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Data Collection and Annotation Strategies for Computer Vision


The key to success in any computer vision project starts with effective data collection and annotation strategies. The quality of the data directly impacts model performance, so it's important to understand the best practices related to data collection and data annotation.

Every consideration regarding the data should closely align with your project's goals. Changes in your annotation strategies could shift the project's focus or effectiveness and vice versa. With this in mind, let's take a closer look at the best ways to approach data collection and annotation.

Setting Up Classes and Collecting Data

Collecting images and video for a computer vision project involves defining the number of classes, sourcing data, and considering ethical implications. Before you start gathering your data, you need to be clear about:

Choosing the Right Classes for Your Project

One of the first questions when starting a computer vision project is how many classes to include. You need to determine the class membership, which is involves the different categories or labels that you want your model to recognize and differentiate. The number of classes should be determined by the specific goals of your project.

For example, if you want to monitor traffic, your classes might include "car," "truck," "bus," "motorcycle," and "bicycle." On the other hand, for tracking items in a store, your classes could be "fruits," "vegetables," "beverages," and "snacks." Defining classes based on your project goals helps keep your dataset relevant and focused.

When you define your classes, another important distinction to make is whether to choose coarse or fine class counts. 'Count' refers to the number of distinct classes you are interested in. This decision influences the granularity of your data and the complexity of your model. Here are the considerations for each approach:

  • Coarse Class-Count: These are broader, more inclusive categories, such as "vehicle" and "non-vehicle." They simplify annotation and require fewer computational resources but provide less detailed information, potentially limiting the model's effectiveness in complex scenarios.
  • Fine Class-Count: More categories with finer distinctions, such as "sedan," "SUV," "pickup truck," and "motorcycle." They capture more detailed information, improving model accuracy and performance. However, they are more time-consuming and labor-intensive to annotate and require more computational resources.

Something to note is that starting with more specific classes can be very helpful, especially in complex projects where details are important. More specific classes lets you collect more detailed data, and gain deeper insights and clearer distinctions between categories. Not only does it improve the accuracy of the model, but it also makes it easier to adjust the model later if needed, saving both time and resources.

Sources of Data

You can use public datasets or gather your own custom data. Public datasets like those on Kaggle and Google Dataset Search Engine offer well-annotated, standardized data, making them great starting points for training and validating models.

Custom data collection, on the other hand, allows you to customize your dataset to your specific needs. You might capture images and videos with cameras or drones, scrape the web for images, or use existing internal data from your organization. Custom data gives you more control over its quality and relevance. Combining both public and custom data sources helps create a diverse and comprehensive dataset.

Avoiding Bias in Data Collection

Bias occurs when certain groups or scenarios are underrepresented or overrepresented in your dataset. It leads to a model that performs well on some data but poorly on others. It's crucial to avoid bias so that your computer vision model can perform well in a variety of scenarios.

Here is how you can avoid bias while collecting data:

  • Diverse Sources: Collect data from many sources to capture different perspectives and scenarios.
  • Balanced Representation: Include balanced representation from all relevant groups. For example, consider different ages, genders, and ethnicities.
  • Continuous Monitoring: Regularly review and update your dataset to identify and address any emerging biases.
  • Bias Mitigation Techniques: Use methods like oversampling underrepresented classes, data augmentation, and fairness-aware algorithms.

Following these practices helps create a more robust and fair model that can generalize well in real-world applications.

What is Data Annotation?

Data annotation is the process of labeling data to make it usable for training machine learning models. In computer vision, this means labeling images or videos with the information that a model needs to learn from. Without properly annotated data, models cannot accurately learn the relationships between inputs and outputs.

Types of Data Annotation

Depending on the specific requirements of a computer vision task, there are different types of data annotation. Here are some examples:

  • Bounding Boxes: Rectangular boxes drawn around objects in an image, used primarily for object detection tasks. These boxes are defined by their top-left and bottom-right coordinates.
  • Polygons: Detailed outlines for objects, allowing for more precise annotation than bounding boxes. Polygons are used in tasks like instance segmentation, where the shape of the object is important.
  • Masks: Binary masks where each pixel is either part of an object or the background. Masks are used in semantic segmentation tasks to provide pixel-level detail.
  • Keypoints: Specific points marked within an image to identify locations of interest. Keypoints are used in tasks like pose estimation and facial landmark detection.

Types of Data Annotation

Common Annotation Formats

After selecting a type of annotation, it's important to choose the appropriate format for storing and sharing annotations.

Commonly used formats include COCO, which supports various annotation types like object detection, keypoint detection, stuff segmentation, panoptic segmentation, and image captioning, stored in JSON. Pascal VOC uses XML files and is popular for object detection tasks. YOLO, on the other hand, creates a .txt file for each image, containing annotations like object class, coordinates, height, and width, making it suitable for object detection.

Techniques of Annotation

Now, assuming you've chosen a type of annotation and format, it's time to establish clear and objective labeling rules. These rules are like a roadmap for consistency and accuracy throughout the annotation process. Key aspects of these rules include:

  • Clarity and Detail: Make sure your instructions are clear. Use examples and illustrations to understand what's expected.
  • Consistency: Keep your annotations uniform. Set standard criteria for annotating different types of data, so all annotations follow the same rules.
  • Reducing Bias: Stay neutral. Train yourself to be objective and minimize personal biases to ensure fair annotations.
  • Efficiency: Work smarter, not harder. Use tools and workflows that automate repetitive tasks, making the annotation process faster and more efficient.

Regularly reviewing and updating your labeling rules will help keep your annotations accurate, consistent, and aligned with your project goals.

Let's say you are ready to annotate now. There are several open-source tools available to help streamline the data annotation process. Here are some useful open annotation tools:

  • Label Studio: A flexible tool that supports a wide range of annotation tasks and includes features for managing projects and quality control.
  • CVAT: A powerful tool that supports various annotation formats and customizable workflows, making it suitable for complex projects.
  • Labelme: A simple and easy-to-use tool that allows for quick annotation of images with polygons, making it ideal for straightforward tasks.

LabelMe Overview

These open-source tools are budget-friendly and provide a range of features to meet different annotation needs.

Some More Things to Consider Before Annotating Data

Before you dive into annotating your data, there are a few more things to keep in mind. You should be aware of accuracy, precision, outliers, and quality control to avoid labeling your data in a counterproductive manner.

Understanding Accuracy and Precision

It's important to understand the difference between accuracy and precision and how it relates to annotation. Accuracy refers to how close the annotated data is to the true values. It helps us measure how closely the labels reflect real-world scenarios. Precision indicates the consistency of annotations. It checks if you are giving the same label to the same object or feature throughout the dataset. High accuracy and precision lead to better-trained models by reducing noise and improving the model's ability to generalize from the training data.

Example of Precision

Identifying Outliers

Outliers are data points that deviate quite a bit from other observations in the dataset. With respect to annotations, an outlier could be an incorrectly labeled image or an annotation that doesn't fit with the rest of the dataset. Outliers are concerning because they can distort the model's learning process, leading to inaccurate predictions and poor generalization.

You can use various methods to detect and correct outliers:

  • Statistical Techniques: To detect outliers in numerical features like pixel values, bounding box coordinates, or object sizes, you can use methods such as box plots, histograms, or z-scores.
  • Visual Techniques: To spot anomalies in categorical features like object classes, colors, or shapes, use visual methods like plotting images, labels, or heat maps.
  • Algorithmic Methods: Use tools like clustering (e.g., K-means clustering, DBSCAN) and anomaly detection algorithms to identify outliers based on data distribution patterns.

Quality Control of Annotated Data

Just like other technical projects, quality control is a must for annotated data. It is a good practice to regularly check annotations to make sure they are accurate and consistent. This can be done in a few different ways:

  • Reviewing samples of annotated data
  • Using automated tools to spot common errors
  • Having another person double-check the annotations

If you are working with multiple people, consistency between different annotators is important. Good inter-annotator agreement means that the guidelines are clear and everyone is following them the same way. It keeps everyone on the same page and the annotations consistent.

While reviewing, if you find errors, correct them and update the guidelines to avoid future mistakes. Provide feedback to annotators and offer regular training to help reduce errors. Having a strong process for handling errors keeps your dataset accurate and reliable.

Domande frequenti

Here are some questions that might encounter while collecting and annotating data:

  • Q1: What is active learning in the context of data annotation?

    • A1: Active learning in data annotation is a technique where a machine learning model iteratively selects the most informative data points for labeling. This improves the model's performance with fewer labeled examples. By focusing on the most valuable data, active learning accelerates the training process and improves the model's ability to generalize from limited data.

Overview of Active Learning

  • Q2: How does automated annotation work?

    • A2: Automated annotation uses pre-trained models and algorithms to label data without needing human effort. These models, which have been trained on large datasets, can identify patterns and features in new data. Techniques like transfer learning adjust these models for specific tasks, and active learning helps by selecting the most useful data points for labeling. However, this approach is only possible in certain cases where the model has been trained on sufficiently similar data and tasks.
  • Q3: How many images do I need to collect for YOLOv8 custom training?

    • A3: For transfer learning and object detection, a good general rule of thumb is to have a minimum of a few hundred annotated objects per class. However, when training a model to detect just one class, it is advisable to start with at least 100 annotated images and train for around 100 epochs. For complex tasks, you may need thousands of images per class to achieve reliable model performance.

Share Your Thoughts with the Community

Bouncing your ideas and queries off other computer vision enthusiasts can help accelerate your projects. Here are some great ways to learn, troubleshoot, and network:

Where to Find Help and Support

  • GitHub Issues: Visit the YOLOv8 GitHub repository and use the Issues tab to raise questions, report bugs, and suggest features. The community and maintainers are there to help with any issues you face.
  • Ultralytics Discord Server: Join the Ultralytics Discord server to connect with other users and developers, get support, share knowledge, and brainstorm ideas.

Documentazione ufficiale

  • Ultralytics YOLOv8 Documentation: Refer to the official YOLOv8 documentation for thorough guides and valuable insights on numerous computer vision tasks and projects.


By following the best practices for collecting and annotating data, avoiding bias, and using the right tools and techniques, you can significantly improve your model's performance. Engaging with the community and using available resources will keep you informed and help you troubleshoot issues effectively. Remember, quality data is the foundation of a successful project, and the right strategies will help you build robust and reliable models.

Created 2024-05-31, Updated 2024-06-10
Authors: glenn-jocher (2), abirami-vina (1)