Creative YUV (CYUV) stream format for AVI ----------------------------------------- Dr. Tim Ferguson, 2001. Originally designed for the Creative Labs Video Blaster, the Creative CYUV video codec stores frames in a DPCM coded YUV411 format. Encoding and decoding of frames was originally performed by the vblaster.drv Windows driver. This codec looks to have been designed for simplicity and speed. The CYUV codec is a very simple predictive coder. A video frame is coded in the YUV colour space, with the chrominance components subsampled by four in the horizontal direction (YUV411). That is, for four horizontal Y pixels there are only one U and V pixels: +----+----+----+----+ +---+ +---+ | y0 | y1 | y2 | y3 | | u | | v | +----+----+----+----+ +---+ +---+ Note this is different to the YUV420 formats used by the ITU standards (MPEG, etc). Predictive coding is reset at the start of each scan line, with the very first Y, U and V component pixels being coded using the upper four bits of their absolute values. These provide the initial predictions for the remainder of the scanline. Each subsequent Y, U and V pixels in the scanline are coded using the difference between the last decoded pixel and the current pixel of the same component (DPCM). This difference is known as a prediction error (ie: the previous pixel is used as a prediction for the current pixel). The prediction error values are coded using three 16 entry tables of signed values found at the start of the frame. The first of the three tables appears to be unused (see Note 1). The second table is used to code the prediction errors for the Y component and the third table is used to code the prediction errors for both the U and V components. These tables allow for either linear or non-linear quantisation of the prediction errors. Since there are 16 entries per table, this requires four bits to index. Thus four bits per component are used, which are packed into bytes as follows: +----+----+ +----+----+ +----+----+ +--+--+ +--+--+ +--+--+ | U1 | Y1 | | V1 | Y2 | | Y4 | Y3 | |U2|Y5| |V2|Y6| |Y8|Y7| UY VY ... +----+----+ +----+----+ +----+----+ +--+--+ +--+--+ +--+--+ 7 4 0 7 4 0 7 4 0 Therefore, four YUV411 pixels are coded into three bytes providing six bits per pixel. If the above diagram were the start of a scanline, the Y1, U1 and V1 values would be coded using the upper four bits of their absolute values (initial predictions). The Y2, Y3, Y4,... values would then be coded as prediction errors using table two, and the U2, U3, ... and V2, V3, ... values would be coded as prediction errors using table three. The overall format of a CYUV coded frame is as follows: +-----------------------+ | Prediction Table 1 | 16 bytes +-----------------------+ | Prediction Table 2 | 16 bytes +-----------------------+ | Prediction Table 3 | 16 bytes +-----------------------+ | . . . | (width * height * 6)/8 bytes DPCM coded YUV pixels | . . . | +-----------------------+ Note 1: This format was tested using the xanim (http://xanim.va.pubnix.com/) video player. Therefore, the format may not be strictly compatible with the old vblaster.drv. This has not been tested. --------------------------------------------------------------------- This document was written by Dr. Tim Ferguson, 2001. For more details, on this and other codecs, including source code, visit: http://www.csse.monash.edu.au/~timf/videocodec.html To contact me, email: timf@csse.monash.edu.au ---------------------------------------------------------------------
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