Internet Engineering Task Force                 Michael F. Speer
Audio-Video Transport Working Group             Don Hoffman
draft-ietf-avt-cellb-profile-02.txt             Sun Microsystems, Inc.
                                                November 11, 1994
                                                Expires: 5/31/94


                RTP Encapsulation of CellB Video Encoding

                          Status of this Memo

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                                Abstract

        This note describes a packetization scheme for the CellB video
        encoding using RTP.  This document is meant for implementors of
        video applications that use RTP and CellB.



















draft-ietf-avt-cellb-profile-02.txt                             [Page 1]

INTERNET-DRAFT               CellB Profile                 November 1994


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 Structure of the CellB Video Stream

The CellB bytestream consists of cell codes, skip codes and
quantization-table specific codes.  These are now described.

2.1 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 s