Barycentric clipping in the renderer and flat shading

Summary:
Updates to the Renderer to enable barycentric clipping. This is important when there is blurring in the rasterization step.

Also added support for flat shading.

Reviewed By: jcjohnson

Differential Revision: D19934259

fbshipit-source-id: 036e48636cd80d28a04405d7a29fcc71a2982904

tests/test_mesh_rendering_utils.py

@@ -0,0 +1,24 @@
+#!/usr/bin/env python3
+# Copyright (c) Facebook, Inc. and its affiliates. All rights reserved.
+
+
+import unittest
+import torch
+
+from pytorch3d.renderer.mesh.utils import _clip_barycentric_coordinates
+
+
+class TestMeshRenderingUtils(unittest.TestCase):
+    def test_bary_clip(self):
+        N = 10
+        bary = torch.randn(size=(N, 3))
+        # randomly make some values negative
+        bary[bary < 0.3] *= -1.0
+        # randomly make some values be greater than 1
+        bary[bary > 0.8] *= 2.0
+        negative_mask = bary < 0.0
+        positive_mask = bary > 1.0
+        clipped = _clip_barycentric_coordinates(bary)
+        self.assertTrue(clipped[negative_mask].sum() == 0)
+        self.assertTrue(clipped[positive_mask].gt(1.0).sum() == 0)
+        self.assertTrue(torch.allclose(clipped.sum(dim=-1), torch.ones(N)))