Project line to pixmap edges when out of area

This is achieved by finding the intersection point of the line formed by
the previous point and current mouse position, and each edge in
succession. The edge closest to the mouse position is chosen, and the
projection of the position on it is selected.

canvas.py

@@ -23,7 +23,6 @@ class Canvas(QWidget):
         self.line = Shape(line_color=self.line_color)
         self.mouseButtonIsPressed=False #when it is true and shape is selected , move the shape with the mouse move event
         self.prevPoint=QPoint()
-        
         self.scale = 1.0
         self.pixmap = None
 
@@ -50,21 +49,24 @@ class Canvas(QWidget):
                 self.repaint()
             return
 
-        if self.current and self.startLabeling: 
+        # Polygon drawing.
+        if self.current and self.startLabeling:
             pos = self.transformPos(ev.posF())
-            # Don't allow the user to draw outside of the pixmap area.
-            # FIXME: Project point to pixmap's edge when getting out too fast
+            color = self.line_color
             if self.outOfPixmap(pos):
-                return ev.ignore()
-            if len(self.current) > 1 and self.closeEnough(pos, self.current[0]):
+                # Don't allow the user to draw outside the pixmap.
+                # Project the point to the pixmap's edges.
+                pos = self.intersectionPoint(pos)
+            elif len(self.current) > 1 and self.closeEnough(pos, self.current[0]):
                 # Attract line to starting point and colorise to alert the user:
-                self.line[1] = self.current[0]
-                self.line.line_color = self.current.line_color
-            else:
-                self.line[1] = pos
-                self.line.line_color = self.line_color
-            return self.repaint()
-            
+                # TODO: I would also like to highlight the pixel somehow.
+                pos = self.current[0]
+                color = self.current.line_color
+            self.line[1] = pos
+            self.line.line_color = color
+            self.repaint()
+            return
+
         if self.selectedShape:
             if self.prevPoint:
                     point=QPoint(self.prevPoint)
@@ -181,6 +183,53 @@ class Canvas(QWidget):
         #print "d %.2f, m %d, %.2f" % (d, m, d - m)
         return distance(p1 - p2) < self.epsilon
 
+    def intersectionPoint(self, mousePos):
+        # Cycle through each image edge in clockwise fashion,
+        # and find the one intersecting the current line segment.
+        # http://paulbourke.net/geometry/lineline2d/
+        size = self.pixmap.size()
+        points = [(0,0),
+                  (size.width(), 0),
+                  (size.width(), size.height()),
+                  (0, size.height())]
+        x1, y1 = self.current[-1].x(), self.current[-1].y()
+        x2, y2 = mousePos.x(), mousePos.y()
+        d, i, (x, y) = min(self.intersectingEdges((x1, y1), (x2, y2), points))
+        x3, y3 = points[i]
+        x4, y4 = points[(i+1)%4]
+        if (x, y) == (x1, y1):
+            # Handle cases where previous point is on one of the edges.
+            if x3 == x4:
+                return QPointF(x3, min(max(0, y2), max(y3, y4)))
+            else: # y3 == y4
+                return QPointF(min(max(0, x2), max(x3, x4)), y3)
+        return QPointF(x, y)
+
+
+    def intersectingEdges(self, (x1, y1), (x2, y2), points):
+        """For each edge formed by `points', yield the intersection
+        with the line segment `(x1,y1) - (x2,y2)`, if it exists.
+        Also return the distance of `(x2,y2)' to the middle of the
+        edge along with its index, so that the one closest can be chosen."""
+        for i in xrange(4):
+            x3, y3 = points[i]
+            x4, y4 = points[(i+1) % 4]
+            denom = (y4-y3) * (x2 - x1) - (x4 - x3) * (y2 - y1)
+            nua = (x4-x3) * (y1-y3) - (y4-y3) * (x1-x3)
+            nub = (x2-x1) * (y1-y3) - (y2-y1) * (x1-x3)
+            if denom == 0:
+                # This covers two cases:
+                #   nua == nub == 0: Coincident
+                #   otherwise: Parallel
+                continue
+            ua, ub = nua / denom, nub / denom
+            if 0 <= ua <= 1 and 0 <= ub <= 1:
+                x = x1 + ua * (x2 - x1)
+                y = y1 + ua * (y2 - y1)
+                m = QPointF((x3 + x4)/2, (y3 + y4)/2)
+                d = distance(m - QPointF(x2, y2))
+                yield d, i, (x, y)
+
 
     # These two, along with a call to adjustSize are required for the
     # scroll area.