add color field in instances and visualize based on preset colors

Summary:
Pull Request resolved: #3549

Currently D2 (11528ce) and D2 (11528ce083dc9ff83ee3a8f9086a1ef54d2a402f)GO uses a box IoU based method to assign id and color for each instance. It is useful but not flexible. This diff allows user to define their own preset colors for each instance. By doing this, some advanced tracking method can be applied before get into the Visualizer.

Reviewed By: FCChen

Differential Revision: D31366663

fbshipit-source-id: e04d26475c2935439a045e96c28f4391e21d40f1

Author: wenliangzhao2018

detectron2/utils/video_visualizer.py

@@ -72,6 +72,7 @@ def draw_instance_predictions(self, frame, predictions):
         scores = predictions.scores if predictions.has("scores") else None
         classes = predictions.pred_classes.numpy() if predictions.has("pred_classes") else None
         keypoints = predictions.pred_keypoints if predictions.has("pred_keypoints") else None
+        colors = predictions.COLOR if predictions.has("COLOR") else [None] * len(predictions)
 
         if predictions.has("pred_masks"):
             masks = predictions.pred_masks
@@ -82,10 +83,11 @@ def draw_instance_predictions(self, frame, predictions):
             masks = None
 
         detected = [
-            _DetectedInstance(classes[i], boxes[i], mask_rle=None, color=None, ttl=8)
+            _DetectedInstance(classes[i], boxes[i], mask_rle=None, color=colors[i], ttl=8)
             for i in range(num_instances)
         ]
-        colors = self._assign_colors(detected)
+        if not predictions.has("COLOR"):
+            colors = self._assign_colors(detected)
 
         labels = _create_text_labels(classes, scores, self.metadata.get("thing_classes", None))