Check a map's colours under colour-vision deficiency

Simulate how the map's fill colours look with deuteranopia, protanopia and tritanopia – the Machado, Oliveira & Fernandes (2009) model, through the optional colorspace – and report the smallest colour difference between adjacent classes under each. Two classes a reader cannot tell apart are two classes the map does not have. Roughly one man in twelve has a red-green deficiency.

Usage

check_palette(p, threshold = 10)

Arguments

p

A map from one of the package's map verbs, or a character vector of colours in class order.

threshold

The smallest acceptable difference between adjacent classes (default 10). Below it, a warning of class countryatlas_palette_cvd names the vision type and the two classes.

Value

A tibble with one row per vision type (normal, deuteranopia, protanopia, tritanopia): min_delta_e, and between, the two classes that come closest.

Details

The difference is CIE 1976 \(\Delta E^*_{ab}\), the distance between the two colours in CIELAB. A difference under about 10 is hard to tell apart at the size of a small country on a map; the package's own defaults (the viridis family) clear it under all three simulations.

References

Machado, G. M., Oliveira, M. M. & Fernandes, L. A. F. (2009). A physiologically-based model for simulation of color vision deficiency. IEEE Transactions on Visualization and Computer Graphics 15(6), 1291-1298. doi:10.1109/TVCG.2009.113

Crameri, F., Shephard, G. E. & Heron, P. J. (2020). The misuse of colour in science communication. Nature Communications 11, 5444. doi:10.1038/s41467-020-19160-7

Examples

# \donttest{
if (requireNamespace("colorspace", quietly = TRUE)) {
  snap <- countryatlas::world_snapshot$countries
  check_palette(world_map(attach_geometry(snap), gdp_per_capita))
  # A rainbow fails all three simulations:
  check_palette(grDevices::rainbow(7))
}
#> Warning: Adjacent classes are hard to tell apart under "deuteranopia", "protanopia", and
#> "tritanopia".
#> • deuteranopia, protanopia, and tritanopia: 2 and 3, 2 and 3, and 3 and 4
#>   (difference 8.8, 7.2, and 3.5).
#> ℹ Use fewer classes, or a palette whose lightness changes monotonically, such
#>   as "viridis" or "cividis".
#> # A tibble: 4 × 3
#>   vision       min_delta_e between
#>   <chr>              <dbl> <chr>  
#> 1 normal             46.2  3 and 4
#> 2 deuteranopia        8.82 2 and 3
#> 3 protanopia          7.21 2 and 3
#> 4 tritanopia          3.47 3 and 4
# }