glPointParameterSGIS(3G) OpenGL Reference glPointParameterSGIS(3G)
NAME
glPointParameterfSGIS, glPointParameterfvSGIS - set point parameters
C SPECIFICATION
void glPointParameterfSGIS( GLenum pname,
GLfloat param )
PARAMETERS
pname The symbolic name of a point parameter. One of
GLPOINTSIZEMINSGIS or GLPOINTSIZEMAXSGIS or
GLPOINTFADETHRESHOLDSIZESGIS.
param The parameter value.
C SPECIFICATION
void glPointParameterfvSGIS( GLenum pname,
const GLfloat *params )
PARAMETERS
pname
The symbolic name of a point parameter. One of
GLPOINTSIZEMINSGIS or GLPOINTSIZEMAXSGIS or
GLPOINTFADETHRESHOLDSIZESGIS or GLDISTANCEATTENUATIONSGIS.
params
A pointer to an array where the values of the parameters are stored.
DESCRIPTION
glPointParameterSGIS is part of the SGISpointparameters extension. It
supports additional geometric characteristics of points. It allows the
size of a point to decrease, as the the distance of the point from the
eye increases. It provides control over the mapping from the point size
to the raster point area and point transparency, in order to increase the
dynamic range of the raster brightness of points.
The derived point size is defined to be the size provided with
glPointSize modulated with a distance attenuation factor.
The distance attenuation function coefficients, namely a, b, and c in:
dist_atten(d) = 1 / (a + b x d + c x d^2)
are defined by the GLDISTANCEATTENUATIONSGIS parameters. By default a
= 1, b = 0, and c = 0.
Let size be the point size provided with glPointSize, let dist be the
distance of the point from the eye, and let threshold be the threshold
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glPointParameterSGIS(3G) OpenGL Reference glPointParameterSGIS(3G)
size defined by the GLPOINTFADETHRESHOLDSIZE parameter. The derived
point size is given by:
derived_size = size x sqrt(dist_atten(dist))
Note that when default values are used, the above formula reduces to:
derived_size = size
If the derived point size is larger than the upper bound defined by the
GLPOINTSIZEMAXSGIS parameter, or smaller than the lower bound defined
by the GLPOINTSIZEMINSGIS parameter, the derived size is clamped to
the upper and lower bounds respectively.
If the derived point size is larger than the threshold size defined by
the GLPOINTFADETHRESHOLDSIZESGIS parameter, the derived point size
is used as the diameter of the rasterized point, and the alpha component
is intact. Otherwise, the threshold size is set to be the diameter of
the rasterized point, while the alpha component is modulated accordingly,
to compensate for the larger area.
The diameter of the rasterized point is given by:
derived_size derived_size >= threshold
diameter =
threshold Otherwise
The alpha component of the rasterized point is given by:
alpha_in derived_size >= threshold
alpha =
alpha_in x (derived_size / threshold)^2 Otherwise
where alpha_in is point alpha possibly modified by lighting.
NOTES
The parameter GLPOINTFADETHRESHOLDSIZESGIS has no effect on non
multisampled points. All other parameters affect non multisampled points
as well as multisampled points,
The threshold defined by GLPOINTFADETHRESHOLDSIZESGIS is not clamped
to the minimum or maximum point sizes.
ERRORS
GLINVALIDENUM is generated if pname is not one of the allowable values.
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glPointParameterSGIS(3G) OpenGL Reference glPointParameterSGIS(3G)
GLINVALIDVALUE is generated if pname is GLPOINTSIZEMINSGIS or
GLPOINTFADETHRESHOLDSIZESGIS, and params is less than zero.
GLINVALIDVALUE is generated if pname is GLPOINTSIZEMAXSGIS and
params is less than or equal to zero.
ASSOCIATED GETS
glGet with argument GLPOINTSIZEMINSGIS
glGet with argument GLPOINTSIZEMAXSGIS
glGet with argument GLPOINTFADETHRESHOLDSIZESGIS
glGet with argument GLDISTANCEATTENUATIONSGIS
SEE ALSO
glPointSize.
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