Index for Section 3G

Alphabetical listing for G

Bottom of page

glStencilFunc(3G)
NAME
  glStencilFunc - set function and reference value for stencil testing

SYNOPSIS
  void glStencilFunc(
	  GLenum func,
	  GLint ref,
	  GLuint mask );

PARAMETERS
  func
      Specifies the test function. Eight tokens are valid: GL_NEVER, GL_LESS,
      GL_LEQUAL, GL_GREATER, GL_GEQUAL, GL_EQUAL, GL_NOTEQUAL, and GL_ALWAYS.
      The initial value is GL_ALWAYS.

  ref Specifies the reference value for the stencil test.  ref is clamped to
      the range [0,2^n - 1], where n is the number of bitplanes in the
      stencil buffer. The initial value is 0.

  mask
      Specifies a mask that is ANDed with both the reference value and the
      stored stencil value when the test is done. The initial value is all
      1's.

DESCRIPTION
  Stenciling, like depth-buffering, enables and disables drawing on a per-
  pixel basis. You draw into the stencil planes using GL drawing primitives,
  then render geometry and images, using the stencil planes to mask out
  portions of the screen. Stenciling is typically used in multipass rendering
  algorithms to achieve special effects, such as decals, outlining, and
  constructive solid geometry rendering.

  The stencil test conditionally eliminates a pixel based on the outcome of a
  comparison between the reference value and the value in the stencil buffer.
  To enable and disable the test, call glEnable() and glDisable() with
  argument GL_STENCIL_TEST. To specify actions based on the outcome of the
  stencil test, call glStencilOp().

  func is a symbolic constant that determines the stencil comparison
  function. It accepts one of eight values, shown in the following list.  ref
  is an integer reference value that is used in the stencil comparison. It is
  clamped to the range [0,2^n - 1], where n is the number of bitplanes in the
  stencil buffer.  mask is bitwise ANDed with both the reference value and
  the stored stencil value, with the ANDed values participating in the
  comparison.

  If stencil represents the value stored in the corresponding stencil buffer
  location, the following list shows the effect of each comparison function
  that can be specified by func.  Only if the comparison succeeds is the
  pixel passed through to the next stage in the rasterization process (see
  glStencilOp()). All tests treat stencil values as unsigned integers in the
  range [0,2^n - 1], where n is the number of bitplanes in the stencil
  buffer.

  The following values are accepted by func:

  GL_NEVER
      Always fails.

  GL_LESS
      Passes if ( ref & mask ) < ( stencil & mask ).

  GL_LEQUAL
      Passes if ( ref & mask ) <= ( stencil & mask ).

  GL_GREATER
      Passes if ( ref & mask ) > ( stencil & mask ).

  GL_GEQUAL
      Passes if ( ref & mask ) >= ( stencil & mask ).

  GL_EQUAL
      Passes if ( ref & mask ) = ( stencil & mask ).

  GL_NOTEQUAL
      Passes if ( ref & mask ) <> ( stencil & mask ).

  GL_ALWAYS
      Always passes.

NOTES
  Initially, the stencil test is disabled. If there is no stencil buffer, no
  stencil modification can occur and it is as if the stencil test always
  passes.

ERRORS
  GL_INVALID_ENUM is generated if func is not one of the eight accepted
  values.

  GL_INVALID_OPERATION is generated if glStencilFunc() is executed between
  the execution of glBegin() and the corresponding execution of glEnd().

ASSOCIATED GETS
       glGet() with argument GL_STENCIL_FUNC
       glGet() with argument GL_STENCIL_VALUE_MASK
       glGet() with argument GL_STENCIL_REF
       glGet() with argument GL_STENCIL_BITS
       glIsEnabled() with argument GL_STENCIL_TEST

SEE ALSO
  glAlphaFunc(3), glBlendFunc(3), glDepthFunc(3), glEnable(3),
  glIsEnabled(3), glLogicOp(3), glStencilOp(3),

Index for Section 3G

Alphabetical listing for G

Top of page