Thursday, 29th September 2005

Fire Track (on the BBC Micro) is probably the fastest, hardest, most adrenaline-packed space shooter I've ever played!
I'm playing my way through the extended version to try and work out the game mechanics a bit more. I've been using an ancient "infinite lives" cheat disk with it, but even so... whew, this game is tough.
I have saved state images for Levels 1 to 22, though, at last. My nerves are shot to pieces, in any case!

It appears that I have to rethink the way enemies are handled. Currently, I store a list of 8 enemies and assume they all follow the same pattern. Wrong. For most of the game, you can have lots of enemies on-screen, each from a different pattern! Or the patterns can overlap, even. What I think I'll do instead is have a list of enemies with ID (attack type - I'll use the sprite number), (X,Y) coordinate (if it's (0,0) it means the slot is free, no enemy) then 5 bytes of extra storage that the enemy attack handler for that enemy can do whatever the Hell they like with. If I have a timer that decreases and fires off the next enemy whenever it underflows (either a predetermined one from a table of enemy attacks or just picking a random one - or, for some of the bonus levels, none at all). Plus, I can rewrite it without lots of use of the slow IX register.

I have, however, added mine support. There's a function to add a mine at a particular (x,y), where it gets set up correctly and will drift towards you (the DX and DY are set up when you add the mine). They aren't part of the collision detection routines yet (I'll add them with the enemy system rewrite) but already scare me wink.gif

mines_2.png mines_1.png mines_3.png

The above is the worst-case scenario - there are not usually more than 4-5 mines on screen at any one time.

As you can probably also see, there is still no black bar at the top of the display. I can't quite work out how to enable/handle line interrupts from the VDP, but have found a nice part of the VRAM that isn't being written to that is just the right size for use as an alternate name table. The way this'll work is that I switch the background layer to another location in VRAM for which tiles to use that is all blacked out, wait until I hit a scanline 8 pixels down then switch back to the name table which contains my real background layer. I don't have to deal with cutting out any sprites as I have 8 sprites dedicated to the score at the top, and as you probably know by now from my constant moaning about it there is an 8-sprite-per-scanline limit hence my score will stop anything else from being drawn in the sprite layer at the top.

Finally, here's a picture of the biggest pest I know - linkified for the spider-haters. These buggers are not very large (5-8cms, biggest approaching 10cms) but very annoying - they seem to like following set paths, and will run around back and forth on the edge of my vision. This particular one was very reluctant to be picked up and lobbed out of the window...

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