Temporary fix for mesh rasterization bug for traingles partially behind the camera

Summary: A triangle is culled if any vertex in a triangle is behind the camera.  This fixes incorrect rendering of triangles that are partially behind the camera, where screen coordinate calculations are strange.  It doesn't work for triangles that are partially behind the camera but still intersect with the view frustum.

Reviewed By: nikhilaravi

Differential Revision: D22856181

fbshipit-source-id: a9cbaa1327d89601b83d0dfd3e4a04f934a4a213

tests/test_rasterize_meshes.py

@@ -552,6 +552,10 @@ class TestRasterizeMeshes(TestCaseMixin, unittest.TestCase):
             + (zbuf1 * grad_zbuf).sum()
             + (bary1 * grad_bary).sum()
         )
+
+        # avoid gradient error if rasterize_meshes_python() culls all triangles
+        loss1 += grad_var1.sum() * 0.0
+
         loss1.backward()
         grad_verts1 = grad_var1.grad.data.clone().cpu()
 
@@ -563,6 +567,10 @@ class TestRasterizeMeshes(TestCaseMixin, unittest.TestCase):
             + (zbuf2 * grad_zbuf).sum()
             + (bary2 * grad_bary).sum()
         )
+
+        # avoid gradient error if rasterize_meshes_python() culls all triangles
+        loss2 += grad_var2.sum() * 0.0
+
         grad_var1.grad.data.zero_()
         loss2.backward()
         grad_verts2 = grad_var2.grad.data.clone().cpu()