Even more display tweaks, looks even better. Still not perfect...

More tweaks to the display mapping, and it's improved even more. There was an offset added in the core (arcade) code when adding the CUR command to the DVG display list that must be peculiar to the vector hardware; I needed to remove that offset to enable objects to use the entire 192 lines of the display. Norbert didn't have this issue because he all-but ignores the display list, except for rendering text and his seemingly arbitrary offset (after scaling) accounts for it - and now makes sense of course.

I've also added clipping to the screen for all objects except the exploding ship. My explosion rendering code differs quite a bit from Norbert's; he uses a generic pixel-plot routine that handles the clipping (it'll render pixels outside the visible display on line 191, which is odd). I will probably not bother with clipping until I optimise the graphics for the Coco3.

After a few more hours of coding and comparing the Atari and Coco3 versions, I'm convinced there's still an issue with collision-detection (apparent in the video below). I'm reasonably sure it's not the display mapping now as sometimes the shots go straight through the middle of the large saucer, and occasionally you can't seem to hit the smallest asteroids. I simply cannot reproduce either bug on Norbert's emulator.

So now I need to go back and review the collision code which is not altogether surprising since it pretty much "worked" straight away. In fact I'm hoping that is the issue because otherwise everything else seems spot-on now, and I can definitely move on to Coco3 optimisations once this issue is sorted - something I've been itching to do for some time now.