Graphics Reference
In-Depth Information
In this equation,
m
is maximum depth slope of the triangle and is
calculated as
2
2
m =
(
∂z/∂x
+
∂z/∂y
)
m
can also be calculated as max {|
∂z
/
∂x
|,
|
∂z/∂y|
}.
The slope terms
∂z/∂x
and
∂z/∂y
are calculated by the OpenGL ES
implementation during the triangle rasterization stage.
r
is an implementation-defined constant and represents the smallest value
that can produce a guaranteed difference in depth value.
Polygon offset can be enabled or disabled using
glEnable(GL_POLYGON_OFFSET_FILL)
and
glDisable(GL_POLYGON_OFFSET_FILL)
, respectively.
With polygon offset enabled, the code for triangles rendered by
Figure 7-11 is as follows:
const float polygonOffsetFactor = -l.Of;
const float polygonOffsetUnits = -2.Of;
glClear ( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT );
// load vertex shader
// set the appropriate transformation matrices
// set the vertex attribute state
// draw the SMALLER quad
glDrawArrays ( GL_TRIANGLE_FAN, 0, 4 );
// set the depth func to <= as polygons are coplanar
glDepthFunc ( GL_LEQUAL );
glEnable ( GL_POLYGON_OFFSET_FILL );
glPolygonOffset ( polygonOffsetFactor, polygonOffsetUnits );
// set the vertex attribute state
// draw the LARGER quad
glDrawArrays ( GL_TRIANGLE_FAN, 0, 4 );
Occlusion queries use query objects to track any fragments or samples
that pass the depth test. This approach can be used for a variety of
techniques, such as visibility determination for a lens flare effect as well
