In Figure 4 showing the per-fragment pipeline, the Pixel Ownership Test is where fragments are thrown away if they fall outside the window's drawable region. The drawable region of a window is often called the window's clip.
When the X server performs 2D X rendering, the X server can perform window clipping in software based on its special knowledge of every window's clip. But in GTX-RD and GT-XRD architectures, typically the hardware must enforce window clipping because the Pixel Ownership Test occurs late in the graphics pipeline. Also when using direct rendering, OpenGL programs do not generally have knowledge of their current window clip. In these cases, window clipping is typically supported in the hardware with the hardware clipping information updated by the X server as the window layout changes.
There are two common approaches to supporting window clipping in hardware:
while clip rectangles
work only for simple clips. Clip ID planes are generally slower than clip
rectangles since every fragment clipped using clip ID planes must be
tested against the pixel's clip ID that must be retrieved from frame
buffer memory. Because the case of an unobscured window is the most
important performance case and arbitrary clips must be supported,
a hybrid approach combining both clip rectangles and clip ID planes is
common. Figure 7 shows clip rectangles and clip
ID planes used simultaneously.
The clip ID and clip rectangle hardware state must be maintained and updated as necessary by the X server when window clips change. Note that there are a limited number of clip ID values so the X server must be ready to virtualize clip ID usage if too many windows have complex clips that require clip IDs.