Details
The HCL (hue-chroma-luminance) color model is a perceptual color model obtained by using polar coordinates in CIE LUV space (i.e., polarLUV), where steps of equal size correspond to approximately equal perceptual changes in color. By taking polar coordinates the resulting three dimensions capture the three perceptual axes very well: hue is the type of color, chroma the colorfulness compared to the corresponding gray, and luminance the brightness. This makes it relatively easy to create balanced palettes through trajectories in this HCL space. In contrast, in the more commonly-used HSV (hue-saturation-value) model (a simple transformation of RGB), the three axes are confounded so that luminance changes along with the hue leading to very unbalanced palettes (see rainbow_hcl for further illustrations).
Three types of palettes are derived based on the HCL model:
-
Qualitative: Designed for coding categorical information, i.e., where no particular ordering of categories is available and every color should receive the same perceptual weight.
-
Sequential: Designed for coding ordered/numeric information, i.e., where colors go from high to low (or vice versa).
-
Diverging: Designed for coding numeric information around a central neutral value, i.e., where colors diverge from neutral to two extremes.
The corresponding functions are qualitative_hcl, sequential_hcl, and diverging_hcl. Their construction principles are explained in more detail below. At the core is the luminance axis (i.e., light-dark contrasts): These are easily decoded by humans and matched to high-low differences in the underlying data. Therefore, sequential_hcl palettes are always based on a monotonic luminance sequence whereas the colors in a qualitative_hcl palette all have the same luminance. Finally, diverging_hcl palettes use the same monotonic luminance sequence in both “arms” of the palette, i.e., correspond to two balanced sequential palettes diverging from the same neutral value. The other two axes, hue and chroma, are used to enhance the luminance information and/or to further discriminate the color.
All three palette functions specify trajectories in HCL space and hence need either single values or intervals of the coordinates h, c, l. Their interfaces are always designed such that h, c, l can take vector arguments (as needed) but alternatively or additionally h1/h2, c1/cmax/c2, and l1/l2 can be specified. If so, the latter coordinates overwrite the former.
qualitative_hcl distinguishes the underlying categories by a sequence of hues while keeping both chroma and luminance constant to give each color in the resulting palette the same perceptual weight. Thus, h should be a pair of hues (or equivalently h1 and h2 can be used) with the starting and ending hue of the palette. Then, an equidistant sequence between these hues is employed, by default spanning the full color wheel (i.e, the full 360 degrees). Chroma c (or equivalently c1) and luminance l (or equivalently l1) are constants.
sequential_hcl codes the underlying numeric values by a monotonic sequence of increasing (or decreasing) luminance. Thus, the l argument should provide a vector of length 2 with starting and ending luminance (equivalently, l1 and l2 can be used). Without chroma (i.e., c = 0), this simply corresponds to a grayscale palette like gray.colors. For adding chroma, a simple strategy would be to pick a single hue (via h or h1) and then decrease chroma from some value (c or c1) to zero (i.e., gray) along with increasing luminance. For bringing out the extremes (a dark high-chroma color vs. a light gray) this is already very effective. For distinguishing also colors in the middle two strategies can be employed: (a) Hue can be varied as well by specifying an interval of hues in h (or beginning hue h1 and ending hue h2). (b) Instead of a decreasing chroma a triangular chroma trajectory can be employed from c1 over cmax to c2 (or equivalently a vector c of length 3). This yields high-chroma colors in the middle of the palette that are more easily distinguished from the dark and light extremes. Finally, instead of employing linear trajectories, power transformations are supported in chroma and luminance via a vector power (or separate p1 and p2). If power[2] (or p2) for the luminance trajectory is missing, it defaults to power[1]/p1 from the chroma trajectory.
diverging_hcl codes the underlying numeric values by a triangular luminance sequence with different hues in the left and in the right arm of the palette. Thus, it can be seen as a combination of two sequential palettes with some restrictions: (a) a single hue is used for each arm of the palette, (b) chroma and luminance trajectory are balanced between the two arms, (c) the neutral central value has zero chroma. To specify such a palette a vector of two hues h (or equivalently h1 and h2), either a single chroma value c (or c1) or a vector of two chroma values c (or c1 and cmax), a vector of two luminances l (or l1 and l2), and power parameter(s) power (or p1 and p2) are used. For more flexible diverging palettes without the restrictrictions above (and consequently more parameters) divergingx_hcl is available. For backward compatibility, diverge_hcl is a copy of diverging_hcl.
To facilitate using HCL-based palettes a wide range of example palettes are provided in the package and can be specified by a name instead of a set of parameters/coordinates. The examples have been taken from the literature and many approximate color palettes from other software packages such as ColorBrewer.org (RColorBrewer), CARTO colors (rcartocolor), scico, or virids. The function hcl_palettes can be used to query the available pre-specified palettes. It comes with a print method (listing names and types), a summary method (additionally listing the underlying parameters/coordinates), and a plot method that creates a swatchplot with suitable labels.
