Correctly create reference image silhouette

Summary: Previously the tutorial code assumed that the reference image had a black background, resulting in an empty silhouette mask. Since the background is white, this change allows the model to find the correct silhouette mask.

Reviewed By: nikhilaravi, sbranson

Differential Revision: D23502202

fbshipit-source-id: c3a570f93efd480323f27cb081db0a9fb54be219

Author: Amitav Baruah

docs/tutorials/camera_position_optimization_with_differentiable_rendering.ipynb

@@ -350,8 +350,8 @@
     "        self.device = meshes.device\n",
     "        self.renderer = renderer\n",
     "        \n",
-    "        # Get the silhouette of the reference RGB image by finding all the non zero values. \n",
-    "        image_ref = torch.from_numpy((image_ref[..., :3].max(-1) != 0).astype(np.float32))\n",
+    "        # Get the silhouette of the reference RGB image by finding all non-white pixel values. \n",
+    "        image_ref = torch.from_numpy((image_ref[..., :3].max(-1) != 1).astype(np.float32))\n",
     "        self.register_buffer('image_ref', image_ref)\n",
     "        \n",
     "        # Create an optimizable parameter for the x, y, z position of the camera. \n",