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colorspace: A Python Toolbox for Colors and Palettes

Achim Zeileis
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Python package ‘colorspace’ with tools for manipulating and assessing colors and palettes is now available from PyPI, accompanied by a documentation web page and an arXiv paper.

Citation

Reto Stauffer, Achim Zeileis (2024). “colorspace: A Python Toolbox for Manipulating and Assessing Colors and Palettes.” arXiv.org E-Print Archive arXiv:2407.19921 [cs.GR]. doi:10.48550/arXiv.2407.19921

Abstract

The Python colorspace package provides a toolbox for mapping between different color spaces which can then be used to generate a wide range of perceptually-based color palettes for qualitative or quantitative (sequential or diverging) information. These palettes (as well as any other sets of colors) can be visualized, assessed, and manipulated in various ways, e.g., by color swatches, emulating the effects of color vision deficiencies, or depicting the perceptual properties. Finally, the color palettes generated by the package can be easily integrated into standard visualization workflows in Python, e.g., using matplotlib, seaborn, or plotly.

Software

Motivation

Color is an integral element of visualizations and graphics and is essential for communicating (scientific) information. However, colors need to be chosen carefully so that they support the information displayed for all viewers (see e.g., Tufte 1990; Ware 2004; Wilke 2019). Therefore, suitable color palettes have been proposed in the literature (e.g., Brewer 1999; Ihaka 2003; Crameri, Shephard, and Heron 2020) and many software packages transitioned to better color defaults over the last decade. A prominent example from the Python community is matplotlib 2.0 (Hunter, Dale, Firing, Droettboom, and the Matplotlib Development Team 2017) which replaced the classic “jet” palette (a variation of the infamous “rainbow”) by the perceptually-based “viridis” palette. Hence a wide range of useful palettes for different purposes is provided in a number of Python packages today, including cmcramery (Rollo 2024), colormap (Cokelaer 2024), colormaps (Patel 2024), matplotlib (Hunter 2007), palettable (Davis 2023), or seaborn (Waskom 2021).

However, in most graphics packages colors are provided as a fixed set. While this makes it easy to use them in different applications, it is usually not easy to modify the perceptual properties or to set up new palettes following the same principles. The colorspace package addresses this by supporting color descriptions using different color spaces (hence the package name), including some that are based on human color perception. One notable example is the Hue-Chroma-Luminance (HCL) model which represents colors by coordinates on three perceptually-based axes: Hue (type of color), chroma (colorfulness), and luminance (brightness). Selecting colors along paths along these axes allows for intuitive construction of palettes that closely match many of the palettes provided in the packages listed above.

In addition to functions and interactive apps for HCL-based colors, the colorspace package also offers functions and classes for handling, transforming, and visualizing color palettes (from any source). In particular, this includes the simulation of color vision deficiencies (Machado Oliviera, and Fernandes 2009) but also contrast ratios, desaturation, lightening/darkening, etc.

The colorspace Python package was inspired by the eponymous R package (Zeileis, Fisher, Hornik, Ihaka, McWhite, Murrell, Stauffer, and Wilke 2020). It comes with extensive documentation at https://retostauffer.github.io/python-colorspace/, including many practical examples. Selected highlights are presented in the following.

Key functionality

HCL-based color palettes

The key functions and classes for constructing color palettes using hue-chroma-luminance paths (and then mapping these to hex codes) are:

These functions provide a range of named palettes inspired by well-established packages but actually implemented using HCL paths. Additionally, the HCL parameters can be modified or new palettes can be created from scratch.

As an example, the figure below depicts color swatches for four viridis variations. The first pal1 sets up the palette from its name. It is identical to the second pal2 which employes the HCL specification directly: The hue ranges from purple (300) to yellow (75), colorfulness (chroma) increases from 40 to 95, and luminance (brightness) from dark (15) to light (90). The power parameter chooses a linear change in chroma and a slightly nonlinear path for luminance.

In pal3 and pal4 the most HCL properties are kept the same but some are modified: pal3 uses a triangular chroma path from 40 via 90 to 20, yielding muted colors at the end of the palette. pal4 just changes the starting hue for the palette to green (200) instead of purple. All four palettes are visualized by the swatchplot function from the package.

The objects returned by the palette functions provide a series of methods, e.g., pal1.settings for displaying the HCL parameters, pal1(3) for obtaining a number of hex colors, or pal1.cmap() for setting up a matplotlib color map, among others.

from colorspace import palette, sequential_hcl, swatchplot

pal1 = sequential_hcl(palette = "viridis")
pal2 = sequential_hcl(h = [300, 75], c = [40, 95], l = [15, 90],
                      power = [1., 1.1])
pal3 = sequential_hcl(palette = "viridis", cmax = 90,  c2 = 20)
pal4 = sequential_hcl(palette = "viridis", h1 = 200)

swatchplot({"Viridis (and altered versions of it)": [
               palette(pal1(7), "By name"), 
               palette(pal2(7), "By hand"),
               palette(pal3(7), "With triangular chroma"),
               palette(pal4(7), "With smaller hue range")
           ]}, figsize = (8, 1.75));

An overview of the named HCL-based palettes in colorspace is depicted below.

from colorspace import hcl_palettes
hcl_palettes(plot = True, figsize = (20, 15))

Palette visualization and assessment

To better understand the properties of palette pal4, defined above, the following figure shows its HCL spectrum (left) and the corresponding path through the HCL space (right).

The spectrum in the first panel shows how the hue (right axis) changes from about 200 (green) to 75 (yellow), while chroma and luminance (left axis) increase from about 20 to 95. Note that the kink in the chroma curve for the greenish colors occurs because such dark greens cannot have higher chromas when represented through RGB-based hex codes. The same is visible in the second panel where the path moves along the outer edge of the HCL space.

pal4.specplot(figsize = (5, 5));
pal4.hclplot(n = 7, figsize = (5, 5));

Color vision deficiency

Another important assessment of a color palette is how well it works for viewers with color vision deficiencies. This is exemplified below by depicting a demo plot (heatmap) under “normal” vision (left), deuteranomaly (colloquially known as “red-green color blindness”, center), and desaturated (gray scale, right). The palette in the top row is the traditional fully-saturated RGB rainbow, deliberately selected here as a palette with poor perceptual properties. It is contrasted with a perceptually-based sequential blue-yellow HCL palette in the bottom row.

The sequential HCL palette is monotonic in luminance so that it is easy to distinguish high-density and low-density regions under deuteranomaly and desaturation. However, the rainbow is non-monotonic in luminance and parts of the red-green contrasts collapse under deuteranomaly, making it much harder to interpret correctly.

from colorspace import rainbow, sequential_hcl
col1 = rainbow(end = 2/3, rev = True)(7)
col2 = sequential_hcl("Blue-Yellow", rev = True)(7)

from colorspace import demoplot, deutan, desaturate
import matplotlib.pyplot as plt

fig, ax = plt.subplots(2, 3, figsize = (9, 4))
demoplot(col1, "Heatmap", ax = ax[0,0], ylabel = "Rainbow", title = "Original")
demoplot(col2, "Heatmap", ax = ax[1,0], ylabel = "HCL (Blue-Yellow)")
demoplot(deutan(col1), "Heatmap", ax = ax[0,1], title = "Deuteranope")
demoplot(deutan(col2), "Heatmap", ax = ax[1,1])
demoplot(desaturate(col1), "Heatmap", ax = ax[0,2], title = "Desaturated")
demoplot(desaturate(col2), "Heatmap", ax = ax[1,2])
plt.show()

Integration with Python graphics packages

To illustrate that colorspace can be easily combined with different graphics workflows in Python, the code below shows a heatmap (two-dimensional histogram) from matplotlib and multi-group density from seaborn. The code below employs an example data set from the package (using pandas) with daily maximum and minimum temperature. For matplotlib the colormap (.cmap(); LinearSegmentedColormap) is extracted from the adapted viridis palette pal3 defined above. For seaborn the hex codes from a custom qualitative palette are extracted via .colors(4).

from colorspace import dataset, qualitative_hcl
import matplotlib.pyplot as plt
import seaborn as sns

df = dataset("HarzTraffic")

fig = plt.hist2d(df.tempmin, df.tempmax, bins = 20,
                 cmap = pal3.cmap().reversed())
plt.title("Joint density daily min/max temperature")
plt.xlabel("minimum temperature [deg C]")
plt.ylabel("maximum temperature [deg C]")
plt.show()

pal = qualitative_hcl("Dark 3", h1 = -180, h2 = 100)
g = sns.displot(data = df, x = "tempmax", hue = "season", fill = "season",   
                kind = "kde", rug = True, height = 4, aspect = 1,
                palette = pal.colors(4))
g.set_axis_labels("temperature [deg C]")              
g.set(title = "Distribution of daily maximum temperature given season")
plt.show()

Dependencies and availability

The colorspace is available from PyPI at https://pypi.org/project/colorspace. It is designed to be lightweight, requiring only numpy (Harris et al. 2020) for the core functionality. Only a few features rely on matplotlib, imageio (Klein et al. 2024), and pandas (The Pandas Development Team 2024). More information and an interactive interface can be found on https://hclwizard.org/. Package development is hosted on GitHub at https://github.com/retostauffer/python-colorspace. Bug reports, code contributions, and feature requests are warmly welcome.

References

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