glStencilOp(3gl)

1GLSTENCILOP(3G)                                                GLSTENCILOP(3G)
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NAME

6       glStencilOp - set stencil test actions
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C SPECIFICATION

10       void glStencilOp( GLenum fail,
11                         GLenum zfail,
12                         GLenum zpass )
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PARAMETERS

16       fail   Specifies  the  action to take when the stencil test fails.  Six
17              symbolic constants are accepted: GL_KEEP,  GL_ZERO,  GL_REPLACE,
18              GL_INCR, GL_DECR, and GL_INVERT. The initial value is GL_KEEP.
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20       zfail  Specifies  the  stencil action when the stencil test passes, but
21              the depth test fails.  zfail accepts the same symbolic constants
22              as fail. The initial value is GL_KEEP.
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24       zpass  Specifies  the stencil action when both the stencil test and the
25              depth test pass, or when the  stencil  test  passes  and  either
26              there is no depth buffer or depth testing is not enabled.  zpass
27              accepts the same symbolic constants as fail. The  initial  value
28              is GL_KEEP.
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DESCRIPTION

31       Stenciling,  like  depth-buffering,  enables  and disables drawing on a
32       per-pixel basis.  You draw into the stencil  planes  using  GL  drawing
33       primitives,  then  render geometry and images, using the stencil planes
34       to mask out portions of the screen.  Stenciling is  typically  used  in
35       multipass  rendering  algorithms  to  achieve  special effects, such as
36       decals, outlining, and constructive solid geometry rendering.
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38       The stencil test conditionally eliminates a pixel based on the  outcome
39       of a comparison between the value in the stencil buffer and a reference
40       value. To enable and disable the test, call glEnable and glDisable with
41       argument GL_STENCIL_TEST; to control it, call glStencilFunc.
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43       glStencilOp  takes  three  arguments  that indicate what happens to the
44       stored stencil value while stenciling is enabled.  If the stencil  test
45       fails,  no  change  is  made to the pixel's color or depth buffers, and
46       fail specifies what happens to the stencil buffer contents.   The  fol‐
47       lowing six actions are possible.
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49       GL_KEEP         Keeps the current value.
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51       GL_ZERO         Sets the stencil buffer value to 0.
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53       GL_REPLACE      Sets  the  stencil buffer value to ref, as specified by
54                       glStencilFunc.
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56       GL_INCR         Increments the current stencil buffer value.  Clamps to
57                       the maximum representable unsigned value.
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59       GL_DECR         Decrements the current stencil buffer value.  Clamps to
60                       0.
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62       GL_INVERT       Bitwise inverts the current stencil buffer value.
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64       Stencil buffer values are treated as unsigned  integers.   When  incre‐
65       mented  and  decremented,  values are clamped to 0 and 2n−1, where n is
66       the value returned by querying GL_STENCIL_BITS.
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68       The other two arguments to glStencilOp specify stencil  buffer  actions
69       that depend on whether subsequent depth buffer tests succeed (zpass) or
70       fail (zfail) (see
71       glDepthFunc).  The actions are specified using the  same  six  symbolic
72       constants  as  fail.  Note that zfail is ignored when there is no depth
73       buffer, or when the depth buffer is not enabled.  In these cases,  fail
74       and  zpass  specify  stencil  action  when  the  stencil test fails and
75       passes, respectively.
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NOTES

78       Initially the stencil test is disabled.  If there is no stencil buffer,
79       no  stencil  modification  can  occur and it is as if the stencil tests
80       always pass, regardless of any call to glStencilOp.
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ERRORS

83       GL_INVALID_ENUM is generated if fail, zfail,  or  zpass  is  any  value
84       other than the six defined constant values.
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86       GL_INVALID_OPERATION  is  generated  if glStencilOp is executed between
87       the execution of glBegin and the corresponding execution of glEnd.
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ASSOCIATED GETS

90       glGet with argument GL_STENCIL_FAIL
91       glGet with argument GL_STENCIL_PASS_DEPTH_PASS
92       glGet with argument GL_STENCIL_PASS_DEPTH_FAIL
93       glGet with argument GL_STENCIL_BITS
94       glIsEnabled with argument GL_STENCIL_TEST
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