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supercoiled

OpenGL Texturing problems [UV vs. STRQ]

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Heya people! I've got a serious problem when i try to texture polygons in 3D space. The textures get 100% right displayed if the polygons occure almost on the XZ-plane, but if they appear to be plane in front of the viewer it seems like one span of the texture gets clamped all over the primitive. The texture coordinates provided by the mesh i try to display are 3 U and 3 V coordinates per polygon and that makes me confused too, since i always were only confronted with 1 U and 1 V per primitive. I even do not know how to apply them to OpenGL's STRQ texturing system. (Although i know what texture coordinates mean in general.) I even tried to handle it through automatic texture coordinate generation using glTexGen with GL_OBJECT_LINEAR as technique used over each of the coordinates. It's not because i want you to fix my code, but it would be nice if someone could explain what i'm making wrong. Thanks for every constructive reply! Greetings, supercoiled

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Quote:
Original post by supercoiled
...
I've got a serious problem when i try to texture polygons in 3D space. The textures get 100% right displayed if the polygons occure almost on the XZ-plane, but if they appear to be plane in front of the viewer it seems like one span of the texture gets clamped all over the primitive.
...
I'm not sure what you mean by that. Could you post a screenshot of the problem (use some image hosting site like ImageShack)?
Quote:
Original post by supercoiled
...
The texture coordinates provided by the mesh i try to display are 3 U and 3 V coordinates per polygon and that makes me confused too, since i always were only confronted with 1 U and 1 V per primitive. I even do not know how to apply them to OpenGL's STRQ texturing system. (Although i know what texture coordinates mean in general.)
...
OpenGL's STRQ texture coordinates is just a different naming convention; U and V correspond to OpenGL's S and T coordinates, respectively, so there's no need to convert them.

Only having 1 UV set per primitive is kind of odd, I think you probably mean 1 UV per vertex. My guess is that the 3 UVs per polygon means you are given triangles with 1 UV for each vertex in the triangle.

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I thought UV and ST could probably be the same, but i never saw a matching description.
The problem is that most polygons of the scene provide 4 vertices each, but only these three UV.

Here we go:
Scene.jpg
It's a bit too much blended, but i think you can see what i mean, the wall to the left is clamped and not repated.

Thanks for the reply!

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It looks like you are correctly setting one texture coordinate, but the other one is fixed at some value. Post the code where you generate/load the texture coords, where you draw the mesh, and check if you are setting the texture matrix anywhere.

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Well, this is a noob's work, so expect the most messup of OpenGL :)
Ok, when initializing the commonly used OpenGL states:

glTexGeni( GL_S, GL_TEXTURE_GEN_MODE, GL_OBJECT_LINEAR );
glTexGeni( GL_T, GL_TEXTURE_GEN_MODE, GL_OBJECT_LINEAR );
// I don't know if this is okay or causing the problem:
glTexGeni( GL_R, GL_TEXTURE_GEN_MODE, GL_OBJECT_LINEAR );


When rendering 3D, i enable automatic texture coordinate generation:
These states get disabled when rendering in 2D.

glEnable( GL_TEXTURE_GEN_S );
// Same thing for T and R.



// Each polygon gets passed as triangle fan (<- problem?)
glBegin( GL_TRIANGLE_FAN );
// In this case i tried many different usages of glTexCoord, i now stuck at
glTexCoord2f( 0.F, 0.F );
// because i think it should work when using GL_OBJECT_LINEAR, even when it looks ugly
for( unsigned int i = 0U; i < Polygon->Vertex.Count(); i++ )
{
glVertex3fv( (GLfloat*)Polygon->Vertex[i] );
}
glEnd( );


The problem could be that i do not load and use all UVs supported by the mesh,
because i don't know how they could comply (3 UVs to 4 vertices).
I also do not modify the texture matrix.

The mesh i load is in plain text (.t3d). Per polygon it mostly provides 4 vertices (each X,Y,Z),one U and one V coordinate, setup of 3 components each.

Thanks people!

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Ah okay. The TextureU and TextureV properties in Unreal's T3D format are vectors which define a coordinate system for applying the texture (source). You can probably find the exact equation to calculate each vertex's texture coordinate somewhere in an Unreal wiki or elsewhere on the net. If you can't, then I guess I could take a stab at it but it might not be correct.

Note that any texture coordinates you specify for a texture unit (with glTexCoord* for example) are ignored if texgen is enabled on that unit, so you shouldn't be using texgen. With GL_OBJECT_LINEAR texgen mode enabled I would expect an effect like that image if you aren't changing the GL_OBJECT_PLANE parameter.

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First of all, thanks for helping me solving this issue!

I took a watch at it and i know what you mean, but i didn't found out a fairly right method to get it solved. I even tried to take a look into the public sources of Unreal's OpenGL driver, but the relevant parts are not directly included, so i don't have a clue about it. But it shouldn't be that hard...

So i'm still trying to figure it out, thanks anyway. If i got it i will drop a line!

Greetings.

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