Media Conversion Recipes

Command-line recipes for converting audio, video, and scanned images in bulk. Install the tools with apt install ffmpeg potrace imagemagick on Ubuntu, or nix profile add nixpkgs#ffmpeg nixpkgs#potrace nixpkgs#imagemagick with Nix (nix profile install on older Nix releases).

Make a Video from One Image and an Audio File

Turn a podcast episode or recording into an MP4 for video platforms by looping a single still image over the audio:

ffmpeg -loop 1 -framerate 1 -i picture.png -i audio.wav \
  -c:v libx264 -tune stillimage -pix_fmt yuv420p -vf scale=1280:720 \
  -c:a aac -b:a 192k -shortest -movflags +faststart episode.mp4

• -loop 1 -framerate 1 repeats the image at one frame per second, which keeps the file small.

• -tune stillimage and -pix_fmt yuv420p give an H.264 stream that plays everywhere.

• -shortest ends the video when the audio ends.

• -movflags +faststart lets playback start before the whole file downloads.

For smaller files on players that support HEVC, use -c:v libx265 instead (and drop -tune stillimage, which is x264-only).

Convert All Files in a Directory Tree

Convert every FLAC file under the current directory to AAC, recreating the same folder structure under an output directory:

find . -type f -iname '*.flac' -exec sh -c '
  for f; do
    out="$0/${f%.*}.m4a"
    mkdir -p "$(dirname "$out")"
    ffmpeg -nostdin -loglevel error -n -i "$f" -c:a aac -b:a 256k "$out"
  done
' <OUTPUT_DIR> {} +

Run it from the top of the source tree. <OUTPUT_DIR> is passed to the inner shell as $0. -n skips files that already exist, so you can safely re-run it after an interruption. -nostdin keeps ffmpeg from swallowing the file list. The .m4a container plays more widely than raw .aac.

Change the Container Without Re-encoding

Copy every stream into a different container format, for example MKV to MP4. This takes seconds because nothing is decoded:

ffmpeg -i input.mkv -map 0 -c copy output.mp4

If ffmpeg reports that a codec isn't supported in the target container (common with subtitles in MP4), drop that stream type, for example with -map 0 -map -0:s.

Vectorize Scanned TIFF Images to PDF

Trace black-and-white scans (line drawings, signatures, plans) into scalable vector PDFs. potrace reads bitmap formats such as PBM, so convert each TIFF to a thresholded bitmap first:

for f in *.[Tt][Ii][Ff]; do
  magick "$f" -colorspace Gray -threshold 50% "${f%.*}.pbm"
  potrace -b pdf -o "${f%.*}.pdf" "${f%.*}.pbm"
  rm "${f%.*}.pbm"
done

Adjust -threshold if thin lines vanish (raise it) or the background speckles (lower it). Use -b svg instead to get SVG files for editing in Inkscape. On ImageMagick 6, use convert instead of magick.