Convert RGB to Grayscale in ImageMagick command-line

Question

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/#grays

Answer 1

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

Answer 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

Answer 3

I use convert mostly to convert colour pictures of documents into grey-scale pdf documents in order to perform OCR. My best results are using Rec709Luminance. So I recommend

convert colourpicture.png -grayscale Rec709Luminance greyscalepicture.png

Short command, nice outputs.

Answer 4

Seems like you are taking the red channel to do that, on convert test.png -colorspace OHTA -channel r -separate +channel gray_average.png i prefer the green channel (i heard that way works on tv sice ancient days, maybe the best)

Answer 5

To batch convert images in Fish shell:

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

Answer 6

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