transcode-jpeg-to-jxl-lossless

Losslessly Transcode JPEG to JPEG XL

Walk a directory of JPEG files and convert them to JPEG XL (.jxl) format using lossless encoding. cjxl automatically detects JPEG bitstream and preserves it without re-compression. Reports byte savings; explains how to recover the original JPEG if needed.

When to use

• Archiving JPEG photo libraries to save space (typically 15–25% reduction) without image degradation.
• Preparing a photo collection for long-term storage where compatibility with JPEG is not required.
• Building a smaller-footprint JXL archive that can still yield the original JPEG via djxl --jpeg.

Inputs to gather

• Source directory — folder containing JPEG files (.jpg or .jpeg). Default: current directory.
• Recursive? Ask whether to process subdirectories. Default: no (flat, current folder only).
• Output location — optional. Default: same directory as originals (.jxl files sit alongside .jpg).
• Dry run first? Recommend a preview pass showing the count and byte savings estimate, before committing to the conversion.

Procedure

1. Check tooling

command -v cjxl || echo "cjxl not installed — install with: sudo apt install libjxl-tools"

Abort if missing.

2. Preview

Show the user what will happen:

# Count JPEGs in scope
find <target> -maxdepth <1|999> -type f \
  \( -iname '*.jpg' -o -iname '*.jpeg' \) | tee /tmp/jpegs.txt | wc -l

# Estimate original size
find <target> -maxdepth <1|999> -type f \
  \( -iname '*.jpg' -o -iname '*.jpeg' \) -exec du -ch {} + | tail -1

Print:

• Number of JPEG files found.
• Total original size.
• Estimated output size (advise 20% reduction as a rough baseline).
• Confirm before proceeding.

3. Transcode

Use -d 0 (lossless) and a reasonable effort (suggest 6–7 for balance):

Folder (non-recursive):

find <target> -maxdepth 1 -type f \
  \( -iname '*.jpg' -o -iname '*.jpeg' \) \
  -exec cjxl -d 0 -e 7 {} {.}.jxl \;

Recursive: