Kostya: fill description

2024-09-18T13:32:10Z

fill description

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* Company: Access Software
* Used in: [https://www.mobygames.com/game/1454/the-pandora-directive/ The Pandora Directive]

This is an animation format that uses 16-bit static Huffman coding along with the fill/skip/pattern block coding.

File header (all values are little-endian):
4 bytes - "H2O\x01"
4 bytes - frame width
4 bytes - frame height
4 bytes - number of frames
48 bytes - unknown

Each frame starts with 24-bit full frame size and 8-bit flags telling which kinds of payloads frame has. Each payload starts with 32-bit size so it can be skipped. Known payloads are:
* <code>0x80</code> - new frame parameters (first two 32-bit words are new frame width and height)
* <code>0x40</code> - code definitions
* <code>0x01</code> - palette update
* <code>0x02</code> - video data

Please note that video data comes last.

=== Code definitions ===
This data defines canonical Huffman codes. It starts with two bytes telling the shortest and the longest code lengths. Then for each of the possible code lengths there is a 16-bit number telling how many codes have such length followed by the 16-bit values for those codes.

The defined codebook is used to unpack video data before processing it further.

=== Palette update ===
Palette updates are transmitted as one or more regions. First 16-bit value tells how many regions are total in the update. Each region start with a byte telling how many colours to skip from the end of the previous region (or 0 for the first region) and a byte telling how many colours are there in the region (0 means all 256 colours). Then a standard VGA palette follows.

=== Video decompression ===
After unpacking data as a stream of 16-bit values, those values are used as opcodes and data to reconstruct 4x4 blocks.

Top four bits of the opcode represent an operation:
* 0-3 - fill blocks. Bits 8-13 of the opcode (i.e. including the operation bits) represent run length (0-59 = 1-60, 60-63 = 128,256,512,1024), bits 0-7 are fill value (0 means skip instead of fill);
* 4 - two-colour pattern fill. Bits 0-11 of the opcode tell how many blocks to process this way (minus one). Then for each block there's 16-bit mask and two colours (16-bit value re-interpreted as two bytes);
* 5 - masked update. Bits 0-11 of the opcode tell how many blocks to process this way (minus one). Then for each block there's 16-bit mask telling which bits of the block to update and new pixel values packed in 16-bit elements (for odd number of pixels last value is stored in its own 16-bit value);
* other values are invalid.

[[Category:Game Formats]]

H2O - Revision history

Kostya: fill description