mmyolo/docs/en/advanced_guides/data_flow.md

Mixed image data augmentation update

Mixed image data augmentation is similar to Mosaic and MixUp, in which the annotation information of multiple images needs to be fused during the runtime. In the OpenMMLab data augmentation pipeline, the other indexes of the dataset are generally not available. In order to achieve the above function, in the MultiImageMixDataset the concept of dataset wrapper is proposed in YOLOX, which is reproduced in MMDet.

MultiImageMixDataset dataset wrapper will include some data augmentation method such as Mosaic and RandAffine, while CocoDataset will also include the pipeline to achieve the img and annotation loading function. through this way we can achieve mix data augmentation quickly. The configuration method is as follows：

train_pipeline = [
    dict(type='Mosaic', img_scale=img_scale, pad_val=114.0),
    dict(
        type='RandomAffine',
        scaling_ratio_range=(0.1, 2),
        border=(-img_scale[0] // 2, -img_scale[1] // 2)),
    dict(
        type='MixUp',
        img_scale=img_scale,
        ratio_range=(0.8, 1.6),
        pad_val=114.0),
    ...
]
train_dataset = dict(
    # use MultiImageMixDataset wrapper to support mosaic and mixup
    type='MultiImageMixDataset',
    dataset=dict(
        type='CocoDataset',
        pipeline=[
            dict(type='LoadImageFromFile'),
            dict(type='LoadAnnotations', with_bbox=True)
        ]),
    pipeline=train_pipeline)

But above method will cause a problem: to the users who are not familiar with MMDet ,they will forget to match data augmentation methods like Mosaic together with MultiImageMixDataset, which could extremely increase the Complexity, and it could be hard to understand.

To solve this problem we make a simplification in MMYOLO, which directly make pipeline catch the dataset ，and make the data augmentation methods like Mosaic be achieved and used as random flip, without data wrapper anymore. The new configuration method is as follows:

pre_transform = [
    dict(type='LoadImageFromFile'),
    dict(type='LoadAnnotations', with_bbox=True)
]
train_pipeline = [
    *pre_transform,
    dict(
        type='Mosaic',
        img_scale=img_scale,
        pad_val=114.0,
        pre_transform=pre_transform),
    dict(
        type='mmdet.RandomAffine',
        scaling_ratio_range=(0.1, 2),
        border=(-img_scale[0] // 2, -img_scale[1] // 2)),
    dict(
        type='YOLOXMixUp',
        img_scale=img_scale,
        ratio_range=(0.8, 1.6),
        pad_val=114.0,
        pre_transform=pre_transform),
    ...
]

A more complex YOLOv5-m configuration include MixUp is shown as follows:

mosaic_affine_pipeline = [
    dict(
        type='Mosaic',
        img_scale=img_scale,
        pad_val=114.0,
        pre_transform=pre_transform),
    dict(
        type='YOLOv5RandomAffine',
        max_rotate_degree=0.0,
        max_shear_degree=0.0,
        scaling_ratio_range=(1 - affine_scale, 1 + affine_scale),
        border=(-img_scale[0] // 2, -img_scale[1] // 2),
        border_val=(114, 114, 114))
]

# enable mixup
train_pipeline = [
    *pre_transform, *mosaic_affine_pipeline,
    dict(
        type='YOLOv5MixUp',
        prob=0.1,
        pre_transform=[*pre_transform, *mosaic_affine_pipeline]),
    dict(
        type='mmdet.Albu',
        transforms=albu_train_transforms,
        bbox_params=dict(
            type='BboxParams',
            format='pascal_voc',
            label_fields=['gt_bboxes_labels', 'gt_ignore_flags']),
        keymap={
            'img': 'image',
            'gt_bboxes': 'bboxes'
        }),
    dict(type='YOLOv5HSVRandomAug'),
    dict(type='mmdet.RandomFlip', prob=0.5),
    dict(
        type='mmdet.PackDetInputs',
        meta_keys=('img_id', 'img_path', 'ori_shape', 'img_shape', 'flip',
                   'flip_direction'))
]

It is very easy to be achieved, just pass the object of Dataset to the pipeline.

def prepare_data(self, idx) -> Any:
   """Pass the dataset to the pipeline during training to support mixed
   data augmentation, such as Mosaic and MixUp."""
   if self.test_mode is False:
        data_info = self.get_data_info(idx)
        data_info['dataset'] = self
        return self.pipeline(data_info)
    else:
        return super().prepare_data(idx)