Shortcuts

Tutorial 4: Customize Data Pipelines

In this tutorial, we will introduce some methods about the design of data pipelines, and how to customize and extend your own data pipelines for the project.

Design of Data Pipelines

Following typical conventions, we use Dataset and DataLoader for data loading with multiple workers. Dataset returns a dict of data items corresponding the arguments of models’ forward method. Since the data in action recognition & localization may not be the same size (image size, gt bbox size, etc.), The DataContainer in MMCV is used to help collect and distribute data of different sizes. See here for more details.

The data preparation pipeline and the dataset is decomposed. Usually a dataset defines how to process the annotations and a data pipeline defines all the steps to prepare a data dict. A pipeline consists of a sequence of operations. Each operation takes a dict as input and also output a dict for the next operation.

We present a typical pipeline in the following figure. The blue blocks are pipeline operations. With the pipeline going on, each operator can add new keys (marked as green) to the result dict or update the existing keys (marked as orange). pipeline figure

The operations are categorized into data loading, pre-processing and formatting.

Here is a pipeline example for TSN.

img_norm_cfg = dict(
    mean=[123.675, 116.28, 103.53], std=[58.395, 57.12, 57.375], to_bgr=False)
train_pipeline = [
    dict(type='SampleFrames', clip_len=1, frame_interval=1, num_clips=3),
    dict(type='RawFrameDecode', io_backend='disk'),
    dict(type='Resize', scale=(-1, 256)),
    dict(
        type='MultiScaleCrop',
        input_size=224,
        scales=(1, 0.875, 0.75, 0.66),
        random_crop=False,
        max_wh_scale_gap=1),
    dict(type='Resize', scale=(224, 224), keep_ratio=False),
    dict(type='Flip', flip_ratio=0.5),
    dict(type='Normalize', **img_norm_cfg),
    dict(type='FormatShape', input_format='NCHW'),
    dict(type='Collect', keys=['imgs', 'label'], meta_keys=[]),
    dict(type='ToTensor', keys=['imgs', 'label'])
]
val_pipeline = [
    dict(
        type='SampleFrames',
        clip_len=1,
        frame_interval=1,
        num_clips=3,
        test_mode=True),
    dict(type='RawFrameDecode', io_backend='disk'),
    dict(type='Resize', scale=(-1, 256)),
    dict(type='CenterCrop', crop_size=224),
    dict(type='Normalize', **img_norm_cfg),
    dict(type='FormatShape', input_format='NCHW'),
    dict(type='Collect', keys=['imgs', 'label'], meta_keys=[]),
    dict(type='ToTensor', keys=['imgs'])
]
test_pipeline = [
    dict(
        type='SampleFrames',
        clip_len=1,
        frame_interval=1,
        num_clips=25,
        test_mode=True),
    dict(type='RawFrameDecode', io_backend='disk'),
    dict(type='Resize', scale=(-1, 256)),
    dict(type='TenCrop', crop_size=224),
    dict(type='Normalize', **img_norm_cfg),
    dict(type='FormatShape', input_format='NCHW'),
    dict(type='Collect', keys=['imgs', 'label'], meta_keys=[]),
    dict(type='ToTensor', keys=['imgs'])
]

We have supported some lazy operators and encourage users to apply them. Lazy ops record how the data should be processed, but it will postpone the processing on the raw data until the raw data forward Fuse stage. Specifically, lazy ops avoid frequent reading and modification operation on the raw data, but process the raw data once in the final Fuse stage, thus accelerating data preprocessing.

Here is a pipeline example applying lazy ops.

train_pipeline = [
    dict(type='SampleFrames', clip_len=32, frame_interval=2, num_clips=1),
    dict(type='RawFrameDecode', decoding_backend='turbojpeg'),
    # The following three lazy ops only process the bbox of frames without
    # modifying the raw data.
    dict(type='Resize', scale=(-1, 256), lazy=True),
    dict(
        type='MultiScaleCrop',
        input_size=224,
        scales=(1, 0.8),
        random_crop=False,
        max_wh_scale_gap=0,
        lazy=True),
    dict(type='Resize', scale=(224, 224), keep_ratio=False, lazy=True),
    # Lazy operator `Flip` only record whether a frame should be fliped and the
    # flip direction.
    dict(type='Flip', flip_ratio=0.5, lazy=True),
    # Processing the raw data once in Fuse stage.
    dict(type='Fuse'),
    dict(type='Normalize', **img_norm_cfg),
    dict(type='FormatShape', input_format='NCTHW'),
    dict(type='Collect', keys=['imgs', 'label'], meta_keys=[]),
    dict(type='ToTensor', keys=['imgs', 'label'])
]

For each operation, we list the related dict fields that are added/updated/removed below, where * means the key may not be affected.

Data loading

SampleFrames

  • add: frame_inds, clip_len, frame_interval, num_clips, *total_frames

DenseSampleFrames

  • add: frame_inds, clip_len, frame_interval, num_clips, *total_frames

PyAVDecode

  • add: imgs, original_shape

  • update: *frame_inds

DecordDecode

  • add: imgs, original_shape

  • update: *frame_inds

OpenCVDecode

  • add: imgs, original_shape

  • update: *frame_inds

RawFrameDecode

  • add: imgs, original_shape

  • update: *frame_inds

Pre-processing

RandomCrop

  • add: crop_bbox, img_shape

  • update: imgs

RandomResizedCrop

  • add: crop_bbox, img_shape

  • update: imgs

MultiScaleCrop

  • add: crop_bbox, img_shape, scales

  • update: imgs

Resize

  • add: img_shape, keep_ratio, scale_factor

  • update: imgs

Flip

  • add: flip, flip_direction

  • update: imgs, label

Normalize

  • add: img_norm_cfg

  • update: imgs

CenterCrop

  • add: crop_bbox, img_shape

  • update: imgs

ThreeCrop

  • add: crop_bbox, img_shape

  • update: imgs

TenCrop

  • add: crop_bbox, img_shape

  • update: imgs

Formatting

ToTensor

  • update: specified by keys.

ImageToTensor

  • update: specified by keys.

Transpose

  • update: specified by keys.

Collect

  • add: img_metas (the keys of img_metas is specified by meta_keys)

  • remove: all other keys except for those specified by keys

It is noteworthy that the first key, commonly imgs, will be used as the main key to calculate the batch size.

FormatShape

  • add: input_shape

  • update: imgs

Extend and Use Custom Pipelines

  1. Write a new pipeline in any file, e.g., my_pipeline.py. It takes a dict as input and return a dict.

    from mmaction.datasets import PIPELINES
    
    @PIPELINES.register_module()
    class MyTransform:
    
        def __call__(self, results):
            results['key'] = value
            return results
    
  2. Import the new class.

    from .my_pipeline import MyTransform
    
  3. Use it in config files.

    img_norm_cfg = dict(
         mean=[123.675, 116.28, 103.53], std=[58.395, 57.12, 57.375], to_rgb=True)
    train_pipeline = [
        dict(type='DenseSampleFrames', clip_len=8, frame_interval=8, num_clips=1),
        dict(type='RawFrameDecode', io_backend='disk'),
        dict(type='MyTransform'),       # use a custom pipeline
        dict(type='Normalize', **img_norm_cfg),
        dict(type='FormatShape', input_format='NCTHW'),
        dict(type='Collect', keys=['imgs', 'label'], meta_keys=[]),
        dict(type='ToTensor', keys=['imgs', 'label'])
    ]
    
Read the Docs v: latest
Versions
latest
stable
1.x
v1.0.0rc1
dev-1.x
Downloads
pdf
html
epub
On Read the Docs
Project Home
Builds

Free document hosting provided by Read the Docs.