GLclampf ref )

PARAMETERS
       func  Specifies  the  alpha  comparison  function.   Symbolic  constants
             GL_NEVER, GL_LESS, GL_EQUAL, GL_LEQUAL,  GL_GREATER,  GL_NOTEQUAL,
             GL_GEQUAL,  and  GL_ALWAYS  are  accepted.  The  initial  value is
             GL_ALWAYS.

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

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

       func and ref specify the conditions under which the pixel is drawn.  The
       incoming  alpha value is compared to ref using the function specified by
       func.  If the value passes the  comparison,  the  incoming  fragment  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:

       GL_NEVER          Never passes.

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

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

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

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

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

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

       GL_ALWAYS         Always passes (initial value).

       glAlphaFunc  operates on all pixel write operations, including those re‐
       sulting from the scan conversion of points, lines,  polygons,  and  bit‐
       glGet with argument GL_ALPHA_TEST_REF
       glIsEnabled with argument GL_ALPHA_TEST

SEE ALSO
       glBlendFunc, glClear, glDepthFunc, glEnable, glStencilFunc

                                                                GLALPHAFUNC(3G)