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+ImageMagick recipes I should stuff into config files or scripts, but
+haven't yet.
+* Configuration
+** Allow conversion from/to PDF
+- Open ImageMagick's =policy.xml= configuration file.
+- Locate the =<policy>= tag with a =pattern= including "PDF".
+- Comment it out.
+* Comparing images
+~magick compare~ is a thing; ~magick convert~ can also do [[https://stackoverflow.com/a/33673440][neat stuff]]:
+#+begin_src sh
+convert '(' a.png -flatten -grayscale Rec709Luminance ')' \
+        '(' b.png -flatten -grayscale Rec709Luminance ')' \
+        '(' -clone 0-1 -compose darken -composite ')' \
+        -channel RGB -combine diff.png
+#+end_src
+
+* Adding watermarks
+Someday, in the Glorious Future, every thirdparty that insists on
+having a copy of my personal documents will be legally obligated to
+request it from a trusted government service that steganographizes the
+requester's identity in the files they send.
+
+Until then…
+#+begin_src sh
+#!/bin/bash
+
+set -eu
+
+src=$1
+dst=$2
+label=$3
+
+mark ()
+{
+    local marksrc=$1
+    local markdst=$2
+
+    w=$(identify -format '%w\n' "$marksrc" | head -1)
+    h=$(identify -format '%h\n' "$marksrc" | head -1)
+    textw=$((w/3))
+    texth=$((h/8))
+
+    pointsize=$((80*w/2500))
+    offx=$((100*w/2500))
+    offy=$((400*h/3500))
+
+    convert -size ${textw}x${texth} xc:none                 \
+            -fill '#80808080' -pointsize $pointsize         \
+            -annotate 340x340+${offx}+${offy} "$label"      \
+            miff:- |                                        \
+        composite -tile - "$marksrc" "$markdst"
+}
+
+if [[ "$src" =~ \.pdf ]]
+then
+    # AFAICT if I feed a multi-page PDF to composite, it outputs only
+    # the first page.  Jump through hoops to split and concatenate the
+    # PDF; also, roundtrip through JPG because setting -density (to
+    # preserve a good quality) messes with the size calculations for
+    # the label.
+
+    tmpd=$(mktemp -d)
+    bname=$(basename "$src")
+    qpdf --split-pages "$src" "${tmpd}/$bname"
+
+    shopt -s extglob
+
+    for page in ${tmpd}/${bname/%.pdf/-+([0-9]).pdf}
+    do
+        convert -density 300 -flatten "$page" "${page/.pdf/.jpg}"
+        mark "${page/.pdf/.jpg}" "${page/.pdf/-MARKED.jpg}"
+        convert -density 300 "${page/.pdf/-MARKED.jpg}" "${page/.pdf/-MARKED.pdf}"
+    done
+
+    qpdf --empty --pages ${tmpd}/${bname/%.pdf/-+([0-9])-MARKED.pdf} -- "$dst"
+
+    exit
+fi
+
+mark "$src" "$dst"
+#+end_src