mmdetection/docs/en/advanced_guides/transforms.md

# Data Transforms (Need to update)

## Design of Data transforms pipeline

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.

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

We present a classical 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](../../../resources/data_pipeline.png)

Here is a pipeline example for Faster R-CNN.

```python
train_pipeline = [  # Training data processing pipeline
    dict(type='LoadImageFromFile', backend_args=backend_args),  # First pipeline to load images from file path
    dict(
        type='LoadAnnotations',  # Second pipeline to load annotations for current image
        with_bbox=True),  # Whether to use bounding box, True for detection
    dict(
        type='Resize',  # Pipeline that resize the images and their annotations
        scale=(1333, 800),  # The largest scale of image
        keep_ratio=True  # Whether to keep the ratio between height and width
        ),
    dict(
        type='RandomFlip',  # Augmentation pipeline that flip the images and their annotations
        prob=0.5),  # The probability to flip
    dict(type='PackDetInputs')  # Pipeline that formats the annotation data and decides which keys in the data should be packed into data_samples
]
test_pipeline = [  # Testing data processing pipeline
    dict(type='LoadImageFromFile', backend_args=backend_args),  # First pipeline to load images from file path
    dict(type='Resize', scale=(1333, 800), keep_ratio=True),  # Pipeline that resize the images
    dict(
        type='PackDetInputs',  # Pipeline that formats the annotation data and decides which keys in the data should be packed into data_samples
        meta_keys=('img_id', 'img_path', 'ori_shape', 'img_shape',
                   'scale_factor'))
]
```