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Tutorial 2: Customize Datasets

Data configuration

data in config file is the variable for data configuration, to define the arguments that are used in datasets and dataloaders.

Here is an example of data configuration:

data = dict(
    samples_per_gpu=4,
    workers_per_gpu=4,
    train=dict(
        type='ADE20KDataset',
        data_root='data/ade/ADEChallengeData2016',
        img_dir='images/training',
        ann_dir='annotations/training',
        pipeline=train_pipeline),
    val=dict(
        type='ADE20KDataset',
        data_root='data/ade/ADEChallengeData2016',
        img_dir='images/validation',
        ann_dir='annotations/validation',
        pipeline=test_pipeline),
    test=dict(
        type='ADE20KDataset',
        data_root='data/ade/ADEChallengeData2016',
        img_dir='images/validation',
        ann_dir='annotations/validation',
        pipeline=test_pipeline))

  • train, val and test: The configs to build dataset instances for model training, validation and testing by using build and registry mechanism.

  • samples_per_gpu: How many samples per batch and per gpu to load during model training, and the batch_size of training is equal to samples_per_gpu times gpu number, e.g. when using 8 gpus for distributed data parallel training and samples_per_gpu=4, the batch_size is 8*4=32. If you would like to define batch_size for testing and validation, please use test_dataloader and val_dataloader with mmseg >=0.24.1.

  • workers_per_gpu: How many subprocesses per gpu to use for data loading. 0 means that the data will be loaded in the main process.

Note: samples_per_gpu only works for model training, and the default setting of samples_per_gpu is 1 in mmseg when model testing and validation (DO NOT support batch inference yet).

Note: before v0.24.1, except train, val test, samples_per_gpu and workers_per_gpu, the other keys in data must be the input keyword arguments for dataloader in pytorch, and the dataloaders used for model training, validation and testing have the same input arguments. In v0.24.1, mmseg supports to use train_dataloader, test_dataloader and val_dataloader to specify different keyword arguments, and still supports the overall arguments definition but the specific dataloader setting has a higher priority.

Here is an example for specific dataloader:

data = dict(
    samples_per_gpu=4,
    workers_per_gpu=4,
    shuffle=True,
    train=dict(type='xxx', ...),
    val=dict(type='xxx', ...),
    test=dict(type='xxx', ...),
    # Use different batch size during validation and testing.
    val_dataloader=dict(samples_per_gpu=1, workers_per_gpu=4, shuffle=False),
    test_dataloader=dict(samples_per_gpu=1, workers_per_gpu=4, shuffle=False))

Assume only one gpu used for model training and testing, as the priority of the overall arguments definition is low, the batch_size for training is 4 and dataset will be shuffled, and batch_size for testing and validation is 1, and dataset will not be shuffled.

To make data configuration much clearer, we recommend use specific dataloader setting instead of overall dataloader setting after v0.24.1, just like:

data = dict(
    train=dict(type='xxx', ...),
    val=dict(type='xxx', ...),
    test=dict(type='xxx', ...),
    # Use specific dataloader setting
    train_dataloader=dict(samples_per_gpu=4, workers_per_gpu=4, shuffle=True),
    val_dataloader=dict(samples_per_gpu=1, workers_per_gpu=4, shuffle=False),
    test_dataloader=dict(samples_per_gpu=1, workers_per_gpu=4, shuffle=False))

Note: in model training, default values in the script of mmseg for dataloader are shuffle=True, and drop_last=True, in model validation and testing, default values are shuffle=False, and drop_last=False

Customize datasets by reorganizing data

The simplest way is to convert your dataset to organize your data into folders.

An example of file structure is as followed.

├── data
│   ├── my_dataset
│   │   ├── img_dir
│   │   │   ├── train
│   │   │   │   ├── xxx{img_suffix}
│   │   │   │   ├── yyy{img_suffix}
│   │   │   │   ├── zzz{img_suffix}
│   │   │   ├── val
│   │   ├── ann_dir
│   │   │   ├── train
│   │   │   │   ├── xxx{seg_map_suffix}
│   │   │   │   ├── yyy{seg_map_suffix}
│   │   │   │   ├── zzz{seg_map_suffix}
│   │   │   ├── val

A training pair will consist of the files with same suffix in img_dir/ann_dir.

If split argument is given, only part of the files in img_dir/ann_dir will be loaded. We may specify the prefix of files we would like to be included in the split txt.

More specifically, for a split txt like following,

xxx
zzz

Only data/my_dataset/img_dir/train/xxx{img_suffix}, data/my_dataset/img_dir/train/zzz{img_suffix}, data/my_dataset/ann_dir/train/xxx{seg_map_suffix}, data/my_dataset/ann_dir/train/zzz{seg_map_suffix} will be loaded.

:::{note} The annotations are images of shape (H, W), the value pixel should fall in range [0, num_classes - 1]. You may use 'P' mode of pillow to create your annotation image with color. :::

Customize datasets by mixing dataset

MMSegmentation also supports to mix dataset for training. Currently it supports to concat, repeat and multi-image mix datasets.

Repeat dataset

We use RepeatDataset as wrapper to repeat the dataset. For example, suppose the original dataset is Dataset_A, to repeat it, the config looks like the following

dataset_A_train = dict(
        type='RepeatDataset',
        times=N,
        dataset=dict(  # This is the original config of Dataset_A
            type='Dataset_A',
            ...
            pipeline=train_pipeline
        )
    )

Concatenate dataset

There 2 ways to concatenate the dataset.

  1. If the datasets you want to concatenate are in the same type with different annotation files, you can concatenate the dataset configs like the following.

    1. You may concatenate two ann_dir.

      dataset_A_train = dict(
    type='Dataset_A',
    img_dir = 'img_dir',
    ann_dir = ['anno_dir_1', 'anno_dir_2'],
    pipeline=train_pipeline
)

    2. You may concatenate two split.

      dataset_A_train = dict(
    type='Dataset_A',
    img_dir = 'img_dir',
    ann_dir = 'anno_dir',
    split = ['split_1.txt', 'split_2.txt'],
    pipeline=train_pipeline
)

    3. You may concatenate two ann_dir and split simultaneously.

      dataset_A_train = dict(
    type='Dataset_A',
    img_dir = 'img_dir',
    ann_dir = ['anno_dir_1', 'anno_dir_2'],
    split = ['split_1.txt', 'split_2.txt'],
    pipeline=train_pipeline
)

      In this case, ann_dir_1 and ann_dir_2 are corresponding to split_1.txt and split_2.txt.

  2. In case the dataset you want to concatenate is different, you can concatenate the dataset configs like the following.

    dataset_A_train = dict()
dataset_B_train = dict()

data = dict(
    imgs_per_gpu=2,
    workers_per_gpu=2,
    train = [
        dataset_A_train,
        dataset_B_train
    ],
    val = dataset_A_val,
    test = dataset_A_test
    )

A more complex example that repeats Dataset_A and Dataset_B by N and M times, respectively, and then concatenates the repeated datasets is as the following.

dataset_A_train = dict(
    type='RepeatDataset',
    times=N,
    dataset=dict(
        type='Dataset_A',
        ...
        pipeline=train_pipeline
    )
)
dataset_A_val = dict(
    ...
    pipeline=test_pipeline
)
dataset_A_test = dict(
    ...
    pipeline=test_pipeline
)
dataset_B_train = dict(
    type='RepeatDataset',
    times=M,
    dataset=dict(
        type='Dataset_B',
        ...
        pipeline=train_pipeline
    )
)
data = dict(
    imgs_per_gpu=2,
    workers_per_gpu=2,
    train = [
        dataset_A_train,
        dataset_B_train
    ],
    val = dataset_A_val,
    test = dataset_A_test
)

Multi-image Mix Dataset

We use MultiImageMixDataset as a wrapper to mix images from multiple datasets. MultiImageMixDataset can be used by multiple images mixed data augmentation like mosaic and mixup.

An example of using MultiImageMixDataset with Mosaic data augmentation:

train_pipeline = [
    dict(type='LoadImageFromFile'),
    dict(type='LoadAnnotations'),
    dict(type='RandomMosaic', prob=1),
    dict(type='Resize', img_scale=(1024, 512), keep_ratio=True),
    dict(type='RandomFlip', prob=0.5),
    dict(type='Normalize', **img_norm_cfg),
    dict(type='DefaultFormatBundle'),
    dict(type='Collect', keys=['img', 'gt_semantic_seg']),
]

train_dataset = dict(
    type='MultiImageMixDataset',
    dataset=dict(
        classes=classes,
        palette=palette,
        type=dataset_type,
        reduce_zero_label=False,
        img_dir=data_root + "images/train",
        ann_dir=data_root + "annotations/train",
        pipeline=[
            dict(type='LoadImageFromFile'),
            dict(type='LoadAnnotations'),
        ]
    ),
    pipeline=train_pipeline
)