How do I convert a RGB image (3 channels) to a grayscale one, using the (r+g+b)/3 method? I look through an examples page: http://www.imagemagick.org/Usage/color_mods/#grayscale but the desired method:
convert test.png -fx '(r+g+b)/3' gray_fx_average.png
gave me a wrong result - the resulted image has still 3 channels.
You can check this by running a command: identify -format "%[colorspace] <== %f\n" *.png.
convert <img_in> -set colorspace Gray -separate -average <img_out> gives the best result for any image for me.
Update: As of 2025, the equivalent command in modern versions of ImageMagick is magick <img_in> -colorspace Gray -separate -evaluate-sequence Mean <img_out>. However, most of the flags can be omitted since they're the default behavior already, so the 2025 command can be simplified to just magick <img_in> -colorspace Gray <img_out>.