NAME    [Toc]    [Back]

       glAlphaFunc - specify the alpha test function

SYNOPSIS    [Toc]    [Back]

       void glAlphaFunc(
               GLenum func,
               GLclampf ref );

PARAMETERS    [Toc]    [Back]

       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.  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    [Toc]    [Back]

       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 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:  Never  passes.   Passes if the incoming
       alpha value is less than the reference value.   Passes  if
       the  incoming alpha value is equal to the reference value.
       Passes if the incoming alpha value is less than  or  equal
       to  the  reference  value.   Passes  if the incoming alpha
       value is greater than the reference value.  Passes if  the
       incoming  alpha value is not equal to the reference value.
       Passes if the incoming alpha  value  is  greater  than  or
       equal  to  the  reference  value.   Always passes (initial
       value).

       glAlphaFunc() operates  on  all  pixel  write  operations,
       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    [Toc]    [Back]

       Alpha testing is performed only in RGBA mode.

ERRORS    [Toc]    [Back]

       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    [Toc]    [Back]

       glGet() with argument GL_ALPHA_TEST_FUNC
       glGet() with argument GL_ALPHA_TEST_REF
       glIsEnabled() with argument GL_ALPHA_TEST

SEE ALSO    [Toc]    [Back]

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



                                                  glAlphaFunc(3G)