examples: add a tiny widget with custom shape and custom collide_point method.

examples/widgets/customcollide.py

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+'''
+Custom shape & collide widget
+=============================
+
+This is a Triangle widget with a triangle shape based on 3 points (p1, p2, p3),
+plus a custom collision function.
+
+The p1, p2, p3 are automatically calculated from the position and the size of
+the Widget bounding box. We are using them to draw the triangle shape.
+(Please note in the kv the special case for Scatter.)
+
+Then, we need to setup a new collision function to collide only on the triangle.
+We are using a external method that will check if a point is inside a polygon
+(we consider our triangle as a polygon).
+'''
+
+
+import kivy
+kivy.require('1.0.8')
+
+from kivy.uix.scatter import Scatter
+from kivy.properties import ListProperty
+from kivy.lang import Builder
+
+
+Builder.load_string('''
+<Triangle>:
+    # example for doing a triangle
+    # this will automatically recalculate pX from pos/size
+    p1: 0, 0
+    p2: self.width, 0
+    p3: self.width / 2, self.height
+
+    # If you use a Widget instead of Scatter as base class, you need that:
+    #p1: self.pos
+    #p2: self.right, self.y
+    #p3: self.center_x, self.top
+
+    # draw something
+    canvas:
+        Color:
+            rgb: 1, 0, 0
+        Triangle:
+            points: self.p1 + self.p2 + self.p3
+''')
+
+
+def point_inside_polygon(x, y, poly):
+    '''Taken from http://www.ariel.com.au/a/python-point-int-poly.html
+    '''
+    n = len(poly)
+    inside = False
+    p1x = poly[0]
+    p1y = poly[1]
+    for i in xrange(0, n + 2, 2):
+        p2x = poly[i % n]
+        p2y = poly[(i + 1) % n]
+        if y > min(p1y, p2y):
+            if y <= max(p1y, p2y):
+                if x <= max(p1x, p2x):
+                    if p1y != p2y:
+                        xinters = (y - p1y) * (p2x - p1x) / (p2y - p1y) + p1x
+                    if p1x == p2x or x <= xinters:
+                        inside = not inside
+        p1x, p1y = p2x, p2y
+    return inside
+
+
+class Triangle(Scatter):
+    p1 = ListProperty([0, 0])
+    p2 = ListProperty([0, 0])
+    p3 = ListProperty([0, 0])
+
+    def collide_point(self, x, y):
+        x, y = self.to_local(x, y)
+        return point_inside_polygon(x, y,
+                self.p1 + self.p2 + self.p3)
+
+if __name__ == '__main__':
+    from kivy.base import runTouchApp
+    runTouchApp(Triangle(size_hint=(None, None)))