Mean background-subtracted brightness of the pieces in a set of squares

White pieces are light-filled and black pieces dark-filled, so once the square background is removed a white piece leaves a much higher (less negative) mean residual than the same black piece. This holds across piece sets because it is a property of the pieces, not of the artwork style.

Usage

squares_mean_brightness(squares, idx)

Arguments

squares

A list of 64 magick square images, row-major, top row first.

idx

1-based indices of the squares to measure.

Value

The mean residual over occupied squares, or NA_real_ if none are occupied.