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                John McCarthy's Constant Slope Texture Mapping

                                  (JTEXTURE)

  Texture mapping coded by:

  Written by: John McCarthy (aka Flynn)
              1316 Redwood Lane
              Pickering, Ontario.
              Canada, Earth, Milky Way (for those out-of-towners)
              L1X 1C5

  email: BRIANMCC@IDIRECT.COM
     or: BRIAN.MCCARTHY@CANREM.COM
     or: RY80@JECALPHA.KA.SUB.ORG

  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!

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        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.

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

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