Image Resizer

How It Works

This tool runs entirely in your browser. Your images are never sent to a server — all processing happens client-side using wasm-vips, a WebAssembly build of libvips, the same high-performance image processing library used by sharp (Node.js) and many server-side pipelines.

Loading the engine

On first visit, the browser downloads a ~4.5 MB WebAssembly module (vips.wasm) from a CDN and compiles it inside a Web Worker. This keeps the UI responsive while the engine initializes. Once loaded, the status badge turns green and the Resize button becomes active. The browser caches the WASM file, so subsequent visits load almost instantly.

Reading the file

When you upload or drop an image, the browser's FileReader API reads it as both an ArrayBuffer (sent to the worker for processing) and a data URL (used for the preview thumbnail). The raw bytes are passed directly to libvips, which can decode JPEG, PNG, WebP, TIFF, GIF, and many other formats natively.

Resizing

The worker calls vips.Image.thumbnailBuffer(), which is significantly more efficient than a naïve decode-then-resize pipeline. For JPEG inputs, it exploits the format's built-in 2×/4×/8× shrink-on-load, drastically reducing memory use and CPU time for large downscales. The resampling kernel is Lanczos3, a high-quality windowed sinc filter that preserves sharp edges and fine detail far better than the bilinear interpolation used by the HTML Canvas API.

Format conversion & encoding

After resizing, libvips encodes the result into the chosen format:

• PNG — Lossless compression using the DEFLATE algorithm. Every pixel is preserved exactly, which makes files larger but guarantees no quality loss. The quality slider has no effect.
• JPEG — Lossy compression with optimized Huffman coding enabled. The quality slider controls quantization — lower values discard more frequency coefficients, producing smaller files with more visible artifacts.
• WebP — A modern format using predictive coding and arithmetic entropy coding, typically achieving 25–35% smaller files than JPEG at equivalent visual quality. The quality slider works the same way as for JPEG.

EXIF metadata & ICC profiles

When resizing, EXIF data (camera model, GPS coordinates, date/time, etc.) is stripped from the output for privacy, while ICC color profiles are preserved so images maintain accurate color rendering across displays. This is controlled via the ForeignKeep.icc flag in libvips.

Strip EXIF Only (lossless)

The Strip EXIF Only button removes EXIF metadata from a JPEG without re-encoding. It works at the binary level: the JPEG file is parsed segment-by-segment, and any APP1/Exif segments are removed while the compressed image data is left completely untouched. This means zero generation loss — every pixel remains bit-identical to the original. The file size will decrease slightly (by the size of the removed metadata). This is ideal when you just want to strip GPS coordinates or camera info before sharing a photo.

Download

The encoded bytes are transferred from the worker back to the main thread, wrapped in a Blob, and turned into an object URL (URL.createObjectURL). Clicking Download triggers the browser's native file-save dialog.

Privacy

Your image data never leaves the browser. The WASM engine, Web Worker, and all encoding happen in local memory. Refreshing or closing the page discards all data. The only network request is fetching the wasm-vips library itself from a CDN.