Skipped Block Data Format
It is a standard feature of Indeo codecs to mark certain 4 x 4 pixel "blocks" of data in the input buffer as "skipped," or not to be copied. This is done to reduce bus writes, and to speed frame updates. At the end of the IF09 buffer is an array of bytes which denotes these skipped blocks, which are encoded in the lower nibble of each byte. The least significant bit of each byte represents the flag to skip the top row of the corresponding 4x4 block, bit 1 is the second row, etc. Thus, if a color converter is interested only in 4x4 blocks that can be skipped completely, it should compare this byte to the value 0xF. If a converter is able to take advantage of DWORD skips, it may do so by checking the individual bit flags. The upper nibble of each byte in the skipped block array is considered reserved set to zero and may be used for later extensions to the Indeo format. The first byte in the skipped block array corresponds to the upper left corner block of the image, with blocks proceeding left to right and top to bottom.