Reading back an integer texture (texture_integer)

OpenGL OpenGL: Advanced Coding
1

Hello!

I want to read back a texture from the GPU and I’m using glGetTexImage for that. While I managed to get it to work for floating-point textures GL_RGBA32F_ARB, I have quite some difficulties reading back an integer GL_RGBA32UI_EXT texture.

Here’s the test case I’m using:

  • initialization

float:
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA32F_ARB, width, height, 0, GL_RGB, GL_FLOAT, 0);

int:
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA32UI_EXT, width, height, 0, GL_RGBA_INTEGER_EXT, GL_UNSIGNED_INT, 0);

  • attach the texture to the FBO

  • just clear the screen with some color
    glClearColorIuiEXT(1, 2, 3, 4);
    glClear(GL_COLOR_BUFFER_BIT);

  • read back
    GLuint *arr = new GLuint[width * height * 4];
    for (int i = 0; i < width * height * 4; i++)
    arr[i] = 3;
    glGetTexImage(GL_TEXTURE_2D, 0, GL_RGBA, GL_UNSIGNED_INT, arr);

Results:
for the floating-point case - all values of arr are 0’s
for the unsigned int one - all values remain 3

So, glGetTexImage doesn’t change the array at all :stuck_out_tongue:

The extension is reported to be supported:
glewGetExtension(“GL_EXT_texture_integer”) return GL_TRUE

Would appreciate any help!

Thanks,
Petar

2

Getting any errors?

On a quick scan I didn’t see anything in the spec about GetTexImage. You’d normally see dedicated entrypoints or a section beginning with something like, “Accepted by the XXX of YYY”, with a list of valid enums to use with this existing function or that.

Maybe I missed something.

3

The spec is broken.

4

Actually, I do get an error when using glGetTexImage() with the integer texture - invalid operation.
The glGetTexImage spec says this means that it is executed in a glBegin/glEnd block, but this is not the case. And with the float texture, I don’t get any errors.

Does this mean I cannot use glGetTexImage with GL_RGBA32UI textures? If so, is there any other way I can read back the texture?

Also, what does the spec is broken mean? It actually should or shouldn’t support glGetTexImage?

Thanks

5

With the help of some people on the IRC channel, we found where the problem was:
glGetTexImage(GL_TEXTURE_2D, 0, GL_RGBA, GL_UNSIGNED_INT, arr);

GL_RGBA should be replaced by GL_RGBA_INTEGER_EXT

This is not documented in the spec for glGetTexImage(), but for glReadPixels(), so one has to extrapolate from there :slight_smile:

Thanks for the help!

6

If you have to extrapolate, the spec is broken. The purpose of the spec is so driver writers and application writers have a defined API and behavior to work with.