glalphafunc(3G) — IRIX 6.5.3f



glAlphaFunc(3G)                OpenGL Reference                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, GLLESS, GLEQUAL, GLLEQUAL, GLGREATER, GLNOTEQUAL,
           GLGEQUAL, and GLALWAYS are accepted.  The default function is
           GLALWAYS.


     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 GLALPHATEST.)

     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:

     GLNEVER          Never passes.

     GLLESS           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.

     GLLEQUAL         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.




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glAlphaFunc(3G)                OpenGL Reference                glAlphaFunc(3G)



     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.

     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
     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.

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


SEE ALSO
     glBlendFunc, glClear, glDepthFunc, glEnable, glStencilFunc

























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