pdfcairo terminal; seams between bars in the colorbox

[Dima K. <gn...@di...>]

Hi. I'm seeing ugly lines between the bars in the colorbox when making
.pdf files. Trivial plot to demo this:

  set terminal pdf
  set output "/tmp/tst.pdf"
  plot 5 with points palette

This isn't a new issue, and I was wondering if anybody here had insight
about fixing it.

gp_cairo.c says this:

  /* By default, Cairo uses an antialiasing algorithm which may
   * leave a seam between polygons which share a common edge.
   * Several solutions allow to workaround this behaviour :
   * - don't antialias the polygons
   *   Problem : aliased lines are ugly
   * - stroke on each edge
   *   Problem : stroking is a very time-consuming operation
   * - draw without antialiasing to a separate context of a bigger size
   *   Problem : not really in the spirit of the rest of the drawing.
   * - enlarge the polygons so that they overlap slightly
   *   Problem : It is really more time-consuming that it may seem.
   *   It implies inspecting each corner to find which direction to move it
   *   (making the difference between the inside and the outside of the polygon).
   * - using CAIRO_OPERATOR_SATURATE
   *   Problem : for each set of polygons, we have to draw front-to-back
   *   on a separate context and then copy back to this one.
   *   Time-consuming but probably less than stroking all the edges.
   *
   * The last solution is implemented if plot->polygons_saturate is set to TRUE
   * Otherwise the default (antialiasing but may have seams) is used.
   */

And the gnuplot code today already does the "enlarge the polygons so
that they overlap slightly" thing. color.c has:

  draw_inside_colorbox_bitmap_smooth() {
    ...
    for (i = 0, xy2 = xy_from; i < steps; i++) {

	xy = xy2;
	xy2 = xy_from + (int) (xy_step * (i + 1));

        ...
	if (color_box.rotation == 'v') {
	    corners[0].y = corners[1].y = xy;
	    corners[2].y = corners[3].y = GPMIN(xy_to,xy2+1);
	} else {
	    corners[0].x = corners[3].x = xy;
	    corners[1].x = corners[2].x = GPMIN(xy_to,xy2+1);
	}
    ...
    }

Note the "+1". I still see the ugly bars in the resulting .pdf file
though. I can run experiments, but before I do anything, does anybody
have a comment? The draw_inside_colorbox_bitmap_smooth() stuff changed
in 2020, so maybe looking deeper at the effects of that patch would be
an interesting thing to do.

Thanks