OME-Zarr tools for the Vesuvius scrolls

What is OME-Zarr?

Zarr is a file format that facilitates efficient access to huge data sets, by sub-dividing the data into what in zarr are called "chunks".

The zarr format also specifies how to store hierarchical sets ("groups") of zarr data volumes, and it supports metadata files.

OME-Zarr is a layer on top of zarr that allows multi-resolution data volumes in zarr format to be stored and accessed in a standardized way. OME-Zarr does this by specifying a convention for structuring the zarr groups and metadata that hold these data volumes.

The zarr-python library provides an API that allows the programmer to easily retrieve zarr and OME-Zarr data, as if it were stored in a numpy array, thus hiding the complexity of the actual storage mechanism.

OME-Zarr and Neuroglancer

Neuroglancer is a program, developed at Google (though not an official Google product), that allows the efficient exploration of data volumes stored in various formats, including the OME-Zarr format. By retrieving only the necessary data, at the necessary resolution, Neuroglancer lets the user move easily and efficiently through enormous data volumes.

OME-Zarr and khartes

Brett Olsen (active on the scrollprize.org Discord server) has recently added code that allows khartes to access TIFF files as if they were zarr data stores. This includes the 500x500x500 grid files, created by @spelufo (same Discord server) that are available in each scroll's volume_grid directory on the scrollprize.org data server. As a result, khartes users can now navigate through an entire scroll volume, instead of having to create a limited-size khartes data volume in advance. However, the data in this case is not multi-resolution, so navigating the data is not as efficient as with Neuroglancer.

A next step will be to allow khartes to access zarr data stores directly, and eventually to take advantage of OME-Zarr multi-resolution data stores.

What scripts are in this repository?

In order for khartes to eventually work with multi-resolution scroll data in OME-Zarr format, there needs to be a way to convert the existing scroll data to this format. So the main script in this repository is scroll_to_ome, which performs this conversion.

Another script, ppm_to_layers, reads a .ppm file and zarr-format scroll data, and uses these to create a flattened layer volume that is as similar as possible to the one created by vc_layers_from_ppm. This was written to test how well the zarr format works in practice, and to learn how best to take advantage of the zarr format's capabilities.

The rest of this README

Here is what you will find in the rest of this README file.

• A guide to installing the scripts.
• Detailed instructions on how to use scroll_to_ome to create an OME-Zarr data store from a scroll's TIFF files.
• A further discussion of OME-Zarr.
• Not-very-detailed instructions on how to run ppm_to_layers
• A beginner's guide to setting up Neuroglancer to view the OME-Zarr files created by scroll_to_ome
• A discussion of compressed TIFF files, including those in the masked_volumes directory

Installation

Only a few packages need to be installed in order to run scroll_to_ome:

• zarr
• tifffile
• scikit-image

When these packages are installed, other required packages, such as numpy, will automatically be brought in, so they are not explicitly listed here. The file anaconda_installation.yml lists the conda commands that will import these packages, if you are using anaconda.

To install with conda (anaconda), execute these commands, replacing yourEnvName with a name for the conda environment you like conda env create -n yourEnvName -f environment.yml conda activate yourEnvName

You can also use pip to install the requirements with pip install -r requirements.txt ideally also in a conda environment you have already created

User guide for scroll_to_ome

scroll_to_ome converts a scroll (more precisely, the z-slice TIFF files that store the scroll data) into a multi-resolution OME-Zarr data store. scroll_to_ome takes a number of arguments, but only two are mandatory: the name of the directory that contains the scroll TIFF files, and the name of the directory where the OME-Zarr multi-resolution data will be stored.

For example:

python scroll_to_ome.py /mnt/vesuvius/Scroll1.volpkg/volumes/20230205180739 /mnt/vesuvius/scroll1.zarr

There are a number of options that modify the default behavior, but before I list them, you need to have a little more background information on zarr and OME-Zarr.

In the zarr data format, the data volume is decomposed into 'chunks', which are 3D rectangles of data. Zarr provides many ways to store these chunks, but for the case we are interested in, each chunk is stored in a separate file; these files thus provide rapid access to data anywhere in the volume. All chunks are the same size. A typical chunk size is 128x128x128 voxels, but chunks can be non-cubical; 137x67x12, for instance, is a valid (though not necessarily recommended) chunk size.

OME-Zarr provides a way to store multi-resolution data: that is, multiple copies of the same data volume, sampled at different resolutions. scroll_to_ome by default creates volumes at 6 different resolutions (including the original full-resolution volume); each volume has half the resolution of the previous one. Thus, with the default settings, the 6th volume has 1/32 the resolution of the original. That represents a reduction of 32x32x32 = 32,768 in the size of the data volume, reducing a 1 Tb data volume to 33 Mb.

One interesting feature of zarr is that if a chunk contains only zeros, the file containing that chunk is not created at all. This is not of much significance in the case of the original scroll files, which have almost no zeros, but @james darby (on the Discord server) has created a "masked" version of Scroll 1, where all the pixels outside of the actual scroll are set to zero. This saves a lot of space in the OME-Zarr data store.

So now you know a little bit more about how zarr and OME-Zarr works, enough to be able to run scroll_to_ome.

If you type python scroll_to_ome.py --help you will see all of the options that are available. The options default to reasonable values, so if you leave them all alone, you will get a good result.

But in case you like tweaking things, what follows is a description of each option.

• chunk_size: The size of the chunks that will be created. scroll_to_ome only creates cubical chunks, so if you choose, say, a chunk-size of 128, then the chunks will be 128x128x128 in size. All resolutions in the multi-resolution data set will be created with the same chunk size.

  In theory, smaller chunks should load more quickly, because there is less data to transfer, but there may be a fixed overhead cost per chunk as well. I've chosen 128 as the default, but 64 and 256 are also worth considering.

• nlevels: "Level" refers to how many levels of resolution will be created in the multi-resolution OME-Zarr data store. The default, 6, means that in addition to the full-resolution zarr store, 5 more will be created at successively lower resolutions.

  scroll_to_ome always reduces the resolution by a factor of 2 from one level to the next, along all 3 axes, so each level is approximately 8 times smaller in data volume than the previous level (approximately, because the chunks may not line up exactly with the data volume boundaries).

• max_gb: By default, scroll_to_ome uses as much memory as it needs to run efficiently; this is approximately chunk size times TIFF size. So for instance, if each input TIFF file is 185 Mb, and the chunk size is 128, the program will need about 24 Gb of RAM to run efficiently.

  If your computer is not able to provide as much memory as needed, or if page faults slow your computer down, you can specify the maximum memory size (in Gb) that should be used. Setting this means that the TIFF files will need to be read repeatedly in order to conserve memory, which will slow down the conversion process significantly.

• zarr_only: This is a flag indicating that only a single full-resolution zarr data store should be created, rather than an OME-Zarr multi-resolution set of zarr stores.

• overwrite: By default, scroll_to_ome will not overwrite an existing zarr data store. Setting this flag allows overwriting; use it with care!

• algorithm: There exist a number of algoithms for down-sampling the data from one resolution level to the next. By default, scroll_to_ome takes the mean of the 8 higher-resolution voxels that will be combined into 1 lower-resolution voxel.

  A second option is 'nearest', which simply selects the value of one of the high-resolution voxels and assigns it to the lower-resolution one. This tends to create rather jittery looking images.

  The third option is 'gaussian', which uses a Gaussian weighing function to smooth the data before sub-sampling. This seems to create an overly smooth sub-sampled image, but perhaps this would be improved by choosing a different sigma factor. In any case, the calculations significantly slow down the conversion process.

• ranges: This option allows you to limit the xyz range of the TIFF data that will be converted. One reason you might want to do this is to test your parameters before committing yourself to a long conversion run. Ranges are given for the three axes, with commas separating the three ranges.

  For example, 1000:2000,1500:4000,2000:7000

  The order is x,y,z where the x and y axes correspond to the x and y axes in a single TIFF file, and z corresponds to the number of the TIFF file. The format describing the range along each axis is similar to the python "slice" format: min:max, where the extracted data covers the range min, min+1, min+2, ..., max-3, max-2, max-1.

  As with the slice format, you can omit part of the range, so that :5 is the same as 0:5, 5: is the same as 5 to the end, and : is the same as the entire range. This last example is useful when you only want to limit the ranges on some of the axes, for instance :,:,:1000 will take the full xy extent of the first 1000 TIFF files.

  The python slice format permits a third parameter, the step size, but slice-to-ome only allows a step size of 1.

• first_new_level: This option allows you to go back to an existing OME-Zarr data store and add new levels of resolution. To see which resolution levels already exist, cd into the data store and look at the existing directories there. You should see one directory per level, starting with 0, which is the full-resolution zarr data store, then 1, 2, etc. As the name of the option suggests, pass it the number of the first level that is not already there. You may also need to set nlevels to a different value.

OME-Zarr conversion: Time and space

In general, the multi-resolution data store created by scroll_to_ome takes up about 20% more space than the original TIFF files. That is, if the scroll TIFF files occupy 1 Tb, then the OME-Zarr data store will occupy about 1.2 Tb.

The reason: The full-resolution zarr data store will take up about as much space as the TIFF files, since it is essentially a rearrangement of the existing scroll data (but there is an exception discussed below!). Then each successive zarr data store in the multi-resolution series takes up about 1/8 of the space of the previous one, since the resolution reduction is a factor of 2 along each axis. So the total storage space required, compared to just the space required by the full-resolution data store, is 1 + 1/8 + 1/64 + ..., which is somewhat less than 1.2.

It is important to know that in some cases, the zarr data store can take up much less space than the original TIFF files! This is thanks to one of the data compression methods used by zarr.

Zarr offers a number of data compression methods; most of them do not perform well on scroll data. That is why scroll_to_ome does not compress the data that is stored in the zarr data volumes. However, one method that works well, on certain data sets, is that the zarr library can be set to not create chunks that consist entirely of zeros. That is, when writing chunks, if the zarr library detects that the chunk consists of all zeros, then that chunk is simply not written to disk. Later, when reading the data store, if the zarr library detects that that chunk is missing, it assumes that the chunk was all zeros.

The original scroll data does not contain many zeros, so this simple compression scheme might seem irrelevant. However, @james darby has created modified TIFFs for some scrolls, available in the scroll's masked_volumes directory, where pixels outside of the scroll itself have been set to zero. These modified TIFFs are smaller than the original TIFFs, since a compression scheme has been used to hide the zeros. So a zarr data store created from the masked_volumes TIFFs will not be any smaller than the masked_volumes directory itself. However, it will be significantly smaller than a zarr data store created from the original non-masked TIFFs, with no loss of information inside the scroll itself.

(The very last section of this README file contains some further information on dealing with compressed TIFF files, and masked_volumes files in particular).

Another note: the max_gb flag may be useful on machines with limited RAM, but you should test on a limited number of slices before processing the entire scroll volume. Depending on the speed of your swap disk versus your data disk, it may be faster to use swap space than to use the max_gb flag to limit memory usage.

As for time requirements, I only have one data point. Using a chunk size of 128, on a laptop with 32 Gb RAM (so the max_gb flag did not need to be used), with the TIFFs and the OME data store on an external USB 3 SSD, the conversion took less than 24 hours, but more than 12.

OME-Zarr naming conventions

The zarr format allows multiple data volumes to be nested hierarchically in a single zarr directory. In fact, as mentioned before, this is how the OME-Zarr format works: it is simply a convention for how the multi-resolution data volumes should be arranged in a hierarchy within a single zarr directory.

This means that multi-resolution OME-Zarr data stores, as well as single high-resolution zarr data stores, are both located in directories that by convention end with a .zarr suffix. One way to see whether a .zarr directory contains a single data volume, or a set of multi-resolution data volumes, is to go into the directory.

If inside the directory you see a file named .zarray, then that directory contains a single data volume.

If you see files named .zattrs and .zgroup, then the directory probably contains OME-Zarr data.

If you have an OME-Zarr directory, but all you want is the high-resolution zarr data volume, go into the OME-Zarr directory and look for the sub-directory named 0. Although the name of the 0 directory does not end in the conventional .zarr, it is in fact a zarr data store, containing the high-resolution zarr data.

User guide for ppm_to_layers

Like vc_layers_from_ppm, ppm_to_layers takes a .ppm file (generated by vc_render), and the scroll data, and from these creates a flattened layer volume (a directory full of TIFFs, one per z value in the flattened volume). The main difference between the two programs is that vc_layers_from_ppm uses the scroll TIFF files, whereas ppm_to_layers uses the full-resolution zarr data.

ppm_to_layers requires the user to provide the name of a .ppm file, and the name of a zarr data store containing a single data volume. Note that giving the name of an OME-Zarr directory won't work. However, as the "OME-Zarr naming convention" section above indicates, you can simply append a /0 (or \0 on Windows) to the name of the OME-Zarr directory, and then ppm_to_layers should work.

Sometimes, in addition to the flattened data volume, you might want to create a single TIFF file representing a stack of the flattened data, which is generated by finding the maximum value at each uv pixel over a range of layers. This is equivalent to the single TIFF file created by vc_render. To create such a file, simply put the desired name of this file on the command line, following the names of the .ppm file, the zarr directory, and the output TIFF directory.

If you type python ppm_to_layers.py --help you will see all of the options that are available. The options default to reasonable values, so if you leave them all alone, you will get a good result.

• number_of_layers: This is the number of flattened layers (TIFF files) that will be created. 65 is the default used by vc_layers_from_ppm and vc_render. The two VC programs provide additional flexibility in terms of layer spacing, and whether the layers will be symmetrical or assymetrical around the center, but these options currently do not exist in ppm_to_layers.

• number_of_stacked_layers: This is the number of layers that will be stacked together to form a single stacked TIFF file. The stacked TIFF file is similar to the one created by vc_render. I have not yet figured out the exact number that vc_render uses, but the resulting TIFF file is fairly similar. vc_render provides a number of options on how to stack the layers together (find the maximum, take the average, etc.), but ppm_to_layers provides only one option: take the maximum value at each uv location, which is the default used by vc_render.

• block_size is an advanced option. The algorithm used by ppm_to_layers breaks the PPM data into smaller blocks, to conserve memory. This number specifies the size of each block.

• zarr_cache_max_size_gb is an advanced option. The zarr data, as it is read, is stored in a cache. Ideally, the cache should be large enough so that there are few cache misses, but not so large that it wastes memory. In limited experiments, the default size (8 Gb) has met these criteria.

Viewing OME-Zarr data with Neuroglancer: A beginner's guide

Neuroglancer (https://github.com/google/neuroglancer) is a browser-based application for viewing volumetric data. Although it is hosted under Google's github account, it is not an official Google product.

Neuroglancer is able to browse OME-Zarr data stores, taking advantage of the multi-resolution data contain there.

In this section, I will show you how to set up Neuroglancer to browse an OME-Zarr data store on your computer.

The main thing to understand is that as a web-based application, Neuroglancer must be run via a web server. Neuroglancer also expects the OME-Zarr data to be served by a web server. Fortunately, these two servers are pretty easy to set up.

1. Build and start Neuroglancer.

   1. Download or clone Neuroglancer from github: https://github.com/google/neuroglancer
   2. cd into the neuroglancer directory.
   3. Follow the instructions in the Building section of the Neuroglancer README file. If your experience is like mine, after you type npm i, you will at one point see some dire security warnings flash by. To me this suggests that you might not want to expose your local Neuroglancer server to the public internet.
   4. Start the local server (npm run dev-server), checking first to make sure that you are still in the neuroglancer directory.
   5. You should now be able to use a browser on your local machine to access Neuroglancer on http://localhost:8080

2. Start a data server.

   1. In a different terminal window, cd to the directory that you want to serve data from. This might be the directory that contains your OME-Zarr .zarr directory.
   2. Use python to run the script called cors_webserver.py, which is in the neuroglancer directory. This is to avoid problems with CORS (https://en.wikipedia.org/wiki/Cross-origin_resource_sharing), which you would probably be happier not knowing anything about.
   3. By default, the data server is now making data available on http://localhost:9000 .

3. Go to the Neuroglancer window that is in your browser.

   1. From the list on the right, select zarr2:// (Zarr v2 data source). This will put zarr2:// into the Source line on the top right.
   2. From the new list on the right, select http:// The Source line will now show zarr2://http://
   3. Now you will type directly into the source line. Just after the current contents, type localhost:9000/ (don't forget the trailing slash!) so the source line should show: zarr2://http://localhost:9000/
   4. At this point you should see a list of the contents of the directory where your data server is running, and in the window where you started the data server, you should see signs that the server is indeed serving data.
   5. Select your OME-Zarr directory.
   6. The list may show the contents of this directory (0/, 1/, etc), but don't click on any of these! Instead, while your cursor is still in the Source window, hit Enter.
   7. If all goes well, Neuroglancer should now show you some information about your data volume. Almost there!
   8. At the bottom of the information, there should be a yellow bar that reads: Create as image layer. Click this.
   9. Your data should now begin showing up in the Neuroglancer window. Happy exploring!
   10. If the data did not show up, go to the window where you started your data server, and look for clues...

So that is how to use Neuroglancer to view your data.

Noate that if you are running on Windows, you might want to monitor whether your anti-virus real-time check is slowing the connection between your data server and the browser.

One thing to keep in mind, if your data volumes are located on a password-protected web server, is that Neuroglancer is not set up to work with http password protection, so you cannot use it to directly view password-protected data. In this case, you will need to run, on your local machine, a proxy server which downloads data from the password-protected server as needed.
Then point Neuroglancer to this local proxy server. And if you figure out how to make a proxy server, please submit a Pull Request!

Compressed TIFFs and the masked_volumes directory

As mentioned earlier, the OME-Zarr data stores will take up much less space if they are made from "masked" TIFF files, where the pixels outside of the scroll itself have been set to zero. For some scrolls, such TIFF files have been been created, and have been put in a directory called masked_volumes, which is parallel to the volumes directory where the original TIFFs are stored.

There is a pitfall that you should be aware of, however, when using the masked TIFFs in masked_volumes: the effect of compression on read speed.

Because these TIFFs are compressed, they are slower to read. The slowdown depends on the read speed of the disk, and on the speed of the CPU, but on my computer, with an external SSD, reading a compressed file is 5 times slower than reading an uncompressed file.

This means that if you are thinking of reading the compressed masked TIFF files more than once (for instance, if you will be running scroll_to_ome with several different chunk sizes), and if you have sufficient storage space, you should consider decompressing the compressed masked_volumes TIFF files. (Important note: the TIFF files in volumes are not compressed, so this discussion does not apply to them).

A script decompress_tiffs.py is provided for this purpose.