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Network Working Group                                      M. SpeerRequest for Comment: 2029                                D. HoffmanCategory: Standards Track                    Sun Microsystems, Inc.                                                       October 1996RTP Payload Format of Sun's CellB Video EncodingStatus of this Memo   This document specifies an Internet standards track protocol for the   Internet community, and requests discussion and suggestions for   improvements.  Please refer to the current edition of the "Internet   Official Protocol Standards" (STD 1) for the standardization state   and status of this protocol.  Distribution of this memo is unlimited.Abstract   This memo describes a packetization scheme for the CellB video   encoding. The scheme proposed allows applications to transport CellB   video flows over protocols used by RTP.  This document is meant for   implementors of video applications that want to use RTP and CellB.1. Introduction   The Cell image compression algorithm is a variable bit-rate video   coding scheme.  It provides "high" quality, low bit-rate image   compression at low computational cost.   The bytestream that is   produced by the Cell encoder consists of instructional codes and   information about the compressed image.   For futher information on Cell compression technology, refer to [1].   Currently, there are two versions of the Cell compression technology:   CellA and CellB.  CellA is primarily designed for the encoding of   stored video intended for local display, and will not be discussed in   this memo.   CellB, derived from CellA, has been optimized for network-based video   applications.  It is computationally symmetric in both encode and   decode.  CellB utilizes a fixed colormap and vector quantization   techniques in the YUV color space to achieve compression.Speer & Hoffman             Standards Track                     [Page 1]

RFC 2029                   RTP Payload Format               October 19962. Network Packetization and Encapsulation2.1 RTP Usage   The RTP timestamp is in units of 90KHz. The same timestamp value is   used for all packet payloads of a frame.  The RTP maker bit denotes   the end of a frame.2.2 CellB Header   The packetization of the CellB bytestream is designed to make the   resulting packet stream robust to packet loss.  To achieve this goal,   an additional header is added to each RTP packet to uniquely identify   the location of the first cell of the packet within the current   frame.  In addition, the width and height of the frame in pixels is   carried in each CellB packet header.  Although the size can only   change between frames, it is carried in every packet to simplify the   packet encoding.    0                   1                   2                   3    0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |       Cell X Location         |      Cell Y Location          |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |       Width of Image          |      Height of Image          |   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+   |                     Compressed CellB Data                     |   |                             ....                              |   +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+   All fields are 16-bit unsigned integers in network byte order, and   are placed at the beginning of the payload for each RTP packet.  The   Cell X and the Cell Y Location coordinates are expressed as cell   coordinates, not pixel coordinates. Since cells represent 4x4 blocks   of pixels, the X or Y dimension of the cell coordinates range in   value from 0 through 1/4 of the of the same dimension in pixel   coordinates.2.3 Packetization Rules   A packet can be of any size chosen by the implementor, up to a full   frame.  All multi-byte codes must be completely contained within a   packet.  In general, the implementor should avoid packet sizes that   result in fragmentation by the network.Speer & Hoffman             Standards Track                     [Page 2]

RFC 2029                   RTP Payload Format               October 19963. References   1.      "Cell Image Compression Byte Stream Description,"ftp://playground.sun.com:/pub/multimedia/video/cellbytestream.ps.Z   2.      Turletti, T., and C. Huitema, "RTP Payload Format           for H.261 Video Streams",RFC 2032, October 1996.   3.      Schulzrinne, H., Casner, S., Frederick, R., and           V. Jacobson, "RTP: A Transport Protocol for Real-Time           Applications",RFC 1889, January 1996.   4.      Schulzrinne, H., "RTP Profile for Audio and Video           Conferences with Minimal Control",RFC 1890,           January 1996.4 Authors' Addresses   Michael F. Speer   Sun Microsystems Computer Corporation   2550 Garcia Ave MailStop UMPK14-305   Mountain View, CA 94043   Voice: +1 415 786 6368   Fax: +1 415 786 6445   EMail: michael.speer@eng.sun.com   Don Hoffman   Sun Microsystems Computer Corporation   2550 Garcia Ave MailStop UMPK14-305   Mountain View, CA 94043   Voice: +1 415 786 6370   Fax: +1 415 786 6445   EMail: don.hoffman@eng.sun.comSpeer & Hoffman             Standards Track                     [Page 3]

RFC 2029                   RTP Payload Format               October 1996Appendix A - Structure of the CellB Video Stream   The CellB bytestream consists of cell codes, skip codes and   quantization-table specific codes.  These are now described.A.1 CellB Cell Code   Cell codes are 4 bytes in length, and describe a 4x4 pixel cell.   There are two possible luminance (Y) levels for each cell, but only   one pair of chrominance (UV) values.  The CellB cell code is shown   below:    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   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |0 M M M M M M M M M M M M M M M|U V U V U V U V|Y Y Y Y Y Y Y Y|   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                4x4 Bitmask             U/V Code       Y/Y Code   The first two bytes of the cell code are a bitmask.  Each bit in the   mask represents a pixel in a 16-pixel cell.  Bit 0 represents the   value of the upper right-hand pixel of the cell, and subsequent bits   represent the pixels in row-major order.  If a pixel's bit is set in   the 4x4 Bitmask, then the pixel will be rendered with the pixel value   <Y(1), U, V>.  If the pixel's bit is not set in the bitmask, then the   pixel's value will be rendered with the value <Y(0), U, V>.  The   bitmask for the cell is normalized so that the most significant bit   is always 0 (i.e., corresponding to <Y(0), U, V>).   The U/V field of the cell code represents the chrominance component.   This code is in index into a table of vectors that represents two   independent components of chrominance.   The Y/Y field of the cell code represents two luminance values (Y(0)   and Y(1)).  This code is an index into a table of two-compoment   luminance vectors.   The derivation of the U/V and Y/Y tables is outside the scope of this   memo. A complete discussion can be found in [1].Speer & Hoffman             Standards Track                     [Page 4]

RFC 2029                   RTP Payload Format               October 1996A.2 CellB Skip Code   The single byte CellB skip code tells the CellB decoder to skip the   next S+1 cells in the current video frame being decoded.  The maximum   number of cells that can be skipped is 32.  The format of the skip   code is shown below.                         0 1 2 3 4 5 6 7                        +-+-+-+-+-+-+-+-+                        |1 0 0 S S S S S|                        +-+-+-+-+-+-+-+-+A.3 CellB Y/Y Table Code   The single byte "new Y/Y table" code is used to tell the decoder that   the next 512 bytes are a new Y/Y quantization table.  The code and   the representation of the table are shown below.  The sample   encoder/decoder pair in this document do not implement this feature   of the CellB compression.  However, future CellB codecs may implement   this feature.                         0 1 2 3 4 5 6 7                        +-+-+-+-+-+-+-+-+                        |1 1 1 1 1 1 1 0|                        +-+-+-+-+-+-+-+-+   The format of the new Y/Y table is:     0                   1                   2                   3     0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+    |    Y1_000     |    Y2_000     |   Y1_001      |   Y2_001      |    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                .                .                .     0                   1                   2                   3     0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+    |    Y1_254     |    Y2_254     |   Y1_255      |   Y2_255      |    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+Speer & Hoffman             Standards Track                     [Page 5]

RFC 2029                   RTP Payload Format               October 1996A.4 CellB U/V Table Code   The single byte "new U/V table" code is used to tell the decoder that   the next 512 bytes represent a new U/V quantization table.  The code   is shown below.  The sample encoder/decoder pair provided in this   document do not implement this feature of the CellB compression.   However, future CellB codecs may implement this feature.                         0 1 2 3 4 5 6 7                        +-+-+-+-+-+-+-+-+                        |1 1 1 1 1 1 1 1|                        +-+-+-+-+-+-+-+-+   The bytes of the new U/V quantization table are arranged as:     0                   1                   2                   3     0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+    |    U_000      |    V_000      |   U_001       |   V_001       |    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                .                .                .     0                   1                   2                   3     0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+    |    U_254      |    V_254      |   U_255       |   V_255       |    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+Appendix B - Availability of CellB   It is the viewpoint of Sun Microsystems, Inc, that CellB is   publically available for use without any license.Speer & Hoffman             Standards Track                     [Page 6]