Add base class

Summary:
Pull Request resolved: facebookresearch#3835

As base class for D2 (facebookresearch@11528ce) tracking module

Reviewed By: zhanghang1989

Differential Revision: D32671844

fbshipit-source-id: a83c3629b7cc326333be7cccf14c1ff95bcfe433

detectron2/tracking/base_tracker.py

@@ -0,0 +1,57 @@
+#!/usr/bin/env python3
+# Copyright 2004-present Facebook. All Rights Reserved.
+from ..structures import Instances
+from detectron2.utils.registry import Registry
+from ..config.config import CfgNode as CfgNode_
+from detectron2.config import configurable
+
+
+TRACKER_HEADS_REGISTRY = Registry("TRACKER_HEADS")
+TRACKER_HEADS_REGISTRY.__doc__ = """
+Registry for tracking classes.
+"""
+
+
+class BaseTracker(object):
+    """
+    A parent class for all trackers
+    """
+    @configurable
+    def __init__(self, **kwargs):
+        self._prev_instances = None  # (D2)instances for previous frame
+        self._matched_idx = set()  # indices in prev_instances found matching
+        self._matched_ID = set()  # idendities in prev_instances found matching
+        self._untracked_prev_idx = set()  # indices in prev_instances not found matching
+        self._id_count = 0  # used to assign new id
+
+    @classmethod
+    def from_config(cls, cfg: CfgNode_):
+        raise NotImplementedError("Calling BaseTracker::from_config")
+
+    def update(self, predictions: Instances) -> Instances:
+        """
+        Args:
+            predictions: D2 Instances for predictions of the current frame
+        Return:
+            D2 Instances for predictions of the current frame with ID assigned
+
+        _prev_instances and instances will have the following fields:
+          .pred_boxes               (shape=[N, 4])
+          .scores                   (shape=[N,])
+          .pred_classes             (shape=[N,])
+          .pred_keypoints           (shape=[N, M, 3], Optional)
+          .pred_masks               (shape=List[2D_MASK], Optional)   2D_MASK: shape=[H, W]
+          .ID                       (shape=[N,])
+
+        N: # of detected bboxes
+        H and W: height and width of 2D mask
+        """
+        raise NotImplementedError("Calling BaseTracker::update")
+
+
+def build_tracker_head(cfg: CfgNode_) -> BaseTracker:
+    """
+    Build a semantic segmentation head from `cfg.MODEL.SEM_SEG_HEAD.NAME`.
+    """
+    name = cfg.TRACKING.TRACKER_NAME
+    return TRACKER_HEADS_REGISTRY.get(name)(cfg)