The 'offset' argument description is blank ...
maybe 'additive adjustment to each of the (red, green, blue, alpha)
values defining the colors, after adjustment by the corresponding
\code{.f} factor' ...?
This is the relevant code:
x <- col2rgb(col, alpha = TRUE)/255
x[] <- pmax(0, pmin(1,
transform %*% x +
matrix(offset, nrow = 4L, ncol = ncol(x))))
rgb(x[1L,], x[2L,], x[3L,], x[4L,])
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