[Bug fix] box points ordering (#1205)

* fix sort_points

* remove functools

* add test cases

* add descriptions for coordinates

* del

Co-authored-by: xinyu <wangxinyu2017@gmail.com>

mmocr/utils/box_util.py

@@ -1,5 +1,7 @@
 # Copyright (c) OpenMMLab. All rights reserved.
-import functools
+import math
+import operator
+from functools import reduce
 
 import numpy as np
 
@@ -160,8 +162,14 @@ def bezier_to_polygon(bezier_points, num_sample=20):
 
 
 def sort_points(points):
-    """Sort arbitory points in clockwise order. Reference:
-    https://stackoverflow.com/a/6989383.
+    """Sort arbitrary points in clockwise order in Cartesian coordinate, you
+    may need to reverse the output sequence if you are using OpenCV's image
+    coordinate.
+
+    Reference:
+    https://github.com/novioleo/Savior/blob/master/Utils/GeometryUtils.py.
+
+    Warning: This function can only sort convex polygons.
 
     Args:
         points (list[ndarray] or ndarray or list[list]): A list of unsorted
@@ -170,30 +178,13 @@ def sort_points(points):
     Returns:
         list[ndarray]: A list of points sorted in clockwise order.
     """
-
     assert is_type_list(points, np.ndarray) or isinstance(points, np.ndarray) \
         or is_2dlist(points)
-
-    points = np.array(points)
-    center = np.mean(points, axis=0)
-
-    def cmp(a, b):
-        oa = a - center
-        ob = b - center
-
-        # Some corner cases
-        if oa[0] >= 0 and ob[0] < 0:
-            return 1
-        if oa[0] < 0 and ob[0] >= 0:
-            return -1
-
-        prod = np.cross(oa, ob)
-        if prod > 0:
-            return 1
-        if prod < 0:
-            return -1
-
-        # a, b are on the same line from the center
-        return 1 if (oa**2).sum() < (ob**2).sum() else -1
-
-    return sorted(points, key=functools.cmp_to_key(cmp))
+    center_point = tuple(
+        map(operator.truediv,
+            reduce(lambda x, y: map(operator.add, x, y), points),
+            [len(points)] * 2))
+    return sorted(
+        points,
+        key=lambda coord: (180 + math.degrees(
+            math.atan2(*tuple(map(operator.sub, coord, center_point))))) % 360)

tests/test_utils/test_box.py

@@ -124,5 +124,27 @@ def test_sort_points():
     points = [[1, 1], [1, -1], [-1, 1], [-1, -1]]
     assert np.allclose(target, sort_points(points))
 
+    points = [[0.5, 0.3], [1, 0.5], [-0.5, 0.8], [-0.1, 1]]
+    target = [[-0.5, 0.8], [-0.1, 1], [1, 0.5], [0.5, 0.3]]
+    assert np.allclose(target, sort_points(points))
+
+    points = [[0.5, 3], [0.1, -0.2], [-0.5, -0.3], [-0.7, 3.1]]
+    target = [[-0.5, -0.3], [-0.7, 3.1], [0.5, 3], [0.1, -0.2]]
+    assert np.allclose(target, sort_points(points))
+
+    points = [[1, 0.8], [0.8, -1], [1.8, 0.5], [1.9, -0.6], [-0.5, 2],
+              [-1, 1.8], [-2, 0.7], [-1.6, -0.2], [-1, -0.5]]
+    target = [[-1, -0.5], [-1.6, -0.2], [-2, 0.7], [-1, 1.8], [-0.5, 2],
+              [1, 0.8], [1.8, 0.5], [1.9, -0.6], [0.8, -1]]
+    assert np.allclose(target, sort_points(points))
+
+    # concave polygon may failed
+    points = [[1, 0], [-1, 0], [0, 0], [0, -1], [0.25, 1], [0.75, 1],
+              [-0.25, 1], [-0.75, 1]]
+    target = [[-1, 0], [-0.75, 1], [-0.25, 1], [0, 0], [0.25, 1], [0.75, 1],
+              [1, 0], [0, -1]]
+    with pytest.raises(AssertionError):
+        assert np.allclose(target, sort_points(points))
+
     with pytest.raises(AssertionError):
         sort_points([1, 2])