subpix(3G)

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

FORTRAN 77 SPECIFICATION
     subroutine subpix(b)
     logical b

PARAMETERS
     b   expects either .FALSE. or .TRUE..

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

          .TRUE. positions screen vertices exactly.

DESCRIPTION
     subpix controls the placement of point, line, and polygon vertices in
     screen coordinates.  By default subpix 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 pntsmo and linesm).
     Thus subpix is typically set to .TRUE.  while smooth points or smooth
     lines are being drawn.

     In addition to its effect on vertex position, subpix 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 subpix.)
     Thus subpixel positioned lines produce better results when logico or
     blendf 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:

          call ortho2(left-0.5,right+0.5,bottom-0.5,top+0.5)
          call viewpo(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
     linesm, pntsmo
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 subpix.  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.