ImageMagick (and GraphicsMagick)

• imagemagick.org, the current 7.x line
• legacy.imagemagick.org, the 6.x line, end of life announced for 2029-04-01
• GraphicsMagick

On ImageMagick 7 the single magick command replaces the old per-tool binaries, so the recipes below use magick, magick identify, magick mogrify, magick montage. On ImageMagick 6 drop the magick prefix and call convert, identify, mogrify, montage directly. The old names survive in 7 as symbolic links to magick, but the porting guide asks for magick identify and friends and is explicit about one form: do not write magick convert, which switches the legacy parser back on. Debian 13 and Ubuntu 25.10 and later ship the 7.x line; Debian 12 and Ubuntu 24.04 LTS still ship 6.9. Everything below was run on 7.1.2-34, and the gotchas also on 6.9.11-60 and 6.9.12-98.

Cookbook

Inspect before you touch anything

magick identify -format "%f: %wx%h %m %b q=%Q\n" *.jpg

One line per file with name, dimensions, format, byte size, and the quality the JPEG was saved at. Read it before a batch: re-encoding at a higher quality than the source only adds bytes. Drop the -format string to get the full verbose dump of a single file.

Batch-optimise a folder of JPEGs for the web

mkdir -p output
for i in *.jpg; do
    magick "$i" -auto-orient -strip -interlace Plane \
        -sampling-factor 4:2:0 -quality 75 -resize '1800x>' "output/$i"
done

-auto-orient rotates the pixels according to the EXIF orientation tag before -strip throws that tag away, which is the whole point of putting it first (see the gotchas). -strip drops metadata, -interlace Plane makes the JPEG progressive (identify -verbose then reports Interlace: JPEG), -sampling-factor 4:2:0 applies the chroma subsampling every browser expects, -quality 75 sets the JPEG quality, and -resize '1800x>' caps the width at 1800px while leaving smaller images alone. Quote the geometry, or the shell reads > as a redirection.

Measured on synthetic test images: a 2400x1600 source of 609kB comes out 1800x1200 and 125kB, and an 800x600 source stays 800x600 and drops from 86kB to 29kB.

Add -gaussian-blur 0.05 when the source is grainy: on a deliberately noisy 2000x1400 image the same pipeline gave 290kB with it against 548kB without. On a clean gradient the two outputs were byte for byte identical, so it buys nothing there.

Convert a whole folder without the loop

magick mogrify -format webp -quality 80 -path output *.jpg

mogrify is ImageMagick’s built-in batch tool, so no shell loop is needed. -format webp writes .webp copies, -path output sends them to output/ instead of next to the sources. Careful: mogrify without -format or -path overwrites the originals in place.

Decode less to go faster

magick -define jpeg:size=800x600 huge.jpg -resize 400x thumb.jpg

The JPEG decoder scales the image while reading it, so the full resolution is never decompressed. Keep the hint at twice the target size or so, to leave the resampling something to work with. On a 6000x4000 source of 7.7MB, the same 400x267 thumbnail took 0.12s instead of 0.45s. This is a setting, not an operator, so it goes before the input file.

Flatten a transparent PNG onto a JPEG

magick logo.png -background white -flatten logo.jpg

JPEG has no alpha channel, so transparent pixels come out black: verified, the corner pixel of a transparent PNG written straight to JPEG reads srgb(0,0,0). -background white -flatten composites the image onto a solid canvas first. Swap white for any colour, for instance -background '#f5f5f5'. On ImageMagick 6.7.5 and later, -background white -alpha remove does the same job without merging layers (both give a white corner pixel here).

Trim surrounding whitespace

magick scan.png -trim +repage cropped.png

-trim removes a uniform border, like the white margin around a scan or the padding around a logo. +repage resets the virtual canvas afterwards. Add -fuzz 5% before -trim when the border is not perfectly uniform, a scan’s off-white margin for instance. Keep the fuzz below the contrast of what you want to keep: a #fefefe block inside a white border trims to 101x41 without fuzz, and with -fuzz 5% it counts as border too and the whole image trims away.

Build a contact sheet

magick montage *.jpg -tile 4x -geometry 240x+8+8 -title "Shoot 2026" contact.jpg

montage tiles thumbnails into one overview image: 4 columns (-tile 4x, rows auto-computed), each thumbnail capped at 240px wide with an 8px gap on every side (-geometry). Handy to eyeball a whole shoot on a single page.

Gotchas

Everything here was reproduced on 7.1.2-34, and on 6.9.11-60 and 6.9.12-98.

-strip rotates photos, silently

A 400x200 JPEG carrying EXIF orientation 6 (RightTop) is displayed 200x400 by every viewer and browser. Now:

• magick in.jpg -strip out.jpg gives 400x200 with no orientation tag, so the photo is suddenly on its side. No warning, and the pixels are fine, which is why this survives review.
• magick in.jpg -auto-orient -strip out.jpg gives 200x400, upright, no tag. This is the one you want.
• -resize alone keeps the tag (200x100, still RightTop), so resizing without stripping is safe.
• -thumbnail is -resize that also throws profiles away, the orientation tag with them (200x100, no tag), so it carries the same trap.

Put -auto-orient first in any pipeline that removes metadata.

-resize enlarges unless you stop it

-resize 1800x on an 800x600 source produces 1800x1350, and the “optimised” file comes out three times the size of the original (86kB to 269kB, measured). 1800x> only ever shrinks, 1800x< only ever enlarges. Both need quoting in the shell.

-quality on a PNG is not what you think

PNG is lossless. Quality 00 and quality 95 on the same source gave 8302 and 8221 bytes, with zero differing pixels (compare -metric AE returns 0). The number only picks a zlib level and a row filter. On the same image as JPEG, quality 50 against 95 is 2.4kB against 9.7kB.

Blur geometry is radius first, and sigma defaults to 1

In -gaussian-blur radiusxsigma, a bare number is the radius: -gaussian-blur 0.05 is exactly -gaussian-blur 0.05x1 (identical bytes), while -gaussian-blur 0x0.05 is a different, almost invisible blur. Any radius below 1 ends up with the same minimal kernel, so 0.05 and 0.5 produced byte-identical output. The usual form is 0xsigma, which lets ImageMagick derive the radius from sigma.

-trim leaves the old canvas behind

A 400x300 image trimmed to 101x101 keeps page=+150+100 and a 400x300 virtual canvas. Add a border afterwards and the offset is still there. PNG and GIF carry that offset in the file, so the surprise turns up later, in a viewer or in the next command. +repage resets it.

mogrify really does overwrite

Same file in, same file out: a 609kB source became 11kB in place. There is no undo. -path or -format, or work on a copy.

The security policy blocks more than you expect

Distributions ship their own policy.xml (/etc/ImageMagick-6/policy.xml on Ubuntu up to 24.04, /etc/ImageMagick-7/policy.xml on 26.04). Read it with magick identify -list policy and the limits with magick identify -list resource. Ubuntu 22.04 is the strictest:

• PDF, PS and EPS coders are off: attempt to perform an operation not allowed by the security policy 'PDF'. Note that a 0 byte output file is still created.
• The @list.txt syntax, which reads filenames from a file, is covered by a path policy on the @* pattern. On 24.04 and 26.04 it is refused with not allowed by the security policy '@list.txt'; on 22.04 (6.9.11-60) the same policy line is there and the list is read anyway.
• Width and height are capped at 16KP, area at 128MP, memory at 256MiB. A 20000x100 canvas gives width or height exceeds limit, while 12000x12000 gives cache resources exhausted, which reads like a machine problem and is a policy line.
• -limit cannot go above the policy: -limit memory 1GiB left the memory limit at 256MiB.

Moving from 6 to 7

From the porting guide: version 7 walks the command line strictly left to right, so an operator placed before any input image is an error, where version 6 accepted it (convert -resize 50% in.jpg out.jpg does resize on 6.9.11-60). Version 7 also enables HDRI by default, so results can shift slightly (-clamp brings pixels back in range), defaults -channel to RGBA instead of RGB (-channel RGB restores the old behaviour), and represents grayscale with a single channel.

GraphicsMagick

GraphicsMagick is a lighter fork with mostly the same options behind a single gm binary: prefix each command with gm, so gm convert in.jpg -resize 1800x out.jpg, gm identify photo.png, gm montage .... There is no unified magick command, and it lags on newer formats (thinner WebP and HEIC support), but it stays fast and steady for the classic resize and convert jobs.

See also

• Shell script for the loop and quoting habits the batch recipes rely on.