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
	     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.

     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	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_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.






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glPointParameterSGIS(3G)       OpenGL Reference	      glPointParameterSGIS(3G)



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




SEE ALSO
     glPointSize.












































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