FAQ
Should I use magick or convert?
Use magick (as in magick convert ... or the shorthand
magick in.jpg out.png) on any current install — it's the unified entry
point introduced in ImageMagick 7 and is what the project recommends. The
standalone convert binary still exists on many distros for backward
compatibility and behaves the same way, but it may eventually be removed,
and on some systems (notably some Windows setups) convert resolves to an
unrelated built-in tool.
What's the difference between -resize and -thumbnail?
They both scale an image, but -thumbnail is a faster, lower-quality
shortcut intended for generating small previews: it skips some filtering
steps and automatically strips embedded color profiles and metadata.
Use -resize when output quality matters (e.g. a resized photo for
publishing) and -thumbnail when generating many small preview images
quickly.
Can I resize a JPEG without any quality loss?
Not with a true resize — resizing always resamples pixels, and
re-encoding a JPEG is inherently lossy. What you can do losslessly is
crop and rotate by multiples of 90°, using -crop together with
+repage, since those operations can work on whole JPEG blocks without
re-encoding pixel data. For genuinely lossless resizing, convert to a
lossless format (PNG, WebP lossless) first.
Why do I get a "not authorized" / policy error converting a PDF?
Many distributions ship a policy.xml (usually at
/etc/ImageMagick-7/policy.xml) that disables certain coders — PDF and
PS/EPS in particular — by default, as a security hardening measure
against maliciously crafted files (this followed a real vulnerability
class dubbed "ImageTragick" in 2016). If you trust your inputs, find the
relevant <policy domain="coder" rights="none" pattern="PDF" /> line and
change rights="none" to rights="read|write".
Why did my GIF lose its animation after editing?
Most single-image operations (like a plain -resize) only touch the
first frame by default. Add -coalesce before the operation to expand
every frame to full size first, and -layers optimize after it to
re-compress the result back into an efficient animated GIF:
magick in.gif -coalesce -resize 50% -layers optimize out.gif
ImageMagick is using huge amounts of memory/disk on a large image. Why?
ImageMagick decodes images to an uncompressed pixel cache, which can be
far larger than the source file, and it will spill that cache to disk
(/tmp by default) once memory limits are hit. For very large images or
batch jobs, raise or lower the relevant limits explicitly with
-limit memory 2GiB / -limit disk 4GiB, or process files one at a time
instead of globbing an entire folder into one command.