Image Manipulations


Starfish offers several options for cropping image data, both out-of-memory on load and in-memory. These can be useful for (1) restricting the size of a large image to load on demand and (2) removing edge effects after image processing approaches have been applied.

Crop on Load

The first opportunity to subset data is during loading. Data is not loaded until get_image() is called on a FieldOfView. Here we demonstrate how to reduce the (y, x) size of the ImageStack that is downloaded. Note that all of the data for the complete tiles must still be downloaded, but only the cropped dimensions will be loaded into memory.

For this example we’ll use a very small test image from an in-situ sequencing experiment.

import starfish
from starfish.core.imagestack.parser.crop import CropParameters

experiment: starfish.Experiment =
field_of_view: starfish.FieldOfView = experiment["fov_001"]


  Primary Image: <slicedimage.TileSet (z: 1, r: 4, c: 4, x: 200, y: 140)>
  Auxiliary Images:
    nuclei: <slicedimage.TileSet (z: 1, r: 1, c: 1, x: 200, y: 140)>
    dots: <slicedimage.TileSet (z: 1, r: 1, c: 1, x: 200, y: 140)>

printing the FieldOfView shows that the test dataset is (140, 200)` pixels in (y, x). We’ll load just the first (100, 80) pixels to demonstrate starfish’s crop-on-load functionality.

y_slice = slice(0, 100)
x_slice = slice(0, 80)
image: starfish.ImageStack = field_of_view.get_image("primary", x=x_slice, y=y_slice)



<starfish.ImageStack (r: 4, c: 4, z: 1, y: 100, x: 80)>

Note the reduced size of the image.

Selecting Images

Once an image has been loaded into memory as an ImageStack object, it is also possible to crop the ImageStack or select a subset of the images associated with the rounds, channels, and z-planes of the experiment.

Here, we demonstrate selecting the last 50 pixels of (x, y) for rounds 2 and 3 using the ImageStack.sel() method.

from starfish.types import Axes

cropped_image: starfish.ImageStack = image.sel(
    {Axes.ROUND: (2, 3), Axes.X: (30, 80), Axes.Y: (50, 100)}


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<starfish.ImageStack (r: 2, c: 4, z: 1, y: 50, x: 50)>


In addition to selecting specific tiles, starfish can also project along an axis of the ImageStack, thereby reducing the size of the dimension to one. For example, projecting along the z-axis using the max function will reduce a 3D image volume into a 2D image where the intensity value at each (y, x) pixel is equal to the maximum of the voxel intensities at (y, x) of the original image volume. This is a common approach for data that has relatively few spots. So long as the projection is unlikely to produce overlapping spots, projecting the data in this way can dramatically reduce processing time, as 2D algorithms are typically much faster than their 3D counterparts.

Because the example image that we’ve downloaded has only one Axes.ZPLANE, we will instead demonstrate the use of reduce() by projecting over Axes.CH to produce an image of all the spots that appear in any channel for each round.

from starfish.image import Filter

projected_image: starfish.ImageStack = image.reduce({Axes.CH}, func="max")

To demonstrate the effect, the below figure displays each channel of round 1 in the left and center columns, and the maximum projection on the right.

import matplotlib.pyplot as plt
import xarray as xr

# select an image for plotting in 2d
round_1_ch_0: xr.DataArray = image.sel({Axes.CH: 0, Axes.ROUND: 1}).xarray.squeeze()
round_1_ch_1: xr.DataArray = image.sel({Axes.CH: 1, Axes.ROUND: 1}).xarray.squeeze()
round_1_ch_2: xr.DataArray = image.sel({Axes.CH: 2, Axes.ROUND: 1}).xarray.squeeze()
round_1_ch_3: xr.DataArray = image.sel({Axes.CH: 3, Axes.ROUND: 1}).xarray.squeeze()
round_1_proj: xr.DataArray = projected_image.sel({Axes.ROUND: 1}).xarray.squeeze()

# plot the images
f, ((ax1, ax2, ax3), (ax4, ax5, ax6)) = plt.subplots(nrows=2, ncols=3)
ax1.set_title("round 1\nchannel 0")
ax2.set_title("round 1\nchannel 1")
ax4.set_title("round 1\nchannel 2")
ax5.set_title("round 1\nchannel 3")

ax3.set_title("round 1\nmaximum projection")

# we're not using the 6th plot

# fix matplotlib whitespace


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