vips-wasm

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

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

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

argv[0]: "thumbnail"    (program name)
argv[1]: <width>        (pixels, required)
argv[2]: <suffix>       (".jpg", ".png", ".webp", ".avif", ".jxl")
argv[3]: <height>       (pixels, 0 = preserve aspect ratio; must be > 0 for crop/force)
argv[4]: <quality>      (1-100, 0 = encoder default; applies to JPEG/WebP/AVIF/JXL)
argv[5]: <strip>        (1 = strip metadata, 0 = keep)
argv[6]: <mode>         (optional: "fit" (default), "crop" or "force")

argv[6] may be omitted; the 5-argument form behaves as fit.

crop and force drop VIPS_SIZE_DOWN on purpose: with it, an input smaller than the box would come out smaller than requested. They exit with an error if height is 0.

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.

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,
    Mode:         govips.ModeFit, // default; or ModeCrop / ModeForce
})

// Exactly 300x200, centre-cropped. Height must be > 0 for crop and force,
// otherwise Resize returns ErrInvalidOptions.
out, err = govips.Resize(ctx, inputBytes, govips.ResizeOptions{
    Width:  300,
    Height: 200,
    Format: govips.FormatWebP,
    Mode:   govips.ModeCrop,
})

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

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
deno run -A deno/run.ts input.jpg output.webp 300 200 --mode=crop

The runner takes --mode=fit|crop|force (default fit) and passes it as argv[6]. crop and force need a height > 0.

• Docker

./build.sh

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

go build -o vips-thumb .

The following libraries are statically linked into the WASM binary: