Lossless image compressor

Lossless compression rewrites a file so it occupies less space while every pixel value survives exactly. Decode the result and you get the original back, byte for byte in the pixel data. Nothing is approximated, and no artefact is introduced at any zoom level.

That guarantee is the whole point, and it is also the whole limitation: an algorithm forbidden from discarding anything can only find savings in redundancy that is genuinely there. Where a lossy encoder can throw away detail you would never notice and shrink a photograph by 90%, a lossless one can only rearrange.

JPEG is a lossy format by construction. Its compression step quantises frequency coefficients and throws information away — that is not a setting, it is the design. There is no quality value at which a JPEG encoder becomes lossless.

Tools that advertise "lossless JPEG compression" are usually doing one of two different things: stripping metadata without touching the image, or losslessly rearranging the existing JPEG data with a tool such as jpegtran. Both are legitimate; neither is what most people picture, and both save far less than the headline suggests. We would rather not offer a JPEG option here than let the word "lossless" do work it cannot support.

If you need genuinely lossless output from a photograph, the answer is PNG or lossless WebP — and a much larger file.

Three places, in rough order of size. First, metadata: colour profile comments, text chunks, editing history and timestamps can be a surprising fraction of a small file. Second, a more efficient encode than the original tool produced — browsers pick reasonable filters, and some software does not. Third, converting to lossless WebP, which is typically 25% smaller than PNG for the same image with the same alpha channel.

What you will not see is the 70–80% figure that lossy tools quote. If a lossless compressor claims that, it is quantising the palette — reducing the image to 256 colours — which is a lossy operation whatever it is called. This tool does not do that, which is why its numbers are more modest and genuinely reversible.

Lossless compression rewrites the file so it takes up less space while every pixel value survives exactly. Decode the result and you get the original back, byte for byte. PNG, GIF and lossless WebP work this way, and so does a ZIP archive.

Lossy compression throws information away on purpose. JPEG converts the image to a frequency representation, then discards the high-frequency detail your eye is least likely to miss. The result cannot be reversed: the discarded detail is gone. That is also why lossy formats reach file sizes lossless ones cannot approach — a photo saved as PNG is routinely five to ten times larger than the same photo saved as JPEG at a sensible quality.

Neither one is better. Photographs, with their smooth gradients and noise, suit lossy encoding. Screenshots, logos, line art, charts and anything with large flat areas and hard edges suit lossless encoding, because lossy encoders produce visible haloes around sharp boundaries.