fast-resize

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>