1GLALPHAFUNC(3G)                  OpenGL Manual                 GLALPHAFUNC(3G)
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NAME

6       glAlphaFunc - specify the alpha test function
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C SPECIFICATION

9       void glAlphaFunc(GLenum func, GLclampf ref);
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PARAMETERS

12       func
13           Specifies the alpha comparison function. Symbolic constants
14           GL_NEVER, GL_LESS, GL_EQUAL, GL_LEQUAL, GL_GREATER, GL_NOTEQUAL,
15           GL_GEQUAL, and GL_ALWAYS are accepted. The initial value is
16           GL_ALWAYS.
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18       ref
19           Specifies the reference value that incoming alpha values are
20           compared to. This value is clamped to the range 0 1, where 0
21           represents the lowest possible alpha value and 1 the highest
22           possible value. The initial reference value is 0.
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DESCRIPTION

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

74       Alpha testing is performed only in RGBA mode.
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ERRORS

77       GL_INVALID_ENUM is generated if func is not an accepted value.
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79       GL_INVALID_OPERATION is generated if glAlphaFunc is executed between
80       the execution of glBegin() and the corresponding execution of glEnd().
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ASSOCIATED GETS

83       glGet() with argument GL_ALPHA_TEST_FUNC
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85       glGet() with argument GL_ALPHA_TEST_REF
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87       glIsEnabled() with argument GL_ALPHA_TEST
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SEE ALSO

90       glBlendFunc(), glClear(), glDepthFunc(), glEnable(), glStencilFunc()
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93       Copyright © 1991-2006 Silicon Graphics, Inc. This document is licensed
94       under the SGI Free Software B License. For details, see
95       http://oss.sgi.com/projects/FreeB/.
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AUTHORS

98       opengl.org
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102opengl.org                        07/13/2018                   GLALPHAFUNC(3G)
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