vips-wasm

libvips compiled to standalone WASM for image thumbnailing. No native dependencies — runs in any WASI-compatible runtime.

Download

Pre-built vips-thumbnail.wasm is available from GitHub Releases.

WASM Interface

Protocol

• Input: image bytes piped to stdin
• Output: thumbnail bytes written to stdout
• Errors: diagnostic messages on stderr

Arguments (argv)

argv[0]: "thumbnail"    (program name)
argv[1]: <width>        (pixels, required)
argv[2]: <suffix>       (".jpg", ".png", ".webp", ".avif")
argv[3]: <height>       (pixels, 0 = preserve aspect ratio)
argv[4]: <quality>      (1-100, 0 = encoder default; applies to JPEG/WebP/AVIF)
argv[5]: <strip>        (1 = strip metadata, 0 = keep)

Aspect ratio is always preserved: the result is constrained within the requested width/height box (or width-only when height=0). Images are always scaled down (VIPS_SIZE_DOWN), so inputs smaller than the target box are returned unchanged.

Supported formats

Format	Read	Write	Suffix
JPEG	yes	yes	.jpg
PNG	yes	yes	.png
WebP	yes	yes	.webp
AVIF	yes	yes	.avif
HEIF/HEIC	yes	no (decode-only)	n/a

Emscripten host imports

The WASM module uses WASI preview1 syscalls plus Emscripten-specific imports for setjmp/longjmp error handling:

• invoke_* trampolines (signatures auto-detected from the WASM import list)
• setThrew(threw, value)
• _emscripten_throw_longjmp()
• emscripten_stack_get_current() → i32
• _emscripten_stack_restore(sp)

Host runtimes must implement these. See the reference implementations below.

Reference implementations

Go (wazero)

main.go — full wazero host with:

• Automatic invoke_* trampoline registration from WASM imports
• setjmp/longjmp via panic/recover with sentinel value
• Persistent compilation cache (~/.cache/vips-thumb/)
• Output format detection from file extension

Library package: govips

out, err := govips.Resize(ctx, inputBytes, govips.ResizeOptions{
    Width:        300,
    Height:       0,
    Format:       govips.FormatWebP, // required
    Quality:      80,
    KeepMetadata: false,
})

go build -o vips-thumb .
./vips-thumb input.jpg output.jpg 300
./vips-thumb input.jpg output.webp 300 200
./vips-thumb -q 80 input.jpg output.webp 300
./vips-thumb -keep-metadata input.jpg output.jpg 300

Deno

deno/run.ts — WebAssembly.instantiate with:

• Custom minimal WASI preview1 shim
• Native JS try/catch for setjmp/longjmp
• Proxied late-binding for circular import dependencies

deno run -A deno/run.ts input.jpg output.jpg 300
deno run -A deno/run.ts input.jpg output.webp 300 200 80
deno run -A deno/run.ts input.jpg output.jpg 300 0 0 --keep-metadata

Building from source

Prerequisites

• Docker

Build the WASM

./build.sh

Output: vips-thumbnail.wasm (~9 MB)

Build the Go CLI

go build -o vips-thumb .

Libraries

The following libraries are statically linked into the WASM binary:

Library	Purpose
zlib-ng	Compression
libffi	Foreign function interface (for GLib)
GLib	Core utilities for libvips
Expat	XML parsing
libexif	EXIF metadata
Little-CMS 2	ICC color management
MozJPEG	JPEG codec
libpng	PNG codec
libwebp	WebP codec
libde265	H.265 decoder (for HEIF)
libaom	AV1 codec (for AVIF)
libheif	HEIF/AVIF container
libvips	Image processing