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glAlphaFunc.3G - Man Page

specify the alpha test function

C Specification

void glAlphaFunc( GLenum func,

	GLclampf ref )



Specifies the alpha comparison function. Symbolic constants GL_NEVER, GL_LESS, GL_EQUAL, GL_LEQUAL, GL_GREATER, GL_NOTEQUAL, GL_GEQUAL, and GL_ALWAYS are accepted. The initial value is GL_ALWAYS.


Specifies the reference value that incoming alpha values are compared to. This value is clamped to the range [0, 1], where 0 represents the lowest possible alpha value and 1 the highest possible value. The initial reference value is 0.


The alpha test discards fragments depending on the outcome of a comparison between an incoming fragment's alpha value and a constant reference value. glAlphaFunc specifies the reference value and the comparison function. The comparison is performed only if alpha testing is enabled. By default, it is not enabled.  (See  glEnable and glDisable of GL_ALPHA_TEST.)

func and ref specify the conditions under which the pixel is drawn. The incoming alpha value is compared to ref using the function specified by func. If the value passes the comparison, the incoming fragment is drawn if it also passes subsequent stencil and depth buffer tests.  If the value fails the comparison, no change is made to the frame buffer at that pixel location. The comparison functions are as follows:


Never passes.


Passes if the incoming alpha value is less than the reference value.


Passes if the incoming alpha value is equal to the reference value.


Passes if the incoming alpha value is less than or equal to the reference value.


Passes if the incoming alpha value is greater than the reference value.


Passes if the incoming alpha value is not equal to the reference value.


Passes if the incoming alpha value is greater than or equal to the reference value.


Always passes (initial value).

glAlphaFunc operates on all pixel write operations, including those resulting from the scan conversion of points, lines, polygons, and bitmaps, and from pixel draw and copy operations. glAlphaFunc does not affect screen clear operations.


Alpha testing is performed only in RGBA mode.


GL_INVALID_ENUM is generated if func is not an accepted value.

GL_INVALID_OPERATION is generated if glAlphaFunc is executed between the execution of glBegin and the corresponding execution of glEnd.

Associated Gets

glGet with argument GL_ALPHA_TEST_FUNC
glGet with argument GL_ALPHA_TEST_REF
glIsEnabled with argument GL_ALPHA_TEST

See Also

glBlendFunc(3G), glClear(3G), glDepthFunc(3G), glEnable(3G), glStencilFunc(3G)

Referenced By