how to use glCopyImage2D

Question

I'm trying something like

glEnable(texture_2d)
glBindTexture
glCopyTexImage2D
glDisable(GL_TEXTURE_2D);

I think glCopyTexImage2D won't work with a non-power of two image, so that's one problem; I've also tried glReadPixels, but it's too slow for my purposes.

Washer

Do you know if the problem is in creating the texture or later rendering it? Does glCopyTexImage2D followed by glGetTexImage yield the same result as glReadPixels? (not recommended as a long-term solution for performance reasons, but as a troubleshoot step to see what's going on with the texture) – Kerstin

For the power-of-2 restriction, you can use glTexImage2D to allocate the texture with a power-of-2 size and passing NULL for the data, then glCopyTexSubImage2D which allows non power-of-2 dimensions. – Kerstin

Answer 1

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.

Apostrophize

Sounds like I should look into framebuffer objects... Thanks – Washer

Answer 2

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.

Ramentum

Answer 3

glCopyTexImage2D does work with NPOT image.

NPT image (non-power of two) is limited supported by OpenGLES 2/OpenGL 1 or WebGL, In OpenGLES 3/OpenGL 2 or later it is fully supported.

If you want to copy color attachment of fbo to newTexture.

glBindFramebuffer(GL_FRAMEBUFFER, fbo);
glReadBuffer(GL_COLOR_ATTACHMENT0);

glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_2D, newTexture);
glTexImage2D(bindTarget, 0, GL_RGBA, width, height, 0, GL_RGBA, GL_UNSIGNED_BYTE, NULL);

glCopyTexSubImage2D(target, level, 0, 0, 0, 0, width, height);

NPT image will output black color in fragment shader sampling if texture mipmap, magnification filter and repeat mode setting is wrong.

Detect

Answer 4

To help figure out if the "non-power of two" thing is a problem, use glGetError() like this:

printf("error: %#06x\n", glGetError());

Put that in different places in your code to make sure what line is causing the problem, then check the error code here: https://www.khronos.org/opengl/wiki/OpenGL_Error

To copy a texture I did:

glCopyTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, 0, 0, TEXTURE_WIDTH, TEXTURE_HEIGHT, 0);
glGenerateMipmap(GL_TEXTURE_2D);

after binding the texture. Check the docs on those two functions for more info.

Hasp