Constant Slope Texture Mapping
by John McCarthy

The following is the README.DOC file located inside the zip file. There is a link at the bottom of this page to download the zip file with all the source code and example files. Please note that the sample file is written in Watcom C and requires dos4gw.exe in order to run.

As a note, John was the person who wrote that 3D Texture mapped "Asteroids" game called Outer Ridge.

Texture mapping coded by:

Home phone,(905) 831-1944 Canada.
Work phone, 0721 9203183 Germany.

Yes, I actually live in Canada, and work in Germany. I'm always willing to talk but don't call at 2 am ok!

Send me a postcard from someplace near where you live.

Wanna help write an Operating System? Call or write to me today!

There are a billion worthless articles about "Can texture mapping be done with constant slope?" and "How can I do texture mapping?" It is sickening to look at the crap written by Micheal Abrash and actually have people think "Wow, he's smart". He's a dork, (sorry Micheal). He writes great sounding titles for his articles, but it takes about the same intellegance to write texture mapping as it does to flush a toilette.

Ok, to close the issue once and for all. Here is a routine I wrote a while back to demonstrate the simplicity of it all. The routine does not perform the "2 divisions per scanline" crap, but uses the slope of the triangle through the center as the interior texture slope for the entire triangle.

It's so simple:

mid = distance across center of triangle:

      0
"A"  /|
    / |
   /  |
  /___|
 1\mid|q
   \  |
    \ |
     \|
      2

slope.right            = (x.2 - x.0) / (y.2 - y.0)
mid                    = slope.right * (y.1 - y.0) - x.1

exteroir.slope.x.right = (tx.2 - tx.0) / (y.2 - y.0)
exteroir.tx.q          = tx.0 + exteroir.slope.x.right * (y.1 - y.0)
interior.slope.x       = (exteroir.tx.q - tx.1) / mid

exteroir.slope.y.right = (ty.2 - ty.0) / (y.2 - y.0)
exteroir.ty.q          = ty.0 + exteroir.slope.y.right * (y.1 - y.0)
interior.slope.y       = (exteroir.ty.q - ty.1) / mid

The only real things need from the above formulas are:

exteroir.slope.x.right      interior.slope.x
exteroir.slope.y.right      interior.slope.y

"B" type triangle (see code) have similar formulas, just opposite sides.

Please excuse the sloppy C code as this was done over a weekend, back when I was new to C. Please also excuse the use of globals in the .ASM as we all know every piece of code we ever write should never contain a global variable. (globals = yuk)

Hope you enjoy the code, and can find it useful, entertaining, or whatever. This code is provided free, and you may do with it whatever you want. Destroy it or make DOOM - (yeah right). If you use it in something cool, like a demo or a game, it would be kind of nice if you sent me a copy - it only costs 1 disk + postage.

I give this code away, since I now have much better code, with much more flexability and smoothness. The TEXT.ASM code was just a test program.

The constant slope method of drawing gouraud shaded triangles, or for texture mapping also works great for perspective texturing. The cube demo I have supplied has no perspective applied to it, and therefore looks quite wobbley, but don't fret - simply by cutting triangles into smaller triangles, (using some sort of tolerance value) makes the triangles look perspectivly correct. I don't have a fancy demo to show you, maybe next month. But it looks great. I don't think there is any real need to perform true perspective texturing after seeing the excellent pseudo perspective effects triangle splitting can give. Try it - I'll have a demo of this for you next month.

Wanna help write an Operating System? (905) 831-1944, Canada.

If you have a comment about this page please send e-mail to: David S. Issel

Last update: 12/18/95

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Date this article was posted to GameDev.net: 7/16/1999
(Note that this date does not necessarily correspond to the date the article was written)

See Also:
Texture Mapping

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