Fix for issue [Cairo]: Arrows have a "gap" (#561)

The problem to fix is that the arrow drawn by draw_arrow1() in src/fl_symbols.cxx
displays a faint clear line between the stem and head of the arrow with the Cairo
graphics driver.

This occurs because draw_arrow1() draws the arrow in 2 steps (a rectangle +
a triangle) and the Cairo driver is configured to use antialiasing when filling
polygons. The antialiasing produces the faint line between stem and head.

Why does draw_arrow1() draw a rectangle + a triangle rather than a
7-vertex polygon? That's because the X11 graphics driver fails with its polygon-
drawing function when the polygon is also rotated: the polygon is drawn
empty.

We want to keep using antialiasing under Cairo for polygons because
the result is better with non horizontal/vertical polygon edges.

This implementation changes function draw_arrow1() which draws
the arrow as a 7-vertex filled polygon except when the graphics driver
returns false for its virtual member function can_fill_non_convex_polygon().
In that situation, draw_arrow1() draws, as before, a rectangle + a triangle.
The new, virtual member function can_fill_non_convex_polygon() returns
true except for the X11 graphics driver. Therefore, draw_arrow1() is effectively
unchanged under the X11 driver.

2 files changed, 5 insertions, 0 deletions

diff --git a/src/drivers/Xlib/Fl_Xlib_Graphics_Driver.H b/src/drivers/Xlib/Fl_Xlib_Graphics_Driver.H
index 899d39819..016ace499 100644
--- a/src/drivers/Xlib/Fl_Xlib_Graphics_Driver.H
+++ b/src/drivers/Xlib/Fl_Xlib_Graphics_Driver.H
@@ -193,6 +193,7 @@ protected:
   void end_loop();
   void end_polygon();
   void end_complex_polygon();
+  bool can_fill_non_convex_polygon();
   void gap();
   virtual void ellipse_unscaled(double xt, double yt, double rx, double ry);
   virtual void arc_unscaled(int x, int y, int w, int h, double a1, double a2);
diff --git a/src/drivers/Xlib/Fl_Xlib_Graphics_Driver_vertex.cxx b/src/drivers/Xlib/Fl_Xlib_Graphics_Driver_vertex.cxx
index 3b43a360a..1b6e7b430 100644
--- a/src/drivers/Xlib/Fl_Xlib_Graphics_Driver_vertex.cxx
+++ b/src/drivers/Xlib/Fl_Xlib_Graphics_Driver_vertex.cxx
@@ -76,6 +76,10 @@ void Fl_Xlib_Graphics_Driver::end_complex_polygon() {
   if (n>2) XFillPolygon(fl_display, fl_window, gc_, short_point, n, 0, 0);
 }
 
+bool Fl_Xlib_Graphics_Driver::can_fill_non_convex_polygon() {
+  return false;
+}
+
 // shortcut the closed circles so they use XDrawArc:
 // warning: these do not draw rotated ellipses correctly!
 // See fl_arc.c for portable version.