import torch
import torch.nn.functional as F
from mmdet.core import bbox_overlaps
from ..builder import HEADS
from .retina_head import RetinaHead
EPS = 1e-12
[docs]@HEADS.register_module()
class FreeAnchorRetinaHead(RetinaHead):
"""FreeAnchor RetinaHead used in https://arxiv.org/abs/1909.02466.
Args:
num_classes (int): Number of categories excluding the background
category.
in_channels (int): Number of channels in the input feature map.
stacked_convs (int): Number of conv layers in cls and reg tower.
Default: 4.
conv_cfg (dict): dictionary to construct and config conv layer.
Default: None.
norm_cfg (dict): dictionary to construct and config norm layer.
Default: norm_cfg=dict(type='GN', num_groups=32,
requires_grad=True).
pre_anchor_topk (int): Number of boxes that be token in each bag.
bbox_thr (float): The threshold of the saturated linear function. It is
usually the same with the IoU threshold used in NMS.
gamma (float): Gamma parameter in focal loss.
alpha (float): Alpha parameter in focal loss.
""" # noqa: W605
def __init__(self,
num_classes,
in_channels,
stacked_convs=4,
conv_cfg=None,
norm_cfg=None,
pre_anchor_topk=50,
bbox_thr=0.6,
gamma=2.0,
alpha=0.5,
**kwargs):
super(FreeAnchorRetinaHead,
self).__init__(num_classes, in_channels, stacked_convs, conv_cfg,
norm_cfg, **kwargs)
self.pre_anchor_topk = pre_anchor_topk
self.bbox_thr = bbox_thr
self.gamma = gamma
self.alpha = alpha
[docs] def loss(self,
cls_scores,
bbox_preds,
gt_bboxes,
gt_labels,
img_metas,
gt_bboxes_ignore=None):
"""Compute losses of the head.
Args:
cls_scores (list[Tensor]): Box scores for each scale level
Has shape (N, num_anchors * num_classes, H, W)
bbox_preds (list[Tensor]): Box energies / deltas for each scale
level with shape (N, num_anchors * 4, H, W)
gt_bboxes (list[Tensor]): each item are the truth boxes for each
image in [tl_x, tl_y, br_x, br_y] format.
gt_labels (list[Tensor]): class indices corresponding to each box
img_metas (list[dict]): Meta information of each image, e.g.,
image size, scaling factor, etc.
gt_bboxes_ignore (None | list[Tensor]): specify which bounding
boxes can be ignored when computing the loss.
Returns:
dict[str, Tensor]: A dictionary of loss components.
"""
featmap_sizes = [featmap.size()[-2:] for featmap in cls_scores]
assert len(featmap_sizes) == len(self.anchor_generator.base_anchors)
anchor_list, _ = self.get_anchors(featmap_sizes, img_metas)
anchors = [torch.cat(anchor) for anchor in anchor_list]
# concatenate each level
cls_scores = [
cls.permute(0, 2, 3,
1).reshape(cls.size(0), -1, self.cls_out_channels)
for cls in cls_scores
]
bbox_preds = [
bbox_pred.permute(0, 2, 3, 1).reshape(bbox_pred.size(0), -1, 4)
for bbox_pred in bbox_preds
]
cls_scores = torch.cat(cls_scores, dim=1)
bbox_preds = torch.cat(bbox_preds, dim=1)
cls_prob = torch.sigmoid(cls_scores)
box_prob = []
num_pos = 0
positive_losses = []
for _, (anchors_, gt_labels_, gt_bboxes_, cls_prob_,
bbox_preds_) in enumerate(
zip(anchors, gt_labels, gt_bboxes, cls_prob, bbox_preds)):
with torch.no_grad():
if len(gt_bboxes_) == 0:
image_box_prob = torch.zeros(
anchors_.size(0),
self.cls_out_channels).type_as(bbox_preds_)
else:
# box_localization: a_{j}^{loc}, shape: [j, 4]
pred_boxes = self.bbox_coder.decode(anchors_, bbox_preds_)
# object_box_iou: IoU_{ij}^{loc}, shape: [i, j]
object_box_iou = bbox_overlaps(gt_bboxes_, pred_boxes)
# object_box_prob: P{a_{j} -> b_{i}}, shape: [i, j]
t1 = self.bbox_thr
t2 = object_box_iou.max(
dim=1, keepdim=True).values.clamp(min=t1 + 1e-12)
object_box_prob = ((object_box_iou - t1) /
(t2 - t1)).clamp(
min=0, max=1)
# object_cls_box_prob: P{a_{j} -> b_{i}}, shape: [i, c, j]
num_obj = gt_labels_.size(0)
indices = torch.stack([
torch.arange(num_obj).type_as(gt_labels_), gt_labels_
],
dim=0)
object_cls_box_prob = torch.sparse_coo_tensor(
indices, object_box_prob)
# image_box_iou: P{a_{j} \in A_{+}}, shape: [c, j]
"""
from "start" to "end" implement:
image_box_iou = torch.sparse.max(object_cls_box_prob,
dim=0).t()
"""
# start
box_cls_prob = torch.sparse.sum(
object_cls_box_prob, dim=0).to_dense()
indices = torch.nonzero(box_cls_prob, as_tuple=False).t_()
if indices.numel() == 0:
image_box_prob = torch.zeros(
anchors_.size(0),
self.cls_out_channels).type_as(object_box_prob)
else:
nonzero_box_prob = torch.where(
(gt_labels_.unsqueeze(dim=-1) == indices[0]),
object_box_prob[:, indices[1]],
torch.tensor([
0
]).type_as(object_box_prob)).max(dim=0).values
# upmap to shape [j, c]
image_box_prob = torch.sparse_coo_tensor(
indices.flip([0]),
nonzero_box_prob,
size=(anchors_.size(0),
self.cls_out_channels)).to_dense()
# end
box_prob.append(image_box_prob)
# construct bags for objects
match_quality_matrix = bbox_overlaps(gt_bboxes_, anchors_)
_, matched = torch.topk(
match_quality_matrix,
self.pre_anchor_topk,
dim=1,
sorted=False)
del match_quality_matrix
# matched_cls_prob: P_{ij}^{cls}
matched_cls_prob = torch.gather(
cls_prob_[matched], 2,
gt_labels_.view(-1, 1, 1).repeat(1, self.pre_anchor_topk,
1)).squeeze(2)
# matched_box_prob: P_{ij}^{loc}
matched_anchors = anchors_[matched]
matched_object_targets = self.bbox_coder.encode(
matched_anchors,
gt_bboxes_.unsqueeze(dim=1).expand_as(matched_anchors))
loss_bbox = self.loss_bbox(
bbox_preds_[matched],
matched_object_targets,
reduction_override='none').sum(-1)
matched_box_prob = torch.exp(-loss_bbox)
# positive_losses: {-log( Mean-max(P_{ij}^{cls} * P_{ij}^{loc}) )}
num_pos += len(gt_bboxes_)
positive_losses.append(
self.positive_bag_loss(matched_cls_prob, matched_box_prob))
positive_loss = torch.cat(positive_losses).sum() / max(1, num_pos)
# box_prob: P{a_{j} \in A_{+}}
box_prob = torch.stack(box_prob, dim=0)
# negative_loss:
# \sum_{j}{ FL((1 - P{a_{j} \in A_{+}}) * (1 - P_{j}^{bg})) } / n||B||
negative_loss = self.negative_bag_loss(cls_prob, box_prob).sum() / max(
1, num_pos * self.pre_anchor_topk)
# avoid the absence of gradients in regression subnet
# when no ground-truth in a batch
if num_pos == 0:
positive_loss = bbox_preds.sum() * 0
losses = {
'positive_bag_loss': positive_loss,
'negative_bag_loss': negative_loss
}
return losses
[docs] def positive_bag_loss(self, matched_cls_prob, matched_box_prob):
"""Compute positive bag loss.
:math:`-log( Mean-max(P_{ij}^{cls} * P_{ij}^{loc}) )`.
:math:`P_{ij}^{cls}`: matched_cls_prob, classification probability of matched samples.
:math:`P_{ij}^{loc}`: matched_box_prob, box probability of matched samples.
Args:
matched_cls_prob (Tensor): Classification probability of matched
samples in shape (num_gt, pre_anchor_topk).
matched_box_prob (Tensor): BBox probability of matched samples,
in shape (num_gt, pre_anchor_topk).
Returns:
Tensor: Positive bag loss in shape (num_gt,).
""" # noqa: E501, W605
# bag_prob = Mean-max(matched_prob)
matched_prob = matched_cls_prob * matched_box_prob
weight = 1 / torch.clamp(1 - matched_prob, 1e-12, None)
weight /= weight.sum(dim=1).unsqueeze(dim=-1)
bag_prob = (weight * matched_prob).sum(dim=1)
# positive_bag_loss = -self.alpha * log(bag_prob)
return self.alpha * F.binary_cross_entropy(
bag_prob, torch.ones_like(bag_prob), reduction='none')
[docs] def negative_bag_loss(self, cls_prob, box_prob):
"""Compute negative bag loss.
:math:`FL((1 - P_{a_{j} \in A_{+}}) * (1 - P_{j}^{bg}))`.
:math:`P_{a_{j} \in A_{+}}`: Box_probability of matched samples.
:math:`P_{j}^{bg}`: Classification probability of negative samples.
Args:
cls_prob (Tensor): Classification probability, in shape
(num_img, num_anchors, num_classes).
box_prob (Tensor): Box probability, in shape
(num_img, num_anchors, num_classes).
Returns:
Tensor: Negative bag loss in shape (num_img, num_anchors, num_classes).
""" # noqa: E501, W605
prob = cls_prob * (1 - box_prob)
# There are some cases when neg_prob = 0.
# This will cause the neg_prob.log() to be inf without clamp.
prob = prob.clamp(min=EPS, max=1 - EPS)
negative_bag_loss = prob**self.gamma * F.binary_cross_entropy(
prob, torch.zeros_like(prob), reduction='none')
return (1 - self.alpha) * negative_bag_loss