FIX: do not simplify path in LineCollection.get_segments

tacaswell · tacaswell · commit 557064218c8d · 2021-07-06T15:50:24.000-04:00
Internally all Collection flavors boil down to calling renderer.draw_path_collection and the sub-classes primarily provide nicer user-facing APIs to fabricate the paths that will be passed down to the renderer. In LineCollection rather than tracking both the user supplied data and the internal Path objects, we just keep the Path objects and re-extract segments on demand. To do this we use Path.iter_segments with defaults to asking the path if it should simplify the path (that is drop points that do not matter which is in turn defined by if the deflection away from "straight" is greater than some threshold). The Path objects we are holding have values in data-space, but the default value of "what is the threshold for 'not mattering'" is tuned to make sense in to pixel space (1/9). By passing `simplify=False` to `iter_segments` we over-ride the Path object's notion of if it should be simplified (which by default is controlled by the simplify threshold (rcParams['path.simplify_threshold']), rcParams['path.simplify'] , how long the path is, and if there are any quadratic or Bézier codes in the path) to never simplify. In this application we never want to simplify because we do not know enough of the context when this is called to know what the "right" simplification threshold is and because we expect `set_segments` and `get_segments` to round-trip. closes #20551
diff --git a/lib/matplotlib/collections.py b/lib/matplotlib/collections.py
@@ -1461,7 +1461,14 @@ def get_segments(self):
         segments = []
 
         for path in self._paths:
-            vertices = [vertex for vertex, _ in path.iter_segments()]
+            vertices = [
+                vertex
+                for vertex, _
+                # Never simplify here, we want to get the data-space values
+                # back and there in no way to know the "right" simplification
+                # threshold so never try.
+                in path.iter_segments(simplify=False)
+            ]
             vertices = np.asarray(vertices)
             segments.append(vertices)
 
diff --git a/lib/matplotlib/tests/test_collections.py b/lib/matplotlib/tests/test_collections.py
@@ -1039,3 +1039,12 @@ def test_quadmesh_cursor_data():
         x, y = ax.transData.transform([-1, 101])
         event = MouseEvent('motion_notify_event', fig.canvas, x, y)
         assert qm.get_cursor_data(event) is None
+
+
+def test_get_segments():
+    segments = np.tile(np.linspace(0, 1, 256), (2, 1)).T
+    lc = LineCollection([segments])
+
+    readback, = lc.get_segments()
+    # these should comeback un-changed!
+    assert np.all(segments == readback)
