import torch
import torch.nn as nn
import torch.nn.functional as F
from mmcv.cnn import ConvModule, DepthwiseSeparableConvModule
from mmcv.runner import force_fp32
from mmdet.core import (build_anchor_generator, build_assigner,
build_bbox_coder, build_sampler, multi_apply)
from ..builder import HEADS
from ..losses import smooth_l1_loss
from .anchor_head import AnchorHead
# TODO: add loss evaluator for SSD
[docs]@HEADS.register_module()
class SSDHead(AnchorHead):
"""SSD head used in https://arxiv.org/abs/1512.02325.
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: 0.
feat_channels (int): Number of hidden channels when stacked_convs
> 0. Default: 256.
use_depthwise (bool): Whether to use DepthwiseSeparableConv.
Default: False.
conv_cfg (dict): Dictionary to construct and config conv layer.
Default: None.
norm_cfg (dict): Dictionary to construct and config norm layer.
Default: None.
act_cfg (dict): Dictionary to construct and config activation layer.
Default: None.
anchor_generator (dict): Config dict for anchor generator
bbox_coder (dict): Config of bounding box coder.
reg_decoded_bbox (bool): If true, the regression loss would be
applied directly on decoded bounding boxes, converting both
the predicted boxes and regression targets to absolute
coordinates format. Default False. It should be `True` when
using `IoULoss`, `GIoULoss`, or `DIoULoss` in the bbox head.
train_cfg (dict): Training config of anchor head.
test_cfg (dict): Testing config of anchor head.
init_cfg (dict or list[dict], optional): Initialization config dict.
""" # noqa: W605
def __init__(self,
num_classes=80,
in_channels=(512, 1024, 512, 256, 256, 256),
stacked_convs=0,
feat_channels=256,
use_depthwise=False,
conv_cfg=None,
norm_cfg=None,
act_cfg=None,
anchor_generator=dict(
type='SSDAnchorGenerator',
scale_major=False,
input_size=300,
strides=[8, 16, 32, 64, 100, 300],
ratios=([2], [2, 3], [2, 3], [2, 3], [2], [2]),
basesize_ratio_range=(0.1, 0.9)),
bbox_coder=dict(
type='DeltaXYWHBBoxCoder',
clip_border=True,
target_means=[.0, .0, .0, .0],
target_stds=[1.0, 1.0, 1.0, 1.0],
),
reg_decoded_bbox=False,
train_cfg=None,
test_cfg=None,
init_cfg=dict(
type='Xavier',
layer='Conv2d',
distribution='uniform',
bias=0)):
super(AnchorHead, self).__init__(init_cfg)
self.num_classes = num_classes
self.in_channels = in_channels
self.stacked_convs = stacked_convs
self.feat_channels = feat_channels
self.use_depthwise = use_depthwise
self.conv_cfg = conv_cfg
self.norm_cfg = norm_cfg
self.act_cfg = act_cfg
self.cls_out_channels = num_classes + 1 # add background class
self.anchor_generator = build_anchor_generator(anchor_generator)
self.num_anchors = self.anchor_generator.num_base_anchors
self._init_layers()
self.bbox_coder = build_bbox_coder(bbox_coder)
self.reg_decoded_bbox = reg_decoded_bbox
self.use_sigmoid_cls = False
self.cls_focal_loss = False
self.train_cfg = train_cfg
self.test_cfg = test_cfg
# set sampling=False for archor_target
self.sampling = False
if self.train_cfg:
self.assigner = build_assigner(self.train_cfg.assigner)
# SSD sampling=False so use PseudoSampler
sampler_cfg = dict(type='PseudoSampler')
self.sampler = build_sampler(sampler_cfg, context=self)
self.fp16_enabled = False
def _init_layers(self):
"""Initialize layers of the head."""
self.cls_convs = nn.ModuleList()
self.reg_convs = nn.ModuleList()
# TODO: Use registry to choose ConvModule type
conv = DepthwiseSeparableConvModule \
if self.use_depthwise else ConvModule
for channel, num_anchors in zip(self.in_channels, self.num_anchors):
cls_layers = []
reg_layers = []
in_channel = channel
# build stacked conv tower, not used in default ssd
for i in range(self.stacked_convs):
cls_layers.append(
conv(
in_channel,
self.feat_channels,
3,
padding=1,
conv_cfg=self.conv_cfg,
norm_cfg=self.norm_cfg,
act_cfg=self.act_cfg))
reg_layers.append(
conv(
in_channel,
self.feat_channels,
3,
padding=1,
conv_cfg=self.conv_cfg,
norm_cfg=self.norm_cfg,
act_cfg=self.act_cfg))
in_channel = self.feat_channels
# SSD-Lite head
if self.use_depthwise:
cls_layers.append(
ConvModule(
in_channel,
in_channel,
3,
padding=1,
groups=in_channel,
conv_cfg=self.conv_cfg,
norm_cfg=self.norm_cfg,
act_cfg=self.act_cfg))
reg_layers.append(
ConvModule(
in_channel,
in_channel,
3,
padding=1,
groups=in_channel,
conv_cfg=self.conv_cfg,
norm_cfg=self.norm_cfg,
act_cfg=self.act_cfg))
cls_layers.append(
nn.Conv2d(
in_channel,
num_anchors * self.cls_out_channels,
kernel_size=1 if self.use_depthwise else 3,
padding=0 if self.use_depthwise else 1))
reg_layers.append(
nn.Conv2d(
in_channel,
num_anchors * 4,
kernel_size=1 if self.use_depthwise else 3,
padding=0 if self.use_depthwise else 1))
self.cls_convs.append(nn.Sequential(*cls_layers))
self.reg_convs.append(nn.Sequential(*reg_layers))
[docs] def forward(self, feats):
"""Forward features from the upstream network.
Args:
feats (tuple[Tensor]): Features from the upstream network, each is
a 4D-tensor.
Returns:
tuple:
cls_scores (list[Tensor]): Classification scores for all scale
levels, each is a 4D-tensor, the channels number is
num_anchors * num_classes.
bbox_preds (list[Tensor]): Box energies / deltas for all scale
levels, each is a 4D-tensor, the channels number is
num_anchors * 4.
"""
cls_scores = []
bbox_preds = []
for feat, reg_conv, cls_conv in zip(feats, self.reg_convs,
self.cls_convs):
cls_scores.append(cls_conv(feat))
bbox_preds.append(reg_conv(feat))
return cls_scores, bbox_preds
[docs] def loss_single(self, cls_score, bbox_pred, anchor, labels, label_weights,
bbox_targets, bbox_weights, num_total_samples):
"""Compute loss of a single image.
Args:
cls_score (Tensor): Box scores for eachimage
Has shape (num_total_anchors, num_classes).
bbox_pred (Tensor): Box energies / deltas for each image
level with shape (num_total_anchors, 4).
anchors (Tensor): Box reference for each scale level with shape
(num_total_anchors, 4).
labels (Tensor): Labels of each anchors with shape
(num_total_anchors,).
label_weights (Tensor): Label weights of each anchor with shape
(num_total_anchors,)
bbox_targets (Tensor): BBox regression targets of each anchor wight
shape (num_total_anchors, 4).
bbox_weights (Tensor): BBox regression loss weights of each anchor
with shape (num_total_anchors, 4).
num_total_samples (int): If sampling, num total samples equal to
the number of total anchors; Otherwise, it is the number of
positive anchors.
Returns:
dict[str, Tensor]: A dictionary of loss components.
"""
loss_cls_all = F.cross_entropy(
cls_score, labels, reduction='none') * label_weights
# FG cat_id: [0, num_classes -1], BG cat_id: num_classes
pos_inds = ((labels >= 0) & (labels < self.num_classes)).nonzero(
as_tuple=False).reshape(-1)
neg_inds = (labels == self.num_classes).nonzero(
as_tuple=False).view(-1)
num_pos_samples = pos_inds.size(0)
num_neg_samples = self.train_cfg.neg_pos_ratio * num_pos_samples
if num_neg_samples > neg_inds.size(0):
num_neg_samples = neg_inds.size(0)
topk_loss_cls_neg, _ = loss_cls_all[neg_inds].topk(num_neg_samples)
loss_cls_pos = loss_cls_all[pos_inds].sum()
loss_cls_neg = topk_loss_cls_neg.sum()
loss_cls = (loss_cls_pos + loss_cls_neg) / num_total_samples
if self.reg_decoded_bbox:
# When the regression loss (e.g. `IouLoss`, `GIouLoss`)
# is applied directly on the decoded bounding boxes, it
# decodes the already encoded coordinates to absolute format.
bbox_pred = self.bbox_coder.decode(anchor, bbox_pred)
loss_bbox = smooth_l1_loss(
bbox_pred,
bbox_targets,
bbox_weights,
beta=self.train_cfg.smoothl1_beta,
avg_factor=num_total_samples)
return loss_cls[None], loss_bbox
[docs] @force_fp32(apply_to=('cls_scores', 'bbox_preds'))
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) == self.anchor_generator.num_levels
device = cls_scores[0].device
anchor_list, valid_flag_list = self.get_anchors(
featmap_sizes, img_metas, device=device)
cls_reg_targets = self.get_targets(
anchor_list,
valid_flag_list,
gt_bboxes,
img_metas,
gt_bboxes_ignore_list=gt_bboxes_ignore,
gt_labels_list=gt_labels,
label_channels=1,
unmap_outputs=False)
if cls_reg_targets is None:
return None
(labels_list, label_weights_list, bbox_targets_list, bbox_weights_list,
num_total_pos, num_total_neg) = cls_reg_targets
num_images = len(img_metas)
all_cls_scores = torch.cat([
s.permute(0, 2, 3, 1).reshape(
num_images, -1, self.cls_out_channels) for s in cls_scores
], 1)
all_labels = torch.cat(labels_list, -1).view(num_images, -1)
all_label_weights = torch.cat(label_weights_list,
-1).view(num_images, -1)
all_bbox_preds = torch.cat([
b.permute(0, 2, 3, 1).reshape(num_images, -1, 4)
for b in bbox_preds
], -2)
all_bbox_targets = torch.cat(bbox_targets_list,
-2).view(num_images, -1, 4)
all_bbox_weights = torch.cat(bbox_weights_list,
-2).view(num_images, -1, 4)
# concat all level anchors to a single tensor
all_anchors = []
for i in range(num_images):
all_anchors.append(torch.cat(anchor_list[i]))
# check NaN and Inf
assert torch.isfinite(all_cls_scores).all().item(), \
'classification scores become infinite or NaN!'
assert torch.isfinite(all_bbox_preds).all().item(), \
'bbox predications become infinite or NaN!'
losses_cls, losses_bbox = multi_apply(
self.loss_single,
all_cls_scores,
all_bbox_preds,
all_anchors,
all_labels,
all_label_weights,
all_bbox_targets,
all_bbox_weights,
num_total_samples=num_total_pos)
return dict(loss_cls=losses_cls, loss_bbox=losses_bbox)