Skip to content

Volume annotation

Volume Annotations & Proof-Reading

In addition to skeleton annotations, WEBKNOSSOS also supports volume/segmentation annotations. With this type of annotation, you can label groups of voxels with efficient drawing tools.


Select one of the drawing tools from the toolbar or toggle through with the keyboard shortcut W.

  • Move: Navigate around the dataset.
  • Trace: Draw outlines around the voxel you would like to label.
  • Brush: Draw over the voxels you would like to label. Adjust the brush size with SHIFT + Mousewheel.
  • Erase (Trace/Brush): Draw over the voxels you would like to erase. Adjust the brush size with SHIFT + Mousewheel.
  • Fill Tool: Flood-fills the clicked region with a volume annotation until it hits the next segment boundary (or the outer edge of your viewport). All adjacent voxels with the same voxel id as the clicked voxel will be changed to the active segment ID. Useful to either fill a hole in a segment or to relabel a segment with a different ID/color.
  • Segment Picker: Click on any segment to select its label ID as the active segment ID and continue any volume annotation operation with that ID.
  • Quick Select: Draw a rectangle over a segment to annotate it automatically. The tool can operate in two different modes. If the "AI" button in the toolbar is activated, a machine-learning model is used to infer the selection. If the AI button is disabled, the tool operates on the intensity data of the visible color layer and automatically fills out the segment starting from the center of the rectangle. Next to the tool, there is a settings button which allows to enable a preview mode and to tweak some other parameters. If the preview is enabled, the parameters can be fine-tuned while the preview updates instantly.

When using the trace or brush tool, a label can be added with Left Mouse Drag. Erasing is possible with the dedicated erase tools or with CTRL + Shift + Left Mouse Drag.

If you have enabled Classic Controls in the settings sidebar, erasing is also possible with Right Mouse Drag within the brush and trace tool (otherwise, right-clicking is mapped to open a context menu).

The following interactions and modifiers are available when working with the volume annotation tools:

  • Create New Segment ID: Creates a new segment ID for labeling. Note the little color indicator in the top right corner of the button visualizing the current color of the active segment ID. Read the explanation for the largest segment id here to understand how new IDs are generated.
  • Change Brush Size: Changes the size of the brushing tool.
  • Overwrite Everything: When labeling with the brush/trace tool, you can annotate every available voxel without any regard if it was already labeled as a different segment or whether it is unlabelled. This allows you to draw over existing segments.
  • Only Overwrite Empty Areas: In contrast to the Overwrite Everything modifier, the forces the brush & trace tools to only label voxels without any segment ID ("empty areas"). This is useful when annotating segments that directly touch each other to avoid accidental overwrites.
  • 2D Fill/ 3D Fill: Modifies the flood filling tool to work in 2D (in-plane only) or 3D (volumetric fill/re-labeling). 3D flood fill is constrained to a small, regional bounding box for performance reasons. Read more about flood fills below.

Adding labels with the Trace tool Adding labels with the Brush tool Removing labels with the Brush tool

In the Segmentation tab on the right-hand side panel, you can see the segment IDs which are available in your annotation. You can rename segments as needed.

The active segment ID under the cursor can be found in the status bar at the bottom of the screen or through the context-sensitive menu on right-click.

Segments List

The right-hand side panel offers a Segments tab that lists segments and allows to edit these. A segment is added to the list as soon as it was clicked in the data viewport. The following functionality is available for each segment:

  • jumping to the segment (via left-click; this uses the position at which the segment was initially registered)
  • naming the segment
  • loading 3D meshes for the segments (ad-hoc and precomputed if available)
  • download of 3D meshes
  • changing the color of the segment
  • activating the segment id (so that you can annotate with that id)

Segments Tab

Merging volume annotation with fallback data

After finishing the annotation of a volume layer with a fallback layer, the combined state of these layers can be materialized into a new dataset. For this, go to the layer settings in the left border tab. On the top right of the volume layer is the following button:

Icon to open the materialize volume annotation modal

This button opens up a modal that starts a long-running job which will materialize the volume annotation.

Proof-Reading and Merging Segments

See the section on proofreading.

Volume Flood Fills

WEBKNOSSOS supports volumetric flood fills (3D) to relabel a segment with a new ID. Instead of having the relabel segment slice-by-slice, WEBKNOSSOS can do this for you. This operation allows you to fix both split and merge errors:

  • For split errors: Combine two segments by relabeling one segment with the ID of the other. Since this operation is fairly compute-intensive you might be better of with the Merger Mode, explained above.
  • For merge errors: You have to manually split two segments at their intersection/border, e.g. a cell boundary. Use the eraser brush and make sure to establish a clear cut between both segments on a slice-by-slice basis. Both segments must not touch any longer. Create a new segment ID from the toolbar and apply it to one of the partial segments that you just divided.

Due to performance reasons, 3D flood-fills only work in a small, local bounding box. WEBKNOSSOS will add a bounding box around the affected area. To truly propagate the new segment ID(s) throughout a whole dataset, you can trigger a WEBKNOSSOS job to apply this change globally. From the BBox tab in the right-hand menu, press the "Globalize Flood-Fill" button. Make sure to do all local fill operations first and apply them all at once.

Check the Processing Jobs page from the Admin menu at the top of the screen to track progress or cancel the operation. The finished, processed dataset will appear as a new dataset in your dashboard.

Volume Interpolation

When using the brush or trace tool, you can use the Volume Interpolation feature for faster annotation speed (in a task context, this feature has to be enabled explicitly). Simply label a segment in one slice (e.g., z=10), move forward by a few slices (e.g., z=14) and label the segment there. Now, you can click the "Interpolate" button (or use the shortcut V) to interpolate the segment between the annotated slices (e.g., z=11, z=12, z=13).

Note that it is recommended to proofread the interpolated slices afterward since the interpolation is a heuristic.

Volume Extrusion

Similar to the above interpolation feature, you can also extrude the currently active segment. This means, that you can label a segment on one slice (e.g., z=10), move a few slices forward (e.g., z=12) and copy the segment to the relevant slices (e.g., z=11, z=12). The extrusion can be triggered by using the extrude button in the toolbar (also available as a dropdown next to the interpolation/extrusion button).

Mappings / On-Demand Agglomeration

With WEBKNOSSOS it is possible to apply a precomputed agglomeration file to re-map/combine over-segmented volume annotations on-demand. Instead of having to materialize one or more agglomeration results as separate segmentation layers, ID mappings allow researchers to apply and compare different agglomeration strategies of their data for experimentation.

This feature works well with automated machine learning segmentation workflows. We typically produce several agglomeration results based on different prediction and size thresholds leading to several possible segmentations based on one initial over-segmentation. We load these ID maps into WEBKNOSSOS to quickly review these results in an interactive session.

Mapping files are automatically identified by WEBKNOSSOS when being placed in a mappings folder within the segmentation folder. All available mappings can be activated from a dropdown under each Segmentation layer. Due to their file size, mappings are fetched on demand before being applied. Users can easily switch between several mappings and WEBKNOSSOS will update accordingly.

Mapping files are stored as JSON or HDF5 files. Read the section on data formats for more information on the file formats.

Download File Format

Volume annotations can be downloaded and imported using ZIP files that contain WKW datasets. The ZIP archive contains one NML file that holds meta information including the dataset name and the user's position. Additionally, there is another embedded ZIP file that contains the volume annotations in WKW file format.


In contrast to on-disk WKW datasets, the WKW files in downloaded volume annotations only contain a single 32^3 bucket in each file. Therefore, also the addressing of the WKW files (e.g. z48/y5444/x5748.wkw) is in steps of 32 instead of 1024. # A ZIP file containing the volume annotation
├─ # Container for WKW dataset
│ └─ 1 # Magnification step folder
│   ├─ z48
│   │ ├─ y5444
│   │ │ └─ x5748.wkw # Actual WKW bucket file (32^3 voxel)
│   │ └─ y5445/...
│   ├─ z49/...
│   └─ header.wkw # Information about the WKW files
└─ volumetracing.nml # Annotation metadata NML file

After unzipping the archives, the WKW files can be read or modified with the WKW libraries that are available for Python, MATLAB, and other languages.

Hybrid Annotations

A hybrid annotation contains both skeleton and volume annotations. This is the default behavior when creating a new WEBKNOSSOS annotation.

With hybrid annotations, you can use an existing skeleton as a guide to support volume annotation tasks. Alternatively, comments on skeleton nodes can be used to label/mark specific cells and positions during a volume annotation.

WEBKNOSSOS also supports pure skeleton or pure volume annotations for dedicated tasks/projects or backward compatibility. Those can be converted to a hybrid annotation, by clicking the Convert to Hybrid button in the info tab. This conversion cannot be reversed.