Myst WDIB resources - Revision history

Clone2727: the LZ decompression info has moved to the Mohawk Bitmaps page

2009-08-17T20:48:13Z

the LZ decompression info has moved to the Mohawk Bitmaps page

TitoDalCanton: a bit of restyling

2008-09-04T13:21:12Z

a bit of restyling

Clone2727: oops!

2008-09-03T20:48:26Z

oops!

TitoDalCanton: cosmetic changes

2008-09-02T16:56:33Z

cosmetic changes

Clone2727: some more notes

2008-08-22T00:34:06Z

some more notes

Clone2727: err... oops

2008-07-19T21:42:40Z

err... oops

Clone2727: adding the details of the Myst WDIB compression

2008-07-19T21:41:49Z

adding the details of the Myst WDIB compression

New page

{{Myst}}
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.

{| class="structure"
|-
|unsigned short||uncompressed size
|-
|variable||compressed data
|}

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.

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.

For example:
<pre>
0xf7 // decoder byte (11110111b)
0x87 // absolute byte
0x73 // absolute byte
0x27 // 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
0x32 // absolute byte
0x00 // absolute byte
0x4e // absolute byte
</pre>

From there, see the wikipedia page on the [http://en.wikipedia.org/wiki/BMP_file_format Bitmap file format].

@@ Line 2: / Line 2: @@
 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 [http://en.wikipedia.org/wiki/LZ77 LZ77] with a 0x400 byte ring-buffer. Once uncompressed, 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. It is in little endian form. Thanks to Petroff Heroj and Ron Hayter for working out the compression.
+The format is similar to [http://en.wikipedia.org/wiki/LZ77 LZ77] with a 0x400 byte ring buffer. 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. 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:
@@ Line 13: / Line 13: @@
 |}
-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. Also, the ring-buffer should start out as all zeroes (as the offsets will reference any part of the buffer).
+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.
-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.
+* 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.
 For example:

@@ Line 1: / Line 1: @@
 {{Myst}}
-This page documents the WDIB image format for all images in the original Windows version of Myst and some images in Myst Masterpiece Edition. It is similar to LZ77 with a 0x400 byte ring-buffer. Once uncompressed, it is a plain Windows BMP and the bitmap is not compressed. 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.
+WDIB resources store compressed images in the original Windows version of Myst, some images in Myst Masterpiece Edition, and cursors.
 {| class="structure"
@@ Line 9: / Line 12: @@
 |variable||compressed data
 |}
 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. Also, the ring-buffer should start out as all zeroes (as the offsets will reference any part of the buffer).
@@ Line 28: / Line 30: @@
 x4e                     // absolute byte
 </pre>

@@ Line 2: / Line 2: @@
 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 [http://en.wikipedia.org/wiki/LZ77 LZ77] with a 0x400 byte ring buffer. 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. It is in little endian form. Thanks to Petroff Heroj and Ron Hayter for working out the compression.
+The data is laid out like this:
-−
-The data is laid out like that:
 {| class="structure"
@@ Line 13: / Line 11: @@
 |}
-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.
+For the compressed data, it uses the same [[Mohawk Bitmaps#LZ Compression|LZ Compression]] as some tBMP's!
-* 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:
+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.
-<pre>
-xf7                     // decoder byte (11110111b)
-x87                     // absolute byte
-x73                     // absolute byte
-x27                     // absolute byte
-x0b                     // byte 1 of the run data: length = 2 + 3 = 5 (first 6 bits + 3)
-xa9                     // byte 2 of the run data: offset = 0x3a9 + 0x42 = 0x3eb
-x27                     // absolute byte
-x32                     // absolute byte
-x00                     // absolute byte
-x4e                     // absolute byte
-</pre>

@@ Line 8: / Line 8: @@
 {| class="structure"
 |-
-|unsigned short||uncompressed size
+|unsigned long||uncompressed size
 |-
 |variable||compressed data