Tuesday, 8 August 2017


I've been digging into Norbert's Atari 800XL Asteroids Emulator and trying to ascertain if/where offsets are used for various objects. To re-iterate; the position of an object is relative to the first point in the draw list of vectors that comprise the object. OTOH the position of a bitmap is (always) relative to one corner of the bounding rectangle. So in theory, displaying bitmaps in place of vector objects should require an offset for each object.

I had luck with the player ship and (all) shots. The ship appears to be a lot closer to its true location now, as evidenced by the much reduced incidence of asteroids merely passing close-by and destroying your ship. Shots are definitely much more accurate - if not perfect - as hitting the smaller asteroids requires you to, well, actually hit them!

In order to maintain the current performance, the offsets are coded directly into the compiled sprites, rather than simply offsetting the object's coordinates and thus requiring additional calculations. It did unfortunately necessitate one extra conditional branch in the ship rendering dispatcher because of an odd-valued X offset, but there was no avoiding it.

Characters don't appear to have any offsets applied, as far as I can see. As for asteroids & saucers - eek! There's a few tables with small values that could feasibly contain offsets (one, for example, is indexed by asteroid shape and size) but I'm not sure I'm going to be able to reverse-engineer exactly how it all works as it looks to my untrained eye that it starts to get into display lists. And those routines appear to be working directly on the vector hardware 1024x1024 coordinates as well.

To further add a spanner in the works, played against the Atari version there do still appear to be some inaccuracies between ship and shot and ship and asteroid, albeit subtle. They'll be fun to track down...

Anyway, I'm happy with the progress thus far, and to be honest, it's probably not going to be noticeable to any but the hard-core Asteroids experts out there... and they're not likely to be playing it on the IIGS I wouldn't think. I will look further into it, and perhaps it's a good excuse to crack open Norbert's C64 version which might be a little easier (for me) to follow.

However, I might actually move on now to the ship explosion and then flicker and sound before returning to this issue. I did notice tonight that with only a few small asteroids on the screen, the game is definitely running way too fast, so I'll look at throttling as well.

Discussions on the IIGS FB page tonight have me interested already in another project, though I would only consider it after I've done a proper feasibility study and I have collaborators. I have to admit that I even loaded up the ROM in IDAPro and had a quick look at the start-up code in my lunch break.

But at this stage it's still only a slight possibility and I still have other plans for Asteroids before I put it to bed.

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