Polyline annotation is an annotation type that marks a continuous line or path in an image by placing a series of connected points, without closing the shape. It is used for features that are lines rather than regions, like lane markings, roads, and wires.
A polyline is an open sequence of connected vertices, a line, not an enclosed area. Polyline annotation traces linear features by clicking points along them, and unlike a polygon, the path does not close back on itself.
It is the natural label for anything best described as a line: lane markings and road edges in autonomous driving, cracks in inspection imagery, wires and pipes, or skeletal connections. Each polyline carries a class and can have attributes, a lane's type or a wire's status, and curved features are approximated with more points.
Key takeaways
A polyline is an open connected line, not a closed shape like a polygon.
It labels linear features, lane lines, road edges, cracks, and wires, where area is not the point.
More points trace curves more faithfully, trading precision for labeling effort.
What polyline annotation provides
How polylines compare to polygons and boxes.
How polylines compare to polygons and boxes.
Term
Shape
Used for
Polyline
Open connected line
Lanes, roads, wires, paths
Polygon
Closed outline
Object regions and shapes
Bounding box
Rectangle
Object location and extent
How it works
Annotators click along the feature to drop vertices and adjust them to follow it, adding points on curves. In FiftyOne, polylines render as line overlays with their classes and attributes, so you can review lane and path annotations against the image and against model predictions.
Why it matters
Polylines capture the geometry that lane and path perception depends on, because a self-driving model needs the line of the lane, not a box around it. Polyline quality is dominated by endpoint and topology decisions, not by how neatly the middle is traced. Where a lane line starts and stops under occlusion, whether two markings connect at an intersection, these consistency calls determine whether the model learns coherent road structure, and they are the source of most disagreement and error, far more than vertex placement along the clear middle stretch.
Frequently asked questions
What is the difference between a polyline and a polygon?
A polyline is an open line. A polygon is a closed outline enclosing a region.
What is polyline annotation used for?
Linear features like lane markings, road edges, cracks, and wires.
How are curves handled in polylines?
With more vertices, because more points approximate a curve more closely.