Friday, 6 June 2014

Mysteries of Apple II artifacts

I took a look at the MESS Apple II video driver, and discovered that the artifact emulation (and there's no way of knowing exactly how accurate that is) is definitely more complicated than my quick 'n' dirty on-the-fly conversion.

So with this knowledge in mind, I hacked the MESS algorithm into my graphics conversion utility and managed to produce colourized versions of the title and tile graphics. Unfortunately the 2BPP title data is slightly larger than I bargained for, so I'll need to find a better compression algorithm, but that's for later. Here's the new colour title screen running on the Coco3, which in theory is identical to that rendered in the MESS Apple II driver:

4-colour title screen - in theory identical to Apple II display

And here is a screenshot from my utility, showing the original and colourized tile graphics:

The monochrome and 4-colour tiles, ready for import

So I now generate three (3) variations of the Lode Runner graphics; 1BPP and both monochrome and 4-colour 2BPP formats. I can build any variation simply by commenting (or not) each of a pair of .define statements.

The other area I've been working on today is the generation of the graphic data .BIN file. With thanks to Jason Law, I have a single .BIN file that contains the title and tile graphics data, that is generated not from binary data but in fact assembled from source .db statements. No more generating segments and binary data from a utility; Jason pointed out that it could all be done with .org statements in a single .ASM file! It also means that the memory map (including the graphics data addresses) can be defined in a single location in the source file - which of course is easy to tweak now!

Jason has also been hacking my code (in a good way) and has advised me on how to center the display on the Coco screen, and also get the code running on a 128KB machine. I'll be implementing his suggestions soon!

But for now, I need to finish the 4BPP tile import and then update the rendering routines to suit.

[And on this note, I've realised that the 4BPP mode will, in fact, affect the collision-detection algorithm used by Lode Runner. That's because I'm actually adding pixels to emulate the artifacting on the Apple. However, I would claim that it will not have any significant impact on the game-play.]

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