Encryption/software decompression in GBA games

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Main - ROM Hacking - Encryption/software decompression in GBA games

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Techokami

Posted on 03-04-07 09:22 PM

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I believe the art for Pac-Man and enemies are encrypted, or compressed with a format other than LZSS, in the horrible GBA port of Pac-Man World 2. The graphics appear in VRAM as directly-accessed, uncompressed art, yet when you look in the ROM for uncompressed art, you do not find the desired graphics. Which is a big shame, as the graphics are the only things that don't suck horridly in this so-called "port".

So, what kind of encryption format (or software-driven compression format) would be feasible to use in a GBA game so that you can decrypt (or decompress) individual frames of a multi-frame sprite in real-time without affecting performance?

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interdpth

Posted on 03-04-07 09:37 PM

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If I remember I think the games were just using a variant of PocketNES or something since it's open source. :/

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Techokami

Posted on 03-04-07 11:34 PM

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Pac-Man World 2? It's not an NES-to-GBA game, it's a poorly-coded remake of the 3D platformer from the PS2/GCN/PC onto the GBA as an isometric platformer with good graphics.

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Tweaker

Posted on 03-05-07 12:22 AM

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Do the graphics appear to be 3D, or are you SURE they're actual sprite graphics? If the former is true, it's very possible that the game may be using a software based 3D renderer, converting polygons in realtime to uncompressed bitmaps and loading them to VRAM.

I'm not sure how practical compressing constantly updated art on the GBA is anyway - can it handle it well without lagging? I've not messed with GBA much.

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interdpth

Posted on 03-05-07 12:45 AM

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To check for LZ77 Compression or any compression Open the rom in VBA options -> Logger

Check SWI and DMA3

SWI are the software interrupts where hardware decomp is done. So might have some luck in there.

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Techokami

Posted on 03-05-07 12:55 AM

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Sprites are most definately not 3D-like. Here's a sample:

The GBA's ARM processor can't do on-the-fly LZSS decompression without a serious speed hit, by the way. Speaking of which, yes I did search the ROM for LZSS compressed data - I found some background stuff, but that's it. No dot muncher with legs.

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Dwedit

Posted on 03-07-07 08:36 PM

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LZSS decompression is actually about 50% the speed of straight memcopy, it's the fastest way to decompress data.

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Main - ROM Hacking - Encryption/software decompression in GBA games

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