glPointParameterSGIS(3G) OpenGL Reference glPointParameterSGIS(3G)
glPointParameterfSGIS, glPointParameterfvSGIS - set point parameters
void glPointParameterfSGIS( GLenum pname,
GLfloat param )
pname The symbolic name of a point parameter. One of
GL_POINT_SIZE_MIN_SGIS or GL_POINT_SIZE_MAX_SGIS or
GL_POINT_FADE_THRESHOLD_SIZE_SGIS.
param The parameter value.
void glPointParameterfvSGIS( GLenum pname,
const GLfloat *params )
pname
The symbolic name of a point parameter. One of
GL_POINT_SIZE_MIN_SGIS or GL_POINT_SIZE_MAX_SGIS or
GL_POINT_FADE_THRESHOLD_SIZE_SGIS or GL_DISTANCE_ATTENUATION_SGIS.
params
A pointer to an array where the values of the parameters are stored.
glPointParameterSGIS is part of the SGIS_point_parameters 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 GL_DISTANCE_ATTENUATION_SGIS 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 GL_POINT_FADE_THRESHOLD_SIZE 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
GL_POINT_SIZE_MAX_SGIS parameter, or smaller than the lower bound defined
by the GL_POINT_SIZE_MIN_SGIS 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 GL_POINT_FADE_THRESHOLD_SIZE_SGIS 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.
The parameter GL_POINT_FADE_THRESHOLD_SIZE_SGIS has no effect on non
multisampled points. All other parameters affect non multisampled points
as well as multisampled points,
The threshold defined by GL_POINT_FADE_THRESHOLD_SIZE_SGIS is not clamped
to the minimum or maximum point sizes.
GL_INVALID_ENUM is generated if pname is not one of the allowable values.
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glPointParameterSGIS(3G) OpenGL Reference glPointParameterSGIS(3G)
GL_INVALID_VALUE is generated if pname is GL_POINT_SIZE_MIN_SGIS or
GL_POINT_FADE_THRESHOLD_SIZE_SGIS, and params is less than zero.
GL_INVALID_VALUE is generated if pname is GL_POINT_SIZE_MAX_SGIS and
params is less than or equal to zero.
glGet with argument GL_POINT_SIZE_MIN_SGIS
glGet with argument GL_POINT_SIZE_MAX_SGIS
glGet with argument GL_POINT_FADE_THRESHOLD_SIZE_SGIS
glGet with argument GL_DISTANCE_ATTENUATION_SGIS
glPointSize.
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