glStencilOp

  • Article

The glStencilOp function sets the stencil test actions.

void glStencilOp(
  GLenum fail,  GLenum zfail,  GLenum zpass);

Parameters

  • fail
    The action to take when the stencil test fails. The following six symbolic constants are accepted:

    Value Meaning
    GL_KEEP Keeps the current value.
    GL_ZERO Sets the stencil buffer value to zero.
    GL_REPLACE Sets the stencil buffer value to ref, as specified by glStencilFunc.
    GL_INCR Increments the current stencil buffer value. Clamps to the maximum representable unsigned value.
    GL_DECR Decrements the current stencil buffer value. Clamps to zero.
    GL_INVERT Bitwise inverts the current stencil buffer value.

  • zfail
    Stencil action when the stencil test passes, but the depth test fails. Accepts the same symbolic constants as fail.

  • zpass
    Stencil action when both the stencil test and the depth test pass, or when the stencil test passes and either there is no depth buffer or depth testing is not enabled. Accepts the same symbolic constants as fail.

Remarks

Stenciling, like z-buffering, enables and disables drawing on a per-pixel basis. You draw into the stencil planes using OpenGL 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 value in the stencil buffer and a reference value. The test is enabled with glEnable and glDisable calls with argument GL_STENCIL, and controlled with glStencilFunc.

The glStencilOp function takes three arguments that indicate what happens to the stored stencil value while stenciling is enabled. If the stencil test fails, no change is made to the pixel's color or depth buffers, and fail specifies what happens to the stencil buffer contents.

Stencil buffer values are treated as unsigned integers. When incremented and decremented, values are clamped to 0 and 2*n* 1, where n is the value returned by querying GL_STENCIL_BITS.

The other two arguments to glStencilOp specify stencil buffer actions should subsequent depth buffer tests succeed (zpass) or fail (zfail). (See glDepthFunc.) They are specified using the same six symbolic constants as fail. Note that zfail is ignored when there is no depth buffer, or when the depth buffer is not enabled. In these cases, fail and zpass specify stencil action when the stencil test fails and passes, respectively.

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

The following functions retrieve information related to glStencilOp:

glGet with argument GL_STENCIL_FAIL

glGet with argument GL_STENCIL_PASS_DEPTH_PASS

glGet with argument GL_STENCIL_PASS_DEPTH_FAIL

glGet with argument GL_STENCIL_BITS

glIsEnabled with argument GL_STENCIL_TEST

Error Codes

The following are the error codes generated and their conditions.

Error code Condition
GL_INVALID_ENUM fail, zfail, or zpass was any value other than the six defined constant values.
GL_INVALID_OPERATION glStencilOp was called between a call to glBegin and the corresponding call to glEnd.

Requirements

**  Windows NT/2000:** Requires Windows NT 3.5 or later.
**  Windows 95/98:** Requires Windows 95 or later. Available as a redistributable for Windows 95.
**  Header:** Declared in Gl.h.
**  Library:** Use Opengl32.lib.

See Also

glAlphaFunc, glBegin, glBlendFunc, glDepthFunc, glEnable, glEnd, glIsEnabled, glLogicOp, glStencilFunc