glAlphaFunc function

The glAlphaFunc function enables your application to set the alpha test function.

Syntax

void WINAPI glAlphaFunc(
   GLenum   func,
   GLclampf ref
);

Parameters

func

The alpha comparison function. The following are the accepted symbolic constants and their meanings.

Value Meaning
GL_NEVER
Never passes.
GL_LESS
Passes if the incoming alpha value is less than the reference value.
GL_EQUAL
Passes if the incoming alpha value is equal to the reference value.
GL_LEQUAL
Passes if the incoming alpha value is less than or equal to the reference value.
GL_GREATER
Passes if the incoming alpha value is greater than the reference value.
GL_NOTEQUAL
Passes if the incoming alpha value is not equal to the reference value.
GL_GEQUAL
Passes if the incoming alpha value is greater than or equal to the reference value.
GL_ALWAYS
Always passes. This is the default.

ref

The reference value to which incoming alpha values are compared. This value is clamped to the range 0 through 1, where 0 represents the lowest possible alpha value and 1 the highest possible value. The default reference is 0.

Return value

This function does not return a value.

Error codes

The following error codes can be retrieved by the glGetError function.

Name Meaning
GL_INVALID_ENUM
func was not an accepted value.
GL_INVALID_OPERATION
The function was called between a call to glBegin and the corresponding call to glEnd.

Error codes

The following error codes can be retrieved by the glGetError function.

Name Meaning
GL_INVALID_ENUM
func was not an accepted value.
GL_INVALID_OPERATION
The function was called between a call to glBegin and the corresponding call to glEnd.

Remarks

The alpha test discards fragments depending on the outcome of a comparison between the incoming fragments' alpha values and a constant reference value. The glAlphaFunc function specifies the reference and comparison function. The comparison is performed only if alpha testing is enabled. (For more information on GL_ALPHA_TEST, see glEnable.)

The func and ref parameters 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 comparison passes, the incoming fragment is drawn, conditional on subsequent stencil and depth-buffer tests. If the comparison fails, no change is made to the framebuffer at that pixel location.

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

Alpha testing is done only in RGBA mode.

The following functions retrieve information related to the glAlphaFunc function:

glGet with argument GL_ALPHA_TEST_FUNC

glGet with argument GL_ALPHA_TEST_REF

glIsEnabled with argument GL_ALPHA_TEST

Requirements

Minimum supported client
Windows 2000 Professional [desktop apps only]
Minimum supported server
Windows 2000 Server [desktop apps only]
Header
Gl.h
Library
Opengl32.lib
DLL
Opengl32.dll

See also

glBegin

glBlendFunc

glClear

glDepthFunc

glEnable

glEnd

glGet

glIsEnabled

glStencilFunc