Tuesday, 17 June 2014

Finally got a-round to it!

Today lunchtime and tonight I tackled the circular wipe function. I'm pleased to say that it's working in both 1BPP and 2BPP mode. Again, somewhat paradoxically, it's (slightly) faster in 2BPP mode.

It's not enough to simply draw concentric circles; that leaves gaps which give rise to the moiré patterns we so often marvelled at in our BASIC type-in listings. So you need to draw thicker circles to overwrite the gaps. Although three (3) pixels is sufficient in this case, in assembler it's a nightmare, and slow. So I settled on 4 pixels - or a whole byte in 2BPP mode - and I defy you to detect a difference with the original Apple version. In fact, I strongly suspect it's doing the same (I haven't reverse-engineered the actual function itself).

After some first-pass optimisations the 2BPP mode is about 15% faster than the Apple version, so I've left it at that. 1BPP mode is roughly the same speed. I won't bother throttling it at all because after a while it is actually a bit annoying (but all part of the experience).

That just leaves sound and high score load/save, before I add all the levels.

Memory is really tight; in fact the stack is perilously close to the data in the latest build. There's still a bit of shuffling and slight memory optimisation I can do, and I'm also considering revisiting the title screen compression. But all this can wait until after the beta.

Unfortunately I didn't get the chance to order my DriveWire cable components today. I'll endeavour to do so tomorrow.

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