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.

1 files changed, 9 insertions, 2 deletions

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;