Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ucbvax!amethyst.bucknell.EDU!hentosh
From: hentosh@amethyst.bucknell.EDU
Newsgroups: comp.sys.apple
Subject: Re:  3-D vector graphics in assembly
Message-ID: <8904220157.AA04218@amethyst>
Date: 22 Apr 89 01:57:07 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 41

>I'm sure most people have seen games like Stellar 7, Arctic Fox, and
>Elite.  While fairly simple, the graphics are nonetheless realistic 3-D.
>Question is, does anybody have source/object/vague general notions of
>how to do 3-D transformations in assembly language?
>
>I have a couple of old Nibble magazines, but those requires some transcen-
>dental functions and square roots (the latter of which is not well handled
>in the GS toolbox... +/- 3...).  Is there a better/faster way?
>
>On a related note, has anybody come up with a faster line drawing routine
>than Applesoft hires?  If you've ever looked at the code, you'd know that
>writing faster code would be difficult... understanding the code is hard
>enough in itself...
>
>fadden@cory.berkeley.edu (Andy McFadden)
I wrote some fast 3d routines awhile ago, I don't have the source code
but the general idea is that you compute the functions for the possible
inputs and then place them into a lookup table. Then if you want a value
just look it up.  Now if do some checking of the position functions you
will see that you don't need much accuracy for the functions so your table
is actually quite small than you would think.

I don't know what routines applesoft uses to draw a line, but a fast one I
know is to take the end point and subtract the other end point from it.
Choose the point that is the lower one to use as your subtrand.
Now the delta_y over delta_x is the slope of the line. Start at the position
that you subtracted and draw a dot. If the delta_x is greater than the
delta_y then draw the next dot to the right of the previous and subtract one
from the delta_x, otherwise draw the next dot above the previous position
and subtract one from the delta_y. repeat until both delta_x and delta_y
equal zero.  This method is fast becuase it doesn't use division. You will
note however it is for lines that only have a positive slope. To do the
negative slope subtract from the delta_y and move down instead of up on your
check.  Also this algorithm is nice in the sense that if you give it the same
two endpoints, it will always draw the same line. (You won't have any stray
pixels if you try to erase a line by drawing another one over it) I
leave the rest to you, have fun.

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