slopes greater than 1 for bresenham algorithm

skais/tests/utils/test_geometry.py

@@ -18,6 +18,13 @@ class TestGeometry(unittest.TestCase):
 
         self.assertListEqual(result, expected)
 
+
+    def test_bresenham_inverted_2(self):
+        result = bresenham(16, 4, 3, 9)
+        expected = [(3, 9), (4, 9), (5, 8), (6, 8), (7, 7), (8, 7), (9, 7), (10, 6), (11, 6), (12, 6), (13, 5), (14, 5),
+                    (15, 4), (16, 4)]
+        self.assertListEqual(result, expected)
+
     def test_bresenham_same_line(self):
         result = bresenham(3, 4, 10, 4)
         expected = [(3, 4), (4, 4), (5, 4), (6, 4), (7, 4), (8, 4), (9, 4), (10, 4)]

skais/utils/geometry.py

@@ -3,28 +3,44 @@ def bresenham(x1, y1, x2, y2):
         tmp = x2
         x2 = x1
         x1 = tmp
-    if y1 > y2:
+        
+
         tmp = y2
         y2 = y1
         y1 = tmp
     
-    pixels = [(x1, y1)]
-    if x1 == x2:
-        x = x1
-        for y in range(y1 + 1, y2 + 1):
-            pixels.append((x, y))
-    else:
-        a = 2*(y2 - y1)
-        b = a - 2*(x2 - x1)
-        p = a - (x2 - x1)
+    dx = int(x2 - x1)
+    dy = int(y2 - y1)
 
+    sx = sy = 1
+    if dx < 0:
+        sx = -1
+    if dy < 0:
+        sy = -1
+
+    x = x1
     y = y1
+
+    pixels = [(x1, y1)]
+    if abs(dx) > abs(dy): # slope < 1
+        p = (2 * abs(dy)) - abs(dx)
+        
         for x in range(x1 + 1, x2 + 1):
             if p < 0:
-                p += a
+                p += 2 * abs(dy)
+            else:
+                y += sy
+                p += (2 * abs(dy)) - (2 * abs(dx))
+            pixels.append((x, y))
+    else: # slope >= 1
+        p = (2 * abs(dx)) - abs(dy)
+        for y in range(y1 + 1, y2 + 1):
+            if p < 0:
+                p += 2 * abs(dx)
             else:
-                y += 1
-                p += b
+                x += sx
+                p += (2 * abs(dx)) - (2 * abs(dy))
             pixels.append((x, y))
+
     return pixels