Difference between revisions of "Myst WDIB resources"

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{{Myst}}
 
{{Myst}}
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This page documents the WDIB image format for all images in the original Windows version of Myst and the cursors in Myst Masterpiece Edition. It is similar to LZ77 with a 0x400 byte ring-buffer. Once uncompressed, it is a plain Windows BMP 8bpp and the bitmap is not compressed. It is in little endian form. Thanks to Petroff Heroj and Ron Hayter for working out the compression.
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WDIB resources store compressed images in the original Windows version of Myst, some images in Myst Masterpiece Edition, and cursors.
  
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The data is laid out like this:
  
 
{| class="structure"
 
{| class="structure"
 
|-
 
|-
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|unsigned short||uncompressed size
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|unsigned long||uncompressed size
 
|-
 
|-
 
|variable||compressed data
 
|variable||compressed data
 
|}
 
|}
  
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For the compressed data, it uses the same [[Mohawk Bitmaps#LZ Compression|LZ Compression]] as some tBMP's!
  
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Until the end of the resource, each run begins with a byte. Each bit (0-7) defines what to do next. A 1 is an absolute byte (which is directly read into the uncompressed buffer) and a 0 represents two bytes to be read in. The front 6 bits (2-7) represent the length of the run minus 3. The last 10 bits represent the offset in the ring-buffer minus 0x42. If the offset is over 0x400, make sure to subtract 0x400 after adding the 0x42 back into the offset.
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Once decoded, a plain uncompressed [http://en.wikipedia.org/wiki/BMP_file_format 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.
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The ring-buffer has a constant size of 0x400 bytes. Remember that once you have decoded 0x400 bytes, it loops around to the beginning, completing the loop.
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For example:
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<pre>
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0xf7                    // decoder byte (11110111b)
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0x87                    // absolute byte
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0x73                    // absolute byte
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0x27                    // absolute byte
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0x0b                    // byte 1 of the run data: length = 2 + 3 = 5 (first 6 bits + 3)
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0xa9                    // byte 2 of the run data: offset = 0x3a9 + 0x42 = 0x3eb
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0x27                    // absolute byte
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0x32                    // absolute byte
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0x00                    // absolute byte
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0x4e                    // absolute byte
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</pre>
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From there, see the wikipedia page on the [http://en.wikipedia.org/wiki/BMP_file_format Bitmap file format].
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Latest revision as of 21:48, 17 August 2009

Myst
Mohawk Overview
CLRC EXIT HINT INIT
MJMP MSND PICT RLST
VIEW WDIB HELP RSFL
Scripts Variables

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

The data is laid out like this:

unsigned long uncompressed size
variable compressed data

For the compressed data, it uses the same LZ Compression as some tBMP's!

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.

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