fglLogicOp(3G)

NAME
     fglLogicOp - specify a logical pixel operation for color index rendering


FORTRAN SPECIFICATION
     SUBROUTINE fglLogicOp( INTEGER*4 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 initial value is GL_COPY.

DESCRIPTION
     fglLogicOp specifies a logical operation that, when enabled, is applied
     between the incoming color index or RGBA color and the color index or
     RGBA color at the corresponding location in the frame buffer. To enable
     or disable the logical operation, call fglEnable and fglDisable using the
     symbolic constant GL_COLOR_LOGIC_OP for RGBA mode or GL_INDEX_LOGIC_OP
     for color index mode. The initial value is disabled for both operations.

                      ______________________________________

                      Opcode             Resulting Operation
                      ______________________________________

                      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

     opcode is a symbolic constant chosen from the list above.  In the
     explanation of the logical operations, s represents the incoming color
     index and d represents the index 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 colors.
NOTES
     Color index logical operations are always supported. RGBA logical
     operations are supported only if the GL version is 1.1 or greater.
     (However, if the GL version is 1.0, then in RGBA mode, fglLogicOp 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
     fglBlendEquationExt).)

     When more than one RGBA color or index buffer is enabled for drawing,
     logical operations are performed separately for each enabled buffer,
     using for the destination value the contents of that buffer (see
     fglDrawBuffer).

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

     GL_INVALID_OPERATION is generated if fglLogicOp is executed between the
     execution of fglBegin and the corresponding execution of fglEnd.

ASSOCIATED GETS
     fglGet with argument GL_LOGIC_OP_MODE.
     fglIsEnabled with argument GL_COLOR_LOGIC_OP or GL_INDEX_LOGIC_OP.



SEE ALSO
     fglAlphaFunc, fglBlendFunc, fglBlendEquationEXT, fglDrawBuffer,
     fglEnable, fglStencilOp