scrmask(3G)

NAME
     scrmask - defines a rectangular screen clipping mask

C SPECIFICATION
     void scrmask(left, right, bottom, top)
     Screencoord left, right, bottom, top;

PARAMETERS
     left     expects the window coordinate of the left-most pixel column
              within the mask region.

     right    expects the window coordinate of the right-most pixel column
              within the mask region.

     bottom   expects the window coordinate of the lowest pixel row within the
              mask region.

     top      expects the window coordinate of the highest pixel row within
              the mask region.

DESCRIPTION
     scrmask defines a subregion of the current viewport that can be updated
     by drawing commands.  Pixels outside this region cannot be modified by
     any drawing commands, including point, line, polygon, character, pixel
     write, and pixel copy commands.  All pixel bitplane buffers, including
     color, depth, accumulation, and stencil buffers, are write protected.
     scrmask operates in all draw modes.

     The enabled subregion is specified as a screen-aligned rectangle in
     window coordinates.  Like viewport, the boundary specification is
     inclusive, so the call scrmask(0,0,0,0) specifies a 1-pixel rectangle in
     the lower-left corner of the window.

     When viewport is called, the screen mask is set to match the newly
     specified viewport.  Any previous scrmask specification is lost.

     scrmask must be specified entirely within the current viewport.

SEE ALSO
     drawmode, getscrmask, viewport

NOTE
     If you set left to be greater than right or bottom to be greater than
     top, all pixels in the viewport are write protected.