RGB10_A2 = RGBA8 on ATI FBO?

[gybe]

I wrote a small test application where I compute the precision of different pixel formats and when I use RGB10_A2 it looks like I have only 8 bits of precisions instead of 10 bits.

Here is my test application:

Test Application

It works fine with the other formats I tested. I have a Radeon X1800 and the latest driver(Cat 6.7). Except for a driver bug, do you have any idea why it's not working?

Here I create the texture:

glGenTextures(1, &m_unTexSceneRGB10_A2);
glBindTexture(GL_TEXTURE_2D, m_unTexSceneRGB10_A2);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
glTexEnvf(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_MODULATE);
glTexImage2D( GL_TEXTURE_2D, 0, GL_RGB10_A2, OFFSCREEN_BUFFER_SIZE, OFFSCREEN_BUFFER_SIZE, 0, GL_RGBA, GL_UNSIGNED_BYTE, NULL );

Here I create the FBO:

glGenRenderbuffersEXT(1, &m_unDepthBufferScene);
glBindRenderbufferEXT( GL_RENDERBUFFER_EXT, m_unDepthBufferScene );
glRenderbufferStorageEXT(GL_RENDERBUFFER_EXT, GL_DEPTH_COMPONENT24, OFFSCREEN_BUFFER_SIZE, OFFSCREEN_BUFFER_SIZE);
 
glGenFramebuffersEXT(1, &m_unFrameBufferSceneRGB10_A2);
glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, m_unFrameBufferSceneRGB10_A2);
glFramebufferTexture2DEXT(GL_FRAMEBUFFER_EXT, GL_COLOR_ATTACHMENT0_EXT, GL_TEXTURE_2D, m_unTexSceneRGB10_A2, 0);
glFramebufferRenderbufferEXT(GL_FRAMEBUFFER_EXT, GL_DEPTH_ATTACHMENT_EXT, GL_RENDERBUFFER_EXT, m_unDepthBufferScene);

[Brian Paul]

The internalFormat parameter to glTexImage2D (and glRenderbufferStorageEXT, etc) is really just a hint to OpenGL about the texture format you'd like to use. OpenGL is free to choose another format that is a reasonably close match.

In an extreme example, if you requested GL_INTENSITY16, OpenGL might actually use GL_RGBA8 setting R=G=B=A for all texels. But when you query the texel component sizes, it'll look like an 8-bit intensity texture.

[gybe]

I can create a RGB10_A2 texture, and I pretty sure Im going to have 10 bits precisions in this situation.

Why is it different on render buffer?

[gybe]

I think I understand what you want to say. Im quite sure RGB10_A2 rendering is supported by ATI GPU. However maybe it's not a bug if they don't implement it, since they still follow the OpenGL spec.