Fast Resize (libvips)

Batch image resize using libvips. Material throughput improvement over ImageMagick on large libraries — typical 5–10× speed-up and lower peak memory.

When to use

• Batch is >500 images.
• The user is feeling ImageMagick's slowness (or is about to).
• The job is "resize this directory" — pure resize without compositing or filters.

Do not use this skill for:

• Small batches (<100 images) — batch-resize is fine, no point swapping engines.
• Resize-with-filter / resize-with-text / multi-op pipelines — ImageMagick is the better tool for compound work.

Inputs

1. Input directory — required.
2. Target size — one of:
   • --width N — resize so width = N, height auto.
   • --height N — resize so height = N, width auto.
   • --max NxM — fit within NxM box (preserves aspect).
   • --exact NxM — force exact dimensions (may distort or crop).
3. Output mode — in-place (overwrite) or --output-dir <path> (default: <input>/resized/).
4. Filter — interpolation kernel: lanczos3 (default — best quality), cubic, linear, nearest.
5. Recursive — --recursive to descend into subdirectories. Default: top-level only.
6. Format preserve — by default keeps source format. --to <ext> to change format on the way out.

Procedure

1. Verify vips is on PATH. If missing, fall back to ImageMagick convert with the same parameters and warn that this will be slower (point at install-deps).
2. Enumerate input files (filter to image extensions: jpg, jpeg, png, webp, tiff, gif, heic, avif, jxl).

3. For each file, build the vips command. Two main forms:

   Fit within box (preserves aspect):

   vipsthumbnail "<input>" --size "<W>x<H>" --output "<output>[Q=85]"

   vipsthumbnail is the right tool for "resize down" — it's optimised for thumbnail-style ops and handles JPEG shrink-on-load (free 8× speedup for 1/8-size targets).

   Exact resize via vips:

   vips resize "<input>" "<output>" <scale-factor> --kernel <kernel>

   For --width/--height, compute scale = target / source. vipsthumbnail is preferred when shrinking; use vips resize when upscaling or when source dimensions vary widely.

4. Run in parallel where safe. vips is internally threaded; running multiple processes via xargs/parallel adds little. One process per CPU core is the sweet spot for large batches.
5. Report progress every N files (every 10% or every 100, whichever is more frequent).

Output

• Resized images at the resolved output paths.
• Summary: <n> files processed, <total-input-MB>MB → <total-output-MB>MB, <elapsed-s>s, avg <ms>ms/image.
• List of any files that failed (corrupt, unsupported format, permission errors).
• If output-dir was used and any source files had identical names from different subdirectories without --recursive flattening logic, flag the collision.

Notes

• vipsthumbnail accepts [Q=N] syntax for JPEG quality on output (default 75; 85 is a good balance).
• Wide colour gamut: vips preserves ICC profiles by default. If the user is processing display-P3 / Adobe RGB images, that's a feature.
• HEIC / AVIF / JXL: vips supports these if compiled with the right libs (apt's libvips-tools includes libheif and libjxl on recent Ubuntu).
• ImageMagick fallback path: if vips is missing, run convert "<input>" -resize <WxH> "<output>" per file. Same output, slower.