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.

4 files changed, 16 insertions, 2 deletions

diff --git a/FL/Fl_Graphics_Driver.H b/FL/Fl_Graphics_Driver.H
index 0f25756d6..e131a2961 100644
--- a/FL/Fl_Graphics_Driver.H
+++ b/FL/Fl_Graphics_Driver.H
@@ -316,6 +316,8 @@ public:
   virtual void fixloop();
   virtual void end_polygon();
   virtual void end_complex_polygon();
+  // default implementation is most probably enough
+  virtual bool can_fill_non_convex_polygon() { return true; }
   virtual void gap();
   virtual void circle(double x, double y, double r);
   virtual void arc(double x, double y, double r, double start, double end);
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.
diff --git a/src/fl_symbols.cxx b/src/fl_symbols.cxx
index 2f86c2394..e6ebb4199 100644
--- a/src/fl_symbols.cxx
+++ b/src/fl_symbols.cxx
@@ -238,8 +238,15 @@ static void draw_search(Fl_Color col)
 static void draw_arrow1(Fl_Color col)
 {
   fl_color(col);
-  BP; vv(-0.8,-0.4); vv(-0.8,0.4); vv(0.0,0.4); vv(0.0,-0.4); EP;
-  BP; vv(0.0,0.8); vv(0.8,0.0); vv(0.0,-0.8); vv(0.0,-0.4); vv(0.0,0.4); EP;
+  if (fl_graphics_driver->can_fill_non_convex_polygon()) {
+    // draw the arrow as a 7-vertex filled polygon
+    BP; vv(-0.8,-0.4); vv(-0.8,0.4); vv(0.0,0.4); vv(0.0,0.8); vv(0.8,0.0);
+    vv(0.0,-0.8); vv(0.0,-0.4); EP;
+  } else {
+    // draw the arrow as a rectangle plus a triangle
+    BP; vv(-0.8,-0.4); vv(-0.8,0.4); vv(0.0,0.4); vv(0.0,-0.4); EP;
+    BP; vv(0.0,0.8); vv(0.8,0.0); vv(0.0,-0.8); vv(0.0,-0.4); vv(0.0,0.4); EP;
+  }
   set_outline_color(col);
   BC; vv(-0.8,-0.4); vv(-0.8,0.4); vv(0.0,0.4); vv(0.0,0.8); vv(0.8,0.0);
       vv(0.0,-0.8); vv(0.0,-0.4); EC;