pptx2png

Render each slide of a PPTX as a PNG using LibreOffice headless in a Chainguard container. Two-stage build: a wolfi-base builder resolves the package closure with apk; the runtime image (glibc-dynamic) ships without a package manager.

Pipeline: soffice --headless converts the deck to PDF (LibreOffice's direct PNG export only renders the first slide), then poppler's pdftoppm rasterizes each page.

Usage

./pptx2png.sh deck.pptx            # PNGs land in ./out at 150 dpi
./pptx2png.sh deck.pptx slides 300 # custom output dir and dpi

The wrapper builds the pptx2png image on first run, then invokes the container with the deck mounted read-only at /in and the output dir at /out. The entrypoint is convert.py (Python 3.13), which drives the conversion synchronously — exit 0 means every slide rendered.

Why LibreOffice-on-distroless works here

The usual advice is that putting LibreOffice in a distroless image is painful, and for the hand-rolled approach it is: COPY its install dir, then chase shared libraries with ldd until it stops crashing. That fails on LibreOffice because it dlopens filters and plugins at runtime that ldd can't see, and the file list changes every release. Guides that mix distros (Debian's LibreOffice on gcr.io/distroless) add glibc/layout mismatches on top.

This build never curates files by hand. The builder stage runs apk add --root /rootfs, which makes apk resolve the entire dependency closure from package metadata into a directory; the runtime stage copies that one directory onto glibc-dynamic. Same Wolfi package universe on both sides, so nothing mismatches — the same idea Chainguard's apko tooling uses, done with stock apk in a Dockerfile.

The distroless sharp edges that remain (each already handled in this repo, listed for the next change):

• Chrooted post-install scripts: apk runs package scripts chrooted in /rootfs; gtk's need /bin/sh there. Hence busybox installs first and is apk del'd after — don't "simplify" that away.
• Triggers don't run: the fontconfig cache is built from the builder via fc-cache --sysroot /rootfs. New font packages get picked up by that same line.
• soffice is a shell script: the runtime has no shell, so convert.py execs soffice.bin directly. That bypasses the normal launch chain — soffice (sh) → oosplash (ELF) → soffice.bin.
• Exit code 81: EXITHELPER_NORMAL_RESTART — benign. soffice.bin exits 81 after initializing its user profile and expects a relaunch, which oosplash normally performs. This no longer occurs in practice: the build stage pre-runs that bootstrap (chrooted into /rootfs) and bakes the initialized profile into the image at /opt/lo-profile, and convert.py copies it into a fresh throwaway profile per run (what makes concurrent runs safe) — so every conversion is a single soffice launch. convert.py still handles 81 with one relaunch purely as a fallback, e.g. if a LibreOffice upgrade invalidates the relocated profile template.
• No shell for anything else either: runtime work must be direct binary execs — no shell=True, no shell-script entrypoints, and docker exec debugging won't work. Debug in the builder stage instead.

Fonts

The image bundles metric-compatible substitutes for the common PowerPoint fonts (Carlito→Calibri, Caladea→Cambria, Liberation→Arial/Times/Courier) plus Noto, Noto Emoji, and DejaVu. If a deck renders with shifted layout, a missing font is the first suspect:

docker run --rm --entrypoint fc-match pptx2png "Calibri"

Add more fonts by appending Wolfi font-* packages to the builder stage's apk add --root list in the Dockerfile.