GLALPHAFUNC(3G)                                                GLALPHAFUNC(3G)



NAME
       glAlphaFunc - specify the alpha test function


C SPECIFICATION
       void glAlphaFunc( GLenum func,
                         GLclampf ref )


PARAMETERS
       func  Specifies  the  alpha  comparison  function.   Symbolic constants
             GLNEVER, GLES, GLEQUAL, GLEQUAL, GLGREATER,  GLNOTEQUAL,
             GLGEQUAL,  and  GLALWAYS  are  accepted.  The  initial value is
             GLALWAYS.

       ref   Specifies the reference value that incoming alpha values are com-
             pared  to.   This  value  is clamped to the range [0, 1], where 0
             represents the lowest possible alpha value and 1 the highest pos-
             sible value.  The initial reference value is 0.

DESCRIPTION
       The alpha test discards fragments depending on the outcome of a compar-
       ison between an incoming fragment's alpha value and a  constant  refer-
       ence value.  glAlphaFunc specifies the reference value and the compari-
       son function.  The comparison is performed only  if  alpha  testing  is
       enabled. By default, it is not enabled.  (See glEnable and glDisable of
       GLALPHATEST.)

       func and ref specify the conditions under which  the  pixel  is  drawn.
       The  incoming  alpha value is compared to ref using the function speci-
       fied by func.  If the value passes the comparison, the  incoming  frag-
       ment  is  drawn  if  it also passes subsequent stencil and depth buffer
       tests.  If the value fails the comparison, no change  is  made  to  the
       frame  buffer  at  that pixel location. The comparison functions are as
       follows:

       GLNEVER          Never passes.

       GLES           Passes if the incoming alpha value is less  than  the
                         reference value.

       GLEQUAL          Passes  if  the  incoming alpha value is equal to the
                         reference value.

       GLEQUAL         Passes if the incoming alpha value is  less  than  or
                         equal to the reference value.

       GLGREATER        Passes  if  the  incoming alpha value is greater than
                         the reference value.

       GLNOTEQUAL       Passes if the incoming alpha value is  not  equal  to
                         the reference value.

       GLGEQUAL         Passes if the incoming alpha value is greater than or
                         equal to the reference value.

       GLALWAYS         Always passes (initial value).

       glAlphaFunc operates on all pixel  write  operations,  including  those
       resulting from the scan conversion of points, lines, polygons, and bit-
       maps, and from pixel draw and copy operations.   glAlphaFunc  does  not
       affect screen clear operations.

NOTES
       Alpha testing is performed only in RGBA mode.

ERORS
       GLINVALIDENUM is generated if func is not an accepted value.

       GLINVALIDOPERATION  is  generated  if glAlphaFunc is executed between
       the execution of glBegin and the corresponding execution of glEnd.

ASOCIATED GETS
       glGet with argument GLALPHATESTFUNC
       glGet with argument GLALPHATESTREF
       glIsEnabled with argument GLALPHATEST

SEE ALSO
       glBlendFunc, glClear, glDepthFunc, glEnable, glStencilFunc



                                                               GLALPHAFUNC(3G)