What is video annotation?

Video annotation is the process of labeling or tagging video data to make it understandable for machine learning models. It involves adding metadata—such as bounding boxes, keypoints, or text labels—to objects, actions, or regions within video frames. This labeled data is then used to train models for tasks like object detection, activity recognition, or motion tracking. Unlike static image annotation, video annotation requires handling temporal continuity, where objects or events may change position, shape, or context over time. For example, annotating a video of a self-driving car’s perspective might involve marking pedestrians, vehicles, and traffic signs across consecutive frames to teach the model how these elements behave in real-world scenarios.

A common use case for video annotation is in training models for autonomous systems. For instance, annotators might label every frame of a dashcam video to identify lanes, obstacles, and traffic lights, enabling a model to learn spatial and temporal relationships. Another example is sports analytics, where annotating player movements and ball trajectories in a soccer match helps models predict strategies or evaluate performance. Techniques like object tracking (following a specific item across frames) or temporal segmentation (marking the start and end of an action, like a tennis serve) are often used. Tools like CVAT, LabelBox, or custom scripts with OpenCV and FFmpeg are typically employed to streamline annotation, often combining manual input with automated interpolation to reduce repetitive work.

Developers implementing video annotation should consider factors like scalability and consistency. Processing hours of video requires efficient storage and retrieval systems, often leveraging cloud services or distributed computing. Consistency across frames is critical—for example, ensuring a car labeled in frame 100 isn’t misidentified in frame 101 due to occlusion or lighting changes. Semi-automated approaches, such as using pre-trained models to suggest annotations (e.g., detecting all faces in a frame) before human review, can save time. Additionally, data formats like JSON or XML for storing annotations must align with model training pipelines. Balancing annotation detail (e.g., pixel-level masks vs. bounding boxes) with computational cost is also key, as overly granular labels may not always improve model performance proportionally to the effort required.