fix in area_polygon

Author: baehrjo

AUTHORS.md

@@ -36,3 +36,4 @@
 - Chen Kasirer <<[email protected]>> [@chenkasirer](https://github.com/chenkasirer)
 - Nickolas Maslarinos <<[email protected]>> [@nmaslarinos](https://github.com/nmaslarinos)
 - Katerina Toumpektsi <<[email protected]>> [@katarametin](https://github.com/katarametin)
+- Joelle Baehr-Bruyere <<[email protected]>> [@baehrjo](https://github.com/baehrjo)

CHANGELOG.md

@@ -16,6 +16,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 * Fixed bug that caused a new-line at the end of the `compas.HERE` constant in IronPython for Mac.
 * Fixed Grasshopper `draw_polylines` method to return `PolylineCurve` instead of `Polyline` because the latter shows as only points.
 * Fixed uninstall post-process.
+* Fixed `area_polygon` that was, in some cases, returning a negative area
 
 ### Removed
 

src/compas/geometry/_core/size.py

@@ -65,7 +65,7 @@ def area_polygon(polygon):
             area += 0.5 * length_vector(n)
         else:
             area -= 0.5 * length_vector(n)
-    return area
+    return abs(area)
 
 
 def area_polygon_xy(polygon):

tests/compas/geometry/test_core.py

@@ -3,7 +3,7 @@
 import pytest
 
 from compas.geometry import Polyhedron
-from compas.geometry import Polygon
+from compas.geometry import Point, Polygon
 from compas.geometry import Rotation
 from compas.geometry import allclose
 from compas.geometry import angle_vectors
@@ -230,6 +230,14 @@ def test_area_triangle(triangle, R):
     assert close(triangle.area, 0.5)
 
 
+def test_area_polygon():
+    # create a test closed (here planar xy) non-convex polygon :
+    polygon = Polygon([Point(-7, -15, 0), Point(-5, 9, 0), Point(13, 0, 0), Point(0, -2, 0), Point(0, -6, 0), Point(-4, -10, 0)])
+    assert area_polygon(polygon) >= 0
+    # the same polygon with vertices list shifted by 3 positions :
+    polygon_ = Polygon([Point(0, -2, 0), Point(0, -6, 0), Point(-4, -10, 0), Point(-7, -15, 0), Point(-5, 9, 0), Point(13, 0, 0)])
+    assert area_polygon(polygon_) >= 0
+
 # ==============================================================================
 # normals
 # ==============================================================================\