tokra wrote:I meant that for each displayed frame only half the lines are drawn at all. You can check this with VICE-C64-emulator and stopping the emulator. Only about half of the visible lines will be shown, that's probably why Elite flickers so much, it's basically 2 x 12 frames instead of 24. Probably they couldn't get to a reasonable speed otherwise.
Actually, it is not quite that simple. The line routine in the C64 version of Elite doesn't simply set the pixels, it inverts them. Thus Elite always first draws all lines, pre-calculates all co-ordinates for the next frame, and then uses the same line routine to delete the lines again from the old co-ordinate set - and then loops for the next frame. Of course this method flickers horribly. For a few lines on screen it is however faster than calculating the co-ordinates, clearing the screen, and then plotting the lines.
Anyway, you'll need at least an off-screen bitmap so you won't see how the lines are plotted. On the C64, you can switch banks when the next frame is completed, so clearing the screen is the only overhead involved. But on the VIC-20, only the built-in RAM can be displayed by the VIC chip. Still, a off-screen bitmap is used in my demo, and copied into the display bitmap with a very fast blitting routine (hint: unrolled loop).
There were some hacks for the C128 and Plus/4 so some people are bound to have reverse engineered this version already.
I have seen them, too. It is not quite clear to me, whether these groups really had reverse engineered the source - or rather "simply" adapted the C64 version at those places where accesses to the VIC-II, and SID chip are made. These ports were made easier by the fact that the VIC-II in the C128 is nearly identical, and TED uses the same bitmap addressing scheme.
A port to the VIC-20 would require a complete re-vamp of the geometry engine, so the different output window is honored. Also the aspect ratio of the pixels is not 2:1, rather 1.67:1 for PAL, and 1.4:1 for NTSC, and this really needs to be taken care of. At least the necessary line routine is already there.
Ian Bell has published the
sources of the Electron version of Elite, and there's also a "
Text Elite" in C. I had taken a look in the assembly sources, and found them rather difficult to follow through, to say the least:
Ian Bell wrote:The BBC Elite sources were initially crammed onto a single sided 40-track disk (100K of data!) and comments and whitespace tended to suffer accordingly. The sources are extremely compact and terse. Some of the routines were developed on an Acorn Atom, hence the preponderance of variable names of the form [letter pair][number].
It's just a Small Matter Of Programming.
OK guys - what else? Doom? MS Flight Simulator? Europa Universalis?
