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GLFOG(3G)							     GLFOG(3G)

NAME
       glFogf, glFogi, glFogfv, glFogiv - specify fog parameters

C SPECIFICATION
       void glFogf( GLenum pname,
	    GLfloat param )
       void glFogi( GLenum pname,
	    GLint param )

       delim $$

PARAMETERS
       pname  Specifies a single-valued fog parameter.	GL_FOG_MODE,
	      GL_FOG_DENSITY, GL_FOG_START, GL_FOG_END, and GL_FOG_INDEX are
	      accepted.

       param  Specifies the value that pname will be set to.

C SPECIFICATION
       void glFogfv( GLenum pname,
	    const GLfloat *params )
       void glFogiv( GLenum pname,
	    const GLint *params )

PARAMETERS
       pname  Specifies a fog parameter.  GL_FOG_MODE, GL_FOG_DENSITY,
	      GL_FOG_START, GL_FOG_END, GL_FOG_INDEX, and GL_FOG_COLOR are
	      accepted.

       params Specifies the value or values to be assigned to pname.
	      GL_FOG_COLOR requires an array of four values.  All other
	      parameters accept an array containing only a single value.

DESCRIPTION
       Fog is initially disabled.  While enabled, fog affects rasterized
       geometry, bitmaps, and pixel blocks, but not buffer clear operations.
       To enable and disable fog, call glEnable and glDisable with argument
       GL_FOG.

       glFog assigns the value or values in params to the fog parameter
       specified by pname.  The following values are accepted for pname:

       GL_FOG_MODE	   params is a single integer or floating-point value
			   that specifies the equation to be used to compute
			   the fog blend factor, $f$.  Three symbolic
			   constants are accepted: GL_LINEAR, GL_EXP, and
			   GL_EXP2.  The equations corresponding to these
			   symbolic constants are defined below.  The initial
			   fog mode is GL_EXP.

       GL_FOG_DENSITY	   params is a single integer or floating-point value
			   that specifies $density$, the fog density used in
			   both exponential fog equations.  Only nonnegative
			   densities are accepted.  The initial fog density is
			   1.

       GL_FOG_START	   params is a single integer or floating-point value
			   that specifies $start$, the near distance used in
			   the linear fog equation.  The initial near distance
			   is 0.

       GL_FOG_END	   params is a single integer or floating-point value
			   that specifies $end$, the far distance used in the
			   linear fog equation.	 The initial far distance is
			   1.

       GL_FOG_INDEX	   params is a single integer or floating-point value
			   that specifies $i sub f$, the fog color index.  The
			   initial fog index is 0.

       GL_FOG_COLOR	   params contains four integer or floating-point
			   values that specify $C sub f$, the fog color.
			   Integer values are mapped linearly such that the
			   most positive representable value maps to 1.0, and
			   the most negative representable value maps to -1.0.
			   Floating-point values are mapped directly.  After
			   conversion, all color components are clamped to the
			   range [0,1].	 The initial fog color is (0, 0, 0,
			   0).

       Fog blends a fog color with each rasterized pixel fragment's
       posttexturing color using a blending factor $f$.	 Factor $f$ is
       computed in one of three ways, depending on the fog mode.  Let $z$ be
       the distance in eye coordinates from the origin to the fragment being
       fogged.	The equation for GL_LINEAR fog is

       f ~=~ {end ~-~ z} over {end ~-~ start}

       The equation for GL_EXP fog is

       f ~=~ e sup {-^(density ~cdot~ z)}

       The equation for GL_EXP2 fog is

       f ~=~ e sup {-^(density ~cdot~ z)} sup 2

       Regardless of the fog mode, $f$ is clamped to the range [0, 1] after it
       is computed.  Then, if the GL is in RGBA color mode, the fragment's
       red, green, and blue colors, represented by $C sub r$, are replaced by

       {C sub r} sup prime ~=~ f^C sub r ~+~ (1 - f)^C sub f

       Fog does not affect a fragment's alpha component.

       In color index mode, the fragment's color index $i sub r$ is replaced
       by

       {i sub r} sup prime ~=~ i sub r ~+~ (1 - f)^i sub f

ERRORS
       GL_INVALID_ENUM is generated if pname is not an accepted value, or if
       pname is GL_FOG_MODE and params is not an accepted value.

       GL_INVALID_VALUE is generated if pname is GL_FOG_DENSITY, and params is
       negative.

       GL_INVALID_OPERATION is generated if glFog is executed between the
       execution of glBegin and the corresponding execution of glEnd.

ASSOCIATED GETS
       glIsEnabled with argument GL_FOG
       glGet with argument GL_FOG_COLOR
       glGet with argument GL_FOG_INDEX
       glGet with argument GL_FOG_DENSITY
       glGet with argument GL_FOG_START
       glGet with argument GL_FOG_END
       glGet with argument GL_FOG_MODE

SEE ALSO
       glEnable(3G)

								 March 1, 2011
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