RFC2029 - RTP Payload Format of Suns CellB Video Encoding

Network Working Group M. Speer

Request for Comment: 2029 D. Hoffman

Category: Standards Track Sun Microsystems, Inc.

October 1996

RTP Payload Format of Sun's CellB Video Encoding

Status 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.

2. Network Packetization and Encapsulation

2.1 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.

3. 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", RFC2032, October 1996.

3. Schulzrinne, H., Casner, S., Frederick, R., and

V. Jacobson, "RTP: A Transport Protocol for Real-Time

Applications", RFC1889, January 1996.

4. Schulzrinne, H., "RTP Profile for Audio and Video

Conferences with Minimal Control", RFC1890,

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.com

Appendix 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 MU V U V U V U VY 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].

A.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

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

A.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.