OpenGL ES 1.1 Combine two textures into one reusable texture

Question

In OpenGL ES 1.1, I would like to take multiple texture Ids and combine them into a single textureId. Then I would be able to use this resulting texture multiple times in the future. My texture sources could be transparent PNGs that I want to stack together. This would be a huge optimization since I wouldn't have to render multiple textures every frame.

I have seen examples like the wiki Texture_Combiners, but it doesn't seem like the results are reusable.

Also, if there is a way to mask an image with another into a reusable texture, that would be extremely helpful too.

Answer 1

What you want to do is render to texture. If you're writing for iOS you're guaranteed that the OES framebuffer extension will be available, so you can use that. If you're writing for Android or another platform then the extension may be available but isn't guaranteed. If it isn't available you can fall back on glCopyTexImage2D.

So in the first case you'd create a frame buffer which has a texture as its colour buffer. Render to that then switch to another frame buffer and you can henceforth draw from the texture.

In the second you'd draw into whatever frame buffer you have, then use glCopyTexImage2D to copy from the current colour buffer into a texture. This will be a little slower because it's a copy, but it'll still probably be a lot faster than reading back the rendered content and then uploading it yourself.

ES 2.0 makes the functions contained in the framebuffer extension mandatory, so ES 2.0 capable GPUs are very likely to support the extension.