Thursday, 12 October 2017

Size does matter

Finally, I've implemented the exploding ship routine! I had even avoided it on the Apple IIGS version which, of course, remains incomplete at this point. It's complicated, it's messy, it's slow - but I get the same results as Norbert, sans graphical bugs.

When it comes time to implement it 'properly' (optimised) on the Coco, I'll likely generate bitmaps of the pieces rather than use a table of pixel coordinates so I can use compiled sprites.

One thing that I did notice, however, on both the Apple IIGS version and the Coco version, is that the screen appears 'compressed' vertically compared to the Atari 800XL version, despite the screen resolution being similar.

Atari 800XL version by Norbert Kehrer

Note particularly the space between rows of text in the high score table.

Coco3 version (WIP) by yours truly

Although, as I said, I had noticed it before, I didn't give it much thought until now, attributing it to larger border areas on both platforms. Aside from reducing the playing area, it does cause the high score table text to overlap. In fact, on the high score entry screen, I (also) patched the message text coordinates to space them out a bit more.

I have confirmed that Norbert hasn't patched the high score display routine at all, so something is amiss. Far from being bad news though, this is actually encouraging, because on the IIGS version I did notice some subtle pixel offsets that didn't look quite right when compared to Norbert's version. I'm hoping then that fixing this issue will not only decompress the screen - and playing area - but also fix those subtle issues!

UPDATE: I've identified the issue with the screen compression, and it's both simpler and a little more complicated than I'd guessed. Asteroids world coordinates are 13 bits (0..8191) which are shifted down to 10 bits (0..1023) - with an X offset - when inserted into the display list for rendering. My rendering routine then takes the Y coordinate, for example, and scales it to (0..191). Norbert, however, doesn't scale the Y coordinate accordingly, but simply shifts it down to 0..255 and adds an offset for rendering, even though the Atari vertical resolution is also 192. That works because the visible vertical resolution on Asteroids isn't the full range 0..1023, but rather closer to 800.

Norbert also handles the objects and text slightly differently, adding a (different) Y offset for the text. It will take some further analysis before I can implement all the subtleties, and I also (now) have to add Y-axis clipping since it's now possible some objects may have a few lines off-screen. At least I've solved the mystery.

UPDATE #2: I've sorted the scaling issue, and the game now uses the full extent of the visible area, and removed my hack for the multi-line high score entry message. I've tweaked a few things to place the score at the very top, and the copyright at the very bottom, like the arcade game. It has, however, introduced a new bug with objects rendered near the bottom of the screen...

Re-scaled Coco3 version, using all 192 lines

I've decided to add all the tweaks before I start optimisations, since it will likely be more difficult to debug/modify once I add things like doulbe-buffering and compiled sprites. I also started to add the ship & shot offsets, but can't for the life of me get the ship erasing properly. I don't really understand why not, since - for now - I've cut-and-paste the render routine, replacing the instructions that write bitmap data with a CLR instruction... simply shouldn't be possible to not work. :(

I'm off interstate for a few days - sans computer - so no new updates for a while...

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