Fix clipping of markers in PDF backend.

QuLogic · QuLogic · commit d13153401b1d · 2020-06-10T14:52:21.000-04:00
The bbox only contains the points of the marker, but the line will extend outside by half the line width, unless it's mitered. The PDF miter limit is 10, so pad by 5*line width (half the miter length). This fixes corners on 'v', '^', '<', '>', 'p', 'h', 'H', 'D', 'd', 'X'. Fixes #9829.
diff --git a/lib/matplotlib/backends/backend_pdf.py b/lib/matplotlib/backends/backend_pdf.py
@@ -1618,7 +1618,14 @@ def markerObject(self, path, trans, fill, stroke, lw, joinstyle,
     def writeMarkers(self):
         for ((pathops, fill, stroke, joinstyle, capstyle),
              (name, ob, bbox, lw)) in self.markers.items():
-            bbox = bbox.padded(lw * 0.5)
+            # bbox wraps the exact limits of the control points, so half a line
+            # will appear outside it. If the join style is miter and the line
+            # is not parallel to the edge, then the line will extend even
+            # further. From the PDF specification, Section 8.4.3.5, the miter
+            # limit is miterLength / lineWidth and from Table 52, the default
+            # is 10. With half the miter length outside, that works out to the
+            # following padding:
+            bbox = bbox.padded(lw * 5)
             self.beginStream(
                 ob.id, None,
                 {'Type': Name('XObject'), 'Subtype': Name('Form'),
diff --git a/lib/matplotlib/tests/test_marker.py b/lib/matplotlib/tests/test_marker.py
@@ -139,3 +139,44 @@ def draw_ref_marker(y, style, size):
 
     ax_test.set(xlim=(-0.5, 1.5), ylim=(-0.5, 1.5))
     ax_ref.set(xlim=(-0.5, 1.5), ylim=(-0.5, 1.5))
+
+
+@check_figures_equal()
+def test_marker_clipping(fig_ref, fig_test):
+    # Plotting multiple markers can trigger different optimized paths in
+    # backends, so compare single markers vs multiple to ensure they are
+    # clipped correctly.
+    marker_count = len(markers.MarkerStyle.markers)
+    marker_size = 50
+    ncol = 7
+    nrow = marker_count // ncol + 1
+
+    width = 2 * marker_size * ncol
+    height = 2 * marker_size * nrow * 2
+    fig_ref.set_size_inches((width / fig_ref.dpi, height / fig_ref.dpi))
+    ax_ref = fig_ref.add_axes([0, 0, 1, 1])
+    fig_test.set_size_inches((width / fig_test.dpi, height / fig_ref.dpi))
+    ax_test = fig_test.add_axes([0, 0, 1, 1])
+
+    for i, marker in enumerate(markers.MarkerStyle.markers):
+        x = i % ncol
+        y = i // ncol * 2
+
+        # Singular markers per call.
+        ax_ref.plot([x, x], [y, y + 1], c='k', linestyle='-', lw=3)
+        ax_ref.plot(x, y, c='k',
+                    marker=marker, markersize=marker_size, markeredgewidth=10,
+                    fillstyle='full', markerfacecolor='white')
+        ax_ref.plot(x, y + 1, c='k',
+                    marker=marker, markersize=marker_size, markeredgewidth=10,
+                    fillstyle='full', markerfacecolor='white')
+
+        # Multiple markers in a single call.
+        ax_test.plot([x, x], [y, y + 1], c='k', linestyle='-', lw=3,
+                     marker=marker, markersize=marker_size, markeredgewidth=10,
+                     fillstyle='full', markerfacecolor='white')
+
+    ax_ref.set(xlim=(-0.5, ncol), ylim=(-0.5, 2 * nrow))
+    ax_test.set(xlim=(-0.5, ncol), ylim=(-0.5, 2 * nrow))
+    ax_ref.axis('off')
+    ax_test.axis('off')
