Add fabric print variant with black replaced by dark grey
When printing on light fabric, solid black bleeds out, making QR codes
hard to scan and text fuzzy. ci/pdf-lighten-black rewrites all vector
pure black in a finished PDF to a dark grey (default 15%, configurable)
via a qpdf QDF round-trip: text and vector graphics stay vector and
embedded raster images are untouched. The CI now additionally produces
target/skeleton_fabric.pdf.
Ported from the DeLiKo@DNB poster project.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Change-Id: I594a1a813284e8d4f9d6c3a97f0d3d56c36162a6
diff --git a/ci/pdf-lighten-black b/ci/pdf-lighten-black
new file mode 100755
index 0000000..e24b565
--- /dev/null
+++ b/ci/pdf-lighten-black
@@ -0,0 +1,41 @@
+#!/bin/sh -eu
+# pdf-lighten-black IN.pdf [OUT.pdf] [GREY]
+#
+# Produce a copy of IN.pdf in which all *vector* pure black is replaced by a
+# dark grey (default 15% = #262626). Intended for printing on fabric, where
+# solid black ink bleeds out, making QR codes hard to scan and text fuzzy.
+#
+# Works on the PDF content streams directly (qpdf QDF round-trip), so text
+# stays text and vector graphics stay vector -- nothing is rasterized and the
+# file works at any print resolution. Colors inside embedded raster images
+# (e.g. screenshots) are not touched.
+#
+# GREY is the target grey level in [0,1) per RGB channel (0.15 = 15% = #262626).
+
+in=$1
+out=${2:-${in%.pdf}_fabric.pdf}
+grey=${3:-0.15}
+
+k=$(awk -v g="$grey" 'BEGIN { printf "%.4g", 1 - g }')
+
+tmpdir=$(mktemp -d)
+trap 'rm -rf "$tmpdir"' EXIT
+
+# QDF mode writes an editable, uncompressed PDF whose stream lengths can be
+# repaired by fix-qdf after editing.
+qpdf --qdf --object-streams=disable "$in" "$tmpdir/in.qdf"
+
+# Rewrite the color-setting operators for pure black:
+# `0 0 0 rg/RG` (DeviceRGB), `0 g/G` (DeviceGray), `0 0 0 sc/scn` variants,
+# and `0 0 0 1 k/K` (DeviceCMYK, mapped to (1-GREY) K).
+# The lookarounds keep numbers like `10 g` or names like `/GS0 gs` intact.
+perl -pe "
+ s/(?<![\\d.])0 0 0 (rg|RG|scn|sc|SCN|SC)(?![\\w])/$grey $grey $grey \$1/g;
+ s/(?<![\\d.])0 (g|G)(?![\\w])/$grey \$1/g;
+ s/(?<![\\d.])0 0 0 1 (k|K)(?![\\w])/0 0 0 $k \$1/g;
+" "$tmpdir/in.qdf" | fix-qdf > "$tmpdir/grey.qdf"
+
+# Rewrite as a normal, compressed PDF.
+qpdf "$tmpdir/grey.qdf" "$out"
+
+echo "pdf-lighten-black: wrote $out (black -> ${grey} grey)" >&2