Bounding box regression loss increases where RPN loss decreases Oriented RCNN

Question

I am playing around with an oriented RCNN with a custom satellite DOTA dataset in MMRotate. When I train the Oriented RCNN, the bounding box regression loss starts very low (loss ~ 0.009) and almost consistently increases when the RPN regression loss decreases! Some facts about the dataset:

• around 60.000 images.
• each images can contain between 0 - 4 objects.
• the images are large (1024x1024) and the objects tend to be very small.

I have looked at my data and angle definitions and am convinced that these are set correctly.

Does anyone have experienced anything similar?

Here is the config for mmrotate oriented rcnn:

_base_ = [
    '../_base_/datasets/ssdd.py', '../_base_/schedules/schedule_3x.py',
    '../_base_/default_runtime.py'
]

angle_version = 'le90'
model = dict(
    type='OrientedRCNN',
    backbone=dict(
        type='ResNet',
        depth=50,
        num_stages=4,
        out_indices=(0, 1, 2, 3),
        frozen_stages=-1,
        norm_cfg=dict(type='BN', requires_grad=True),
        norm_eval=True,
        style='pytorch',
        init_cfg=dict(type='Pretrained', checkpoint='torchvision://resnet50')),
    neck=dict(
        type='FPN',
        in_channels=[256, 512, 1024, 2048],
        out_channels=256,
        num_outs=5),
    rpn_head=dict(
        type='OrientedRPNHead',
        in_channels=256,
        feat_channels=256,
        version=angle_version,
        anchor_generator=dict(
            type='AnchorGenerator',
            scales=[8],
            ratios=[0.5, 1.0, 2.0],
            strides=[4, 8, 16, 32, 64]),
        bbox_coder=dict(
            type='MidpointOffsetCoder',
            angle_range=angle_version,
            target_means=[0.0, 0.0, 0.0, 0.0, 0.0, 0.0],
            target_stds=[1.0, 1.0, 1.0, 1.0, 0.5, 0.5]),
        loss_cls=dict(
            type='CrossEntropyLoss', use_sigmoid=True, loss_weight=1.0),
        loss_bbox=dict(
            type='SmoothL1Loss', beta=0.1111111111111111, loss_weight=1.0)),
    roi_head=dict(
        type='OrientedStandardRoIHead',
        bbox_roi_extractor=dict(
            type='RotatedSingleRoIExtractor',
            roi_layer=dict(
                type='RoIAlignRotated',
                out_size=7,
                sample_num=2,
                clockwise=True),
            out_channels=256,
            featmap_strides=[4, 8, 16, 32]),
        bbox_head=dict(
            type='RotatedShared2FCBBoxHead',
            in_channels=256,
            fc_out_channels=1024,
            roi_feat_size=7,
            num_classes=1,
            bbox_coder=dict(
                type='DeltaXYWHAOBBoxCoder',
                angle_range=angle_version,
                norm_factor=None,
                edge_swap=True,
                proj_xy=True,
                target_means=(.0, .0, .0, .0, .0),
                target_stds=(0.1, 0.1, 0.2, 0.2, 0.1)),
            reg_class_agnostic=True,
            loss_cls=dict(
                type='CrossEntropyLoss', use_sigmoid=False, loss_weight=1.0),
            loss_bbox=dict(type='SmoothL1Loss', beta=1.0, loss_weight=1.0))),
    train_cfg=dict(
        rpn=dict(
            assigner=dict(
                type='MaxIoUAssigner',
                pos_iou_thr=0.7,
                neg_iou_thr=0.3,
                min_pos_iou=0.3,
                match_low_quality=True,
                ignore_iof_thr=-1),
            sampler=dict(
                type='RandomSampler',
                num=256,
                pos_fraction=0.5,
                neg_pos_ub=-1,
                add_gt_as_proposals=False),
            allowed_border=0,
            pos_weight=-1,
            debug=False),
        rpn_proposal=dict(
            nms_pre=2000,
            max_per_img=2000,
            nms=dict(type='nms', iou_threshold=0.8),
            min_bbox_size=0),
        rcnn=dict(
            assigner=dict(
                type='MaxIoUAssigner',
                pos_iou_thr=0.5,
                neg_iou_thr=0.5,
                min_pos_iou=0.5,
                match_low_quality=False,
                iou_calculator=dict(type='RBboxOverlaps2D'),
                ignore_iof_thr=-1),
            sampler=dict(
                type='RRandomSampler',
                num=512,
                pos_fraction=0.25,
                neg_pos_ub=-1,
                add_gt_as_proposals=True),
            pos_weight=-1,
            debug=False)),
    test_cfg=dict(
        rpn=dict(
            nms_pre=2000,
            max_per_img=2000,
            nms=dict(type='nms', iou_threshold=0.8),
            min_bbox_size=0),
        rcnn=dict(
            nms_pre=2000,
            min_bbox_size=0,
            score_thr=0.05,
            nms=dict(iou_thr=0.1),
            max_per_img=2000)))

img_norm_cfg = dict(
    mean=[21.55, 21.55, 21.55], std=[24.42, 24.42, 24.42], to_rgb=True)
train_pipeline = [
    dict(type='LoadImageFromFile'),
    dict(type='LoadAnnotations', with_bbox=True),
    dict(type='RResize', img_scale=(608, 608)),
    dict(
        type='RRandomFlip',
        flip_ratio=[0.25, 0.25, 0.25],
        direction=['horizontal', 'vertical', 'diagonal'],
        version=angle_version),
    dict(type='Normalize', **img_norm_cfg),
    dict(type='Pad', size_divisor=32),
    dict(type='DefaultFormatBundle'),
    dict(type='Collect', keys=['img', 'gt_bboxes', 'gt_labels'])
]
data = dict(
    train=dict(pipeline=train_pipeline, version=angle_version),
    val=dict(version=angle_version),
    test=dict(version=angle_version))

optimizer = dict(lr=0.005)

# evaluation
evaluation = dict(interval=72, metric='mAP')
# learning policy
lr_config = dict(
    policy='step',
    warmup='linear',
    warmup_iters=500,
    warmup_ratio=1.0 / 3,
    step=[65, 71])
runner = dict(type='EpochBasedRunner', max_epochs=72)
checkpoint_config = dict(interval=12)