| ||Definition:|| ||Cell is a compression technique developed by Sun Microsystems Inc. The compression algorithms, the bit-stream definition, and the decompression algorithms are open. That is Sun will tell anybody who is interested about them . Cell compression is similar to MPEG and H.261 in that there is a lot of room for value-add on the compressor end. Getting the highest quality image from a given bit count at a reasonable amount of compute is an art. In addition the bit-stream completely defines the compression format and defines what the decoder must do and there is less art in the docoder.
There are two flavors of Cell: the original called Cell or CellA, and a newer flavor called CellB. CellA is designed for use many times video, where one does not mind that the encoder runs at less than real time. For example, CD-ROM playback, distance learning, video help for applications. CellB is designed for use once video where the encoder must run at real time (interactive) rates. For example, video mail and video conferencing.
Both flavors of cell use the same basic technique of representing each 4x4 pixel block with a 16-bit bitmask and two 8-bit vector quantized codebook indices. This produces a compression of 12-1 (or 8-1) since each 16 pixel block is represented by 32 bits (16-bit mask, and two 8-bit codebook indices). In both flavors, further compression is accomplished by checking current blocks against the spatially equivalent block in the previous frame. If the new block is "close enough" to the old block, the block is coded as a skip code. Consecutive skip codes are run-length encoded for further compression. Modifying the definition of close enough allows one to trade off quality and compression rates. Both version of Cell typically compress video images down to about .75 to .5 bits/pixel.
Both flavors have many similar steps in the early part of compression. For each 4x4 block, the compressor calculates the average luma of the 16 pixels. It then partions the pixels into two groups, those whose luma is above the average and those whose luma is below the average. The compressor sets the 16-bit bitmask based on which pixels are in each partition. The compressor then calculates a color to represent each partition.
In Cell, the compressor calculates an average color of each partion, it then does a vector quantization against the Cell codebook (which is just a color-map). The encoded block is the 16-bit mask and the two 8-bit colormap indices. The compressor maintains statistics about how much error each codebook entry is responsible for and how many times each codebook entry is used. It uses these numbers to adaptively refine the codebook on each frame. Changed codebooks are sent in the bitstream.
In CellB, the compressor calculates the average luma for each partition and the average chroma for the entire block. This gives two colors [Y_lo, Cb_ave, Cr_ave] and [Y_hi, Cb_ave, Cr_ave]. The pair [Y_lo, Y_hi] is vector quantized against the Y/Y codebook and the pair [Cb_ave, Cr_ave] is vector quantized against the Cr/Cb codebook. Here the encoded block is the 16-bit mask and the two 8-bit VQ indices. Both of CellB's codebooks are fixed. This allows both the compressor and decompressor to run at high-speed by using table lookups. Both codebooks are designed with the human visual system in mind. They are not just uniform partition of the Y/Y or Cr/Cb space. Each codebook has fewer than 256 entries.
Cell (or CellA) is supported in XIL 1.0 from SMI. It is part of Solaris 2.2. CellB is supported in XIL 1.1 from SMI. It will be part of Solaris 2.3 when that becomes available. Complete bitstream definitions for both flavors of cell are in the XIL 1.1 programmer's guide. There is some discussion of the CellA bitstream in the XIL 1.0 programmer's guide.
CellB was used for the SMI Scott McNealy holiday broadcast, where he talk to the company in real-time over Sun Wide Area Network. This broadcast reach from Tokyo Japan to Munich Germany with over 3000 known viewers.|