Tutorial 4: Customize Models
We basically categorize model components into 5 types.
- backbone: usually an FCN network to extract feature maps, e.g., ResNet, MobileNet.
- neck: the component between backbones and heads, e.g., FPN, PAFPN.
- head: the component for specific tasks, e.g., bbox prediction and mask prediction.
- roi extractor: the part for extracting RoI features from feature maps, e.g., RoI Align.
- loss: the component in head for calculating losses, e.g., FocalLoss, L1Loss, and GHMLoss.
Develop new components
Add a new backbone
Here we show how to develop new components with an example of MobileNet.
1. Define a new backbone (e.g. MobileNet)
Create a new file mmdet/models/backbones/mobilenet.py.
import torch.nn as nn
from ..builder import BACKBONES
@BACKBONES.register_module()
class MobileNet(nn.Module):
def __init__(self, arg1, arg2):
pass
def forward(self, x): # should return a tuple
pass
2. Import the module
You can either add the following line to mmdet/models/backbones/__init__.py
from .mobilenet import MobileNet
or alternatively add
custom_imports = dict(
imports=['mmdet.models.backbones.mobilenet'],
allow_failed_imports=False)
to the config file to avoid modifying the original code.
3. Use the backbone in your config file
model = dict(
...
backbone=dict(
type='MobileNet',
arg1=xxx,
arg2=xxx),
...
Add new necks
1. Define a neck (e.g. PAFPN)
Create a new file mmdet/models/necks/pafpn.py.
from ..builder import NECKS
@NECKS.register_module()
class PAFPN(nn.Module):
def __init__(self,
in_channels,
out_channels,
num_outs,
start_level=0,
end_level=-1,
add_extra_convs=False):
pass
def forward(self, inputs):
# implementation is ignored
pass
2. Import the module
You can either add the following line to mmdet/models/necks/__init__.py,
from .pafpn import PAFPN
or alternatively add
custom_imports = dict(
imports=['mmdet.models.necks.pafpn.py'],
allow_failed_imports=False)
to the config file and avoid modifying the original code.
3. Modify the config file
neck=dict(
type='PAFPN',
in_channels=[256, 512, 1024, 2048],
out_channels=256,
num_outs=5)
Add new heads
Here we show how to develop a new head with the example of Double Head R-CNN as the following.
First, add a new bbox head in mmdet/models/roi_heads/bbox_heads/double_bbox_head.py.
Double Head R-CNN implements a new bbox head for object detection.
To implement a bbox head, basically we need to implement three functions of the new module as the following.
from mmdet.models.builder import HEADS
from .bbox_head import BBoxHead
@HEADS.register_module()
class DoubleConvFCBBoxHead(BBoxHead):
r"""Bbox head used in Double-Head R-CNN
/-> cls
/-> shared convs ->
\-> reg
roi features
/-> cls
\-> shared fc ->
\-> reg
""" # noqa: W605
def __init__(self,
num_convs=0,
num_fcs=0,
conv_out_channels=1024,
fc_out_channels=1024,
conv_cfg=None,
norm_cfg=dict(type='BN'),
**kwargs):
kwargs.setdefault('with_avg_pool', True)
super(DoubleConvFCBBoxHead, self).__init__(**kwargs)
def forward(self, x_cls, x_reg):
Second, implement a new RoI Head if it is necessary. We plan to inherit the new DoubleHeadRoIHead from StandardRoIHead. We can find that a StandardRoIHead already implements the following functions.
import torch
from mmdet.core import bbox2result, bbox2roi, build_assigner, build_sampler
from ..builder import HEADS, build_head, build_roi_extractor
from .base_roi_head import BaseRoIHead
from .test_mixins import BBoxTestMixin, MaskTestMixin
@HEADS.register_module()
class StandardRoIHead(BaseRoIHead, BBoxTestMixin, MaskTestMixin):
"""Simplest base roi head including one bbox head and one mask head.
"""
def init_assigner_sampler(self):
def init_bbox_head(self, bbox_roi_extractor, bbox_head):
def init_mask_head(self, mask_roi_extractor, mask_head):
def forward_dummy(self, x, proposals):
def forward_train(self,
x,
img_metas,
proposal_list,
gt_bboxes,
gt_labels,
gt_bboxes_ignore=None,
gt_masks=None):
def _bbox_forward(self, x, rois):
def _bbox_forward_train(self, x, sampling_results, gt_bboxes, gt_labels,
img_metas):
def _mask_forward_train(self, x, sampling_results, bbox_feats, gt_masks,
img_metas):
def _mask_forward(self, x, rois=None, pos_inds=None, bbox_feats=None):
def simple_test(self,
x,
proposal_list,
img_metas,
proposals=None,
rescale=False):
"""Test without augmentation."""
Double Head's modification is mainly in the bbox_forward logic, and it inherits other logics from the StandardRoIHead.
In the mmdet/models/roi_heads/double_roi_head.py, we implement the new RoI Head as the following:
from ..builder import HEADS
from .standard_roi_head import StandardRoIHead
@HEADS.register_module()
class DoubleHeadRoIHead(StandardRoIHead):
"""RoI head for Double Head RCNN
https://arxiv.org/abs/1904.06493
"""
def __init__(self, reg_roi_scale_factor, **kwargs):
super(DoubleHeadRoIHead, self).__init__(**kwargs)
self.reg_roi_scale_factor = reg_roi_scale_factor
def _bbox_forward(self, x, rois):
bbox_cls_feats = self.bbox_roi_extractor(
x[:self.bbox_roi_extractor.num_inputs], rois)
bbox_reg_feats = self.bbox_roi_extractor(
x[:self.bbox_roi_extractor.num_inputs],
rois,
roi_scale_factor=self.reg_roi_scale_factor)
if self.with_shared_head:
bbox_cls_feats = self.shared_head(bbox_cls_feats)
bbox_reg_feats = self.shared_head(bbox_reg_feats)
cls_score, bbox_pred = self.bbox_head(bbox_cls_feats, bbox_reg_feats)
bbox_results = dict(
cls_score=cls_score,
bbox_pred=bbox_pred,
bbox_feats=bbox_cls_feats)
return bbox_results
Last, the users need to add the module in
mmdet/models/bbox_heads/__init__.py and mmdet/models/roi_heads/__init__.py thus the corresponding registry could find and load them.
Alternatively, the users can add
custom_imports=dict(
imports=['mmdet.models.roi_heads.double_roi_head', 'mmdet.models.bbox_heads.double_bbox_head'])
to the config file and achieve the same goal.
The config file of Double Head R-CNN is as the following
_base_ = '../faster_rcnn/faster_rcnn_r50_fpn_1x_coco.py'
model = dict(
roi_head=dict(
type='DoubleHeadRoIHead',
reg_roi_scale_factor=1.3,
bbox_head=dict(
_delete_=True,
type='DoubleConvFCBBoxHead',
num_convs=4,
num_fcs=2,
in_channels=256,
conv_out_channels=1024,
fc_out_channels=1024,
roi_feat_size=7,
num_classes=80,
bbox_coder=dict(
type='DeltaXYWHBBoxCoder',
target_means=[0., 0., 0., 0.],
target_stds=[0.1, 0.1, 0.2, 0.2]),
reg_class_agnostic=False,
loss_cls=dict(
type='CrossEntropyLoss', use_sigmoid=False, loss_weight=2.0),
loss_bbox=dict(type='SmoothL1Loss', beta=1.0, loss_weight=2.0))))
Since MMDetection 2.0, the config system supports to inherit configs such that the users can focus on the modification.
The Double Head R-CNN mainly uses a new DoubleHeadRoIHead and a new
DoubleConvFCBBoxHead, the arguments are set according to the __init__ function of each module.
Add new loss
Assume you want to add a new loss as MyLoss, for bounding box regression.
To add a new loss function, the users need implement it in mmdet/models/losses/my_loss.py.
The decorator weighted_loss enable the loss to be weighted for each element.
import torch
import torch.nn as nn
from ..builder import LOSSES
from .utils import weighted_loss
@weighted_loss
def my_loss(pred, target):
assert pred.size() == target.size() and target.numel() > 0
loss = torch.abs(pred - target)
return loss
@LOSSES.register_module()
class MyLoss(nn.Module):
def __init__(self, reduction='mean', loss_weight=1.0):
super(MyLoss, self).__init__()
self.reduction = reduction
self.loss_weight = loss_weight
def forward(self,
pred,
target,
weight=None,
avg_factor=None,
reduction_override=None):
assert reduction_override in (None, 'none', 'mean', 'sum')
reduction = (
reduction_override if reduction_override else self.reduction)
loss_bbox = self.loss_weight * my_loss(
pred, target, weight, reduction=reduction, avg_factor=avg_factor)
return loss_bbox
Then the users need to add it in the mmdet/models/losses/__init__.py.
from .my_loss import MyLoss, my_loss
Alternatively, you can add
custom_imports=dict(
imports=['mmdet.models.losses.my_loss'])
to the config file and achieve the same goal.
To use it, modify the loss_xxx field.
Since MyLoss is for regression, you need to modify the loss_bbox field in the head.
loss_bbox=dict(type='MyLoss', loss_weight=1.0))