Alpha channel to return the mask

Summary: Updated the alpha channel in the `hard_rgb_blend` function to return the mask of the pixels which have overlapping mesh faces.

Reviewed By: bottler

Differential Revision: D29001604

fbshipit-source-id: 22a2173d769f2d3ad34892d68ceb628f073bca22

tests/test_blending.py

@@ -184,8 +184,8 @@ class TestBlending(TestCaseMixin, unittest.TestCase):
             channel_color = blend_params.background_color[i]
             self.assertTrue(images[~is_foreground][..., i].eq(channel_color).all())
 
-        # Examine the alpha channel is correct
-        self.assertTrue(images[..., 3].eq(1).all())
+        # Examine the alpha channel
+        self.assertClose(images[..., 3], (pix_to_face[..., 0] >= 0).float())
 
     def test_sigmoid_alpha_blend_manual_gradients(self):
         # Create dummy outputs of rasterization

tests/test_render_meshes.py

@@ -125,6 +125,10 @@ class TestRenderMeshes(TestCaseMixin, unittest.TestCase):
                     )
                     images, fragments = renderer(sphere_mesh)
                     self.assertClose(fragments.zbuf, rasterizer(sphere_mesh).zbuf)
+                    # Check the alpha channel is the mask
+                    self.assertClose(
+                        images[..., -1], (fragments.pix_to_face[..., 0] >= 0).float()
+                    )
                 else:
                     renderer = MeshRenderer(rasterizer=rasterizer, shader=shader)
                     images = renderer(sphere_mesh)
@@ -165,6 +169,10 @@ class TestRenderMeshes(TestCaseMixin, unittest.TestCase):
                 self.assertClose(
                     fragments.zbuf, rasterizer(sphere_mesh, lights=lights).zbuf
                 )
+                # Check the alpha channel is the mask
+                self.assertClose(
+                    images[..., -1], (fragments.pix_to_face[..., 0] >= 0).float()
+                )
             else:
                 phong_renderer = MeshRenderer(
                     rasterizer=rasterizer, shader=phong_shader