Myst WDIB resources

WDIB resources store compressed images in the original Windows version of Myst, some images in Myst Masterpiece Edition, and cursors.

The format is similar to LZ77 with a 0x400 byte ring buffer. Once decoded, a plain uncompressed Windows BMP is recovered. All in the original Myst are 8bpp. However, some in Myst ME are 24bpp (like the images in Help.dat) but the cursors are still 8bpp. It is in little endian form. Thanks to Petroff Heroj and Ron Hayter for working out the compression.

The data is laid out like that:

unsigned long	uncompressed size
variable	compressed data

Until the end of the resource, each run begins with a byte. Each bit of this byte defines what to do next, starting from the least significant one.

• A 1 means an absolute byte follows. Read a byte from the compressed data and store it directly into the uncompressed buffer.
• A 0 means a length/offset pair follows. Read two bytes b1 and b2 from the compressed data. The most significant 6 bits of b1 represent the length of the run minus 3. The 2 least significant bits of b1 and the whole b2 form together a 10-bit offset into the ring buffer, minus 0x42. At this point copy length bytes from the ring buffer, starting at offset, to the uncompressed buffer. If offset is over 0x400 make sure to subtract 0x400 after adding the 0x42, i.e. loop around to the beginning of the ring buffer.

The ring-buffer should be initialized to all zeroes. Remember to store the uncompressed bytes in the ring buffer as well, looping to the beginning after 0x400 bytes.

For example:

0xf7                     // decoder byte (11110111b)
0x87                     // absolute byte
0x73                     // absolute byte
0x27                     // absolute byte
0x0b                     // byte 1 of the run data: length = 2 + 3 = 5 (first 6 bits + 3)
0xa9                     // byte 2 of the run data: offset = 0x3a9 + 0x42 = 0x3eb
0x27                     // absolute byte
0x32                     // absolute byte
0x00                     // absolute byte
0x4e                     // absolute byte