Date : Mon, 23 Sep 2002 17:12:38 +0100
From : Thomas Harte <thomasharte@...>
Subject: Re[1]: rupture technique
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> >By the way, something which I've always thought would be interesting would
> >be to use this technique to program a new start address every line (or
> >every other if timing conditions require) in order to make a Wolfenstein
> >type display on the side. You'd almost certainly not be able to do the ray
> >cast once per frame with all that going on, but you'd be in the region of
> >5-10fps, surely?
>
> Luckily enough, there is a technique similar to rupture that does exactly
> that, forcing a reload of the start address every fame.
Is that a small typo - and you mean every line? If not, is it at least possible
to reload the counter every other line or whatever? If that isn't possible,
please everyone politely ignore the remainder of this message.
> As for speed, I reckon that a textureless mode 8 raycaster using the above
> technique should run reasonably.
I don't see texturing to be a problem, if colour resolution is low.
Suppose we are in a 160 pixel mode, and we select to scale our slithers
so that the largest is indeed 160 pixels wide. Say that the minimum width
is 2 pixels, and we don't do anything like subpixel accuracy. That gives
only (160-2)/2 possibly sized slithers, which is just 79.
In 40 byte pitch mode, precalculating all the possibilities and putting
them on screen 'for free' simply by playing with the start addresses per
line costs only 3160 bytes, and any combination of texturing/depth lighting/1d
mip mapping that makes sense within 4 colours (40 byte pitch) is usable without
additional runtime cost.
Thats if the texture is 1d anyway. But at a severe stretch (or if you supported
sideways RAM machines only) you could fit 8 such tables into memory and use
a single 8 texel wide texture. Doesn't sound much, but would be okay for
a suitably repetitive texture like brickwork or wooden planks or something.
Presumably you use the VIAs so that a timing heavy section of code for pixel
covered parts of the display and a ray caster during the top/bottom border
and vsync could coexist? In which case I'm aware that the ray casting code
I've seen for the Atari Lynx can do around 10,000 casts per second - which
is a 4Mhz 65SC02 with an external unit providing hardware multiply. I guess
that means around 600-700 cycles per average ray cast on a normal 6502,
or around 560 rays cast per second assuming the normal display area of 256
lines. So, a shade over 2 fps. Although that rises to a little over 6 fps
if you cut the display down to 192 lines, the same height as a ZX Spectrum.
Now . . . . anyone want to give it a go? :)
-Thomas
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