Conversion to monochrome image

Provided is a method of obtaining a monochrome image from a color image, said color image comprising a plurality of color pixels, and having at least two color channels, each pixel of said plurality of color pixels comprising a value for each color channel of said at least two color channels, said monochrome image comprising a plurality of monochrome pixels, each monochrome pixel having a value for a monochrome channel, the method comprising: calculating a local weighting factor for each color channel of each pixel of said color image; calculating said value of said monochrome channel for each pixel as a sum over said at least three color channels of products of said local weighting factor and said value for each color channel.