IMAGE FORMAT LAB
Every “WebP is 30% smaller” claim depends on the picture. Put yours in, sweep the quality, and read the size against a measured similarity score instead of a blog post.
Run the sweep to encode this image at every quality step in each format your browser supports, and measure how close each result is to the original.
SSIM is the structural similarity index: it compares local patterns of brightness and contrast the way an eye does, rather than counting how far individual pixels moved. 1.000 is identical. Above about 0.99 the difference is usually invisible at normal viewing size; below 0.95 you can generally find it without looking hard. It is computed here on the luma channel with the usual 11×11 Gaussian window, and the difference view is the per-pixel error multiplied by eight so you can see where the encoder spent its errors — almost always on edges and fine texture.
Encoding happens in the browser, so the available formats are whatever your browser can write, not what this page wishes it could. That is checked rather than assumed: some browsers accept a request for a format they cannot encode and quietly hand back a PNG, which would show up here as a wildly large “AVIF”. Every encode is verified to be the format that was asked for, and anything else is reported as unsupported.
"The right quality setting is the lowest one where you cannot see the difference. That is measurable."