C SPECIFICATION

       void GLAPIENTRY glFogf( GLenum pname,
                               GLfloat param )
       void GLAPIENTRY glFogi( GLenum pname,
                               GLint param )

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 GLAPIENTRY glFogfv( GLenum pname,
                                const GLfloat *params )
       void GLAPIENTRY 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 param-
              eters accept an array containing only a single value.

DESCRIPTION

       Fog is initially disabled.  While enabled, fog affects rasterized geom-
       etry, 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 speci-
       fied 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_COLOR        params contains four integer or floating-point val-
                           ues that specify Cf, the fog color.  Integer values
                           are  mapped  linearly  such  that the most positive
                           representable value maps to 1.0, and the most nega-
                           tive  representable  value maps to -1.0.  Floating-
                           point values are mapped  directly.   After  conver-
                           sion, 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  posttex-
       turing 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=e_n_e<I>d_n_d<I>s_<I>t_z<I>a_r_t_

                               f=e-(density?z)

                               f=e-(density?z)2


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

                               Cr?=fCr+(1-f)Cf


Fog does not affect a fragment's alpha component.

In color index mode, the fragment's color index ir is replaced by

                                ir?=ir+(1-f)if

       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 exe-
       cution of glBegin and the corresponding execution of glEnd.

                                                                     GLFOG(3G)