Reverse Python Colormaps? Learn This One Trick! (Visual)

Reverse Python Colormaps: A Simple Visual Guide to Reversing & Truncating

This guide will walk you through the process of reversing and truncating Python colormaps using matplotlib, a popular data visualization library. We will focus on using the core matplotlib functionalities without relying on external dependencies beyond numpy. The primary goal is to provide a visual understanding of the process, enhancing comprehension.

Understanding Python Colormaps

A colormap is a mapping from data values to colors. In matplotlib, colormaps are used to visualize data sets where color represents the magnitude of the data points. For example, in a heatmap, higher values might be represented by warmer colors (like red), while lower values are represented by cooler colors (like blue).

Default Colormaps in Matplotlib

Matplotlib comes with a wide variety of built-in colormaps, such as ‘viridis’, ‘magma’, ‘coolwarm’, and ‘jet’. These colormaps are designed to be perceptually uniform, which means that equal changes in data values should correspond to equal changes in perceived color intensity. This helps prevent misinterpretation of the data.

Accessing Colormaps

You can access these colormaps using the matplotlib.cm module. Here’s how you can load a colormap:

import matplotlib.pyplot as plt
import matplotlib.cm as cm
import numpy as np

# Load the 'viridis' colormap
viridis = cm.get_cmap('viridis', 256) # 256 is the number of discrete colors

Reversing Colormaps

Sometimes, you might want to reverse a colormap. This might be necessary to match conventions in your field, or to simply improve the visual clarity of your data representation. The core idea behind reversing a colormap is to simply invert the order of the colors within the colormap.

The "_r" Suffix

The simplest way to reverse a colormap in matplotlib is to add "_r" to the colormap name.

# Reverse the 'viridis' colormap
viridis_reversed = cm.get_cmap('viridis_r', 256)

This creates a new colormap object that has the same colors as the original colormap, but in the reverse order.

Programmatically Reversing a Colormap

You can also programmatically reverse a colormap by manipulating its colors attribute if you need more granular control, or if you are working with a custom colormap.

# Programmatically reverse a colormap
viridis = cm.get_cmap('viridis', 256)
viridis_reversed_colors = viridis(np.arange(viridis.N)[::-1])
viridis_reversed = cm.colors.ListedColormap(viridis_reversed_colors)

Here’s a breakdown:

1. viridis(np.arange(viridis.N)): This generates an array of colors from the viridis colormap. viridis.N provides the number of colors defined in the colormap (usually 256). np.arange() creates a sequence of numbers from 0 to viridis.N - 1. These numbers are then used as indices to sample the colors from the colormap.
2. [::-1]: This reverses the order of the indices.
3. viridis(...): We get the color values using the reversed indices.
4. cm.colors.ListedColormap(...): This creates a new colormap object from the reversed list of colors.

Truncating Colormaps

Truncating a colormap means selecting a subset of the colors from the original colormap. This can be useful when you want to focus on a particular range of data values or remove visually distracting colors from the ends of the colormap.

Adjusting the "vmin" and "vmax" Parameters

The most straightforward way to truncate a colormap is to adjust the vmin (minimum value) and vmax (maximum value) parameters when plotting your data. These parameters tell matplotlib which data values should correspond to the beginning and end of the colormap.

# Generate some sample data
data = np.random.rand(10, 10)

# Plot the data using a truncated colormap
plt.imshow(data, cmap='viridis', vmin=0.2, vmax=0.8)
plt.colorbar()
plt.title("Truncated Colormap (vmin=0.2, vmax=0.8)")
plt.show()

In this example, only the colors corresponding to data values between 0.2 and 0.8 will be used. Values below 0.2 will be mapped to the lowest color in the colormap, and values above 0.8 will be mapped to the highest color.

Creating a Truncated Colormap Object

You can create a new colormap object that represents a truncated version of the original colormap. This allows you to reuse the truncated colormap multiple times without having to specify the vmin and vmax parameters each time.

from matplotlib.colors import LinearSegmentedColormap

def truncate_colormap(cmap, minval=0.0, maxval=1.0, n=256):
"""Truncates a colormap between specified values."""
new_cmap = LinearSegmentedColormap.from_list(
'trunc({n},{a:.2f},{b:.2f})'.format(n=cmap.name, a=minval, b=maxval),
cmap(np.linspace(minval, maxval, n)))
return new_cmap

# Truncate the viridis colormap
truncated_viridis = truncate_colormap(cm.get_cmap('viridis'), minval=0.2, maxval=0.8)

# Plot the data using the truncated colormap
plt.imshow(data, cmap=truncated_viridis)
plt.colorbar()
plt.title("Truncated Colormap Object")
plt.show()

This code defines a function truncate_colormap that takes a colormap, a minimum value (minval), and a maximum value (maxval) as input, and returns a new colormap object that represents the truncated colormap. It uses LinearSegmentedColormap.from_list to create the new colormap from a list of colors sampled from the original colormap between minval and maxval.

Combining Reversing and Truncating

You can combine both techniques to create a reversed and truncated colormap. First reverse the colormap using the _r suffix or programmatic reversing, then truncate it using the vmin/vmax parameters or the truncate_colormap function.

# Reverse and truncate the colormap
truncated_viridis_reversed = truncate_colormap(cm.get_cmap('viridis_r'), minval=0.2, maxval=0.8)

# Plot the data using the reversed and truncated colormap
plt.imshow(data, cmap=truncated_viridis_reversed)
plt.colorbar()
plt.title("Reversed and Truncated Colormap")
plt.show()

This first reverses the ‘viridis’ colormap, and then truncates the reversed colormap between 0.2 and 0.8.

Hopefully, this little trick made working with python colormap reversing colors and truncating a bit easier. Now go forth and make some awesome visuals!