glStencilFunc
NAME
glStencilFunc - set function and reference value for stencil testingC SPECIFICATION
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 |
| 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 on glStencilFunc
Stenciling, like depth-buffering, enables and disables drawing on a per-pixel basis. GL drawing primitives are used to draw into the stencil planes which can then be used to mask out portions of the screen when rendering geometry and images. 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 glDisable with argument GL_STENCIL_TEST. To specify actions based on the outcome of the stencil test, call glStencilOp. (glStencilFunc)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
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
The following values are accepted by func: (glStencilFunc)
- 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_FUNCglGet with argument GL_STENCIL_VALUE_MASK
glGet with argument GL_STENCIL_REF
glGet with argument GL_STENCIL_BITS
glIsEnabled with argument GL_STENCIL_TEST
Example of glStencilFunc
void RenderScence(void){
GLdouble dRadius = 0.1;
GLdouble dAngle;
glClearColor(0.0f,0.0f,1.0f,0.0f);
glClearStencil(0.0f);
glEnable(GL_STENCIL_TEST);
glClear(GL_COLOR_BUFFER | GL_STENCIL_BUFFER_BIT);
glStencilFunc(GL_NEVER,0x0,0x0);
glStencilOp(GL_INCR , GL_INCR , GL_INCR);
glColor3f(1.0f,1.0f,1.0f);
glBegin(GL_LINE_STRIP);
for( dAngle = 0 ; dAngle < 400 ; dAngle += 0.1)
{
glVertex2d( dRadius * cos(dAngle) , dRadius * sin( dAngle));
dRadius *= 1.002;
}
glEnd();
glStencilFunc(GL_NOTEQUAL , 0x1, 0x1 );
glStencilOp(GL_KEEP,GL_KEEP,GL_KEEP);
glColor3f(1.0f,0.0f,0.0f);
glRectf(x,y, x + rsize, y - rsize);
glSwapBuffer();
}
