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Dave007as

3D Texture transformation

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Dave007as    122
Hi everybody, I've been working on this problem for almost 2 weeks now and it's driving me crazy! Maybe you guys can help me. I have a 3D texture that I map to a series of quads and I manipulate the texture matrix in order to rotate the texture (instead of chaning the modelview matrix). So far so good. However, I realized that calling glRotate3f(90, 0.0, 0.0, 1.0) does not rotate my texture in counterclockwise direction around the z-axis, but in clockwise direction. The other two axis seem to be fine. I can't figure out what the problem is, but I wrote a little test program in which I apply exactly the same operations to a 3d vertex by manipulating the modelview matrix and everything works fine. I have a feeling that something is wrong with my way of mapping texel coords to quads... Here some part of my code
glMatrixMode ( GL_TEXTURE );
glLoadIdentity();
// shift origin to center of texture
glTranslatef ( 0.5, 0.5, 0.5 );
// apply transformation
glRotatef(90, 0.0, 0.0, 1.0);
// shift back
glTranslatef ( -0.5, -0.5, -0.5 );
   
glMatrixMode ( GL_MODELVIEW );
glLoadIdentity();

float tex_z;
int num_quads = 200;

glBegin(GL_QUADS);
for(int z = num_quads - 1; z >= 0; z --)
{		
    tex_z = (GLfloat)z / (GLfloat) num_quads;		

    glTexCoord3f( (GLfloat) 0.0, (GLfloat) 0.0, tex_z );      		    
    glVertex3f( -win_w / 2.0, -win_h / 2.0, 0 );
    
    glTexCoord3f( (GLfloat) 1.0, (GLfloat) 0.0, tex_z );    
    glVertex3f(  win_w / 2.0, -win_h / 2.0, 0 );

    glTexCoord3f( (GLfloat) 1.0, (GLfloat) 1.0, tex_z );
    glVertex3f(  win_w / 2.0,  win_h / 2.0, 0 );

    glTexCoord3f( (GLfloat) 0.0, (GLfloat) 1.0, tex_z );
    glVertex3f( -win_w / 2.0,  win_h / 2.0, 0 );
}
glEnd();

Everything seems pretty straight forward, but it's the first time I work with 3D textures, so hopefully you guys can help me. Is it possible that the texture coordinate system is oriented differently? Thank, Dave

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Dave007as    122
Ok,

let me explain it in a different and simpler way:

When using 3d textures, no matter how I map texel coords to quad vertices, one dimension always has the wrong orientation. In case of the code posted, it's the z axis, meaning that glRotate(20, 0.0, 0.0, 1.0) rotates in CLOCKWISE direction instead of counterclockwise. Rotations around the x and y axis are as expected.

Interestingly enough, when using 2D textures glRotate(20, 0.0, 0.0, 1.0) has the desired effect.

Could it be a bug in the openGL implementation (I know it's unlikely)=

Maybe this will help you guys helping me ;-)
Dave

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Dave007as    122
Yes I did. A negativ rotation has the disired effect, which makes me believe that the z axis isn't oriented corretly...maybe because I'm messing something up when mapping texture coordinates. I'm not sure. However, I can't just invert the rotation because I need the texture matrix as an input for some kind of clipping algorithm, which assumes that axis are oriented the way they are supposed to be.

Dave

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