The ANHD chunk consists of the following data structure: UBYTE operation The compression method: =0 set directly (normal ILBM BODY), =1 XOR ILBM mode, =2 Long Delta mode, =3 Short Delta mode, =4 Generalized short/long Delta mode, =5 Byte Vertical Delta mode =6 Stereo op 5 (third party) =74 (ascii 'J') reserved for Eric Graham's compression technique (details to be released later). UBYTE mask (XOR mode only - plane mask where each bit is set =1 if there is data and =0 if not.) UWORD w,h (XOR mode only - width and height of the area represented by the BODY to eliminate unnecessary un-changed data) WORD x,y (XOR mode only - position of rectangular area representd by the BODY) ULONG abstime (currently unused - timing for a frame relative to the time the first frame was displayed - in jiffies (1/60 sec)) ULONG reltime (timing for frame relative to time previous frame was displayed - in jiffies (1/60 sec)) UBYTE interleave (unused so far - indicates how may frames back this data is to modify. =0 defaults to indicate two frames back (for double buffering). =n indicates n frames back. The main intent here is to allow values of =1 for special applications where frame data would modify the immediately previous frame) UBYTE pad0 Pad byte, not used at present. ULONG bits 32 option bits used by options=4 and 5. At present only 6 are identified, but the rest are set =0 so they can be used to implement future ideas. These are defined for option 4 only at this point. It is recommended that all bits be set =0 for option 5 and that any bit settings used in the future (such as for XOR mode) be compatible with the option 4 bit settings. Player code should check undefined bits in options 4 and 5 to assure they are zero. The six bits for current use are: bit # set =0 set =1 =============================================== 0 short data long data 1 set XOR 2 separate info one info list for each plane for all planes 3 not RLC RLC (run length coded) 4 horizontal vertical 5 short info offsets long info offsets UBYTE pad This is a pad for future use for future compression modes.
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