def - other
      other.is_a?(Vips::Image) ?
          subtract(other) : linear(1, Image::smap(other) {|x| x * -1})
    end
def - other
      other.is_a?(Vips::Image) ?
          subtract(other) : linear(1, Image::smap(other) {|x| x * -1})
    end
Subtract an image, constant or array.

 @param other [Image, Real, Array<Real>] Thing to subtract from self
 @return [Image] result of subtraction