luminance plane) uses Huffman table number 0 and quantiz-
ation table number 0 (defined below) and components 1 and 2 (i.e.,
the chrominance planes) use Huffman table number 1 and quantization
table number 1 (defined below).


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Additionally, video is non-interlaced and unscaled (i.e., the aspect
ratio is determined by the image width and height).  The frame rate is
variable and explicit via the RTP timestamp.

Six RTP/JPEG types are currently defined that assume all of the above.
The odd types have different JPEG sampling factors from the even ones:

                        horizontal   vertical
           types   comp  samp. fact. samp. fact.
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |  0/2/4  |  0  |     2     |   1   |
          |  0/2/4  |  1  |     1     |   1   |
          |  0/2/4  |  2  |     1     |   1   |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |  1/3/5  |  0  |     2     |   2   |
          |  1/3/5  |  1  |     1     |   1   |
          |  1/3/5  |  2  |     1     |   1   |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

These sampling factors indicate that the chromanince components of type
0/2/4 video is downsampled horizontally by 2 (often called 4:2:2) while
the chrominance components of type 1/3/5 video are downsampled both
horizontally and vertically by 2 (often called 4:2:0).

The three pairs of types (0/1), (2/3) and (4/5) differ from each other
as follows:

0/1 : No restart markers are present in the entropy data.
      No restriction is placed on the fragmentation of the stream into
      RTP packets.
      The type specific field is unused and must be zero.

2/3 : Restart markers are present in the entropy data.
      The entropy data is preceded by a DRI marker segment, defining the
      restart interval.
      No restriction is placed on the fragmentation of the stream into
      RTP packets.
      The type specific field is unused and must be zero.

4/5 : Restart markers are present in the entropy data.
      The entropy data is preceded by a DRI marker segment, defining the
      restart interval.
      Restart intervals are be sent as separate (possibly multiple) RTP
      packets.
      The type specific field (TSPEC) is used as follows:
          A restart interval count (RCOUNT) is defined, which starts at
          zero, and is incremented for each restart interval in the
          frame.



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          The first packet of a restart interval gets TSPEC = RCOUNT.
          Subsequent packets of the restart interval get TSPEC = 254,
          except the final packet, which gets TSPEC = 255.

Additional types in the range 128-255 may be defined by external means,
such as a session protocol.

Appendix B contains C source code for transforming the RTP/JPEG header
parameters into the JPEG frame and scan headers that are absent from the
data payload.

4.2.  The Q Field

The quantization tables used in the decoding process are algorithmically
derived from the Q field.  The algorithm used depends on the type field
but only one algorithm is currently defined for the two types.

Both type 0 and type 1 JPEG assume two quantizations tables.  These
tables are chosen as follows.  For 1 <= Q <= 99, the Independent JPEG
Group's formula [5] is used to produce a scale factor S as:

        S = 5000 / Q          for  1 <= Q <= 50
          = 200 - 2 * Q       for 51 <= Q <= 99

This value is then used to scale Tables K.1 and K.2 from [1] (saturating
each value to 8-bits) to give quantization table numbers 0 and 1,
respectively.  C source code is provided in Appendix A to compute these
tables.

For Q >= 100, a dynamically defined quantization table is used, which
might be specified by a session setup protocol.  (This session protocol
is beyond the scope of this document).  It is expected that the standard
quantization tables will handle most cases in practice, and dynamic
tables will be used rarely.  Q = 0 is reserved.

4.3.  Fragmentation and Reassembly

Since JPEG frames are large, they must often be fragmented.  Frames
should be fragmented into packets in a manner avoiding fragmentation at
a lower level.  When using restart markers, frames should be fragmented
such that each packet starts with a restart interval (see below).

Each packet that makes up a single frame has the same timestamp.  The
fragment offset field is set to the byte offset of this packet within
the original frame.  The RTP marker bit is set on the last packet in a
frame.

An entire frame can be identified as a sequence of packets beginning



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with a packet having a zero fragment offset and ending with a packet
having the RTP marker bit set.  Missing packets can be detected either
with RTP sequence numbers or with the fragment offset and lengths of
each packet.  Reassembly could be carried out without the offset field
(i.e., using only the RTP marker bit and sequence numbers), but an effi-
cient single-copy implementation would not otherwise be possible in the
presence of misordered packets.  Moreover, if the last packet of the
previous frame (containing the marker bit) were dropped, then a receiver
could not detect that the current frame is entirely intact.

4.4.  Restart Markers

Restart markers indicate a point in the JPEG stream at which the Huffman
codec and DC predictors  are reset, allowing partial decoding starting
at that point.  The use of restart markers allows for robustness in the
face of packet loss.

RTP/JPEG Types 4/5 allow for partial decode of frames, due to the
alignment of restart intervals with RTP packets. The decoder knows it
has a whole restart interval when it gets sequence of packets with
contiguous RTP sequence numbers, starting with TSPEC<254 (RCOUNT) and
either ending with TSPEC==255, or TSPEC<255 and next packet's TSPEC<254
(or end of frame).

It can then decompress the RST interval, and paint it. The X and Y tile
offsets of the first MCU in the interval are given by:-

tile_offset = RCOUNT * restart_interval * 2
x_offset    = tile_offset % frame_width_in_tiles
y_offset    = tile_offset / frame_width_in_tiles

The MCUs in a restart interval may span multiple tile rows.

Decoders can, however, treat types 4/5 as types 2/3, simply reassembling
the entire frame and then decoding.


5.  Security Considerations

Security issues are not discussed in this memo.











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6.  Authors' Addresses

   Lance M. Berc
   Systems Research Center
   Digital Equipment Corporation
   130 Lytton Ave
   Palo Alto CA 94301
   Phone: +1 415 853 2100
   Email: berc@pa.dec.com

   William C. Fenner
   Xerox PARC
   3333 Coyote Hill Road
   Palo Alto, CA 94304
   Phone: +1 415 812 4816
   Email: fenner@cmf.nrl.navy.mil

   Ron Frederick
   Xerox PARC
   3333 Coyote Hill Road
   Palo Alto, CA 94304
   Phone: +1 415 812 4459
   Email: frederick@parc.xerox.com

   Steven McCanne
   Lawrence Berkeley Laboratory
   M/S 46A-1123
   One Cyclotron Road
   Berkeley, CA 94720
   Phone: +1 510 486 7520
   Email: mccanne@ee.lbl.gov




















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7.  References


[1]  ISO DIS 10918-1. Digital Compression and Coding of Continuous-tone
     Still Images (JPEG), CCITT Recommendation T.81.

[2]  William B. Pennebaker, Joan L. Mitchell, JPEG: Still Image Data
     Compression Standard, Van Nostrand Reinhold, 1993.

[3]  Gregory K. Wallace, The JPEG Sill Picture Compression Standard,
     Communications of the ACM, April 1991, Vol 34, No. 1, pp. 31-44.

[4]  The JPEG File Interchange Format.  Maintained by C-Cube Microsys-
     tems, Inc., and available in
     ftp://ftp.uu.net/graphics/jpeg/jfif.ps.gz.

[5]  Tom Lane et. al., The Independent JPEG Group software JPEG codec.
     Source code available in
     ftp://ftp.uu.net/graphics/jpeg/jpegsrc.v5.tar.gz.

Appendix A

The following code can be used to create a quantization table from a Q
factor:

/*
 * Table K.1 from JPEG spec.
 */
static const int jpeg_luma_quantizer[64] = {
        16, 11, 10, 16, 24, 40, 51, 61,
        12, 12, 14, 19, 26, 58, 60, 55,
        14, 13, 16, 24, 40, 57, 69, 56,
        14, 17, 22, 29, 51, 87, 80, 62,
        18, 22, 37, 56, 68, 109, 103, 77,
        24, 35, 55, 64, 81, 104, 113, 92,
        49, 64, 78, 87, 103, 121, 120, 101,
        72, 92, 95, 98, 112, 100, 103, 99
};

/*
 * Table K.2 from JPEG spec.
 */
static const int jpeg_chroma_quantizer[64] = {
        17, 18, 24, 47, 99, 99, 99, 99,
        18, 21, 26, 66, 99, 99, 99, 99,
        24, 26, 56, 99, 99, 99, 99, 99,
        47, 66, 99, 99, 99, 99, 99, 99,
        99, 99, 99, 99, 99, 99, 99, 99,



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        99, 99, 99, 99, 99, 99, 99, 99,
        99, 99, 99, 99, 99, 99, 99, 99,
        99, 99, 99, 99, 99, 99, 99, 99
};

/*
 * Call MakeTables with the Q factor and two int[64] return arrays
 */
void
MakeTables(int q, u_char *lum_q, u_char *chr_q)
{
  int i;
  int factor = q;

  if (q < 1) factor = 1;
  if (q > 99) factor = 99;
  if (q < 50)
    q = 5000 / factor;
  else
    q = 200 - factor*2;

  for (i=0; i < 64; i++) {
    int lq = ( jpeg_luma_quantizer[i] * q + 50) / 100;
    int cq = ( jpeg_chroma_quantizer[i] * q + 50) / 100;

    /* Limit the quantizers to 1 <= q <= 255 */
    if ( lq < 1) lq = 1;
    else if ( lq > 255) lq = 255;
    lum_q[i] = lq;

    if ( cq < 1) cq = 1;
    else if ( cq > 255) cq = 255;
    chr_q[i] = cq;
  }
}


Appendix B

The following routines can be used to create the JPEG marker segments
corresponding to the table-specification data that is absent from the
RTP/JPEG body.


u_char lum_dc_codelens[] = {
        0, 1, 5, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0,
};




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u_char lum_dc_symbols[] = {
        0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11,
};

u_char lum_ac_codelens[] = {
        0, 2, 1, 3, 3, 2, 4, 3, 5, 5, 4, 4, 0, 0, 1, 0x7d,
};

u_char lum_ac_symbols[] = {
        0x01, 0x02, 0x03, 0x00, 0x04, 0x11, 0x05, 0x12,
        0x21, 0x31, 0x41, 0x06, 0x13, 0x51, 0x61, 0x07,
        0x22, 0x71, 0x14, 0x32, 0x81, 0x91, 0xa1, 0x08,
        0x23, 0x42, 0xb1, 0xc1, 0x15, 0x52, 0xd1, 0xf0,
        0x24, 0x33, 0x62, 0x72, 0x82, 0x09, 0x0a, 0x16,
        0x17, 0x18, 0x19, 0x1a, 0x25, 0x26, 0x27, 0x28,
        0x29, 0x2a, 0x34, 0x35, 0x36, 0x37, 0x38, 0x39,
        0x3a, 0x43, 0x44, 0x45, 0x46, 0x47, 0x48, 0x49,
        0x4a, 0x53, 0x54, 0x55, 0x56, 0x57, 0x58, 0x59,
        0x5a, 0x63, 0x64, 0x65, 0x66, 0x67, 0x68, 0x69,
        0x6a, 0x73, 0x74, 0x75, 0x76, 0x77, 0x78, 0x79,
        0x7a, 0x83, 0x84, 0x85, 0x86, 0x87, 0x88, 0x89,
        0x8a, 0x92, 0x93, 0x94, 0x95, 0x96, 0x97, 0x98,
        0x99, 0x9a, 0xa2, 0xa3, 0xa4, 0xa5, 0xa6, 0xa7,
        0xa8, 0xa9, 0xaa, 0xb2, 0xb3, 0xb4, 0xb5, 0xb6,
        0xb7, 0xb8, 0xb9, 0xba, 0xc2, 0xc3, 0xc4, 0xc5,
        0xc6, 0xc7, 0xc8, 0xc9, 0xca, 0xd2, 0xd3, 0xd4,
        0xd5, 0xd6, 0xd7, 0xd8, 0xd9, 0xda, 0xe1, 0xe2,
        0xe3, 0xe4, 0xe5, 0xe6, 0xe7, 0xe8, 0xe9, 0xea,
        0xf1, 0xf2, 0xf3, 0xf4, 0xf5, 0xf6, 0xf7, 0xf8,
        0xf9, 0xfa,
};

u_char chm_dc_codelens[] = {
        0, 3, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0,
};

u_char chm_dc_symbols[] = {
        0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11,
};

u_char chm_ac_codelens[] = {
        0, 2, 1, 2, 4, 4, 3, 4, 7, 5, 4, 4, 0, 1, 2, 0x77,
};

u_char chm_ac_symbols[] = {
        0x00, 0x01, 0x02, 0x03, 0x11, 0x04, 0x05, 0x21,
        0x31, 0x06, 0x12, 0x41, 0x51, 0x07, 0x61, 0x71,
        0x13, 0x22, 0x32, 0x81, 0x08, 0x14, 0x42, 0x91,



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        0xa1, 0xb1, 0xc1, 0x09, 0x23, 0x33, 0x52, 0xf0,
        0x15, 0x62, 0x72, 0xd1, 0x0a, 0x16, 0x24, 0x34,
        0xe1, 0x25, 0xf1, 0x17, 0x18, 0x19, 0x1a, 0x26,
        0x27, 0x28, 0x29, 0x2a, 0x35, 0x36, 0x37, 0x38,
        0x39, 0x3a, 0x43, 0x44, 0x45, 0x46, 0x47, 0x48,
        0x49, 0x4a, 0x53, 0x54, 0x55, 0x56, 0x57, 0x58,
        0x59, 0x5a, 0x63, 0x64, 0x65, 0x66, 0x67, 0x68,
        0x69, 0x6a, 0x73, 0x74, 0x75, 0x76, 0x77, 0x78,
        0x79, 0x7a, 0x82, 0x83, 0x84, 0x85, 0x86, 0x87,
        0x88, 0x89, 0x8a, 0x92, 0x93, 0x94, 0x95, 0x96,
        0x97, 0x98, 0x99, 0x9a, 0xa2, 0xa3, 0xa4, 0xa5,
        0xa6, 0xa7, 0xa8, 0xa9, 0xaa, 0xb2, 0xb3, 0xb4,
        0xb5, 0xb6, 0xb7, 0xb8, 0xb9, 0xba, 0xc2, 0xc3,
        0xc4, 0xc5, 0xc6, 0xc7, 0xc8, 0xc9, 0xca, 0xd2,
        0xd3, 0xd4, 0xd5, 0xd6, 0xd7, 0xd8, 0xd9, 0xda,
        0xe2, 0xe3, 0xe4, 0xe5, 0xe6, 0xe7, 0xe8, 0xe9,
        0xea, 0xf2, 0xf3, 0xf4, 0xf5, 0xf6, 0xf7, 0xf8,
        0xf9, 0xfa,
};

u_char *
MakeQuantHeader(u_char *p, u_char *qt, int tableNo)
{
        *p++ = 0xff;
        *p++ = 0xdb;            /* DQT */
        *p++ = 0;               /* length msb */
        *p++ = 67;              /* length lsb */
        *p++ = tableNo;
        memcpy(p, qt, 64);
        return (p + 64);
}

u_char *
MakeHuffmanHeader(u_char *p, u_char *codelens, int ncodes, u_char *symbols,
                  int nsymbols, int tableNo, int tableClass)
{
        *p++ = 0xff;
        *p++ = 0xc4;            /* DHT */
        *p++ = 0;               /* length msb */
        *p++ = 3 + ncodes + nsymbols; /* length lsb */
        *p++ = tableClass << 4 | tableNo;
        memcpy(p, codelens, ncodes);
        p += ncodes;
        memcpy(p, symbols, nsymbols);
        p += nsymbols;
        return (p);
}




                            Expires 1/1/97                     [Page 13]

Internet Draft         draft-ietf-avt-jpeg-03.txt              July 1996


/*
 * Given an RTP/JPEG type code, q factor, width, and height,
 * generate a frame and scan headers that can be prepended
 * to the RTP/JPEG data payload to produce a JPEG compressed
 * image in interchange format (except for possible trailing
 * garbage and absence of an EOI marker to terminate the scan).
 */
int MakeHeaders(u_char *p, int type, int q, int w, int h)
{
        u_char *start = p;
        u_char lqt[64];
        u_char cqt[64];

        /* convert from blocks to pixels */
        w <<= 3;
        h <<= 3;

        MakeTables(q, lqt, cqt);

        *p++ = 0xff;
        *p++ = 0xd8;            /* SOI */

        p = MakeQuantHeader(p, lqt, 0);
        p = MakeQuantHeader(p, cqt, 1);

        p = MakeHuffmanHeader(p, lum_dc_codelens, sizeof(lum_dc_codelens),
                              lum_dc_symbols, sizeof(lum_dc_symbols), 0, 0);
        p = MakeHuffmanHeader(p, lum_ac_codelens, sizeof(lum_ac_codelens),
                              lum_ac_symbols, sizeof(lum_ac_symbols), 0, 1);
        p = MakeHuffmanHeader(p, chm_dc_codelens, sizeof(chm_dc_codelens),
                              chm_dc_symbols, sizeof(chm_dc_symbols), 1, 0);
        p = MakeHuffmanHeader(p, chm_ac_codelens, sizeof(chm_ac_codelens),
                              chm_ac_symbols, sizeof(chm_ac_symbols), 1, 1);

        *p++ = 0xff;
        *p++ = 0xc0;            /* SOF */
        *p++ = 0;               /* length msb */
        *p++ = 17;              /* length lsb */
        *p++ = 8;               /* 8-bit precision */
        *p++ = h >> 8;          /* height msb */
        *p++ = h;               /* height lsb */
        *p++ = w >> 8;          /* width msb */
        *p++ = w;               /* wudth lsb */
        *p++ = 3;               /* number of components */
        *p++ = 0;               /* comp 0 */
        if (type == 0)
                *p++ = 0x21;    /* hsamp = 2, vsamp = 1 */
        else



                            Expires 1/1/97                     [Page 14]

Internet Draft         draft-ietf-avt-jpeg-03.txt              July 1996


                *p++ = 0x22;    /* hsamp = 2, vsamp = 2 */
        *p++ = 0;               /* quant table 0 */
        *p++ = 1;               /* comp 1 */
        *p++ = 0x11;            /* hsamp = 1, vsamp = 1 */
        *p++ = 1;               /* quant table 1 */
        *p++ = 2;               /* comp 2 */
        *p++ = 0x11;            /* hsamp = 1, vsamp = 1 */
        *p++ = 1;               /* quant table 1 */

        *p++ = 0xff;
        *p++ = 0xda;            /* SOS */
        *p++ = 0;               /* length msb */
        *p++ = 12;              /* length lsb */
        *p++ = 3;               /* 3 components */
        *p++ = 0;               /* comp 0 */
        *p++ = 0;               /* huffman table 0 */
        *p++ = 1;               /* comp 1 */
        *p++ = 0x11;            /* huffman table 1 */
        *p++ = 2;               /* comp 2 */
        *p++ = 0x11;            /* huffman table 1 */
        *p++ = 0;               /* first DCT coeff */
        *p++ = 63;              /* last DCT coeff */
        *p++ = 0;               /* sucessive approx. */

        return (p - start);
};

























                            Expires 1/1/97                     [Page 15]


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