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
             GL_POINT_SIZE_MIN_SGIS or GL_POINT_SIZE_MAX_SGIS or
             GL_POINT_FADE_THRESHOLD_SIZE_SGIS.

     param   The parameter value.



C SPECIFICATION
     void glPointParameterfvSGIS( GLenum pname,
                                  const GLfloat *params )


PARAMETERS
     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.


DESCRIPTION
     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
     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.


NOTES
     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.


ERRORS
     GL_INVALID_ENUM is generated if pname is not one of the allowable values.

     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.


ASSOCIATED GETS
     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




SEE ALSO
     glPointSize.