Index for Section 3G

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glAlphaFunc(3G)
NAME
  glAlphaFunc - specify the alpha test function

SYNOPSIS
  void glAlphaFunc(
	  GLenum func,
	  GLclampf ref );

PARAMETERS
  func
      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.

  ref 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.

DESCRIPTION
  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:

  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 (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.

NOTES
  Alpha testing is performed only in RGBA mode.

ERRORS
  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(3), glClear(3), glDepthFunc(3), glEnable(3), glStencilFunc(3)

Index for Section 3G

Alphabetical listing for G

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