Use Imagemagick to dump the pixels of a grayscale image

Question

I have a sample grayscale file z-gray-1x3.png, consisting of a single row of three pixels with grayscale values 0x00, 0x7f, 0xff:

I tried to write a script using imagemagick, hexdump, sed and tr but got 3-byte pixels instead:

$ magick z-gray-1x3.png -depth 8 RGB:output.txt
$ hexdump -xv output.txt | sed -E 's/^[^ ]+//' | tr -d ' \n'; echo
01017f017f7fffff00ff

(the byte 00 is padding by hexdump to an even number of digits.)

The desired output would be 007f00ff (including padding). Is there a way to modify the imagemagick command to do this?

I really do not understand what you want. But possibly (guess) use magick z-gray-1x3.png -format "%[hex:u.p{0;0}]%[hex:u.p{1;0}]%[hex:u.p{2;0}]" info: | sed 's/\#//g'. The sed is to remove the # sign from before each hex value. This is not tested, so work with the concept. See the very bottom of imagemagick.org/script/fx.php. See also imagemagick.org/script/escape.php — fmw42
– fmw42, Commented Sep 29, 2024 at 5:35
Add -depth 8 to my command above after reading the input image. — fmw42
– fmw42, Commented Sep 29, 2024 at 16:17
fmw42: Thank you for your suggestion, but it didn't work. With -depth 8 added, it gave 010101010101010101, not what I wanted. Let me try again: z-gray-1x3.png has 3 pixels. I wish to dump them to a file of 3 hex bytes. I can use the Mac application GraphicConverter to export z-gray-1x3.png to a "raw" format, consisting of just the three hex bytes. I can then do hexdump -C z-gray-1x3.raw whose first output line includes the 3 bytes I want: 00000000 01 7f ff Can Imagemagick export my image to something like GraphicConverter's "raw" format, and how? — Jonathan Levi
– Jonathan Levi, Commented Sep 29, 2024 at 16:49
If your input is in an image format such as PNG, then it can export as raw. See imagemagick.org/Usage/formats/#rgb — fmw42
– fmw42, Commented Sep 29, 2024 at 19:16
Thank you, that's what I was looking for. hexdump -C z-gray-1x3.gray | head -n 1 | tr -d "\147-\377" | sed 's/.\{10\}\(.*\)/\1/' | tr -d ' .' -> 017fff — Jonathan Levi
– Jonathan Levi, Commented Sep 29, 2024 at 19:51

Mark Setchell · Accepted Answer · 2024-10-18 06:53:35Z

1

I think you want to convert to greyscale, rather than RGB, so that you don't get repeated RGB components, and also output in "plain" hexdump style:

magick 3pixels.png gray:- | xxd -p
017fff

Just in case you use a Q16 version of ImageMagick, add in -depth 8 like this:

magick 3pixels.png -depth 8 gray:- | xxd -p

edited Oct 18, 2024 at 6:53

answered Oct 3, 2024 at 7:08

Mark Setchell

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