Custom detection dataset in PyTorch - ML Experiment: Train & Evaluate

Experiment - Custom detection dataset

Problem:You want to train an object detection model using PyTorch on your own images and labels. Currently, you do not have a dataset class that loads your images and bounding boxes correctly.

Current Metrics:No model training yet because dataset loading is incomplete.

Issue:Without a proper custom dataset class, the model cannot read images and bounding boxes, so training cannot start.

Your Task

Create a PyTorch custom dataset class that loads images and their bounding boxes with labels correctly, so the model can train.

Use PyTorch Dataset class.

Load images from file paths.

Return image tensors and target dictionaries with boxes and labels.

Do not use external dataset libraries.

Hint 1

Hint 2

Hint 3

Hint 4

Solution

PyTorch

import torch
from torch.utils.data import Dataset
from PIL import Image
import os
import torchvision.transforms as T

class CustomDetectionDataset(Dataset):
    def __init__(self, annotations, img_dir, transforms=None):
        """
        annotations: list of dicts, each dict has 'image_id', 'boxes' and 'labels'
        img_dir: directory where images are stored
        transforms: torchvision transforms to apply
        """
        self.annotations = annotations
        self.img_dir = img_dir
        self.transforms = transforms

    def __len__(self):
        return len(self.annotations)

    def __getitem__(self, idx):
        ann = self.annotations[idx]
        img_path = os.path.join(self.img_dir, ann['image_id'])
        img = Image.open(img_path).convert("RGB")

        boxes = torch.as_tensor(ann['boxes'], dtype=torch.float32)  # [[xmin, ymin, xmax, ymax], ...]
        labels = torch.as_tensor(ann['labels'], dtype=torch.int64)  # [label1, label2, ...]

        target = {}
        target['boxes'] = boxes
        target['labels'] = labels

        if self.transforms:
            img = self.transforms(img)

        return img, target

# Example usage:
# annotations = [
#     {'image_id': 'img1.jpg', 'boxes': [[10, 20, 50, 60]], 'labels': [1]},
#     {'image_id': 'img2.jpg', 'boxes': [[15, 25, 55, 65], [30, 40, 70, 80]], 'labels': [2, 3]}
# ]
# img_dir = '/path/to/images'
# transforms = T.ToTensor()
# dataset = CustomDetectionDataset(annotations, img_dir, transforms)
# img, target = dataset[0]
# print(img.shape, target)

Created a PyTorch Dataset subclass named CustomDetectionDataset.

Implemented __init__ to accept annotations, image directory, and optional transforms.

Implemented __len__ to return dataset size.

Implemented __getitem__ to load image, convert to RGB, convert boxes and labels to tensors, and apply transforms.

Returned image tensor and target dictionary with 'boxes' and 'labels' keys.

Results Interpretation

Before: No dataset class, so no data loading possible.

After: Dataset class loads images and bounding boxes correctly, enabling model training.

Creating a custom dataset class in PyTorch is essential to load your own images and annotations properly for object detection tasks.

Bonus Experiment

Add data augmentation transforms like random horizontal flip and color jitter to the dataset.

💡 Hint

Use torchvision.transforms.Compose to combine multiple transforms and apply them in the dataset __getitem__ method.

Practice

1. What is the main purpose of creating a Custom detection dataset in PyTorch?

easy

A. To perform image classification without labels

B. To convert images to grayscale only

C. To generate random images for testing

D. To load images and their bounding boxes with labels for training detection models

Solution

1. Step 1: Understand dataset role in detection

   A detection dataset provides images along with bounding boxes and labels for each object to train detection models.

2. Step 2: Identify correct purpose

   Classification without labels, random image generation, and grayscale conversion do not provide bounding boxes or labels needed for detection tasks.

3. Final Answer:

   To load images and their bounding boxes with labels for training detection models -> Option D

4. Quick Check:

   Detection dataset = images + bounding boxes + labels [OK]

Hint: Detection datasets always include bounding boxes and labels [OK]

Common Mistakes:

• Thinking detection datasets only have images
• Confusing detection with classification datasets
• Ignoring bounding boxes in dataset

2. Which of the following is the correct way to define the __getitem__ method in a custom detection dataset class?

easy

A. def __getitem__(self): return image, label

B. def __getitem__(self, idx): return image, label

C. def __getitem__(self, idx): return image, bounding_boxes, labels

D. def getitem(self, idx): return image, bounding_boxes

Solution

1. Step 1: Recall __getitem__ signature

   The method must accept an index parameter and return image, bounding boxes, and labels for detection.

2. Step 2: Check options for correct syntax and return values

   def __getitem__(self, idx): return image, bounding_boxes, labels correctly uses def __getitem__(self, idx): and returns image, bounding boxes, and labels. Others miss parameters or return incomplete data.

3. Final Answer:

   def __getitem__(self, idx): return image, bounding_boxes, labels -> Option C

4. Quick Check:

   __getitem__ needs idx and returns image + boxes + labels [OK]

Hint: __getitem__ always takes index and returns image with boxes and labels [OK]

Common Mistakes:

• Omitting the idx parameter
• Returning only image and label without boxes
• Using wrong method name without underscores

3. Given this snippet from a custom detection dataset, what will be the output type of dataset[0]?

class CustomDataset(torch.utils.data.Dataset):
    def __getitem__(self, idx):
        image = self.images[idx]
        boxes = self.boxes[idx]
        labels = self.labels[idx]
        target = {"boxes": boxes, "labels": labels}
        return image, target

medium

A. A tuple containing an image tensor and a dictionary with boxes and labels

B. A list containing image, boxes, and labels separately

C. Only the image tensor

D. A dictionary with keys 'image' and 'target'

Solution

1. Step 1: Analyze return statement

   The method returns image, target where target is a dictionary with keys 'boxes' and 'labels'.

2. Step 2: Determine output type

   Returning two items separated by comma means a tuple is returned: (image, target dictionary).

3. Final Answer:

   A tuple containing an image tensor and a dictionary with boxes and labels -> Option A

4. Quick Check:

   Return image, target dict = tuple [OK]

Hint: Comma-separated returns form a tuple in Python [OK]

Common Mistakes:

• Thinking return is a list instead of tuple
• Assuming only image is returned
• Confusing dictionary keys with return structure

4. You wrote this __getitem__ method but get an error when loading data:

def __getitem__(self, idx):
    image = self.images[idx]
    boxes = self.boxes[idx]
    labels = self.labels[idx]
    target = {"boxes": boxes, "labels": labels}
    return image, target

What is the most likely cause of the error?

medium

A. Missing return statement

B. boxes and labels are not converted to tensors

C. image is not normalized

D. Incorrect method name without underscores

Solution

1. Step 1: Check common dataset errors

   Detection models expect bounding boxes and labels as tensors, not raw lists or arrays.

2. Step 2: Identify missing tensor conversion

   If boxes and labels are not converted to tensors, PyTorch will raise errors during training or data loading.

3. Final Answer:

   boxes and labels are not converted to tensors -> Option B

4. Quick Check:

   Boxes and labels must be tensors [OK]

Hint: Always convert boxes and labels to tensors before returning [OK]

Common Mistakes:

• Forgetting to convert boxes and labels to torch tensors
• Assuming image normalization fixes tensor errors
• Using wrong method name causing attribute errors

5. You want to add a transform that flips images horizontally and adjusts bounding boxes accordingly in your custom detection dataset. Which approach is best?

hard

A. Apply the horizontal flip transform to both image and bounding boxes inside __getitem__

B. Flip bounding boxes but not the image

C. Apply the transform only after training the model

D. Apply the horizontal flip transform only to the image inside __getitem__, leave boxes unchanged

Solution

1. Step 1: Understand transform requirements

   When flipping images, bounding boxes must be adjusted to match the new image layout.

2. Step 2: Apply transform to both image and boxes

   Applying the transform only to the image or only to boxes causes mismatch and incorrect training data.

3. Final Answer:

   Apply the horizontal flip transform to both image and bounding boxes inside __getitem__ -> Option A

4. Quick Check:

   Flip image and boxes together for correct data [OK]

Hint: Transform image and boxes together to keep alignment [OK]

Common Mistakes:

• Flipping only image or only boxes
• Applying transforms after training
• Ignoring bounding box coordinate updates