The OpenGL Graphics System: A Specification

4.1. PER-FRAGMENT OPERATIONS 199
Fragment
+
Associated
Data
Framebuffer
Pixel
Ownership
Test
Framebuffer
Blending
(RGBA Only)
Depth buffer
Test
Figure 4.1. Per-fragment operations.
Scissor
Test
Framebuffer
Dithering
4.1 Per-Fragment Operations
Stencil
Test
Framebuffer
Alpha
Test
(RGBA Only)
Logicop
To
Framebuffer
A fragment produced by rasterization with window coordinates of (xw, yw) modifies
the pixel in the framebuffer at that location based on a number of parameters
and conditions. We describe these modifications and tests, diagrammed in
figure 4.1, in the order in which they are performed. Figure 4.1 diagrams these
modifications and tests.
4.1.1 Pixel Ownership Test
The first test is to determine if the pixel at location (xw, yw) in the framebuffer
is currently owned by the GL (more precisely, by this GL context). If it is not,
the window system decides the fate the incoming fragment. Possible results are
that the fragment is discarded or that some subset of the subsequent per-fragment
operations are applied to the fragment. This test allows the window system to
control the GL’s behavior, for instance, when a GL window is obscured.
Version 2.0 - October 22, 2004