update ssd loss, make it faster (#2761)

* update ssd loss, make it faster test=develop

* update comments, test=develop

* ssd loss, fix bug when no object, text=develop

ppdet/modeling/losses/ssd_loss.py

@@ -19,9 +19,9 @@ from __future__ import print_function
 import paddle
 import paddle.nn as nn
 import paddle.nn.functional as F
-import numpy as np
 from ppdet.core.workspace import register
-from ..ops import bipartite_match, box_coder, iou_similarity
+from ..ops import iou_similarity
+from ..bbox_utils import bbox2delta
 
 __all__ = ['SSDLoss']
 
@@ -32,191 +32,131 @@ class SSDLoss(nn.Layer):
     SSDLoss
 
     Args:
-        match_type (str): The type of matching method, should be
-           'bipartite' or 'per_prediction'. None ('bipartite') by default.
-        overlap_threshold (float32, optional): If `match_type` is 'per_prediction',
-            this threshold is to determine the extra matching bboxes based
-            on the maximum distance, 0.5 by default.
+        overlap_threshold (float32, optional): IoU threshold for negative bboxes
+            and positive bboxes, 0.5 by default.
         neg_pos_ratio (float): The ratio of negative samples / positive samples.
-        neg_overlap (float): The overlap threshold of negative samples.
         loc_loss_weight (float): The weight of loc_loss.
         conf_loss_weight (float): The weight of conf_loss.
+        prior_box_var (list): Variances corresponding to prior box coord, [0.1,
+            0.1, 0.2, 0.2] by default.
     """
 
     def __init__(self,
-                 match_type='per_prediction',
                  overlap_threshold=0.5,
                  neg_pos_ratio=3.0,
-                 neg_overlap=0.5,
                  loc_loss_weight=1.0,
-                 conf_loss_weight=1.0):
+                 conf_loss_weight=1.0,
+                 prior_box_var=[0.1, 0.1, 0.2, 0.2]):
         super(SSDLoss, self).__init__()
-        self.match_type = match_type
         self.overlap_threshold = overlap_threshold
         self.neg_pos_ratio = neg_pos_ratio
-        self.neg_overlap = neg_overlap
         self.loc_loss_weight = loc_loss_weight
         self.conf_loss_weight = conf_loss_weight
+        self.prior_box_var = [1. / a for a in prior_box_var]
 
-    def _label_target_assign(self,
-                             gt_label,
-                             matched_indices,
-                             neg_mask=None,
-                             mismatch_value=0):
-        gt_label = gt_label.numpy()
-        matched_indices = matched_indices.numpy()
-        if neg_mask is not None:
-            neg_mask = neg_mask.numpy()
-
-        batch_size, num_priors = matched_indices.shape
-        trg_lbl = np.ones((batch_size, num_priors, 1)).astype('int32')
-        trg_lbl *= mismatch_value
-        trg_lbl_wt = np.zeros((batch_size, num_priors, 1)).astype('float32')
-
-        for i in range(batch_size):
-            col_ids = np.where(matched_indices[i] > -1)
-            col_val = matched_indices[i][col_ids]
-            trg_lbl[i][col_ids] = gt_label[i][col_val]
-            trg_lbl_wt[i][col_ids] = 1.0
-
-        if neg_mask is not None:
-            trg_lbl_wt += neg_mask[:, :, np.newaxis]
-
-        return paddle.to_tensor(trg_lbl), paddle.to_tensor(trg_lbl_wt)
-
-    def _bbox_target_assign(self, encoded_box, matched_indices):
-        encoded_box = encoded_box.numpy()
-        matched_indices = matched_indices.numpy()
-
-        batch_size, num_priors = matched_indices.shape
-        trg_bbox = np.zeros((batch_size, num_priors, 4)).astype('float32')
-        trg_bbox_wt = np.zeros((batch_size, num_priors, 1)).astype('float32')
-
+    def _bipartite_match_for_batch(self, gt_bbox, gt_label, prior_boxes,
+                                   bg_index):
+        """
+        Args:
+            gt_bbox (Tensor): [B, N, 4]
+            gt_label (Tensor): [B, N, 1]
+            prior_boxes (Tensor): [A, 4]
+            bg_index (int): Background class index
+        """
+        batch_size, num_priors = gt_bbox.shape[0], prior_boxes.shape[0]
+        ious = iou_similarity(gt_bbox.reshape((-1, 4)), prior_boxes).reshape(
+            (batch_size, -1, num_priors))
+
+        # Calculate the number of object per sample.
+        num_object = (ious.sum(axis=-1) > 0).astype('int64').sum(axis=-1)
+
+        # For each prior box, get the max IoU of all GTs.
+        prior_max_iou, prior_argmax_iou = ious.max(axis=1), ious.argmax(axis=1)
+        # For each GT, get the max IoU of all prior boxes.
+        gt_max_iou, gt_argmax_iou = ious.max(axis=2), ious.argmax(axis=2)
+
+        # Gather target bbox and label according to 'prior_argmax_iou' index.
+        batch_ind = paddle.arange(
+            0, batch_size, dtype='int64').unsqueeze(-1).tile([1, num_priors])
+        prior_argmax_iou = paddle.stack([batch_ind, prior_argmax_iou], axis=-1)
+        targets_bbox = paddle.gather_nd(gt_bbox, prior_argmax_iou)
+        targets_label = paddle.gather_nd(gt_label, prior_argmax_iou)
+        # Assign negative
+        bg_index_tensor = paddle.full([batch_size, num_priors, 1], bg_index,
+                                      'int64')
+        targets_label = paddle.where(
+            prior_max_iou.unsqueeze(-1) < self.overlap_threshold,
+            bg_index_tensor, targets_label)
+
+        # Ensure each GT can match the max IoU prior box.
         for i in range(batch_size):
-            col_ids = np.where(matched_indices[i] > -1)
-            col_val = matched_indices[i][col_ids]
-            for v, c in zip(col_val.tolist(), col_ids[0]):
-                trg_bbox[i][c] = encoded_box[i][v][c]
-            trg_bbox_wt[i][col_ids] = 1.0
-
-        return paddle.to_tensor(trg_bbox), paddle.to_tensor(trg_bbox_wt)
-
-    def _mine_hard_example(self,
-                           conf_loss,
-                           matched_indices,
-                           matched_dist,
-                           neg_pos_ratio=3.0,
-                           neg_overlap=0.5):
-        pos = (matched_indices > -1).astype(conf_loss.dtype)
+            if num_object[i] > 0:
+                targets_bbox[i] = paddle.scatter(
+                    targets_bbox[i], gt_argmax_iou[i, :int(num_object[i])],
+                    gt_bbox[i, :int(num_object[i])])
+                targets_label[i] = paddle.scatter(
+                    targets_label[i], gt_argmax_iou[i, :int(num_object[i])],
+                    gt_label[i, :int(num_object[i])])
+
+        # Encode box
+        prior_boxes = prior_boxes.unsqueeze(0).tile([batch_size, 1, 1])
+        targets_bbox = bbox2delta(
+            prior_boxes.reshape([-1, 4]),
+            targets_bbox.reshape([-1, 4]), self.prior_box_var)
+        targets_bbox = targets_bbox.reshape([batch_size, -1, 4])
+
+        return targets_bbox, targets_label
+
+    def _mine_hard_example(self, conf_loss, targets_label, bg_index):
+        pos = (targets_label != bg_index).astype(conf_loss.dtype)
         num_pos = pos.sum(axis=1, keepdim=True)
-        neg = (matched_dist < neg_overlap).astype(conf_loss.dtype)
+        neg = (targets_label == bg_index).astype(conf_loss.dtype)
 
-        conf_loss = conf_loss * (1.0 - pos) * neg
+        conf_loss = conf_loss.clone() * neg
         loss_idx = conf_loss.argsort(axis=1, descending=True)
         idx_rank = loss_idx.argsort(axis=1)
         num_negs = []
-        for i in range(matched_indices.shape[0]):
-            cur_idx = loss_idx[i]
+        for i in range(conf_loss.shape[0]):
             cur_num_pos = num_pos[i]
-            num_neg = paddle.clip(cur_num_pos * neg_pos_ratio, max=pos.shape[1])
+            num_neg = paddle.clip(
+                cur_num_pos * self.neg_pos_ratio, max=pos.shape[1])
             num_negs.append(num_neg)
-        num_neg = paddle.stack(num_negs, axis=0).expand_as(idx_rank)
+        num_neg = paddle.stack(num_negs).expand_as(idx_rank)
         neg_mask = (idx_rank < num_neg).astype(conf_loss.dtype)
-        return neg_mask
 
-    def forward(self, boxes, scores, gt_box, gt_class, anchors):
+        return (neg_mask + pos).astype('bool')
+
+    def forward(self, boxes, scores, gt_bbox, gt_label, prior_boxes):
         boxes = paddle.concat(boxes, axis=1)
         scores = paddle.concat(scores, axis=1)
-        prior_boxes = paddle.concat(anchors, axis=0)
-        gt_label = gt_class.unsqueeze(-1)
-        batch_size, num_priors = scores.shape[:2]
-        num_classes = scores.shape[-1] - 1
-
-        def _reshape_to_2d(x):
-            return paddle.flatten(x, start_axis=2)
-
-        # 1. Find matched bounding box by prior box.
-        #   1.1 Compute IOU similarity between ground-truth boxes and prior boxes.
-        #   1.2 Compute matched bounding box by bipartite matching algorithm.
-        matched_indices = []
-        matched_dist = []
-        for i in range(gt_box.shape[0]):
-            iou = iou_similarity(gt_box[i], prior_boxes)
-            matched_indice, matched_d = bipartite_match(iou, self.match_type,
-                                                        self.overlap_threshold)
-            matched_indices.append(matched_indice)
-            matched_dist.append(matched_d)
-        matched_indices = paddle.concat(matched_indices, axis=0)
-        matched_indices.stop_gradient = True
-        matched_dist = paddle.concat(matched_dist, axis=0)
-        matched_dist.stop_gradient = True
-
-        # 2. Compute confidence for mining hard examples
-        # 2.1. Get the target label based on matched indices
-        target_label, _ = self._label_target_assign(
-            gt_label, matched_indices, mismatch_value=num_classes)
-        confidence = _reshape_to_2d(scores)
-        # 2.2. Compute confidence loss.
-        # Reshape confidence to 2D tensor.
-        target_label = _reshape_to_2d(target_label).astype('int64')
-        conf_loss = F.softmax_with_cross_entropy(confidence, target_label)
-        conf_loss = paddle.reshape(conf_loss, [batch_size, num_priors])
-
-        # 3. Mining hard examples
-        neg_mask = self._mine_hard_example(
-            conf_loss,
-            matched_indices,
-            matched_dist,
-            neg_pos_ratio=self.neg_pos_ratio,
-            neg_overlap=self.neg_overlap)
-
-        # 4. Assign classification and regression targets
-        # 4.1. Encoded bbox according to the prior boxes.
-        prior_box_var = paddle.to_tensor(
-            np.array(
-                [0.1, 0.1, 0.2, 0.2], dtype='float32')).reshape(
-                    [1, 4]).expand_as(prior_boxes)
-        encoded_bbox = []
-        for i in range(gt_box.shape[0]):
-            encoded_bbox.append(
-                box_coder(
-                    prior_box=prior_boxes,
-                    prior_box_var=prior_box_var,
-                    target_box=gt_box[i],
-                    code_type='encode_center_size'))
-        encoded_bbox = paddle.stack(encoded_bbox, axis=0)
-        # 4.2. Assign regression targets
-        target_bbox, target_loc_weight = self._bbox_target_assign(
-            encoded_bbox, matched_indices)
-        # 4.3. Assign classification targets
-        target_label, target_conf_weight = self._label_target_assign(
-            gt_label,
-            matched_indices,
-            neg_mask=neg_mask,
-            mismatch_value=num_classes)
-
-        # 5. Compute loss.
-        # 5.1 Compute confidence loss.
-        target_label = _reshape_to_2d(target_label).astype('int64')
-        conf_loss = F.softmax_with_cross_entropy(confidence, target_label)
-
-        target_conf_weight = _reshape_to_2d(target_conf_weight)
-        conf_loss = conf_loss * target_conf_weight * self.conf_loss_weight
-
-        # 5.2 Compute regression loss.
-        location = _reshape_to_2d(boxes)
-        target_bbox = _reshape_to_2d(target_bbox)
-
-        loc_loss = F.smooth_l1_loss(location, target_bbox, reduction='none')
-        loc_loss = paddle.sum(loc_loss, axis=-1, keepdim=True)
-        target_loc_weight = _reshape_to_2d(target_loc_weight)
-        loc_loss = loc_loss * target_loc_weight * self.loc_loss_weight
-
-        # 5.3 Compute overall weighted loss.
-        loss = conf_loss + loc_loss
-        loss = paddle.reshape(loss, [batch_size, num_priors])
-        loss = paddle.sum(loss, axis=1, keepdim=True)
-        normalizer = paddle.sum(target_loc_weight)
-        loss = paddle.sum(loss / normalizer)
+        gt_label = gt_label.unsqueeze(-1).astype('int64')
+        prior_boxes = paddle.concat(prior_boxes, axis=0)
+        bg_index = scores.shape[-1] - 1
+
+        # Match bbox and get targets.
+        targets_bbox, targets_label = \
+            self._bipartite_match_for_batch(gt_bbox, gt_label, prior_boxes, bg_index)
+        targets_bbox.stop_gradient = True
+        targets_label.stop_gradient = True
+
+        # Compute regression loss.
+        # Select positive samples.
+        bbox_mask = (targets_label != bg_index).astype(boxes.dtype)
+        loc_loss = bbox_mask * F.smooth_l1_loss(
+            boxes, targets_bbox, reduction='none')
+        loc_loss = loc_loss.sum() * self.loc_loss_weight
+
+        # Compute confidence loss.
+        conf_loss = F.softmax_with_cross_entropy(scores, targets_label)
+        # Mining hard examples.
+        label_mask = self._mine_hard_example(
+            conf_loss.squeeze(-1), targets_label.squeeze(-1), bg_index)
+        conf_loss = conf_loss * label_mask.unsqueeze(-1).astype(conf_loss.dtype)
+        conf_loss = conf_loss.sum() * self.conf_loss_weight
+
+        # Compute overall weighted loss.
+        normalizer = (targets_label != bg_index).astype('float32').sum().clip(
+            min=1)
+        loss = (conf_loss + loc_loss) / (normalizer + 1e-9)
 
         return loss