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.

Tutorial 4: Customize Data Pipelines
- Design of Data Pipelines
- Extend and Use Custom Pipelines

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¶

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

Import the new class.
```
from .my_pipeline import MyTransform
```

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'])
]