Here are the background bitmaps (or title screens,) taken from dave2 floppy. All of these images were compressed using some flavor of Huffman. Decompression code starts at seg000:7936h, and was quite clean to follow. Figuring that this is the actually unhuff code was quite easy: each compressed file has a nice “HUFF” signature, I looked up the string and from there I just checked cross-reference. There was only one location that had anything to do with the signature string, and that very same function also prints out “Tried to expand a file that isn’t HUFF!” when it fails. So, as they say, X marks the spot.

Rewriting deflate algorithms is always tricky. Even if you know how it is supposed to work, you can never be sure the original coders haven’t tweaked it for one reason or another. When I get to a decompression code, I just copy-paste the assembly code into a clean C file. `Hello, my name is Gil, and I’m a goto user`. The initial code that was written and actually ran, looked completely horrible. The variable names were named after the registers; arrays were just prefixed ‘dummy’. I consider rewriting obscure code a game on its own. You start with something that works okay, but looks bad, and your goal is to make it look good and readable, without breaking what already works.

Within less than an hour I have rewritten the deflation code, got rid of all the assembly leftovers, and as a Good Samaritan, I even commented my code. After unpacking these full screen bitmaps, I noticed a new file format. It begins with “PIC\0“, followed by a short valued 320, and another that is 200. The other 32000 bytes are 320×200 pixels layered as 4 bitplanes (gotta love EGA!)

Coders who have done some EGA work know exactly what pain it is to work with these bit planes. Looking back at this, it is very good I started with the full screen bitmaps; it is much easier to try and figure out what’s going on a big bitmap, rather on a 16×16 sprite. With my swiss-knife uber-library PIL (Python Imaging Library), I hacked up a small script that converts these 32008 files into PNGs.

The decompression code is available for download, and is called decompress.c (well, d’uh).

progpic.dd2- displays player status during load

title1.dd2

title2.dd2

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