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2008-04-17 20:36

doynax
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Registered: Oct 2004
Posts: 212

Your favorite 3d tricks

In the interest of educating newbies like me who don't yet know all the tricks, and detracting attention from that ridiculous multithreading "debate" I thought we could share some tips on how to write a speedy vector part.
Because the depressing fact is that aside from this very forum there seems to be about zero information on how to do 3D on 8-bit systems, unless it's all been hiding in some old diskmag somewhere (and as far as I'm concerned information that isn't indexed by Google might as well not exist).

Now, to start things off..

You can get rid of all those nasty multiplications when building a rotation matrix from Euler angles by exploiting the trigonometric product rules ( 2*cos x*cos y = cos(x - y) + cos(x + y) and friends ). I realize that this is probably common knowledge but its far from obvious and without it those multiplications are real cycle eaters. Oh, and thanks for the tip WVL :)

Another method is an alternative to Bresenham, or fixed point slopes, for deciding whether to continue on the major axis or move diagonally when drawing lines. The idea here is simply that those decisions can easily be precalculated and stuffed into bit arrays. The tricky part here is that you can use SBX to test these bits while letting the same mask for figuring out what pixels to plot in a horizontal run in turn mask our decision bits in X.

In other words:

	lda #%11111111
	sbx #%10000000
	bcs .x1

	eor #%01111111
	eor pixels,y
	sta pixels,y
	iny

	lda #%01111111
.x1	sbx #%01000000
	bcs .x2

	eor #%00111111
	eor pixels,y
	sta pixels,y
	iny

	lda #%00111111
.x2	sbx #%00100000
	bcs .x3
	.
	.
	.

So please post any interesting suggestions you've got. Any efficient and/or elegant methods for doing clipping, transformations, line drawing, lighting, dithering and all the other black arts..

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2008-04-24 18:46

doynax
Account closed

Registered: Oct 2004
Posts: 212

Quoting Oswald

you can only get away with dots/bobs without perspective imho.

I've been trying out a few things in my C prototype and the whole thing is kind of weird.
For low-polygon objects (say a cube or a tetrahedron) you truly *need* a small bit of perspective or the the illusion of 3d falls apart, it doesn't have to be much mind you but it has to be there (about 10% of scaling difference between nearest and farthest points is sufficient). On the other hand if you try let the Z value have more than a 30% effect on more complex objects (an icosahedron or a dodecahedron in my case) then it just looks *wrong*. And here I see no problems whatsoever when dropping Z altogether.

Yet I don't particularly feel like maintaining separate code paths for them. Perhaps the simple objects would look better with a bit of lighting?

Quoting Oswald

I think you are aiming simply too high, your methods of doing 3d actually represent the state of the art way of doing it on the c64, you cant do it much faster. 98% of c64 demos doing 3d do it slower than how you already do it.

Trying to optimize this bloody program has become a matter of diminishing returns, and the transformation code just happens to be the easiest thing to attack right now.
I've just run out of practical ideas on how to squeeze cycles out of the line drawing or filling code and I'm not quite ready to give up yet, which is why I created this thread in the first place I suppose.

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