Add check_palette() for colorblind accessibility

[truenomad]

Summary

sntutils has 27+ built-in palettes used for choropleth maps (get_palette(), auto_bin()), but zero colorblind accessibility checks. ~8% of men have some form of color vision deficiency. We need a function that checks if a palette is safe and optionally fixes it.

Proposed function

check_palette(
  palette,
  n = NULL,
  fix = FALSE,
  severity = 1,
  tolerance = 10
)

Parameters

Param	Type	Default	Description
palette	character	required	Preset name (e.g. "blues") or hex vector
n	integer	NULL	Number of colors to check (passed to get_palette())
fix	logical	FALSE	If TRUE, return an adjusted colorblind-safe palette
severity	numeric	1	Deficiency severity 0–1 (1 = complete)
tolerance	numeric	10	Min perceptual distance threshold (Delta E in CIELab)

Return value

A list with:

• safe — logical, TRUE if palette passes all 3 deficiency types
• colors — the input palette (hex vector)
• min_dist — named numeric vector with min pairwise distance for each type + normal
• details — data frame with per-type results (type, min_dist, pass)
• fixed — (only when fix = TRUE) adjusted hex vector that passes the check

Core logic

Checking:

1. Resolve palette via get_palette(palette, n) to get hex vector
2. For each deficiency type (deutan, protan, tritan):
   a. Simulate with colorspace::deutan(), protan(), tritan()
   b. Convert simulated colors to CIELab
   c. Compute all pairwise Euclidean distances in Lab space
   d. Record minimum distance
3. If all min distances >= tolerance → safe = TRUE

Fixing (when fix = TRUE):

1. Convert input palette to HCL space
2. Keep the hue endpoints (preserve color intent)
3. Spread luminance values evenly from ~30 to ~90
4. Rebuild palette with colorspace::hex(colorspace::polarLUV(L, C, H))
5. Re-check — if still fails, reduce chroma and widen luminance further
6. If that still fails, fall back to colorspace::sequential_hcl(n, palette = "viridis") and warn

Dependency: colorspace

Add to Imports. Provides CVD simulation, HCL conversion, and desaturation — one package handles both checking and fixing.

Files to create/modify

File	Action
R/check_palette.R	Create — new function + internal helpers
tests/testthat/test-check_palette.R	Create — tests
DESCRIPTION	Edit — add colorspace to Imports

Example usage

# check a built-in palette
check_palette("blues", n = 6)

# check custom colors
check_palette(c("#FF0000", "#00FF00", "#0000FF"))

# check and fix
result <- check_palette("reds", n = 5, fix = TRUE)
result$safe
result$fixed

# check with partial severity
check_palette("default", n = 6, severity = 0.5)

Tests to write

1. Known-safe palette (viridis) returns safe = TRUE
2. Known-bad palette (red + green only) returns safe = FALSE
3. fix = TRUE returns a palette that passes the check
4. Custom hex input works
5. Preset name input works (resolves via get_palette())
6. n parameter interpolates correctly before checking
7. severity = 0 always returns safe (no deficiency)
8. Edge cases: single color, two colors
9. Tolerance parameter affects pass/fail threshold
10. Error on invalid inputs (missing palette, bad severity, bad tolerance)