pyramid_sampler

A small utility for taking a 3D zarr image at a single resolution and downsampling to create an image pyramid.

installation

python -m pip install pyramid-sampler

usage

create a test base image

import zarr
from pyramid_sampler import initialize_test_image

# create an on-disk zarr store
zarr_file = "test_image.zarr"
zarr_store = zarr.group(zarr_file)

# write base level 0 image to the specified store and field name
new_field = "field1"
base_res = (1024, 1024, 1024)
chunks = (64, 64, 64)
initialize_test_image(zarr_store, new_field, base_res, chunks=chunks)

initialize_test_image will utilize a dask.delayed workflow, so you can configure a dask client prior to calling initilize_test_iamge if you wish. The resulting base image will reside in zarr_store[new_field][0].

downsampling an image

First initialize a Downsampler instance with the path to the zarr store, the refinement factor to use between image levels, the base image resolution and chunksizes:

from pyramid_sampler import Downsampler

zarr_file = "test_image.zarr"
refinement_factor = (2, 2, 2)
base_res = (1024, 1024, 1024)
chunks = (64, 64, 64)
ds = Downsampler(zarr_file, (2, 2, 2), base_res, chunks)

For now, this assumes your base image will reside in zarr_file/field_name/0. To run the downsampling,

field_to_downsample = "field1"
max_levels = 10
ds.downsample(max_levels, zarr_file, field_to_downsample)

Downsampling will only proceed until a layer is created with a single chunk of size set by the Downsampler and image chunksize, i.e., until base_resolution / refinement_factor**current_level / chunks has a value of 1 in any dimension, or max_levels = log(base_resolution/chunks) / log(refinement_factor) (giving a max level id of max_levels - 1 to account for 0-indexing).

assumptions

Some assumptions in the current algorithm:

• exact chunks only! The base image resoultion and chunks must perfectly subdivide and downsampling must result in an even number of chunks.
• the base field exists and is stored at zarr_store[field][0]
• Only tested with on-disk filestores, but should work for any zarr store.

method

at present, the downsampling simply averages overlapping array elements: for a given image level, L1, the pixels of the higher resoluiton image level L1 - 1 covered by each pixel in L1 are found and averaged. Levels are built up sequentially (i.e., L1 is built from L1 - 1, not the base resolution).

Calculations and chunk-processing are accelerated with dask delayed objects using numba.jit compilation for the pixel-averaging.

developing & contributing

At present, this package is a small utility used for experimentations with zarr files. But contributions are welcome! Open up an issue at https://github.com/data-exp-lab/pyramid_sampler/issues to discuss ideas.