Why does the order of ImageMagick options matter?

Options split into settings, which configure images read or drawn after them, and operators, which act immediately on loaded images. A setting placed after the thing it should configure has no effect.

Why does -density have no effect on my PDF conversion?

It must come before the input filename. -density tells Ghostscript what resolution to rasterise at, so placed afterwards the page has already been rendered at 72 DPI.

Why is my -extent padding the wrong colour?

-background is a setting, so it must appear before -extent. Placed after, the extent has already used the previous background colour.

The order of my options changes the result

ImageMagick options split into settings that configure what comes after, and operators that act immediately. Understanding the difference explains most confusing results.

You moved a flag and the output changed completely, or a flag appeared to do nothing at all. This is the concept that explains most confusing ImageMagick behaviour, and it is barely mentioned in the documentation's introduction.

Two kinds of option

Settings — configure what comes after

-density, -size, -background, -gravity, -fill, -font, -pointsize, -delay, -loop, -quality, -channel, -compose, -bordercolor, -filter.

These change state. They affect images read or drawn after them. Placed too late in the command, they have no effect at all.

Operators — act immediately

-resize, -crop, -rotate, -flip, -composite, -annotate, -border, -extent, -trim, -blur, -negate.

These act on the images already loaded, in the order written, left to right.

Where this bites

-density before the PDF

magick -density 300 in.pdf out.png    # correct — 300 DPI render
magick in.pdf -density 300 out.png    # wrong — rendered at 72, then tagged 300

The second produces a blurry image with misleading metadata. This is the single most common ordering mistake.

-size before a generated source

magick -size 800x600 xc:white out.png    # correct — 800x600
magick xc:white -size 800x600 out.png    # wrong — 1x1 pixel

-background before -extent

magick in.jpg -background white -gravity center -extent 1200x1200 out.jpg  # correct
magick in.jpg -gravity center -extent 1200x1200 -background white out.jpg  # padding is not white

-delay before the frames

magick -delay 8 -loop 0 frame_*.png out.gif    # correct
magick frame_*.png -delay 8 -loop 0 out.gif    # timing ignored

Operators run in sequence

magick in.jpg -resize 400x400 -crop 200x200+0+0 +repage out.jpg
magick in.jpg -crop 200x200+0+0 +repage -resize 400x400 out.jpg

The first scales to 400×400 then takes a 200×200 corner. The second takes a 200×200 corner of the original and scales it up to 400×400. Both are valid; they produce entirely different images.

Settings persist until changed

magick in.jpg -gravity south -annotate +0+20 'Bottom' \
  -gravity north -annotate +0+20 'Top' out.jpg

The first -gravity stays in force until the second replaces it. Forgetting to reset a setting is a common source of "why is my second annotation in the wrong place".

Some settings are cleared with the plus form: -channel A then +channel, -repage then +repage. Generally minus sets and plus unsets.

Parentheses scope the sequence

magick in.jpg '(' logo.png -resize 20% ')' -gravity southeast -composite out.jpg

The -resize 20% inside the parentheses applies only to logo.png. Without them it would apply to whatever images are currently loaded — which here would be both.

A rule of thumb

Read the command left to right as a sequence of instructions to a machine that holds a stack of images. Ask of each flag: is this describing how to do the next thing, or is it doing something now? Settings go before the thing they describe. Operators go where you want them to happen.