[gimp/gimp-2-10] app: in GimpToolCompass, add visual distinction between angle lines

[Ell <ell src gnome org>]

commit 7ab213d9183109359c2afa3239cab465dec19f2a
Author: Ell <ell_se yahoo com>
Date:   Mon Jul 16 02:13:26 2018 -0400

    app: in GimpToolCompass, add visual distinction between angle lines
    
    When using a GimpToolCompass in 3-point mode, add a small gap after
    the angle arc to the line corresponding to the "second" non-origin
    point, so that it's visually distinguishable from the line
    corresponding to the "first" point.  This has significance for the
    measure tool, since it determines the direction of the rotation
    when straightening the image (the first point is rotated toward the
    second point.)
    
    (cherry picked from commit 0f03f9e9f5ed2a39359f3de67b9ff7075850d835)

 app/display/gimptoolcompass.c | 74 ++++++++++++++++++++++++++++++++-----------
 1 file changed, 56 insertions(+), 18 deletions(-)
---
diff --git a/app/display/gimptoolcompass.c b/app/display/gimptoolcompass.c
index 1eb6095d56..f1cba0d312 100644
--- a/app/display/gimptoolcompass.c
+++ b/app/display/gimptoolcompass.c
@@ -50,6 +50,7 @@
 
 
 #define ARC_RADIUS 30
+#define ARC_GAP    (ARC_RADIUS / 2)
 #define EPSILON    1e-6
 
 
@@ -438,12 +439,14 @@ gimp_tool_compass_get_property (GObject    *object,
 static void
 gimp_tool_compass_changed (GimpToolWidget *widget)
 {
-  GimpToolCompass        *compass = GIMP_TOOL_COMPASS (widget);
-  GimpToolCompassPrivate *private = compass->private;
-  GimpDisplayShell       *shell   = gimp_tool_widget_get_shell (widget);
+  GimpToolCompass        *compass       = GIMP_TOOL_COMPASS (widget);
+  GimpToolCompassPrivate *private       = compass->private;
+  GimpDisplayShell       *shell         = gimp_tool_widget_get_shell (widget);
   gdouble                 angle1;
   gdouble                 angle2;
-  gint                    draw_arc = 0;
+  gint                    draw_arc      = 0;
+  gboolean                draw_arc_line = FALSE;
+  gdouble                 arc_line_display_length;
   gdouble                 arc_line_length;
 
   gimp_tool_compass_update_angle (compass, private->orientation, FALSE);
@@ -468,20 +471,55 @@ gimp_tool_compass_changed (GimpToolWidget *widget)
       draw_arc++;
     }
 
-  gimp_canvas_line_set (private->line2,
-                        private->x[0],
-                        private->y[0],
-                        private->x[2],
-                        private->y[2]);
-  gimp_canvas_item_set_visible (private->line2, private->n_points > 2);
-  if (private->n_points > 2 &&
-      gimp_canvas_item_transform_distance (private->line2,
-                                           private->x[0],
-                                           private->y[0],
-                                           private->x[2],
-                                           private->y[2]) > ARC_RADIUS)
+
+  arc_line_display_length = ARC_RADIUS                           +
+                            (GIMP_CANVAS_HANDLE_SIZE_CROSS >> 1) +
+                            ARC_GAP;
+  arc_line_length         = arc_line_display_length              /
+                            hypot (private->radius2.x * shell->scale_x,
+                                  private->radius2.y * shell->scale_y);
+
+  if (private->n_points > 2)
     {
-      draw_arc++;
+      gdouble length = gimp_canvas_item_transform_distance (private->line2,
+                                                            private->x[0],
+                                                            private->y[0],
+                                                            private->x[2],
+                                                            private->y[2]);
+
+      if (length > ARC_RADIUS)
+        {
+          draw_arc++;
+          draw_arc_line = TRUE;
+
+          if (length > arc_line_display_length)
+            {
+              gimp_canvas_line_set (
+                private->line2,
+                private->x[0] + private->radius2.x * arc_line_length,
+                private->y[0] + private->radius2.y * arc_line_length,
+                private->x[2],
+                private->y[2]);
+              gimp_canvas_item_set_visible (private->line2, TRUE);
+            }
+          else
+            {
+              gimp_canvas_item_set_visible (private->line2, FALSE);
+            }
+        }
+      else
+        {
+          gimp_canvas_line_set (private->line2,
+                                private->x[0],
+                                private->y[0],
+                                private->x[2],
+                                private->y[2]);
+          gimp_canvas_item_set_visible (private->line2, TRUE);
+        }
+    }
+  else
+    {
+      gimp_canvas_item_set_visible (private->line2, FALSE);
     }
 
   gimp_canvas_handle_set_position (private->arc,
@@ -502,7 +540,7 @@ gimp_tool_compass_changed (GimpToolWidget *widget)
                         private->x[0] + private->radius2.x * arc_line_length,
                         private->y[0] + private->radius2.y * arc_line_length);
   gimp_canvas_item_set_visible (private->arc_line,
-                                private->n_points == 2 &&
+                                (private->n_points == 2 || draw_arc_line) &&
                                 fabs (angle2) > EPSILON);
 
   gimp_canvas_handle_set_position (private->handles[0],