Before starting this lesson, you should be familiar with:

Learning Objectives

After completing this lesson, learners should be able to:
  • Understand the OME-Zarr image file format

  • Render cloud (S3 object store) hosted OME-Zarr image data

  • Access the pixel values of cloud hosted OME-Zarr image data

  • Apply basic image processing on cloud hosted OME-Zarr image data


Storing TB-sized image data locally and in multiple copies is either not possible or inefficient. Cloud storage enables efficient concurrent access to the same image data by multiple clients (scientists). OME-Zarr is the emerging community standard image file format for cloud (S3 object store) compatible image data storage. Thus it is important to know how to access S3 hosted OME-Zarr in various image analysis and visualisation platforms.

Concept map

graph TD S3("S3 object store") ---|publicly hosts| C("Multi-scale image data chunks") C -->|read into| IV("Image viewer")



OME-Zarr inspection

Inspect the OME-Zarr layout and metadata.

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Check out what we have at our s3 bucket:

mc tree -d 2 s3/ome-zarr-course/
mc ls s3/ome-zarr-course/data/MFF/
mc ls s3/ome-zarr-course/data/JPEG/
mc ls s3/ome-zarr-course/data/ZARR/common/

Check out the multiscales metadata for one of the OME-Zarr datasets we created:

mc cat s3/ome-zarr-course/data/ZARR/common/13457537T.zarr/.zattrs

Check out the array metadata for the highest resolution array:

mc cat s3/ome-zarr-course/data/ZARR/common/13457537T.zarr/0/.zarray

Configure mc for anonymous access to public s3 buckets:

mc alias set s3pub

Have a look at the metadata for a big OME-Zarr data:

mc cat s3pub/i2k-2020/platy-raw.ome.zarr/.zattrs
mc cat s3pub/i2k-2020/platy-raw.ome.zarr/s0/.zarray


Use the ome_zarr tool for the inspection:

ome_zarr info
ome_zarr info

OME-Zarr creation

Create OME-Zarr data from data stored in monolithic file formats.

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First check out what data we have the s3 end:

mc tree -d 2 s3/ome-zarr-course/

There are multiple conversion modes. Let’s try each of them.

Perform parallelised, independent conversion:

batchconvert omezarr -st s3 -dt s3 --drop_series data/MFF data/ZARR/$USER;

This command maps each input file in the data/MFF folder to a single OME-Zarr series, which is then transferred to a user-specific folder. Note that the -st s3 option makes sure that the input path is searched for in the s3 bucket, while -dt s3 triggers the output files to be transferred to the s3 bucket under the output path.

Perform grouped conversion:

batchconvert omezarr -st s3 -dt s3 --drop_series --merge_files --concatenation_order t data/JPEG data/ZARR/$USER;

This conversion mode assumes that the input files are part of the same series and thus will merge them along a specific axis during the conversion process. The --merge_files flag specifies the grouped conversion option and the --concatenation_order t option allows the files to be merged along the time channel.

Check what has changed at the s3 end after the conversion:

mc tree -d 2 s3/ome-zarr-course/
mc ls s3/ome-zarr-course/data/ZARR/$USER/

Optional: Copy the converted Zarr data to the home folder:

mc mirror s3/ome-zarr-course/data/ZARR/$USER ~/data/ZARR;

OME-Zarr visualisation

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Visualise the remote data using Napari together with the napari-ome-zarr plugin.

napari --plugin napari-ome-zarr$USER/xyzct_8bit__mitosis.ome.zarr
napari --plugin napari-ome-zarr$USER/xyz_8bit_calibrated__fib_sem_crop.ome.zarr

Optional: visualise local OME-Zarr data in the same way:

napari --plugin napari-ome-zarr ~/data/ZARR/xyzct_8bit__mitosis.ome.zarr

Optional: visualise big remote OME-Zarr data:

napari --plugin napari-ome-zarr

Note that compared to BigDataViewer, there are more delays with Napari.


  • Run Fiji with MoBIE
  • Open OME-Zarr from S3 in BigDataViewer:
    • [ Plugins › BigDataViewer › OME ZARR › Open OME ZARR From S3... ]
      • S3 URL: Choose one of the above example URLs
      • Log chunk loading
  • Examine the IJ Log window to see:
    • image scale levels
    • transformations from data space to global space
  • One OME-Zarr URL may contain multiple images, you can toggle them on and off:
    • BDV: Press P and use the active checkbox
  • Examine multi-scale chunk loading:
    • [ Fiji > Window > Console ]
    • Move around to load different chunks, e.g.:
      • BDV: Mouse scroll to move along the current viewing axis
      • BDV: Shift x to view along the x-axis.
    • Zoom in to load higher resolution data:
      • BDV: Arrow up


  • Open an image:
    • Go to
    • On to top right in Source, enter, e.g., zarr:// (replace the part behind zarr:// with your URL.
    • Press Enter (multiple times).
  • Navigate around in the sample
    • Zooming seems to require pinching on a trackpad
  • Sharing views:
    • The URL in your browser adapts to your current view
    • Copy and paste the URL to share a view with a collaborator
    • For example, try this one


  • Open Google Chrome on BAND (for some reason vizarr does not work with Firefox on BAND). Google Chrome can be found under the Applications menu at the top left corner of the screen:
    [Applications > internet > Google Chrome]

  • To visualise a self-created OME-Zarr via vizarr, replace the $USER in the following link with your user name, copy-paste the link into the Google Chrome’s search bar and press enter:$USER/xyzct_8bit__mitosis.ome.zarr
    • Note: you can find your user name by entering echo $USER in the BAND terminal.
  • Optional: visualise the following in the same way:
    • 3D EM data:$USER/xyz_8bit_calibrated__fib_sem_crop.ome.zarr
    • A well from an HCS plate:

OME-Zarr segmentation

Segment remotely located OME-Zarr data without explicitly downloading it.

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Examine the dataset that is to be segmented:

mc tree -d 2 s3/ome-zarr-course/data/ZARR/$USER/23052022_D3_0002_positiveCTRL.ome.zarr
mc cat s3/ome-zarr-course/data/ZARR/$USER/23052022_D3_0002_positiveCTRL.ome.zarr/0/.zarray

Also view the data

napari --plugin napari-ome-zarr$USER/23052022_D3_0002_positiveCTRL.ome.zarr;

Perform threshold segmentation on each channel

zseg threshold -r -m otsu -c 1 -ch 0 -n otsu-c1-ch0 --colormap viridis ome-zarr-course/data/ZARR/$USER/23052022_D3_0002_positiveCTRL.ome.zarr;

In this command, the -r flag ensures that the input path is searched at the s3 bucket. The -m option specifies the thresholding algorithm, which in this case is the Otsu algorithm. The -c is a coefficient that is multiplied with the found threshold value to get the effective threshold. The -ch species the channel 0 for segmentation. The -n option specifies the name of the label path created.

Now also segment the other channel:

zseg threshold -r -m otsu -c 1 -ch 1 -n otsu-c1-ch1 --colormap viridis ome-zarr-course/data/ZARR/$USER/23052022_D3_0002_positiveCTRL.ome.zarr;

Note that the -ch argument has been changed.

Have a look at the segmented data

napari --plugin napari-ome-zarr$USER/23052022_D3_0002_positiveCTRL.ome.zarr;

Apply mathematical morphology to the label image

zseg postprocess -r -m binary_opening -f 1,1 -l otsu-c1-ch1 --colormap viridis ome-zarr-course/data/ZARR/$USER/23052022_D3_0002_positiveCTRL.ome.zarr;

Here the -m specifies the postprocessing method; the -f determines the footprint shape. Depending on the shape of the input data, it can be 2 or 3-dimensional. The -l can be used to decide on the name of the label image, that is subjected to the postprocessing.

Now examine the OME-Zarr data:

mc tree -d 2 s3/ome-zarr-course/data/ZARR/$USER/23052022_D3_0002_positiveCTRL.ome.zarr
ome_zarr info$USER/23052022_D3_0002_positiveCTRL.ome.zarr

Also visualise the data:

napari --plugin napari-ome-zarr$USER/23052022_D3_0002_positiveCTRL.ome.zarr;



Follow-up material

Recommended follow-up modules:

Learn more: