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WEBKNOSSOS works great with OME-Zarr datasets, sometimes called next-generation file format (NGFF).

We strongly believe in this community-driven, cloud-native data format for n-dimensional datasets. Zarr is a first-class citizen in WEBKNOSSOS and will likely replace WKW long term.

Zarr datasets can both be uploaded to WEBKNOSSOS through the web uploader or streamed from a remote server or the cloud. When streaming and using several layers, import the first Zarr group and then use the UI to add more URIs/groups.


You can try the OME-Zarr support with the following datasets. Load them in WEBKNOSSOS as a remote dataset:

  • Mouse Cortex Layer 4 EM Cutout over HTTPs
    • Source: Dense connectomic reconstruction in layer 4 of the somatosensory cortex. Motta et al. Science 2019. 10.1126/science.aay3134

Zarr Folder Structure

WEBKNOSSOS expects the following file structure for OME-Zarr (v0.4) datasets:

.                             # Root folder,
│                             # with a flat list of images by image ID.
└── 456.zarr                  # Another image (id=456) converted to Zarr.
    ├── .zgroup               # Each image is a Zarr group, or a folder, of other groups and arrays.
    ├── .zattrs               # Group level attributes are stored in the .zattrs file and include
    │                         # "multiscales" and "omero" (see below). In addition, the group level attributes
    │                         # may also contain "_ARRAY_DIMENSIONS" for compatibility with xarray if this group directly contains multi-scale arrays.
    ├── 0                     # Each multiscale level is stored as a separate Zarr array,
    │   ...                   # which is a folder containing chunk files which compose the array.
    ├── n                     # The name of the array is arbitrary with the ordering defined by
    │   │                     # by the "multiscales" metadata, but is often a sequence starting at 0.
    │   │
    │   ├── .zarray           # All image arrays must be up to 5-dimensional
    │   │                     # with the axis of type time before type channel, before spatial axes.
    │   │
    │   └─ t                  # Chunks are stored with the nested directory layout.
    │      └─ c               # All but the last chunk element are stored as directories.
    │         └─ z            # The terminal chunk is a file. Together the directory and file names
    │            └─ y         # provide the "chunk coordinate" (t, c, z, y, x), where the maximum coordinate
    │               └─ x      # will be dimension_size / chunk_size.
    └── labels
        ├── .zgroup           # The labels group is a container which holds a list of labels to make the objects easily discoverable
        ├── .zattrs           # All labels will be listed in .zattrs e.g. { "labels": [ "original/0" ] }
        │                     # Each dimension of the label (t, c, z, y, x) should be either the same as the
        │                     # corresponding dimension of the image, or 1 if that dimension of the label
        │                     # is irrelevant.
        └── original          # Intermediate folders are permitted but not necessary and currently contain no extra metadata.
            └── 0             # Multiscale, labeled image. The name is unimportant but is registered in the "labels" group above.
                ├── .zgroup   # Zarr Group which is both a multiscaled image as well as a labeled image.
                ├── .zattrs   # Metadata of the related image and as well as display information under the "image-label" key.
                ├── 0         # Each multiscale level is stored as a separate Zarr array, as above, but only integer values
                │   ...       # are supported.
                └── n

See OME-Zarr 0.4 spec for details.

Conversion to Zarr

You can easily convert image stacks manually with the WEBKNOSSOS CLI. The CLI tool expects all image files in a single folder with numbered file names. After installing, you can convert image stacks to Zarr datasets with the following command:

pip install webknossos

webknossos convert \
  --voxel-size 11.24,11.24,25 \
  --name my_dataset \
  --data-format zarr \
  data/source data/target

This snippet converts an image stack that is located in directory called data/source into a Zarr dataset which will be located at data/target. It will create a so called color layer containing your raw greyscale/color image. The supplied --voxel-size is specified in nanometers.

Read the full documentation at WEBKNOSSOS CLI.

Conversion with Python

You can use the free WEBKNOSSOS Python library to convert image stacks to Zarr or integrate the conversion as part of an existing workflow.

import webknossos as wk

def main() -> None:
    """Convert a folder of image files to a WEBKNOSSOS dataset."""
    dataset = wk.Dataset.from_images(
        voxel_size=(11, 11, 11),

    print(f"Saved {} at {dataset.path}.")

    with wk.webknossos_context(token="..."):

if __name__ == "__main__":

Read the full example in the WEBKNOSSOS Python library documentation.

Time-Series and N-Dimensional Datasets

WEBKNOSSOS also supports loading n-dimensional datasets, e.g. 4D = time series of 3D microscopy. This feature in currently only supported for Zarr dataset due to their flexbile structure and design for n-dimensional data.

Performance Considerations

To get the best streaming performance for Zarr datasets consider the following settings.

  • Use chunk sizes of 32 - 128 voxels^3
  • Enable sharding (only available in Zarr 3+)