glPolygonOffsetEXT(3G) OpenGL Reference glPolygonOffsetEXT(3G)
NAME
glPolygonOffsetEXT - set parameters used to offset polygon depth
C SPECIFICATION
void glPolygonOffsetEXT( GLfloat factor,
GLfloat bias )
PARAMETERS
factor specifies a slope scale factor.
bias specifies a constant bias.
The depth values for each subsequent polygon rendered in GLFILL
mode will be offset by the the slope scale factor times the
maximum absolute depth slope of the polygon, plus the bias.
DESCRIPTION
When GLPOLYGONOFFSETEXT is enabled, the depth value of each pixel
fragment generated for a polygon rendered in GLFILL mode is offset by an
amount that depends on the maximum absolute depth slope of the polygon.
This is useful for rendering hidden line images, for applying decals to
surfaces, and for rendering solids with highlighted edges.
For example, an application might attempt to highlight the edges of a
polygon by drawing the polygon with mode GLFILL and then drawing the
polygon again in a different color with mode GLLINE. However, because
of differences in the way lines and polygons are rasterized, the depth
values generated for pixels on a line between two vertices are usually
not the same as the depth values generated for a polygon edge between the
same two vertices. The result is ``stitching'' or other objectionable
rendering artifacts along the edges. This can be eliminated by adding an
appropriate offset to the depth values so that the filled part of the
polygon is cleanly separated from the highlighted edges.
At first glance it might appear sufficient to adjust the polygon depth by
a constant amount. However, it turns out that there is no single
constant offset that works for all the polygons in a scene. The reason
is that the depth value at a given pixel may be derived from a polygon
sample point nearly one full pixel away from the mathematically-precise
edge of the polygon. Therefore, the correct offset must be at least as
large as the maximum absolute change in depth for any one-pixel step on
the face of the polygon. In the following discussion, let dz represent
this change in depth.
In the example above, the width of the lines used to highlight the
polygon edges was not specified. If the width was one, then the analysis
in the previous paragraph shows that an offset equal to dz is sufficient
to keep the line and the polygon edge separated. However, if the width
was two, then it would be necessary to offset at least twice as much as
dz. Therefore, the correct offset must be a multiple of dz, where the
scale factor is a function of the width of the highlight line.
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glPolygonOffsetEXT(3G) OpenGL Reference glPolygonOffsetEXT(3G)
However, even this is not sufficient. If a polygon is at a constant
depth, then dz is zero, so an additional bias is required to ensure that
the highlight lines and the underlying polygon remain separated in that
case.
glPolygonOffsetEXT allows the application to define the proper scale
factor (factor) and additional bias (bias). The value of the offset
derived from these parameters is factor * dz + bias, where the GL
computes dz automatically for each polygon.
Initially GLPOLYGONOFFSETFACTOREXT (the value specified by factor)
and GLPOLYGONOFFSETBIASEXT (the value specified by bias) are both set
to 0.0.
NOTES
Since factor is always used to scale dz, it can be chosen without regard
for the precision with which depth values are represented in the graphics
system. However, if the value chosen for bias is smaller than the
minimum representable difference in depth values, it will have no effect.
This minimum representable difference varies not only with the size of
the depth buffer, but also with other implementation-dependent
parameters, such as the number of guard bits that are used to prevent
overflow during rasterization. 0.00002 (somewhat larger than 1 / 2**16)
should be adequate for most systems, but may be so large that it causes
depth buffering artifacts on others. 0.000001 (somewhat larger than
1 / 2**20) might be a reasonable choice on systems with 24-bit depth
buffers.
glPolygonOffsetEXT is part of the EXTpolygonoffset extension, not part
of the core GL command set. If GLEXTpolygonoffset is included in the
string returned by glGetString, when called with argument GLEXTENSIONS,
extension EXTpolygonoffset is supported by the connection.
ERRORS
GLINVALIDOPERATION is generated if glPolygonOffsetEXT is executed
between the execution of glBegin and the corresponding execution of
glEnd.
ASSOCIATED GETS
glIsEnabled with argument GLPOLYGONOFFSETEXT
glGet with argument GLPOLYGONOFFSETFACTOREXT or
GLPOLYGONOFFSETBIASEXT.
SEE ALSO
glDepthFunc, glDisable, glEnable, glGet, glIsEnabled, glLineWidth,
glReferencePlaneSGIX, glStencilOp, glTexEnv.
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