Convert RGB to Grayscale in ImageMagick command-line

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.

Solution 1:

convert <img_in> -set colorspace Gray -separate -average <img_out> gives the best result for any image for me.

Solution 2:

Using the (r+g+b)/3 method will apply the effects of grayscale, but the image will remain in sRGB (which is the expected behavior for this method). You'll need to specify the desired colorspace along with the -fx command.

convert test.png -fx '(r+g+b)/3' -colorspace Gray gray_fx_average.png

Verify with identify -format "%[colorspace] <== %f\n" gray_fx_average.png

Gray <== gray_fx_average.png

Solution 3:

To batch convert images in Fish shell:

for file in *.jpg; convert -colorspace Gray $file $file; end;

Solution 4:

A few ways to that in Imagemagick command line are:

convert test.png -grayscale average gray_average.png

or

convert test.png -colorspace OHTA -channel r -separate +channel gray_average.png

or

convert test.png -intensity average -colorspace gray gray_average.png

or

convert test.png -colorspace HSI -channel blue -separate +channel gray_average.png

See

https://imagemagick.org/script/command-line-options.php#grayscale https://imagemagick.org/script/command-line-options.php#intensity https://imagemagick.org/script/command-line-options.php#colorspace