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GLALPHAFUNC(3G)                                                                              GLALPHAFUNC(3G)

       glAlphaFunc - specify the alpha test function

       void glAlphaFunc( GLenum func,
                         GLclampf ref )

       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

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

       The  alpha test discards fragments depending on the outcome of a comparison between an incoming frag-ment's fragment's
       ment'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 com-pared compared
       pared  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 func-tions functions
       tions 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 resulting from the scan conver-sion conversion
       sion of points, lines, polygons, and bitmaps, and from pixel draw and copy  operations.   glAlphaFunc
       does not affect screen clear operations.

       Alpha testing is performed only in RGBA mode.

       GL_INVALID_ENUM is generated if func is not an accepted value.

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

       glGet with argument GL_ALPHA_TEST_FUNC
       glGet with argument GL_ALPHA_TEST_REF
       glIsEnabled with argument GL_ALPHA_TEST

       glBlendFunc(3G), glClear(3G), glDepthFunc(3G), glEnable(3G), glStencilFunc(3G)


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