Trim Gouraud triangles that contain NaN

Agg enters an infinite loop if you give it points that are NaN, due to
converting the values to fixed-point integers, and then oscillating
between the large values that result from that conversion.

NaN values may be introduced after transforming the input, so we need to
trim those after transformation. This matches what is done in normal
paths.

Author: QuLogic

lib/matplotlib/tests/test_transforms.py

@@ -142,6 +142,25 @@ def test_pcolormesh_pre_transform_limits():
     assert_almost_equal(expected, ax.dataLim.get_points())
 
 
+def test_pcolormesh_gouraud_nans():
+    np.random.seed(19680801)
+
+    values = np.linspace(0, 180, 3)
+    radii = np.linspace(100, 1000, 10)
+    z, y = np.meshgrid(values, radii)
+    x = np.radians(np.random.rand(*z.shape) * 100)
+
+    fig = plt.figure()
+    ax = fig.add_subplot(111, projection="polar")
+    # Setting the limit to cause clipping of the r values causes NaN to be
+    # introduced; these should not crash but be ignored as in other path
+    # operations.
+    ax.set_rlim(101, 1000)
+    ax.pcolormesh(x, y, z, shading="gouraud")
+
+    fig.canvas.draw()
+
+
 def test_Affine2D_from_values():
     points = np.array([[0, 0],
                        [10, 20],

src/_backend_agg.h

@@ -1193,6 +1193,9 @@ inline void RendererAgg::_draw_gouraud_triangle(PointArray &points,
             tpoints[i][j] = points(i, j);
         }
         trans.transform(&tpoints[i][0], &tpoints[i][1]);
+        if(std::isnan(tpoints[i][0]) || std::isnan(tpoints[i][1])) {
+            return;
+        }
     }
 
     span_alloc_t span_alloc;