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

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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.
This commit is contained in:
ManoloFLTK 2022-11-29 12:28:31 +01:00
parent 7a42a80caf
commit 00884f28e2
4 changed files with 16 additions and 2 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

2
FL/Fl_Graphics_Driver.H
View File

@ -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);

1
src/drivers/Xlib/Fl_Xlib_Graphics_Driver.H
View File

@ -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);

4
src/drivers/Xlib/Fl_Xlib_Graphics_Driver_vertex.cxx
View File

@ -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.

11
src/fl_symbols.cxx
View File

@ -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;