Image segmentation

Updated 3 days ago by Alex Cota


In the Image segmentation tool, each annotated pixel in an image belongs to a single class. It is often used to label images for applications that require high accuracy. The output is a mask that outlines the shape of the object in the image.

Configuring image segmentation

During project setup, you can setup your ontology by adding all of the objects and classifications needed for your project.

Pen tool

The pen drawing tool is designed to be the fastest way to outline objects. It allows you to draw freehand as well as straight lines. You can also use the pen tool to erase, just click the (-) icon in the top bar. Tip: Hold Alt on your keyboard to temporarily switch to the erase mode while you draw.

Layer locking

With layer locking, you are able to continue annotating new objects without overwriting the objects you have already drawn. In the example above, the fish have already been annotated. When the Water class is chosen and the draw to back option is selected the sections of water are annotated behind the fish which essentially locks them from being modified. This can significantly speed up labeling since it is not required to intricately outline around the border of other objects.


  1. Select the object.
  2. Select the draw to back option.
  3. Begin drawing without fear of overwriting objects you have already annotated.

Creating instances

From the labeling interface, you can use the same class for more than one annotation. For example, if there are 5 fish in an image and you would like to assign the "Fish" class to all five, you can create multiple instances of the "Fish" class.

Follow these steps to create multiple instances of the same object:

  1. Select a class and draw the object.
  2. Select the same class again.
  3. Draw the next instance of the object.

Nested classifications

If you have configured the interface to have nested classifications for any of your objects, the labeler will be presented with classification questions after the annotation of the object.

Label format

When you export your labels as a JSON file, the file will be structured as follows. The instanceURI field is unique to the JSON files exported from the Image segmentation tool. The value for instanceURI is a URL to the image mask.

"Label": {
"objects": [{
"featureId": "cjxtm2d32i9aa07940tifrpuh",
"schemaId": "cjxtjkpjai8t80846iwqaa1d8",
"title": "Orange Fish",
"value": "orange_fish",
"color": "#3F51B5",
"instanceURI": "",
"classifications": [{
"featureId": "cjxtm2e8zheyt0863sds45xyx",
"schemaId": "cjxtjkphehbnj0848wknmg44u",
"title": "Is the fish blurry?",
"value": "is_the_fish_blurry?",
"answer": {
"featureId": "cjxtm2ebgi7tx0944gr8jx0lp",
"schemaId": "cjxtjkpfwh4y90721uodxps2b",
"title": "Yes",
"value": "yes"
}, {
"featureId": "cjxtm2kf0i9b207944z3gypsn",
"schemaId": "cjxtjkpjai8t80846iwqaa1d8",
"title": "Orange Fish",
"value": "orange_fish",
"color": "#3F51B5",
"instanceURI": ""
}, {
"featureId": "cjxtm2spbh8xx0721s18nwqq0",
"schemaId": "cjxtjkpjai8t90846yzpbpo7t",
"title": "Blue Fish",
"value": "blue_fish",
"color": "#F4511E",
"instanceURI": ""
}, {
"featureId": "cjxtm3i3ci9cx07940e5t7if8",
"schemaId": "cjxtk7z5qi5s90794v7wvseu6",
"title": "Water",
"value": "water",
"color": "#EF6C00",
"instanceURI": ""
"classifications": []

Convert mask to polygon coordinates

If you need to convert your vector masks to polygon coordinates, you can use this script here. You will need to pip install labelbox first.

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