Question

I’m trying to design an algorithm for selecting photos which are monochrome (the one photographers call “black & white”) and have a single color toning such as sepia. (think as you’ve applied a solid color filter to a monochrome image) If I was after just monochrome only, all I need is to check the saturation which is easy (and I’m currently doing it), but it is unsuccessful at finding solid-color-toned monochrome photos. What could be an approach for that one?

Answer 1

If an image is monochrome, either black and white or sepia, the colors corresponding to each lightness level should be nearly identical. Convert the image colorspace to one which contains a Luminance (Y) or Lightness component, such as YCbCr or HSL, and for each Y/L value look at the variance in the other two channels. If the values cluster together, you have a monochrome image.