Labelbox documentation

Segmentation mask

When you label an image with the Segmentation tool, each annotated pixel in the mask annotation belongs to a single class. It is often used to label images for applications that require high accuracy.

Pen tool

Use the Pen tool to outline items in the image. Hold the cursor down to draw freehand or let the cursor go to draw straight lines between points. Select the (-) icon to draw around an area to erase. The Pen tool is only available when creating Segmentation annotations.


For Segmentation annotations with complex boundaries, using the Superpixel tool first may be more efficient than using the Pen tool alone. Superpixel works by calculating segment clusters of similarly colored pixels in the image. The Superpixel tool is only available when you are creating Segmentation annotations.


In the top toolbar is a slider, which allows you to adjust the size of segment clusters from XS to XL. After you have selected a segment cluster size, choose an object from the left Tools panel and use your cursor to select and classify each segment to be included in that Segmentation annotation. You can then adjust the boundaries using the Pen and Eraser tools.

Calculation time for Superpixel segments increases with image size. We advise using images that are no more than 4000 by 4000 pixels.

Draw over existing objects

"Draw over existing objects" is on by default. When this tool is enabled, drawing a new Segmentation annotation over an existing annotation will overwrite previously classified pixels. When this tool is disabled, a new Segmentation annotation drawn over an existing annotation will be "drawn behind" the existing annotation.


This tool is designed to significantly speed up labeling time since it is not required to intricately outline around the border of other objects. This tool is only available when creating Segmentation annotations.