- specify the alpha test function



C SPECIFICATION


PARAMETERS

       _param1  Specifies  the  alpha  comparison function.  Sym-
                bolic  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.

       _param2  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  highest  possible
                value.  The initial reference value is 0.


DESCRIPTION

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

       _param1 and _param2 specify the conditions under which the
       pixel is drawn.  The incoming alpha value is  compared  to
       _param2  using  the function specified by _param1.  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.

       Alpha testing is performed only in RGBA mode.


ERRORS

       GL_INVALID_ENUM is generated if _param1 is not an accepted
       value.

       GL_INVALID_OPERATION  is  generated if glAlphaFunc is exe-
       cuted between the execution of glBegin and the correspond-
       ing execution of glEnd.


ASSOCIATED GETS

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


SEE ALSO

       glBlendFunc, glClear, glDepthFunc, glEnable, glStencilFunc



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