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  Constant:  _GAND, _GOR, _GPRESET, _GPSET, _GXOR
 
  Include:   <graph.h>
 
  Context:   _putimage, _putimage_w
 
  Synopsis:  Specifies the interaction between the stored image and
             the image that is already on the screen.
 
     The image constants are described below:
 
     Constant     Meaning
 
     _GAND        Transfers the image over an existing image on the
                  screen. The resulting image is the logical-AND
                  product of the two images: points that had the same
                  color in both the existing image and the new one will
                  remain the same color, while points that have
                  different colors are joined by logical-AND.
 
     _GOR         Superimposes the image onto an existing image. The
                  new image does not erase the previous screen
                  contents.
 
     _GPRESET     Transfers the data point-by-point onto the screen.
                  Each point has the inverse of the color attribute it
                  had when taken from the screen by _getimage,
                  producing a negative image.
 
     _GPSET       Transfers the data point-by-point onto the screen.
                  Each point has the exact color attribute it had when
                  taken from the screen by _getimage.
 
     _GXOR        Causes the points on the screen to be inverted where
                  a point exists in the image buffer. This behavior is
                  exactly like that of the cursor: when an image is put
                  against a complex background twice, the background is
                  restored unchanged. This allows you to move an object
                  around without erasing the background. The _GXOR
                  constant is a special mode often used for animation.