📝 Add object detection processing tutorial

Author: nateraw

docs/source/image_process.mdx

@@ -39,7 +39,11 @@ Both parameter values default to 1000, which can be expensive if you are storing
 
 ## Data augmentation
 
-🤗 Datasets can apply data augmentations from any library or package to your dataset. This guide will use the transforms from [torchvision](https://pytorch.org/vision/stable/transforms.html).
+🤗 Datasets can apply data augmentations from any library or package to your dataset.
+
+### Image Classification
+
+First let's see how you can transform image classification datasets. This guide will use the transforms from [torchvision](https://pytorch.org/vision/stable/transforms.html).
 
 <Tip>
 
@@ -88,3 +92,163 @@ Now you can take a look at the augmented image by indexing into the `pixel_value
     <img class="block dark:hidden" src="https://huggingface.co/datasets/huggingface/documentation-images/resolve/main/image_process_jitter.png">
     <img class="hidden dark:block" src="https://huggingface.co/datasets/huggingface/documentation-images/resolve/main/image_process_jitter.png"/>
 </div>
+
+### Object Detection
+
+Next, let's see how to apply transformations to object detection datasets. For this we'll use [Albumentations](https://albumentations.ai/docs/), following their object detection [tutorial](https://albumentations.ai/docs/examples/example_bboxes/).
+
+To run these examples, make sure you have up to date versions of `albumentations` and `cv2` installed:
+
+```
+pip install -U albumentations opencv-python
+```
+
+In this example, the [`cppe-5`](https://huggingface.co/datasets/cppe-5) dataset is used, which is a dataset for identifying medical personal protective equipments (PPEs) in the context of the COVID-19 pandemic.
+
+You can load the dataset and take a look at an example:
+
+```py
+from datasets import load_dataset
+
+>>> ds = load_dataset("cppe-5")
+>>> example = ds['train'][0]
+>>> example
+{'height': 663,
+ 'image': <PIL.JpegImagePlugin.JpegImageFile image mode=RGB size=943x663 at 0x7FC3DC756250>,
+ 'image_id': 15,
+ 'objects': {'area': [3796, 1596, 152768, 81002],
+  'bbox': [[302.0, 109.0, 73.0, 52.0],
+   [810.0, 100.0, 57.0, 28.0],
+   [160.0, 31.0, 248.0, 616.0],
+   [741.0, 68.0, 202.0, 401.0]],
+  'category': [4, 4, 0, 0],
+  'id': [114, 115, 116, 117]},
+ 'width': 943}
+```
+
+The dataset has the following fields:
+
+- `image`: PIL.Image.Image object containing the image.
+- `image_id`: The image ID.
+- `height`: The image height.
+- `width`: The image width.
+- `objects`: a dictionary containing bounding box metadata for the objects present on the image
+  - `id`: the annotation id
+  - `area`: the area of the bounding box
+  - `bbox`: the object's bounding box (in the [coco](https://albumentations.ai/docs/getting_started/bounding_boxes_augmentation/#coco) format)
+  - `category`: the object's category, with possible values including `Coverall (0)`, `Face_Shield (1)`, `Gloves (2)`, `Goggles (3)` and `Mask (4)`
+
+You can visualize the bboxes on the image using some internal torch utilities. But, to do that, you will need to reference the `datasets.ClassLabel` feature associated with the category IDs so you can look up the string labels.
+
+    
+```py
+>>> import torch
+>>> from torchvision.ops import box_convert
+>>> from torchvision.utils import draw_bounding_boxes
+>>> from torchvision.transforms.functional import pil_to_tensor, to_pil_image
+
+>>> categories = ds['train'].features['objects'].feature['category']
+
+>>> boxes_xywh = torch.tensor(example['objects']['bbox'])
+>>> boxes_xyxy = box_convert(boxes_xywh, 'xywh', 'xyxy')
+>>> labels = [categories.int2str(x) for x in example['objects']['category']]
+>>> to_pil_image(
+...     draw_bounding_boxes(
+...         pil_to_tensor(example['image']),
+...         boxes_xyxy,
+...         colors="red",
+...         labels=labels,
+...     )
+... )
+```
+
+<div class="flex justify-center">
+    <img src="https://huggingface.co/datasets/nateraw/documentation-images/resolve/main/visualize_detection_example.png">
+</div>
+
+
+Using `albumentations`, we can apply transforms that will effect the Image while also updating the bounding boxes accordingly. In this case, the image is resized to (480, 480), flipped horizontally, and brightened. 
+
+Note that `albumentations` is expecting the image to be in BGR format, not RGB, so we'll have to convert our image first before applying the transform.
+
+```py
+>>> import albumentations as A
+>>> import numpy as np
+
+>>> transform = A.Compose([
+...     A.Resize(480, 480),
+...     A.HorizontalFlip(p=1.0),
+...     A.RandomBrightnessContrast(p=1.0),
+... ], bbox_params=A.BboxParams(format='coco',  label_fields=['category']))
+
+>>> # RGB PIL Image -> BGR Numpy array
+>>> image = np.flip(np.array(example['image']), -1)
+>>> out = transform(
+...     image=image,
+...     bboxes=example['objects']['bbox'],
+...     category=example['objects']['category'],
+... )
+```
+
+Now when we visualize the result, the image should be flipped but the boxes should still be in the right places.
+
+```py
+>>> image = torch.tensor(out['image']).flip(-1).permute(2, 0, 1)
+>>> boxes_xywh = torch.stack([torch.tensor(x) for x in out['bboxes']])
+>>> boxes_xyxy = box_convert(boxes_xywh, 'xywh', 'xyxy')
+>>> labels = [categories.int2str(x) for x in out['category']]
+>>> to_pil_image(
+...     draw_bounding_boxes(
+...         image,
+...         boxes_xyxy,
+...         colors='red',
+...         labels=labels
+...     )
+... )
+```
+
+<div class="flex justify-center">
+    <img src="https://huggingface.co/datasets/nateraw/documentation-images/resolve/main/visualize_detection_example_transformed.png">
+</div>
+
+Create a function to apply the transform to a batch of examples:
+
+```py
+>>> def transforms(examples):
+...     images, bboxes, categories = [], [], []
+...     for image, objects in zip(examples['image'], examples['objects']):
+...         image = np.array(image.convert("RGB"))[:, :, ::-1]
+...         out = transform(
+...             image=image,
+...             bboxes=objects['bbox'],
+...             category=objects['category']
+...         )
+...         images.append(torch.tensor(out['image']).flip(-1).permute(2, 0, 1))
+...         bboxes.append(torch.tensor(out['bboxes']))
+...         categories.append(out['category'])
+...     return {'image': images, 'bbox': bboxes, 'category': categories}
+```
+
+Use the [`~Dataset.set_transform`] function to apply the transform on-the-fly which consumes less disk space. This function is useful if you only need to access the examples once:
+
+```py
+>>> ds['train'].set_transform(transforms)
+```
+
+Verify the transform is working by visualizing the 10th example:
+
+```py
+>>> example = ds['train'][10]
+>>> to_pil_image(
+...     draw_bounding_boxes(
+...         example['image'],
+...         box_convert(example['bbox'], 'xywh', 'xyxy'),
+...         colors='red',
+...         labels=[categories.int2str(x) for x in example['category']]
+...     )
+... )
+```
+
+<div class="flex justify-center">
+    <img src="https://huggingface.co/datasets/nateraw/documentation-images/resolve/main/visualize_detection_example_transformed_2.png">
+</div>