Set reference for relative position

painter.py

@@ -29,6 +29,9 @@ class Painter(Widget):
         # relative position of mouse when moving
         self.offset=Point(0,0)
 
+        # reference particle from which positions should be computed
+        self.reference=None
+
         # app is used to share information between widgets
         self.app=app
 
@@ -130,6 +133,14 @@ class Painter(Widget):
                 # reset
                 self.set_zoom(1)
 
+            # set reference
+            elif text=="r":
+                if len(self.selected)>0:
+                    self.reference=self.selected[0]
+                else:
+                    self.reference=None
+                self.app.status_bar.draw()
+
     def on_key_up(self, keyboard, keycode):
         if keycode[1]=="shift":
             if 's' in self.modifiers:

status_bar.py

@@ -44,7 +44,10 @@ class Status_bar(Label):
             # number of spaces to align right (use 13 characters to print position)
             spaces=int(self.width/self.char_width)-len(self.raw_text)-13
             if spaces>0:
+                if self.app.painter.reference==None:
                     self.raw_text+=" "*spaces+"({:05.2f},{:05.2f})\n".format(self.app.painter.selected[0].squares[0].pos.x,self.app.painter.selected[0].squares[0].pos.y)
+                else:
+                    self.raw_text+=" "*spaces+"({:05.2f},{:05.2f})\n".format(self.app.painter.selected[0].squares[0].pos.x-self.app.painter.reference.squares[0].pos.x,self.app.painter.selected[0].squares[0].pos.y-self.app.painter.reference.squares[0].pos.y)
 
         # do not wrap
         self.text=self.raw_text[:min(len(self.raw_text),int(self.width/self.char_width))]