Graphics Reference
In-Depth Information
The clipping stage will clip each primitive to the clip volume shown in
Figure 7-8. By “primitive,” here we imply each triangle of a list of separate
triangles drawn using
GL_TRIANGLES
, or a triangle of a triangle strip or a
fan, or a line from a list of separate lines drawn using
GL_LINES
, or a line
of a line strip or line loop, or a specific point in a list of point sprites. For
each primitive type, the following operations are performed:
• Clipping triangles—If the triangle is completely inside the viewing
volume, no clipping is performed. If the triangle is completely
outside the viewing volume, the triangle is discarded. If the triangle
lies partly inside the viewing volume, then the triangle is clipped
against the appropriate planes. The clipping operation will generate
new vertices that are clipped to the plane that are arranged as a
triangle fan.
• Clipping lines—If the line is completely inside the viewing volume,
then no clipping is performed. If the line is completely outside the
viewing volume, the line is discarded. If the line lies partly inside the
viewing volume, then the line is clipped and appropriate new vertices
are generated.
• Clipping point sprites—The clipping stage will discard the point
sprite if the point position lies outside the near or far clip plane or if
the quad that represents the point sprite is outside the clip volume.
Otherwise, it is passed unchanged and the point sprite will be scissored
as it moves from inside the clip volume to the outside, or vice versa.
After the primitives have been clipped against the six clipping planes,
the vertex coordinates undergo perspective division to become
normalized device coordinates. A normalized device coordinate is in the
range -1.0 to +1.0.
Note:
The clipping operation (especially for lines and triangles) can
be quite expensive to perform in hardware. A primitive must be
clipped against six clip planes of the viewing volume, as shown
in Figure 7-8. Primitives that are partly outside the near and far
planes go through the clipping operations. However, primitives
that are partially outside the
x
and
y
planes do not necessarily
need to be clipped. By rendering into a viewport that is bigger
than the dimensions of the viewport specified with
glViewport
,
clipping in the
x
and
y
planes becomes a scissoring operation.
Scissoring is implemented very efficiently by GPUs. This larger
viewport region is called the
guard-band
region. Although OpenGL
ES does not allow an application to specify a guard-band region,
most—if not all—OpenGL ES implementations implement a
guard-band.
