subpixel(3G)

NAME
     subpixel - controls the placement of point, line, and polygon vertices

C SPECIFICATION
     void subpixel(b)
     Boolean b;

PARAMETERS
     b   expects either FALSE or TRUE.

         FALSE forces screen vertices to the centers of pixels (default).

         TRUE positions screen vertices exactly.

DESCRIPTION
     subpixel controls the placement of point, line, and polygon vertices in
     screen coordinates.  By default subpixel is FALSE, causing vertices to be
     snapped to the center of the nearest pixel after they have been
     transformed to screen coordinates.  Vertex snapping introduces artifacts
     into the scan conversion of lines and polygons.  It is especially
     noticable when points or lines are drawn smooth (see pntsmooth and
     linesmooth).  Thus subpixel is typically set to TRUE while smooth points
     or smooth lines are being drawn.

     In addition to its effect on vertex position, subpixel also modifies the
     scan conversion of lines.  Specifically, non-subpixel positioned lines
     are drawn closed, meaning that connected line segments both draw the
     pixel at their shared vertex, while subpixel positioned lines are drawn
     half open, meaning that connected lines segments share no pixels.
     (Smooth lines are always drawn half open, regardless of state of
     subpixel.)  Thus subpixel positioned lines produce better results when
     logicop or blendfunction are used, but will produce different, possibly
     undesirable results in 2-D applications where the endpoints of lines have
     been carefully placed.

     For example, using the standard 2-D projection:

          ortho2(left-0.5,right+0.5,bottom-0.5,top+0.5);
          viewport(left,right,bottom,top);

     subpixel positioned lines match non-subpixel positioned lines pixel for
     pixel, except that they omit either the right-most or top-most pixel.
     Thus the non-subpixel positioned line drawn from (0,0) to (0,2) fills
     pixels (0,0), (0,1), and (0,2), while the subpixel positioned line drawn
     between the same coordinates fills only pixels (0,0) and (0,1).

SEE ALSO
     linesmooth, pntsmooth
NOTES
     This routine does not function on IRIS-4D B or G models.

     The IRIS-4D GT and GTX models do not implement subpixel positioned
     polygons.  They also do not implement subpixel positioned non-smooth
     lines.

     On the Personal Iris, XS, XS24, XZ, Elan and Extreme, polygons are always
     subpixel positioned, regardless of the value of subpixel.  Subpixel
     positioned non-smooth lines are not implemented.

     On Indy, Iris Indigo Entry, and XL systems, subpixel positioned non-
     smooth lines are drawn closed instead of half-open.