OpenGL: How to update subimage of a rectangular texture?

I am trying to update a small square section in a large rectangular texture.

I've tried using glTexSubImage2D with the tar开发者_如何学Pythonget set to GL_TEXTURE_RECTANGLE_ARB, but I'm running into issues. It may just be that I don't know how to use glTexSubImage2D correctly though. The issues may be due to the fact that I'm trying not to load the whole texture into main memory using glTexImage2D before updating the subimage?

Can anyone tell me if it's possible to update a subimage of a rectangular texture without having to read the entire texture into main memory? I see glTexCopyTexSubImage2D... Still wondering if these methods work with rectangular textures though.

1. I'm quite sure you have to at least upload full-size texture even once with glTexImage2D. You can even use a blank array if you don't have any access to whole image at the beginning.

2. glCopyTexSubImage2D is not really like what you want. It copies a specific portion from "frame buffer" to a texture. But, you want to upload from main memory, right?

3. I don't see any reason why rectangular textures cannot be supported by those methods unless there is a broken driver.

4. Consider that rectangular textures do not use texture coordinates in [0..1] range like the other textures. Instead they use [0..Width] and [0..Height] range.