Annotation Tools: Merge, Subtract, Slice, Split, Unify

This guide explains how to efficiently manage and refine your image and video annotations using the Merge, Subtract, Split, Slice, and Unify features. These tools streamline your annotation workflow, especially when dealing with complex objects, overlapping polygons, or model-generated annotations that require manual correction.

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Overview & Key Use Cases

• Merge and Subtract: Combine or separate polygons in images to accurately represent objects. Use these to clean up complex or overlapping polygons, or when an object is split into multiple polygons but should be treated as one.
• Slice: Cut a polygon into separate annotations by drawing a cut line through or inside it. Use this to divide an object into parts, assign a different class to a specific region, or create a hole in a polygon.
• Split and Unify: Divide or combine annotations over time in videos. These are ideal for correcting or refining model-generated results, such as when an object's identity is swapped between instances over time (ID swap). Use Split to separate the annotation at the point of the swap, then Unify to merge the correct segments together, ensuring consistent labeling throughout the video.

Note: All actions described here are fully undo-able, so you can experiment and correct mistakes without worry.

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Hotkeys Summary

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Image Annotation Tools

Merge

Purpose: Combine overlapping or related polygons into a single object.

How to Use:

1. Select the first polygon with the Select tool.
2. Press Shift + M to activate Merge.
3. Click the second polygon to combine them.

Tip: Use this when an object is split into multiple polygons but should be treated as one.

Subtract

Purpose: Remove a polygon from another, creating a precise border between them.

How to Use (Images):

1. Select the polygon you want to remove.
2. Press Shift + S to activate Subtract.
3. Select the polygon you want to cut from.

How to Use (Videos):

1. Navigate to the frame where you want the subtraction to start.
2. Select the polygon you want to remove.
3. Press Shift + S to activate Subtract.
4. Choose how to apply it:
   1. Click the polygon you want to cut from to apply the subtraction to the current frame and all following frames.
   2. Option + click (Mac) or Alt + click (Windows) the polygon you want to cut from to apply the subtraction to the current frame only.

Tip: Useful for separating overlapping objects or cleaning up boundaries.

For more on using the brush tool for polygons, see this article.

Slice

Purpose: Cut a polygon into two or more separate annotations.

How to Use:

1. Select the polygon you want to slice.
2. Press Shift + K to activate Slice.
3. Draw the cut on the selected polygon. You can slice straight across it (for example, vertically) or draw a closed shape inside it.

Behavior:
Slicing across a polygon creates two separate annotations, one on each side of the cut.
Drawing a closed shape inside a polygon creates a new annotation from the inner area.
Each resulting annotation is independent, so you can change its class without affecting the others.

Tip: To create a hole in a polygon, slice out the area you want removed, then delete the new inner annotation. The original polygon will be left with a hole in the middle.

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Video Annotation Tools

Split Annotation

Purpose: Divide a single annotation across multiple frames into separate segments.

How to Use:

1. Select the annotation you wish to split.
2. Press Ctrl + S to activate Split.
3. The annotation will be split at the selected frame(s).

Behavior:

All properties and attributes are preserved in both resulting annotations.

The split annotations may appear on different lines in the timeline, which can cause a visual jump.

Unify Annotation

Purpose: Combine two annotations (even with overlapping frames) into one. For polygons, overlapping frames are supported.

How to Use:

1. Cmd + click to select both annotations (on canvas or timeline).
2. Right-click the timeline and select Unify, or press Ctrl + M.

Behavior:

For polygons, you can unify annotations even if their frames overlap.

The unified annotation will preserve the properties and attributes of the earlier annotation (frame-wise).

Note: Unify with overlapping frames is only supported for polygons.