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# OpenGL texture mapping

Posted on 2000-09-26
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How can I map a texture automatically in OpenGl ???
I tried to use OpenGl's automatic texture coord generation, but it don't seems to work.
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Expert Comment

what object do you have on which you want to apply a texture? Is your question about loading a texture into OpenGL or is it about generating texture coordinates?

Ciao, Mike
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Accepted Solution

TheNeil earned 250 total points
This is not an easy problem. I've used the tutorials (and components) from NeHe and what I came up with two solutions (one nice, easy, and ok, and another which is horendous, 'perfect', and unworkable).

The best way I found was to do this:

Deal with only two dimensions (forget about the third, it makes things way too complex). Using your two chosen dimensions, find the maximum and minimum co-ordinate values in each of them. You can then 'projec' the texture onto the object by simply finding out where the projected shape lies within the rectangle defined by the maximum and minimum values. You then just need to scale everything to be in the range 0to 1.

If you need some code then just ask and I'll see what I can do

The Neil =:)
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Author Comment

Send me code please...
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mad666, you should accept an answer after you have seen the code not before. It could  well be that I had given you better code (although TheNeil's is surely not bad either).
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Expert Comment

Thanks Lischke (I think).

As for some code then this is what I use:

PROCEDURE CalculateMapping(ax, ay, az, bx, by, bz, cx, cy, cz : REAL;
rMaxX, rMinX, rMaxY, rMinY, rMaxZ, rMinZ : REAL;
VAR p1x, p1y, p2x, p2y, p3x, p3y : REAL;
iMapDir : LONGINT);
VAR
rSx : REAL;
rSy : REAL;
rSz : REAL;
BEGIN
rSx := 1 / ABS(rMaxX - rMinX);
rSy := 1 / ABS(rMaxY - rMinY);
rSz := 1 / ABS(rMaxZ - rMinZ);

CASE iMapDir OF
0 : BEGIN     //Side
p1x := (ax - rMinX) * rSx;
p2x := (bx - rMinX) * rSx;
p3x := (cx - rMinX) * rSx;
p1y := (ay - rMinY) * rSy;
p2y := (by - rMinY) * rSy;
p3y := (cy - rMinY) * rSy;
END;
1 : BEGIN     //Top
p1x := (ax - rMinX) * rSx;
p2x := (bx - rMinX) * rSx;
p3x := (cx - rMinX) * rSx;
p1y := (az - rMinZ) * rSz;
p2y := (bz - rMinZ) * rSz;
p3y := (cz - rMinZ) * rSz;
END;
2 : BEGIN     //Front
p1x := (az - rMinZ) * rSz;
p2x := (bz - rMinZ) * rSz;
p3x := (cz - rMinZ) * rSz;
p1y := (ay - rMinY) * rSy;
p2y := (by - rMinY) * rSy;
p3y := (cy - rMinY) * rSy;
END;
END;
END;

It gets called like:

GetXRange(rMinX, rMaxX, iObjectMatch);
GetYRange(rMinY, rMaxY, iObjectMatch);
GetZRange(rMinZ, rMaxZ, iObjectMatch);

CalculateAlphaMapping(<Point 1.x>, <Point 1.y>, <Point 1.z>,
<Point 2.x>, <Point 2.y>, <Point 2.z>,
<Point 3.x>, <Point 3.y>, <Point 3.z>,
rMaxX, rMinX, rMaxY, rMinY, rMaxZ, rMinZ,
px[1], py[1], px[2], py[2], px[3], py[3],
<Mapping Direction>);

The results get placed into the three arrays px, py, and pz (all of which are defined as ARRAY [1..3] OF REAL). These then contain the mapping values for each of the three points. You will need to write the three routines to get your point ranges (in the above code mine are called GetXRange, GetYRange, and GetZRange). These though are a total doddle as they just iterate through your co-ordinate data and find the maximum and minimum values

Hope this is what you're after (and hope it justifies the points)

The Neil =:)
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