alter the way the border of Canvas::Rectangle is drawn to meet the intent that a...

diff --git a/libs/canvas/rectangle.cc b/libs/canvas/rectangle.cc
index 1b75584849ef4500300f7afb06741e6b10518fa6..4afdfbacd4f67c610a446723fbbdd89febf9e645 100644 (file)
--- a/libs/canvas/rectangle.cc
+++ b/libs/canvas/rectangle.cc
@@ -88,36 +88,48 @@ Rectangle::render (Rect const & area, Cairo::RefPtr<Cairo::Context> context) con
 
                setup_outline_context (context);
                
-               /* see the cairo FAQ on single pixel lines to see why we do
-                * the 0.5 pixel additions.
-                */
-
                if (_outline_what == What (LEFT|RIGHT|BOTTOM|TOP)) {
                        
                        context->rectangle (self.x0 + 0.5, self.y0 + 0.5, self.width() - 1.0, self.height() - 1.0);
 
                } else {
 
+                       /* the goal here is that if the border is 1 pixel
+                        * thick, it will precisely align with the corner
+                        * coordinates of the rectangle. So if the rectangle
+                        * has a left edge at 0 and a right edge at 10, then
+                        * the right edge must span -0.5..+0.5, the left edge
+                        * must span 9.5..10.5 (i.e. the single full color
+                        * pixel is precisely aligned with 0 and 10
+                        * respectively).
+                        *
+                        * we have to shift left/up in all cases, which means
+                        * subtraction along both axes.
+                        *
+                        * see the cairo FAQ on single pixel lines to see why we do
+                        * the 0.5 pixel additions.
+                        */
+
                        if (_outline_what & LEFT) {
-                               /* vertical line: move x-coordinate by 0.5 pixels */
-                               context->move_to (self.x0 + 0.5, self.y0);
-                               context->line_to (self.x0 + 0.5, self.y1);
+                               /* vertical line: move x-coordinate left by 0.5 pixels */
+                               context->move_to (self.x0 - 0.5, self.y0);
+                               context->line_to (self.x0 - 0.5, self.y1);
                        }
                        
                        if (_outline_what & TOP) {
-                               /* horizontal line: move y-coordinate by 0.5 pixels */
-                               context->move_to (self.x0, self.y0 + 0.5);
-                               context->line_to (self.x1, self.y0 + 0.5);
+                               /* horizontal line: move y-coordinate up by 0.5 pixels */
+                               context->move_to (self.x0, self.y0 - 0.5);
+                               context->line_to (self.x1, self.y0 - 0.5);
                        }
 
                        if (_outline_what & BOTTOM) {
-                               /* horizontal line: move y-coordinate by 0.5 pixels */
+                               /* horizontal line: move y-coordinate up by 0.5 pixels */
                                context->move_to (self.x0, self.y1 - 0.5);
                                context->line_to (self.x1, self.y1 - 0.5);
                        }
                        
                        if (_outline_what & RIGHT) {
-                               /* vertical line: move x-coordinate by 0.5 pixels */
+                               /* vertical line: move x-coordinate left by 0.5 pixels */
                                context->move_to (self.x1 - 0.5, self.y0);
                                context->line_to (self.x1 - 0.5, self.y1);
                        }