allow changes to background_color in BlendParams (#64)

Summary:
BlendParams background_color is immutable , type hint as a sequence allows setting new values in constructor.
Pull Request resolved: https://github.com/facebookresearch/pytorch3d/pull/64

Reviewed By: bottler

Differential Revision: D20068911

Pulled By: nikhilaravi

fbshipit-source-id: c580a7654dca25629218513841aa16d9d1055588

pytorch3d/renderer/blending.py

@@ -3,7 +3,7 @@
 
 
 import numpy as np
-from typing import NamedTuple
+from typing import NamedTuple, Sequence
 import torch
 
 # Example functions for blending the top K colors per pixel using the outputs
@@ -15,7 +15,7 @@ import torch
 class BlendParams(NamedTuple):
     sigma: float = 1e-4
     gamma: float = 1e-4
-    background_color = (1.0, 1.0, 1.0)
+    background_color: Sequence = (1.0, 1.0, 1.0)
 
 
 def hard_rgb_blend(colors, fragments) -> torch.Tensor:

tests/test_blending.py

@@ -234,3 +234,12 @@ class TestBlending(unittest.TestCase):
 
         self.assertTrue(torch.allclose(dists1.grad, dists2.grad, atol=2e-5))
         self.assertTrue(torch.allclose(zbuf1.grad, zbuf2.grad, atol=2e-5))
+
+    def test_blend_params(self):
+        """Test colour parameter of BlendParams().
+        Assert passed value overrides default value.
+        """
+        bp_default = BlendParams()
+        bp_new = BlendParams(background_color=(0.5, 0.5, 0.5))
+        self.assertEqual(bp_new.background_color, (0.5, 0.5, 0.5))
+        self.assertEqual(bp_default.background_color, (1.0, 1.0, 1.0))