according to the ARB_framebuffer_object spec, using glGetFramebufferAttachmentParameteriv() on GL_FRAMEBUFFER to retrieve the size of the colors like this: GLenum attachment = GL_COLOR_ATTACHMENT0; GLenum pname = GL_FRAMEBUFFER_ATTACHMENT_RED_SIZE; glGetFramebufferAttachmentParameteriv (GL_FRAMEBUFFER, attachment, pname, &red_bits); should be functionally equivalent to: glGetIntegerv (GL_RED_BITS, &red_bits); in Mesa, the former works for any FBO, but it doesn't work for the default onscreen framebuffer.
Added a piglit test case and this commit in mesa: commit 80dfec3e53fd5b5c8c31fb16376c9910258c91b0 Refs: master, <origin/HEAD>, <origin/master>, mesa_7_6_1_rc1-8853-g80dfec3 Author: Kristian Høgsberg <krh@bitplanet.net> AuthorDate: Tue Jun 15 13:07:01 2010 -0400 Commit: Kristian Høgsberg <krh@bitplanet.net> CommitDate: Tue Jun 15 13:28:38 2010 -0400 mesa: Allow querying the system FBO in GetFramebufferAttachmentParameteriv If the default framebuffer is bound to <target>, then <attachment> must be one of FRONT_LEFT, FRONT_RIGHT, BACK_LEFT, BACK_RIGHT, AUXi, DEPTH_BUFFER, or STENCIL_BUFFER, identifying a color buffer, the depth buffer, or the stencil buffer, and <pname> may be FRAMEBUFFER_ATTACHMENT_OBJECT_TYPE or FRAMEBUFFER_ATTACHMENT_OBJECT_NAME. as well as these <pname> values FRAMEBUFFER_ATTACHMENT_RED_SIZE, FRAMEBUFFER_ATTACHMENT_GREEN_SIZE, FRAMEBUFFER_ATTACHMENT_BLUE_SIZE, FRAMEBUFFER_ATTACHMENT_ALPHA_SIZE, FRAMEBUFFER_ATTACHMENT_DEPTH_SIZE, FRAMEBUFFER_ATTACHMENT_STENCIL_SIZE, FRAMEBUFFER_ATTACHMENT_COMPONENT_TYPE, or FRAMEBUFFER_ATTACHMENT_COLOR_ENCODING.
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