OpenGL textures: why does order matter?

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:

+++++++++++++++++++++++++++++++ EDIT +++++++++++++++++++++++++++++++

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).

Answer 2

OpenGL is a state machine, which means that you can pull its levers, turn its knobs, and it will keep those settings, until you change them actively.

However it also manages it's persistent data in objects. Such objects are something abstract, and must not be confused with objects seen on the screen! Now to the outside OpenGL identifies objects by their so called name , a numerical ID. You create a (list of) name(s) – but not the object(s)! – with glGenTextures for texture objects, which are such a kind of OpenGL object.

To maniupulate such an object, OpenGL must first be put into a state that all the following calls to manipulate such objects of that type happen to one particular object. This is done with glBindTexture . After calling glBindTexture all the following calls that manipulate textures happen to that one texture object you've just bound. If the object didn't exist previously, it is created if a new assigned object name is bound for the first time. Now OpenGL uses that particular object.

glTexImage2D is just one of several functions to maniuplate the data of the currently bound textures.

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Otherwise your function points into the right direction. OpenGL has no real initialization phase, you just do things as you go along. And it makes sense to defer loading of data until you need it. But it also makes sense to have multiple iterations over the lists of objects before you actually draw a frame. One of the preparations should be, that you iterate over all objects (now not OpenGL but your's) to test if the data's already loaded. If a significant amount of data's still missing, draw a loading screen instead, so that the user doesn't get the impression your program hangs. Maybe even carry out lengthy loading operations in a separate thread, but with OpenGL this requires some precautions.