Using custom colormaps in napari

Using custom colormaps in napari#

Overview#

Colormaps are an essential part of visualizing your image data, as they can affect how a user perceives your images (see the matplotlib docs for a primer on colormaps). In this notebook, we will define and use custom colormaps in napari. After performing this exercise, you should be able to define a custom colormap and apply it to both existing and new Image layers.

Loading the data#

We will start by loading an image of DAPI stained nuclei. We can use scikit-image’s imread() function to download the data from the link below and load it into a numpy array called nuclei.

from skimage.io import imread

nuclei_url = 'https://raw.githubusercontent.com/alisterburt/napari-workshops/main/napari-workshops/notebooks/data/nuclei.tif'
nuclei = imread(nuclei_url)

Viewing the image#

As we did in the previous notebooks, we can view the image in napari using the napari.view_image() function. Here we set the colormap to blue. We additionally set the contrast limits and select a z slice where the nuclei are in focus.

import napari

viewer = napari.view_image(
    nuclei,
    colormap='blue',
    contrast_limits=(0, 0.4)
);

# choose the z slice where nuclei are in focus
viewer.dims.current_step = (30, 0, 0)

from napari.utils import nbscreenshot

nbscreenshot(viewer)

Inspecting the colormap#

We can inspect the current colormap via the colormap property of the Image layer.

viewer.layers[0].colormap

Colormap(colors=array([[0., 0., 0., 1.],
       [0., 0., 1., 1.]], dtype=float32), name='blue', interpolation=<ColormapInterpolationMode.LINEAR: 'linear'>, controls=array([0., 1.], dtype=float32))

Create the lookup table#

To create the lookup table, we create an array of colors. The first element will map to pixels with the minimum value and the last value will map to the pixels with the maximum value. The colors are represented as RGBA (red, green, blue, alpha) values, where alpha is the opacity (i.e., 0 is completely transparent, 1 is completely opaque). In the example below, the colormap will go from white ([1, 1, 1, 1]) to blue ([0, 0, 1, 1]).

We use numpy linspace to interpolate colors between white and blue for demonstration purposes. However, as we will see below, if you just to linearly interpolate, you can just pass the end points ([1, 1, 1, 1] and [0, 0, 1, 1] in this case) and napari will do the interpolation for you.

import numpy as np

colors = np.linspace(
    start=[1, 1, 1, 1],
    stop=[0, 0, 1, 1],
    num=10,
    endpoint=True
)
print(colors)

[[1.         1.         1.         1.        ]
 [0.88888889 0.88888889 1.         1.        ]
 [0.77777778 0.77777778 1.         1.        ]
 [0.66666667 0.66666667 1.         1.        ]
 [0.55555556 0.55555556 1.         1.        ]
 [0.44444444 0.44444444 1.         1.        ]
 [0.33333333 0.33333333 1.         1.        ]
 [0.22222222 0.22222222 1.         1.        ]
 [0.11111111 0.11111111 1.         1.        ]
 [0.         0.         1.         1.        ]]

Add the new colormap to the layer#

We pass our new colormap to the layer as a dictionary. We use the following keys:

colors: this is the array of colors the comprise the colormap lookup table
name: this is the displayed name of the colormap
interpolation: this is the interpolation mode that is applied to the lookup table. Here we use 'linear' for linear interpolation.

new_colormap = {
    'colors': colors,
    'name': 'white_to_blue',
    'interpolation': 'linear'
}
viewer.layers[0].colormap = new_colormap

nbscreenshot(viewer)

Use a custom colormap when creating a layer#

We can also use a custom colormap when creating a new layer. We define the colormap as before. Note that since we are using linear interpolation, we can just define the end points of the look up table. In our white_to_green colormap, we just define the minimum value (white, [1, 1, 1, 1]) and the maximum value (green, [0, 1, 0, 1]). When constructing the new Image layer, we additionally set the contrast_limits and opacity to make the layer move visible.

# load the image data
membranes_url = 'https://raw.githubusercontent.com/alisterburt/napari-workshops/main/napari-workshops/notebooks/data/cell_membranes.tif'
membranes = imread(membranes_url)

white_green_cmap = {
    'colors': [[1, 1, 1, 1], [0, 1, 0, 1]],
    'name': 'white_to_green',
    'interpolation': 'linear'
}

viewer.add_image(
    membranes,
    colormap=white_green_cmap,
    contrast_limits=(0.02, 0.1),
    opacity=0.7
)

nbscreenshot(viewer)

Conclusions#

In this notebook, we have created and applied custom colormaps to Image layers. For additional information on colormaps in napari, please see the Image layer tutorial.