Fast Thumbnail (vipsthumbnail)

Generate thumbnails at high throughput using vipsthumbnail. Distinct from fast-resize: this is purpose-built for "make small previews of every image in this folder" workflows where output is always a downscale and quality is "good enough for a thumbnail".

When to use

• Building a contact-sheet / web-gallery / index of an image library.
• Pre-rendering thumbnails for a UI / filesystem browser.
• Bulk-shrinking large originals to share / archive.

Do not use this skill when:

• The user wants the same size as the originals — that's a no-op.
• The user is upscaling — use fast-resize (or upscale-image for AI upscale).
• The user wants to keep original quality — this skill assumes lossy thumbnail-grade output.

Inputs

1. Input directory — required.
2. Size — pixel size. Single number is shorthand for NxN (fit within square). Default: 512.
3. Output dir — default: <input>/thumbnails/. The skill never overwrites originals.
4. Format — output format. Default: jpg (best size/quality balance for thumbs). Options: jpg, png, webp, avif.
5. Quality — JPEG/WebP/AVIF quality. Default: 80.
6. Recursive — --recursive to descend. Mirrors subdirectory structure under the output dir.
7. Naming — default: same base name. --prefix <s> or --suffix <s> if collision is a risk.

Procedure

1. Verify vipsthumbnail is on PATH. If missing, point at install-deps. No graceful fallback — if vips isn't installed, the skill's whole reason to exist is gone; tell the user to install or use batch-resize.
2. Enumerate input files (image extensions only).

3. For each file:

   vipsthumbnail "<input>" \
     --size "<size>" \
     --output "<output-dir>/<basename>.<ext>[Q=<quality>]"

   For non-JPEG outputs, the [Q=N] syntax still works for WebP/AVIF.

4. For very large batches, parallelise with find ... | xargs -P <ncpu> -I{} vipsthumbnail .... vips is internally threaded but per-image work is small enough that process-level parallelism wins on >5k batches.
5. Track progress and skipped files (corrupt, wrong format).

Output

• Thumbnails at <output-dir>/.
• Summary: <n> thumbnails, <total-output-MB>MB, <elapsed-s>s, avg <ms>ms/image.
• Failed files listed.

Notes

• vipsthumbnail shrink-on-load: for JPEG sources, it decodes at the smallest DCT scale ≥ target — so generating a 256-px thumbnail from a 4000-px JPEG is much faster than full decode + resize. This is the headline reason to use it.
• Sharpen-on-shrink is on by default (vipsthumbnail applies a mild sharpen after downscale). Add --no-rotate and explicit --linear flags only if Daniel asks; the defaults are sensible for thumbnails.
• For contact-sheet layout (multiple thumbs into a grid image), this skill produces the inputs; combine with ImageMagick montage separately.
• ICC handling: vipsthumbnail strips ICC by default to keep file sizes small (right call for thumbs). If colour-accurate thumbs are needed, pass --export-profile <path-to-srgb.icc>.