Colour / exposure calibration against a reference patch

Source: R/read_plate.R

Rescales the image in linear RGB so that a neutral reference patch photographed in every image (grey card, white standard, colour-checker neutral patch) takes its known CIELAB value. This is a diagonal (von Kries-type) correction: it removes differences in exposure and white balance between photographs, which is essential for comparing absolute \(L^*\) (melanization) across plates, days and cameras.

calibrate_color(plate, region, ref_lab = c(50, 0, 0))

Arguments

plate

A mycohalo_plate.

region

Reference patch as c(x, y, r) in relative image coordinates (fractions of width, height and of the shorter side), e.g. c(0.08, 0.92, 0.03) for a card in the bottom-left corner.

ref_lab

Known CIELAB of the patch (default neutral c(50, 0, 0)).

Value

The calibrated mycohalo_plate (with $calibration gains).

Details

A photographic 18 % grey card has \(L^* \approx 50\); the neutral 5 patch of an X-Rite ColorChecker Classic has \(L^* \approx 51\). The patch must be evenly lit and not clipped.

Examples

sim <- simulate_plate(width = 300, height = 400, seed = 2, exposure = 0.8, grey_card = TRUE)
plate <- read_plate(sim$image)
plate <- calibrate_color(plate, region = sim$truth$grey_card)
#> Warning: 0.44 % of channel values exceed 1 after calibration and were clipped.