I put a hack into gimage_mask_stroke to work around roundoff errors in

paint_core_subsample_mask, addressing Ben Jackson's Stroked pixel bug.

ChangeLog

@@ -1,3 +1,9 @@
+Tue Jan 27 22:31:00 PST 1998 Raph Levien <raph@acm.org>
+
+	* app/gimage_mask.c: hacked around fractional pixel errors in
+ 	paint_core_subsample_mask, fixing the "Stroked pixel bug" posted
+ 	by Ben Jackson on 14 Jan 1998 to gimp-developer.
+
 Tue Jan 27 10:21:00 PST 1998 Raph Levien <raph@acm.org>
 
 	* app/gimage.c: fixed bug in image display with all channels

app/core/gimpimage-mask.c

@@ -573,16 +573,33 @@ gimage_mask_stroke (gimage, drawable)
   non_gui_paint_core.paint_func = gimage_mask_stroke_paint_func;
   gimage_mask_stroking = TRUE;
 
-  non_gui_paint_core.startx = non_gui_paint_core.lastx = (stroke_segs[0].x1 - offx);
+  /* Note added by Raph Levien, 27 Jan 1998
-  non_gui_paint_core.starty = non_gui_paint_core.lasty = (stroke_segs[0].y1 - offy);
+
+     The subtraction of 0.125 is to compensate for imprecision in
+     paint_core_subsample_mask. Ben Jackson posted a patch on 14 Jan
+     1998 to gimp-developers which addresses the imprecision more
+     directly. However, I've chosen this quick hack instead because it
+     is a less drastic change, and has no impact on performance. By
+     contrast, Ben's patch builds 25 rather than 16 subsampled brush
+     masks.
+
+     I'm planning to rework the subsample mechanism anyway to make way
+     for the natural brushes. When that happens, I'll be sure to make
+     it round precisely, at which point these -0.125 offsets can come
+     out.
+
+     */
+
+  non_gui_paint_core.startx = non_gui_paint_core.lastx = (stroke_segs[0].x1 - offx) - 0.125;
+  non_gui_paint_core.starty = non_gui_paint_core.lasty = (stroke_segs[0].y1 - offy) - 0.125;
 
   seg = 0;
   for (i = 0; i < num_strokes; i++)
     {
       while (stroke_segs[seg].x2 != -1)
 	{
-	  non_gui_paint_core.curx = (stroke_segs[seg].x2 - offx);
+	  non_gui_paint_core.curx = (stroke_segs[seg].x2 - offx - 0.125);
-	  non_gui_paint_core.cury = (stroke_segs[seg].y2 - offy);
+	  non_gui_paint_core.cury = (stroke_segs[seg].y2 - offy - 0.125);
 
 	  paint_core_interpolate (&non_gui_paint_core, drawable);
 

app/gimage_mask.c

@@ -573,16 +573,33 @@ gimage_mask_stroke (gimage, drawable)
   non_gui_paint_core.paint_func = gimage_mask_stroke_paint_func;
   gimage_mask_stroking = TRUE;
 
-  non_gui_paint_core.startx = non_gui_paint_core.lastx = (stroke_segs[0].x1 - offx);
+  /* Note added by Raph Levien, 27 Jan 1998
-  non_gui_paint_core.starty = non_gui_paint_core.lasty = (stroke_segs[0].y1 - offy);
+
+     The subtraction of 0.125 is to compensate for imprecision in
+     paint_core_subsample_mask. Ben Jackson posted a patch on 14 Jan
+     1998 to gimp-developers which addresses the imprecision more
+     directly. However, I've chosen this quick hack instead because it
+     is a less drastic change, and has no impact on performance. By
+     contrast, Ben's patch builds 25 rather than 16 subsampled brush
+     masks.
+
+     I'm planning to rework the subsample mechanism anyway to make way
+     for the natural brushes. When that happens, I'll be sure to make
+     it round precisely, at which point these -0.125 offsets can come
+     out.
+
+     */
+
+  non_gui_paint_core.startx = non_gui_paint_core.lastx = (stroke_segs[0].x1 - offx) - 0.125;
+  non_gui_paint_core.starty = non_gui_paint_core.lasty = (stroke_segs[0].y1 - offy) - 0.125;
 
   seg = 0;
   for (i = 0; i < num_strokes; i++)
     {
       while (stroke_segs[seg].x2 != -1)
 	{
-	  non_gui_paint_core.curx = (stroke_segs[seg].x2 - offx);
+	  non_gui_paint_core.curx = (stroke_segs[seg].x2 - offx - 0.125);
-	  non_gui_paint_core.cury = (stroke_segs[seg].y2 - offy);
+	  non_gui_paint_core.cury = (stroke_segs[seg].y2 - offy - 0.125);
 
 	  paint_core_interpolate (&non_gui_paint_core, drawable);