How do I specify texture coordinates for a GL_QUAD_STRIP?

Question

I am trying to understand how to specify texture coordinates for a GL_QUAD_STRIP.

I have managed to get things working for one quad:

float vertices[] = { 0.0f, 0.0f, 1.0f,   +1.0f, 0.0f, 0.0f,  // bottom line
                     0.0f, 1.0f, 1.0f,   +1.0f, 1.0f, 0.0f}; // top line

unsigned int indices[] = {2, 0,  // x =  0
                          3, 1}; // x = +1

float textureCoordinates[] = { 1.0f, 0.0f, 
                               0.0f, 0.0f,
                               1.0f, 1.0f,
                               0.0f, 1.0f};
...

glBindBuffer(GL_ARRAY_BUFFER, 0); // unbinds any buffer object previously bound
glTexCoordPointer(2, GL_FLOAT, 0, textureCoordinates);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, ibufferid);
glDrawElements(GL_QUAD_STRIP, 4, GL_UNSIGNED_INT, BUFFER_OFFSET(0));

And here is how the result looks (white rectangle with image, rest is drawn on to help explain): enter image description here

However I do not understand the logic behind the choice of textureCoordinates[] :-(.

The first texture coordinate is (1,0); I would assume that this corresponds to lower right corner?

Also I would assume that when OpenGL reads the first index: 2, it uses this to look up the vertex: (0,1,1): upper left corner. Next it reads the first texture coordinate: (1,0). But as mentioned above I would assume this to be the lower right corner of the texture !?

However the texture is shown unrotated so this can not be the case!?

I wonder, if glBindBuffer(GL_ARRAY_BUFFER, 0) is called, where is glDrawElements() reading its vertices from? — Victor Zamanian

Victor Zamanian Victor Zamanian · Accepted Answer · 2013-01-07T23:51:42

Just like the vertices, the texture coordinates are also selected based on the indices used by glDrawElements(). So the first texture coordinate is not (1,0), but (1,1) because the first index is 2. Vertices and coordinates would be according to the following table, where i = index, v = vertex and t = texture coordinate. (I'll only take the x and y coordinates into consideration for the vertices, as the z coordinate doesn't really matter in this case.)

i   v     t
2 (0,1) (1,1)
0 (0,0) (1,0)
3 (1,1) (0,1)
1 (1,0) (0,0)

If we draw this on a piece of paper, we can see that this means the coordinates make more sense, since the indices matter. (I recommend that you do this! I had to do that to understand what was going on.) Notice in the table how the y coordinates match perfectly between the vertex and texture coordinate for a given index. But the x coordinates don't match: when the vertex has x = 0, the texture coordinate has x = 1 and vice versa. I assume this would make the image appear mirrored around the y axis instead of rotated in any way. What does the original image look like? Is it mirrored compared to what we see in the image you posted so that the building is on the left? If so, the texture coordinates would be the explanation. In that case, texture coordinate 2 and 3 should switch places.

In case you are curious, you could take a look at the OpenGL 2.1 specification on page 18, Figure 2.5(a), to see why the vertex indices were selected as they were. It would create a quad with vertices specified in a counterclockwise direction when projected on the screen. This is good because the initial value for glFrontFace() is GL_CCW, which means we see the front face of the polygons in the rendered image and the polygons would not have been culled if culling was enabled (see glCullFace()). (Culling is not enabled by default though, so it may or may not have mattered in your case.)

I hope this helped. Do comment if something is unclear!

How do I specify texture coordinates for a GL_QUAD_STRIP?

1 Answers