Question

I am trying to setup a simple function which will make it a lot easier for me to texture map geometry in OpenGL, but for some reason when I’m trying to make a skybox, I am getting a white box instead of the texture mapped geometry. I think that the problemed code lies within the following:

void MapTexture (char *File, int TextNum) {
    if (!TextureImage[TextNum]){
        TextureImage[TextNum]=auxDIBImageLoad(File);
        glGenTextures(1, &texture[TextNum]);
        glBindTexture(GL_TEXTURE_2D, texture[TextNum]);
        glTexImage2D(GL_TEXTURE_2D, 0, 3, TextureImage[TextNum]->sizeX, TextureImage[TextNum]->sizeY, 0, GL_RGB, GL_UNSIGNED_BYTE, TextureImage[TextNum]->data);
        glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MIN_FILTER,GL_LINEAR);
    }
glEnable(GL_TEXTURE_2D);
glBindTexture(GL_TEXTURE_2D, texture[TextNum]);
//glTexImage2D(GL_TEXTURE_2D, 0, 3, TextureImage[TextNum]->sizeX, TextureImage[TextNum]->sizeY, 0, GL_RGB, GL_UNSIGNED_BYTE, TextureImage[TextNum]->data);
//glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MIN_FILTER,GL_LINEAR);

}

The big thing I don’t understand is for some reason the glBindTexture() must come between glGenTextures() and glTexImage2D. If I place it anywhere else, it screws everything up. What could be causing this problem? Sorry if it’s something simple, I’m brand new to openGL.

Below is a screenshot of the whitebox I am talking about:

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EDIT
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After playing around with the code a bit more, i realized that if I added glTexImage2D() and glTexParameteri()after the last glBindTexture() then all the textures load. Why is it that without these two lines most textures would load, and yet there are a few that would not, and why do I have to call glTexImage() for every frame, but only for a few textures?

Answer 1

Yes, order is definitely important.

• glGenTexture creates a texture name.

• glBindTexture takes the texture name generated by glGenTexture, so it can’t be run before glGenTexture.

• glTexImage2D uploads data to the currently bound texture, so it can’t be run before glBindTexture.

The client-side interface to OpenGL is a Big Giant Squggly State Machine. There are an enormous number of parameters and flags that you can change, and you have to be scrupulous to always leave OpenGL in the right state. This usually means popping matrices you push and restoring flags that you modify (at least in OpenGL 1.x).