Image Compressor

This image compressor makes JPG, PNG and WebP files smaller without changing their pixel dimensions, entirely inside your browser. It is for anyone who has hit an upload limit, needs a lighter image for a website or email, or has to get a photo under a size such as 200 KB — and would rather not send private pictures to a server to do it.

JPG and WebP are compressed by re-encoding them at a lower quality with the browser's own encoder. Quality 75, the default, usually cuts a camera JPG substantially with little visible change; lower values shrink the file further and eventually show blocky artefacts. In "Target size (KB)" mode you type the size you need and the tool searches for the highest quality that fits: it tries quality 95 first, then halves the range each time, so it settles in at most 8 encodes instead of trying every value. For PNG the same search runs over palette sizes that are powers of two from 2 to 256 colours (at most 4 encodes). If even quality 1 is too big, it gives you that smallest version and says the target cannot be reached — resizing the image smaller is then the only way further.

PNG needs a different approach. PNG is lossless, and re-saving a PNG from a canvas usually makes it no smaller, often larger. What does work is reducing the number of colours. This tool builds a palette of at most 256 colours with the median-cut algorithm — repeatedly splitting the set of colours in the image where it is widest — and writes an indexed-colour PNG that stores one small palette index per pixel (1, 2, 4 or 8 bits) instead of four full bytes. Optional Floyd–Steinberg dithering spreads each pixel's rounding error to its neighbours so gradients do not band. Fully transparent areas stay exactly transparent. If an image already has 256 colours or fewer (a logo, a screenshot of flat UI), the palette is exact and nothing changes visually. Screenshots, charts and illustrations typically shrink a lot; photos saved as PNG shrink too but are usually better off as JPG.

Every result shows the size before and after and the percentage change. If the compressed version is not smaller than the original — which happens with files that were already well optimised — the tool keeps your original untouched, says so, and offers that for download instead of a worse file.

Metadata is removed as part of the process: EXIF camera data, GPS location, dates and embedded colour profiles are not written to a compressed file, and photos are saved upright. (An original kept because compression did not help is returned exactly as it was, metadata included.) That is often a privacy win, but it means a photo with a wide-gamut colour profile may look slightly different.

Everything happens locally, up to 50 files of 50 MB each per batch; PNGs larger than 40 megapixels are refused with a message, because reducing their colours would need several hundred MB of memory. Your images are decoded, compressed and offered back from the page's memory; you can confirm in your browser's developer tools (Network tab) that no request carries your files.

1. Each file's signature is checked: only real JPG, PNG and WebP files are accepted, whatever their names say. 2. The image is decoded by the browser (EXIF orientation applied) and drawn onto a canvas at its original size. 3. JPG/WebP, quality mode: the canvas is encoded at the chosen quality (default 75). 4. PNG, quality mode: the pixels are read; if there are no more distinct colours than the chosen palette size, that exact palette is used. Otherwise a 5-bit-per-channel RGBA histogram is built and median cut splits it into that many colours (one entry reserved for full transparency); each pixel is mapped to its nearest palette colour, with optional Floyd–Steinberg dithering (7/16, 3/16, 5/16, 1/16). 5. The indexed PNG is written by this page's own encoder: IHDR (colour type 3), PLTE, tRNS only if some colours are translucent, one IDAT compressed with the browser's zlib (CompressionStream "deflate") using filter type None as the PNG specification recommends for indexed images, and IEND. No metadata chunks are written. 6. Target-size mode: a binary search over quality 1–95 (JPG/WebP) or palette sizes 2, 4, 8, … 256 — powers of two — (PNG) keeps the highest setting whose output is at or under the target. 7. The output is compared with the original. If it is not smaller, the original is kept and you are told.