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Computer Visionml~5 mins

Custom object detection dataset in Computer Vision

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Introduction

We create custom object detection datasets to teach computers how to find specific things in images. This helps computers recognize objects that matter to us.

You want a computer to find your own special objects, like your pet or your car.
You have images from your workplace and want to detect tools or products.
You want to build a smart camera that spots certain items in real time.
You need to train a model to detect objects that are not in public datasets.
You want to improve safety by detecting hazards in images from your environment.
Syntax
Computer Vision
Dataset structure:
- Images folder: contains all images (e.g., image1.jpg, image2.jpg)
- Annotations folder: contains label files (e.g., image1.txt, image2.txt)

Annotation format (YOLO style example):
<class_id> <x_center> <y_center> <width> <height>

All values are normalized between 0 and 1 relative to image size.

Annotations must match image filenames exactly except for extension.

Coordinates are usually normalized to keep dataset flexible for different image sizes.

Examples
This means image1.jpg has two objects: class 0 at center (50%,50%) with width 20% and height 30%, and class 1 at center (70%,80%) with width and height 10%.
Computer Vision
Annotations for image1.txt:
0 0.5 0.5 0.2 0.3
1 0.7 0.8 0.1 0.1
Images and labels are stored separately but linked by filename.
Computer Vision
Folder structure:
/images/image1.jpg
/images/image2.jpg
/labels/image1.txt
/labels/image2.txt
Sample Model

This code loads images and their labels from folders. It reads bounding boxes in normalized form and converts them to pixel values. Then it prints the objects found in each image.

Computer Vision
import os
from PIL import Image

def load_dataset(image_dir, label_dir):
    dataset = []
    for filename in os.listdir(image_dir):
        if filename.endswith('.jpg') or filename.endswith('.png'):
            image_path = os.path.join(image_dir, filename)
            label_path = os.path.join(label_dir, filename.rsplit('.',1)[0] + '.txt')
            if os.path.exists(label_path):
                with open(label_path, 'r') as f:
                    labels = [line.strip().split() for line in f.readlines()]
                image = Image.open(image_path)
                width, height = image.size
                objects = []
                for label in labels:
                    class_id, x_c, y_c, w, h = label
                    x_c, y_c, w, h = map(float, (x_c, y_c, w, h))
                    # Convert normalized to pixel coordinates
                    x_center = x_c * width
                    y_center = y_c * height
                    box_width = w * width
                    box_height = h * height
                    objects.append({
                        'class_id': int(class_id),
                        'bbox': [x_center, y_center, box_width, box_height]
                    })
                dataset.append({'image': filename, 'objects': objects})
    return dataset

# Example usage
image_dir = 'images'
label_dir = 'labels'
dataset = load_dataset(image_dir, label_dir)
for data in dataset:
    print(f"Image: {data['image']}")
    for obj in data['objects']:
        print(f" Class {obj['class_id']} at {obj['bbox']}")
OutputSuccess
Important Notes

Make sure your annotation format matches the model you plan to use.

Keep your dataset balanced with enough examples for each class.

Use tools like labelImg or makesense.ai to create annotations easily.

Summary

Custom datasets help computers learn to find your special objects.

Annotations link images to object locations using normalized coordinates.

Organize images and labels carefully for smooth training.

Practice

(1/5)
1. What is the main purpose of creating a custom object detection dataset?
easy
A. To compress images for faster loading
B. To improve image resolution automatically
C. To generate random images for testing
D. To teach a computer to find specific objects in images

Solution

  1. Step 1: Understand the goal of object detection datasets

    Custom object detection datasets are made to help computers learn to recognize and locate specific objects in images.
  2. Step 2: Identify the correct purpose

    Among the options, only teaching a computer to find specific objects matches the goal of such datasets.
  3. Final Answer:

    To teach a computer to find specific objects in images -> Option D
  4. Quick Check:

    Custom dataset purpose = teach detection [OK]
Hint: Custom datasets train models to detect your objects [OK]
Common Mistakes:
  • Confusing dataset purpose with image enhancement
  • Thinking dataset creates images instead of labeling
  • Assuming dataset compresses images
2. Which of the following is the correct format for an annotation line in a YOLO custom dataset label file?
easy
A. class_id x_center y_center width height (all normalized between 0 and 1)
B. image_width image_height class_id x y (pixel values)
C. class_name x_min y_min x_max y_max (absolute pixel coordinates)
D. class_id x_min y_min width height (normalized coordinates)

Solution

  1. Step 1: Recall YOLO annotation format

    YOLO uses normalized coordinates with class_id, x_center, y_center, width, and height all between 0 and 1.
  2. Step 2: Match options to YOLO format

    class_id x_center y_center width height (all normalized between 0 and 1) matches this format exactly; others use pixel values or different coordinate types.
  3. Final Answer:

    class_id x_center y_center width height (all normalized between 0 and 1) -> Option A
  4. Quick Check:

    YOLO labels use normalized center coords [OK]
Hint: YOLO labels use normalized center coordinates [OK]
Common Mistakes:
  • Using pixel coordinates instead of normalized
  • Confusing bounding box corners with center format
  • Mixing class name with class id
3. Given this label file line for an image of size 640x480:
2 0.5 0.5 0.25 0.25
What are the pixel coordinates of the bounding box corners (x_min, y_min, x_max, y_max)?
medium
A. (200, 150, 360, 270)
B. (160, 120, 320, 240)
C. (240, 180, 400, 300)
D. (320, 240, 480, 360)

Solution

  1. Step 1: Calculate center pixel coordinates

    x_center = 0.5 * 640 = 320, y_center = 0.5 * 480 = 240
  2. Step 2: Calculate width and height in pixels

    width = 0.25 * 640 = 160, height = 0.25 * 480 = 120
  3. Step 3: Calculate bounding box corners

    x_min = 320 - 160/2 = 240, y_min = 240 - 120/2 = 180
    x_max = 320 + 160/2 = 400, y_max = 240 + 120/2 = 300
  4. Final Answer:

    (240, 180, 400, 300) -> Option C
  5. Quick Check:

    Center ± half width/height = corners [OK]
Hint: Multiply normalized by image size, then subtract/add half box [OK]
Common Mistakes:
  • Using width/height as full coordinates instead of half
  • Mixing up x and y calculations
  • Forgetting to multiply by image dimensions
4. You have a dataset folder with images and label files. The labels are missing for some images. What is the best way to fix this before training?
medium
A. Remove images without label files from the dataset
B. Create empty label files for those images
C. Rename images to match existing label files
D. Ignore missing labels and train anyway

Solution

  1. Step 1: Understand dataset consistency needs

    Training requires each image to have a matching label file with annotations.
  2. Step 2: Choose the best fix

    Removing images without labels avoids errors and ensures clean training data. Creating empty labels may confuse the model, renaming won't help if labels don't exist, ignoring missing labels causes errors.
  3. Final Answer:

    Remove images without label files from the dataset -> Option A
  4. Quick Check:

    Dataset must have matching image-label pairs [OK]
Hint: Keep only images with labels to avoid training errors [OK]
Common Mistakes:
  • Creating empty label files that confuse training
  • Ignoring missing labels causing crashes
  • Renaming files without creating labels
5. You want to add a new object class to your existing custom dataset. Which steps should you follow to update your dataset correctly?
hard
A. Only add new images with the new class, no config changes needed
B. Add the new class to the class list, label new images with it, and update training config
C. Replace old class labels with the new class in all files
D. Add new class images but keep old labels unchanged without config update

Solution

  1. Step 1: Update class list

    Add the new object class to your dataset's class list file so the model knows about it.
  2. Step 2: Label new images with the new class

    Annotate images containing the new object using the new class id in label files.
  3. Step 3: Update training configuration

    Modify your training config to include the new class count and class names.
  4. Final Answer:

    Add the new class to the class list, label new images with it, and update training config -> Option B
  5. Quick Check:

    Update classes, labels, and config for new objects [OK]
Hint: Update classes, label new data, and config for new objects [OK]
Common Mistakes:
  • Not updating class list causing training errors
  • Forgetting to label new objects
  • Ignoring config updates leading to wrong class counts
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