OpenGL 3D Texture Coordinates

Question

I've got a problem with a 3d texture in OpenGL.

I set the texture via

glTexImage3D(GL_TEXTURE_3D, 0, GL_LUMINANCE, 640, 480, 8, 0, GL_RED, GL_UNSIGNED_BYTE, tex3Ddata);

with tex3Ddata being in total 8 slices of 640x480 bitmaps. Now when I try to access the different slices with texture coordinates, for some reason the images blend into each other, thus I don't properly set the coordinates, yet I do not know why. The texture slices itself are 8bit monochrome.

Code for the displaying:

for(unsigned int i = 0; i < g_numCameras; i++) {
    float tx = ((float)i) / ((float)g_numCameras - 1);
    //tx = 0.5f; //this is for testing only
    float fI = ((float)i) / ((float)g_numCameras);
    if( i < (g_numCameras >> 1)) {
        glTexCoord3f(0.0f, 1.0f, tx); 
        glVertex3f( -1.0f + fI * 4.0f, 0.0f,  0.5f);

        glTexCoord3f(1.0f, 1.0f, tx); 
        glVertex3f( -1.0f + (fI + 1.0f / ((float)g_numCameras)) * 4.0f, 0.0f,  0.5f);

        glTexCoord3f(1.0f, 0.0f, tx); 
        glVertex3f( -1.0f + (fI + 1.0f / ((float)g_numCameras)) * 4.0f,  1.0f,  0.5f);

        glTexCoord3f(0.0f, 0.0f, tx); 
        glVertex3f( -1.0f + fI * 4.0f,  1.0f,  0.5f);
    }
    else {
        fI -= 0.5f;
        glTexCoord3f(0.0f, 1.0f, tx); 
        glVertex3f( -1.0f + fI * 4.0f, -1.0f,  0.5f);

        glTexCoord3f(1.0f, 1.0f, tx); 
        glVertex3f( -1.0f + (fI + 1.0f / ((float)g_numCameras)) * 4.0f, -1.0f,  0.5f);

        glTexCoord3f(1.0f, 0.0f, tx); 
        glVertex3f( -1.0f + (fI + 1.0f / ((float)g_numCameras)) * 4.0f,  0.0f,  0.5f);

        glTexCoord3f(0.0f, 0.0f, tx); 
        glVertex3f( -1.0f + fI * 4.0f,  0.0f,  0.5f);
    }
}

g_numCameras is 8, so I would expect the slices to be accessible via the Z-coordinates 0.0f, 1/7, 2/7, ..., 1.0f. Yet it's always interpolated. I tested with tx=0.5f; as well, but this is also a mix of images. The x/y coordinates work properly, and the 8 quads are placed as expected, just the slicing through the cube doesn't work the way I would have expected it.

Any ideas what I am doing wrong here? (I was not able to find an equivalent answer / example on 3d textures).

I've also tested whether the data is proper by uploading 8 times the same image, and that worked just fine (since interpolation will result in original image, I got the original image there).

Answer 1

What happens to you is, that those texture coordinates don't address the texel centers, but edges between the texels. Let's say you've got a 1D texture of dimension 8

| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |

Then the very first | is at texture coordinate 0.0, and the very last | is at texture coordinate 1.0

0.0                             1.0
0/8                             8/8
 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |

Let me write the whole set of fractions:

0/8 1/8 2/8 3/8 4/8 5/8 6/8 7/8 8/8
 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |

What you want to hit are the texel centers, ie the numbers between the bars. |1| is in the middle between 0/8 to 1/8, ie (0/8+1/8)/2 = 1/16, |2| is between 1/8 and 2/8 = (1/8 + 2/8)/3 = 3/16 and so on.

So the texture coordinates you want to address are 1/16 3/16 5/16 7/16 9/16 11/16 13/16 15/16

or in more general form: (1 + 2*t) / 2*dim

Answer 2

Your expectations are off.

each slice i (indexing from 0) is available at (i+0.5) / num_slices .

0.f means interpolate halfway between the first slice and the border, 1.f means interpolate half-way between the last slice and the border (assuming clamping).

0.5, as it turns out, is exactly in the middle, hence between slices 3 and 4 (if starting to count at 0). So you get a blend between the 2 slices.

Answer 3

Is it acceptable to call the following:

glTexParameteri(GL_TEXTURE_3D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
glTexParameteri(GL_TEXTURE_3D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);

That should turn off interpolation, I think.