OpenGL Y texture coordinates behaving oddly

Question

So basically I am making a 2D game with opengl/c++. I have a quad with a texture mapped on it and because I cant use non power of two images (or at least I shouldnt) I have an image inside a power of two image and I wish to remove the excess image with texture mapping.

GLfloat quadTexcoords[] = {
    0.0, 0.0,
    0.78125, 0.0,
    0.78125, 0.8789,
    0.0, 0.8789
};
glGenBuffers(1, &VBO_texcoords);
glBindBuffer(GL_ARRAY_BUFFER, VBO_texcoords);
glBufferData(GL_ARRAY_BUFFER, sizeof(quadTexcoords), quadTexcoords, GL_DYNAMIC_DRAW);
glBindBuffer(GL_ARRAY_BUFFER, 0);

This is my texture code. Nothing special I know. The X coordinates (0.7256) work fine and it remove the excess image on the right side of the image. However the Y coordinate does not work. I have debugged my game and found the correct coordinate is sent to the shader but it wont map. It seems to be working in reverse sometimes but it is very unclear.

if I give it a coordinate for Y like 20 it repeats multiple times but still leaves a little white line a the top of the quad. I havent got the faintest idea what it could be.

Other details: The image I am trying to map is 800 by 450 and it is wrapped in an image of size 1024 by 512. I scale the quad by the aspect ratio of 800 by 450. I doubt this would make a difference but you never know!

Thanks for your time.

EDIT: here is an example of whats happening.

enter image description here

This is the full image mapped fully (0 to 1 in X and Y) The blue portion is 200 pixels high and the full image is 300 pixels high.

enter image description here

The second image is the image mapped to 2 thirds of the Y axis (i.e. 0 to 0.6666 in Y). This should remove the white at the top but that is not what is happening. I don't think the coordinates are back to front as I got the mapping of several tutorials online.

Define "does not work." What exactly is happening? What is the texture's resolution? — Nicol Bolas
You do know that OpenGL coordinates start in the lower left corner (not upper) and that the Y coordinates increase as you go up? So (0.0,0.0) = lower left, (0.0,1.0) = upper left. I say this because you are mentioning "working in reverse". If you are flipping y-coordinates of your textures weird things can happen. — KillianDS
I am aware of it starting in the lower left and I have accounted for this. You can see the order of the coordinates above. Tell me what you think — DavidColson
"This should remove the white at the top" And what color should be left? — Nicol Bolas
well no white band on the top of the image, so the image would be a white, blue, white flag — DavidColson

datenwolf datenwolf · Accepted Answer · 2012-08-16T18:41:23

It seems to be working in reverse sometimes but it is very unclear.

OpenGL assumes the viewport origin in the lower left and texture coordinates running "along with" flat memory texel values in S, then T direction. In essence this means, that with one of the usual mappings, textures have their origin in the lower left, contrary to the upper left origin found in most image manipulation programs.

So in your case the white margin you see, is simply the padding you probably applied to the texture image at the bottom, instead of the top, where you should put it. Why can't you use NPO2 textures anyway? They're widely supported.

OpenGL Y texture coordinates behaving oddly

2 Answers