glAlphaFunc(3gl)

1GLALPHAFUNC(3G)                                                GLALPHAFUNC(3G)
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NAME

6       glAlphaFunc - specify the alpha test function
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C SPECIFICATION

10       void glAlphaFunc( GLenum func,
11                         GLclampf ref )
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PARAMETERS

15       func  Specifies  the  alpha  comparison  function.   Symbolic constants
16             GL_NEVER, GL_LESS, GL_EQUAL, GL_LEQUAL, GL_GREATER,  GL_NOTEQUAL,
17             GL_GEQUAL,  and  GL_ALWAYS  are  accepted.  The  initial value is
18             GL_ALWAYS.
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20       ref   Specifies the reference value that incoming alpha values are com‐
21             pared  to.   This  value  is clamped to the range [0, 1], where 0
22             represents the lowest possible alpha value and 1 the highest pos‐
23             sible value.  The initial reference value is 0.
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DESCRIPTION

26       The alpha test discards fragments depending on the outcome of a compar‐
27       ison between an incoming fragment's alpha value and a  constant  refer‐
28       ence value.  glAlphaFunc specifies the reference value and the compari‐
29       son function.  The comparison is performed only  if  alpha  testing  is
30       enabled. By default, it is not enabled.  (See glEnable and glDisable of
31       GL_ALPHA_TEST.)
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33       func and ref specify the conditions under which  the  pixel  is  drawn.
34       The  incoming  alpha value is compared to ref using the function speci‐
35       fied by func.  If the value passes the comparison, the  incoming  frag‐
36       ment  is  drawn  if  it also passes subsequent stencil and depth buffer
37       tests.  If the value fails the comparison, no change  is  made  to  the
38       frame  buffer  at  that pixel location. The comparison functions are as
39       follows:
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41       GL_NEVER          Never passes.
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43       GL_LESS           Passes if the incoming alpha value is less  than  the
44                         reference value.
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46       GL_EQUAL          Passes  if  the  incoming alpha value is equal to the
47                         reference value.
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49       GL_LEQUAL         Passes if the incoming alpha value is  less  than  or
50                         equal to the reference value.
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52       GL_GREATER        Passes  if  the  incoming alpha value is greater than
53                         the reference value.
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55       GL_NOTEQUAL       Passes if the incoming alpha value is  not  equal  to
56                         the reference value.
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58       GL_GEQUAL         Passes if the incoming alpha value is greater than or
59                         equal to the reference value.
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61       GL_ALWAYS         Always passes (initial value).
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63       glAlphaFunc operates on all pixel  write  operations,  including  those
64       resulting from the scan conversion of points, lines, polygons, and bit‐
65       maps, and from pixel draw and copy operations.   glAlphaFunc  does  not
66       affect screen clear operations.
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NOTES

69       Alpha testing is performed only in RGBA mode.
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ERRORS

72       GL_INVALID_ENUM is generated if func is not an accepted value.
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74       GL_INVALID_OPERATION  is  generated  if glAlphaFunc is executed between
75       the execution of glBegin and the corresponding execution of glEnd.
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ASSOCIATED GETS

78       glGet with argument GL_ALPHA_TEST_FUNC
79       glGet with argument GL_ALPHA_TEST_REF
80       glIsEnabled with argument GL_ALPHA_TEST
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