Resize image with ImageJ python script

In between the downsizing and upsizing below I want to do other stuff, which I think requires conversion back to an ImagePlus object. Here’s what I have so far:

from ij import IJ

# Opening the image
img = IJ.openImage('path/to/image.tif')

# Downsize image
img = img.getProcessor().resize(200)

# convert img to ImagePlus??
# do other stuff (e.g. apply Weka classifier)

# Upsize image
img = img.getProcessor().resize(2048)

Can someone explain how ImagePlus and ImageProcessor objects interact. Do I need to convert the ImageProcessor object to an ImagePlus object? If so, how?

More generally, does anyone have any advice on translating the java docs for use with python? Everyone seems to refer to these docs but it’s not obvious to me how to use them for python.

Hello holastello -

Let me answer your last question first.

Unlike, for example, the ImageJ Macro (IJM) language, jython
(a flavor of python) doesn’t know anything specific about
ImageJ. Instead, jython is powerful because it knows about
java. Specifically, jython has hooks that let it access java
classes and call their methods. Since ImageJ is written
(nearly entirely) in java, you can use jython to script ImageJ
by instantiating ImageJ classes and manipulating them with
their methods.

So ImageJ’s java api documentation is pretty much precisely
the documentation you want for scripting ImageJ with jython.

(The division of labor is that you use jython syntax for the
“glue” – for loops, if / else, basic lists, and so on. And the
real image processing is done by the ImageJ java classes.)

First you need to poke around to get the lay of the land.
Let’s say that you’ve figured out that ImageJ has an IJ
class – a utility class with a bunch of static methods.

I’ll cheat by telling you that you can open a sample image
with the method:

from ij import IJ ('Blobs (25K)')

But the rest you can get pretty straightforwardly from the

Looking at the IJ.getImage() documentation, we see that
you can get your hands on the open image:

from ij import IJ ('Blobs (25K)')
imp = IJ.getImage()

imp is a jython variable that now refers to an instance of
the java class ImagePlus. This is essentially the same as
saying (in java):

ImagePlus imp = IJ.getImage();

The “glue” (syntax) is jython, but the relevant documentation
is the ImageJ java api documentation. ImagePlus has a
getProcessor() method that returns (a reference to) an
instance of ImageProcessor, and ImageProcessor has a
resize() method that “[r]eturns a new ImageProcessor containing
a scaled copy of this image …”

from ij import IJ ('Blobs (25K)')
imp = IJ.getImage()
ip = imp.getProcessor().resize (128, 127)

So, ip is not imp's ImageProcessor – it’s a new instance.
We can now assign the new ip to imp, modifying the image:

from ij import IJ ('Blobs (25K)')
imp = IJ.getImage()
ip = imp.getProcessor().resize (128, 127)
imp.setProcessor (ip)

In general, the ImageProcessor is the actual image – it
holds the pixels. (In fact, there are specific subclasses,
ByteProcessor, ShortProcessor, etc., that hold the pixel
values stored as java arrays of java primitive types.)

ImagePlus is a wrapper for ImageProcessor that knows
things like how to display an image, and has various kinds
of metadata like the image’s title. (It can also wrap fancier
things like image stacks.) More concretely, ImagePlus has
an ImageProcessor data member.

Not surprisingly, you can construct an ImagePlus from an
ImageProcessor (ImagePlus (String title, ImageProcessor ip)).

Let’s upsize your downsized image and display it:

from ij import IJ ('Blobs (25K)')
imp = IJ.getImage()
ip = imp.getProcessor().resize (128, 127)
imp.setProcessor (ip)
ip2 = ip.resize (512, 508)
from ij import ImagePlus
imp2 = ImagePlus ('upsized image', ip2)

So you don’t convert between ImageProcessors and
ImagePluss, per se – the former is wrapped by the latter.

Depending on what you are doing, to modify an image you
can construct a new ImagePlus that wraps a new or modified
ImageProcessor, you can assign a new or modified
ImageProcessor to an existing ImagePlus, or you can
modify the pixel values of an ImageProcessor in place,
without instantiating a new ImageProcessor or

Thanks, mm

1 Like

For general advice on how to use Jython scripting in Fiji I would advise the associated wiki page and other ressources mentioned in it :wink:

Thank you – this was hugely helpful!