Star gazing

Have to admit I had a few days off doing much on Scramble. I did a little with non-tate mode on the Neo Geo but not enough to have anything worth blogging about.

Curiously there is some sort of 'glitch' in the rendering but I don't even understand the nature of the glitch at this point so I can't explain it. It manifests itself as an occasional dark flicker in a bottom tile of the landscape, which only happens whilst the draw_landscape() function is executing. But the landscape is only ever drawn off the visible area to the right, and once scrolled into the screen, is not updated. Furthermore, if I remove the code that clears the columns before rendering, the flickering seems to disappear. So I'm more inclined to conclude at this stage that it's specifically related to the Neo Geo implementation rather than the race conditions I saw earlier.

Moving on from there, I've been looking at the implementation of the stars in MAME. I should be able to mimic the distribution and the colour set; the precise blinking pattern may be another matter. It does cycle through 4 patterns of different stars being enabled/disabled. I think a close approximation will suffice.

It'll be a matter of generating a set of 16x16 tiles with the star patterns and using palette cycling to control them, perhaps even to enable/disable them as well. Like the tilemap, the stars will require a set of chained sprites in a fixed location. And being at the back in the priority order, I'll have to shuffle the existing sprites up a bit.

That leaves tweaking sprite positions, fixing Neo Geo-related implementation issues (controllers and coin-up)  and maybe look at sound, before I revisit non-tate mode.

I'm still undecided about attempting an Amiga port just now. I did just see reference to a video setting up gcc for Amiga development. I'm no Amiga expert so not sure how difficult scrolling a portion of the display is... but maybe that's a good excuse to learn.

UPDATE: I've fixed sprite and bullet position relative to the tilemap. Still have to tweak tilemap position on the Neo Geo though, so will likely require more tweaking. However I think I've got a bug in the bullet/rocket collision detection routine; occasionally the bullets appear to pass straight through rockets.

UPDATE #2: Had a quick play this morning and can see there's definitely an issue with bullet collision-detection at least, and it's not limited to rockets. Occasionally the bullet will pass straight through objects. A quick look at the code doesn't give any hints.

UPDATE #3: Had a look at the star generation for Galaxian in MAME. Scramble is slightly simpler as it doesn't scroll, but mapping the hardware implementation to the software framework of MAME convolutes the algorithm somewhat. In short, an LFSR generates the star positions and colour index (from a palette of 64), whilst further combinatorial logic controls the 4 cycles of blinking. There are a total of (just) 124 stars on the screen, not all visible at the same time due to the blinking effect.

All 124 stars generated for Scramble

I have actually implemented the same LFSR in the code to generate the star tiles for the Neo Geo, but there are a few unsolved issues that remain. Sprites are limited to 16 colours, whilst stars have 64. There are a number of approaches to that issue, including (effectively) assigning each star its own palette. Then there's the blinking; do I use palette cycling, or have 4 layers of sprites? Food for thought...