how to use glCopyImage2D

If glReadPixels is too slow for you, then glCopyTexImage2D and glCopyTexSubImage2D aren’t going to be a whole lot faster. On platforms with support for framebuffer objects, like iOS, the recommended (i.e. faster) way to get GPU-rendered image data into a texture is to use that texture as the color attachment for a framebuffer object and render directly into it. That said, if you still want to pursue this method, here’s what you need to do to fix it:

First, you’re passing bad arguments to glCopyTexImage2D. The third argument, internalformat, should probably be GL_RGBA instead of 0. If you had called glGetError after calling glCopyTexImage2D, you would probably have gotten GL_INVALID_OPERATION. See the OpenGL ES 1.1 man pages for glCopyTexImage2D and glCopyTexSubImage2D.

Second, as you’ve already observed, glCopyTexImage2D requires its width and height arguments to be power-of-two as well. The correct way to deal with this is to allocate a texture image using glTexImage2D (you can pass NULL for pixels here), then use glCopyTexSubImage2D to copy your framebuffer contents into a rectangle. Note that glCopyTexSubImage2D doesn’t take an internalformat argument—because it’s updating a subrectangle of a texture, it uses the texture’s existing format.

For the record, glGetTexImage doesn’t exist in OpenGL ES 1.1 or 2.0, which is why you’re getting an implicit declaration.

You can check if the video card supports non-power of 2 textures if it supports the ARB_texture_non_power_of_two extension. See here for info.