napari image arithmetic widget

napari is a fast, interactive, multi-dimensional image viewer for python. It uses Qt for the GUI, so it’s easy to extend napari with small, composable widgets created with magicgui. Here we’re going to build this simple image arithmetic widget with a few additional lines of code.

For napari-specific magicgui documentation, see the napari docs



This example demonstrates how to:

⬇️ Create a magicgui widget that can be used in another program (napari)

⬇️ Use an Enum to create a dropdown menu

⬇️ Connect some event listeners to create interactivity.


Code follows, with explanation below… You can also get this example at github.

 1from enum import Enum
 3import numpy
 4import napari
 5from napari.types import ImageData
 7from magicgui import magicgui
 9class Operation(Enum):
10    """A set of valid arithmetic operations for image_arithmetic.
12    To create nice dropdown menus with magicgui, it's best
13    (but not required) to use Enums.  Here we make an Enum
14    class for all of the image math operations we want to
15    allow.
16    """
17    add = numpy.add
18    subtract = numpy.subtract
19    multiply = numpy.multiply
20    divide = numpy.divide
23# here's the magicgui!  We also use the additional
24# `call_button` option
26def image_arithmetic(
27    layerA: ImageData, operation: Operation, layerB: ImageData
28) -> ImageData:
29    """Add, subtracts, multiplies, or divides to image layers."""
30    return operation.value(layerA, layerB)
32# create a viewer and add a couple image layers
33viewer = napari.Viewer()
34viewer.add_image(numpy.random.rand(20, 20), name="Layer 1")
35viewer.add_image(numpy.random.rand(20, 20), name="Layer 2")
37# add our new magicgui widget to the viewer
40# keep the dropdown menus in the gui in sync with the layer model


We’re going to go a little out of order so that the other code makes more sense. Let’s start with the actual function we’d like to write to do some image arithmetic.

the function

Our function takes two numpy arrays (in this case, from Image layers), and some mathematical operation (we’ll restrict the options using an Enum). When called, our function calls the selected operation on the data.

def image_arithmetic(array1, operation, array2):
    return operation.value(array1, array2)

type annotations

magicgui works particularly well with type annotations, and allows third-party libraries to register widgets and behavior for handling their custom types (using magicgui.type_map.register_type()). napari provides support for magicgui by registering a dropdown menu whenever a function parameter is annotated as one of the basic napari Layer types, or, in this case, ImageData indicates we just the data attribute of the layer. Furthermore, it recognizes when a function has a Layer or LayerData return type annotation, and will add the result to the viewer. So we gain a lot by annotating the above function with the appropriate napari types.

from napari.types import ImageData

def image_arithmetic(
    layerA: ImageData, operation: Operation, layerB: ImageData
) -> ImageData:
    return operation.value(layerA, layerB)

the magic part

Finally, we decorate the function with @magicgui and tell it we’d like to have a call_button that we can click to execute the function.

def image_arithmetic(layerA: ImageData, operation: Operation, layerB: ImageData):
    return operation.value(layerA, layerB)

That’s it! The image_arithmetic function is now a FunctionGui that can be shown, or incorporated into other GUIs (such as the napari GUI shown in this example)


While type hints aren’t always required in magicgui, they are recommended (see type inference )… and they are required for certain things, like the Operation(Enum) used here for the dropdown and the napari.types.ImageData annotations that napari has registered with magicgui.

create dropdowns with Enums

We’d like the user to be able to select the operation (add, subtract, multiply, divide) using a dropdown menu. Enums offer a convenient way to restrict values to a strict set of options, while providing name: value pairs for each of the options. Here, the value for each choice is the actual function we would like to have called when that option is selected.

class Operation(enum.Enum):
    add = numpy.add
    subtract = numpy.subtract
    multiply = numpy.multiply
    divide = numpy.divide

add it to napari

When we decorated the image_arithmetic function above, it became a FunctionGui. Napari recognizes this type, so we can simply add it to the napari viewer as follows:



This api has changed slightly with version 0.2.0 of magicgui. See the migration guide if you are migrating from a previous version.

connect event listeners for interactivity

What fun is a GUI without some interactivity? Let’s make stuff happen.

We connect the image_arithmetic.reset_choices function to the event from napari, to make sure that the dropdown menus stay in sync if a layer gets added or removed from the napari window:


An additional offering from magicgui here is that the decorated function also acquires a new attribute “called” that can be connected to callback functions of your choice. Then, whenever the gui widget or the original function are called, the result will be passed to your callback function:

def print_mean(value):
    """Callback function that accepts an event"""
    # the value attribute has the result of calling the function

>>> image_arithmetic()