glLogicOp(3G)		       OpenGL Reference			 glLogicOp(3G)



NAME
     glLogicOp - specify a logical pixel operation for rendering


C SPECIFICATION
     void glLogicOp( GLenum opcode )


PARAMETERS
     opcode  Specifies a symbolic constant that	selects	a logical operation.
	     The following symbols are accepted:  GL_CLEAR, GL_SET, GL_COPY,
	     GL_COPY_INVERTED, GL_NOOP,	GL_INVERT, GL_AND, GL_NAND, GL_OR,
	     GL_NOR, GL_XOR, GL_EQUIV, GL_AND_REVERSE, GL_AND_INVERTED,
	     GL_OR_REVERSE, and	GL_OR_INVERTED.	The default is GL_COPY.

DESCRIPTION
     glLogicOp specifies a logical operation that, when	enabled, combines the
     incoming color or color index and the color or color index	from the frame
     buffer.  Logicops are used	differently in color index mode	and RGBA mode.

     In	color index mode the logical operation is enabled or disabled with
     glEnable and glDisable using the symbolic constant	GL_LOGIC_OP.

     In	RGBA mode glLogicOp is used to specify a blending operation. The
     specified logicop is used if blending is enabled and the blend equation
     is	set to GL_LOGIC_OP. (see glBlendEquationExt).  In RGBA mode each
     component of an incoming fragment's color is combined with	the
     corresponding component of	the color taken	from the color buffer.

     opcode is a symbolic constant chosen from the list	below.	In the
     explanation of the	logical	operations, s represents each of the 4
     incoming color components,	or it represents the color index. d represents
     the corresponding value in	the frame buffer.  Standard C-language
     operators are used.  As these bitwise operators suggest, the logical
     operation is applied independently	to each	bit pair of the	source and
     destination indices or color components.



















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glLogicOp(3G)		       OpenGL Reference			 glLogicOp(3G)



		      _____________________________________
		       |_________________|_________________|
		       |    GL_CLEAR	 |	  0	   |
		       |     GL_SET	 |	  1	   |
		       |    GL_COPY	 |	  s	   |
		       |GL_COPY_INVERTED |	 !s	   |
		       |    GL_NOOP	 |	  d	   |
		       |   GL_INVERT	 |	 ~d	   |
		       |     GL_AND	 |	s & d	   |
		       |    GL_NAND	 |    ~(s & d)	   |
		       |     GL_OR	 |	s | d	   |
		       |     GL_NOR	 |    ~(s | d)	   |
		       |     GL_XOR	 |	s ^ d	   |
		       |    GL_EQUIV	 |    ~(s ^ d)	   |
		       | GL_AND_REVERSE	 |     s & ~d	   |
		       |GL_AND_INVERTED	 |     ~s & d	   |
		       | GL_OR_REVERSE	 |     s | ~d	   |
		       | GL_OR_INVERTED	 |     ~s | d	   |
		       |_________________|_________________|


NOTES
     When more than one	color buffer is	enabled	for drawing, logical
     operations	are done separately for	each enabled buffer, using for the
     destination index the contents of that buffer (see	glDrawBuffer).

     In	RGB mode glLogicOp is part of the EXT_blend_logic_op extension,	not
     part of the core GL command set. If GL_EXT_blend_logic_op is included in
     the string	returned by glGetString, when called with argument
     GL_EXTENSIONS, extension EXT_blend_logic_op is supported by the
     connection.


ERRORS
     GL_INVALID_ENUM is	generated if opcode is not an accepted value.

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


ASSOCIATED GETS
     glGet with	argument GL_LOGIC_OP_MODE
     glIsEnabled with argument GL_LOGIC_OP



SEE ALSO
     glAlphaFunc, glBlendFunc, glBlendEquationEXT, glDrawBuffer, glEnable,
     glStencilOp






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