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
           GL_NEVER, GL_LESS, GL_EQUAL, GL_LEQUAL, GL_GREATER, GL_NOTEQUAL,
           GL_GEQUAL, and GL_ALWAYS are accepted.  The default function is
           GL_ALWAYS.


     ref   Specifies the reference value that incoming alpha values are
           compared to.  This value is clamped to the range 0 through 1, where
           0 represents the lowest possible alpha value and 1 the highest
           possible value.  The default reference is 0.


DESCRIPTION
     The alpha test discards fragments depending on the outcome of a
     comparison between the incoming fragment's alpha value and a constant
     reference value.   glAlphaFunc specifies the reference and comparison
     function.  The comparison is performed only if alpha testing is 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 comparison passes, the incoming fragment is drawn,
     conditional on subsequent stencil and depth buffer tests. If the
     comparison fails, 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.

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

NOTES
     Alpha testing is done only in RGBA mode.

ERRORS
     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.

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