Auto optimize

Auto optimize chooses the output format and the compression quality for each image by measuring the image itself, then lists every decision in a ledger under the queue. The format comes from what the file actually is — how many distinct colours it holds, whether it has transparency, and whether this browser can write AVIF. The quality comes from a search that encodes the image, decodes it back and compares luminance, settling on the lowest quality whose error stays inside the budget you picked.

How it works

  1. Drop your images Add the files. Each one is read once and reduced to a small sample, so changing a setting later re-decides without decoding again.
  2. Pick a quality budget Smallest, Balanced (the default) or Best looking. This is the one number everything else follows from: it sets how far the pixels are allowed to move from the original.
  3. Set the largest edge The default caps the long side at 2400 px, which is usually the biggest saving of all. Clear the field to write every image at the size it arrived.
  4. Read the ledger Under the queue, each row names the chosen format, the chosen quality, the output dimensions and the reason — photograph or flat art, colour count, and whether there is transparency.
  5. Process and download Press process to apply the decisions at full resolution, then download. A batch comes back as a zip.

Questions

How does it decide which format to use?
AVIF first, when the browser can write it and you have left the AVIF toggle on, because it is the smallest of the four and keeps transparency; WebP next for the same reason. Only when neither is available does it fall back — PNG for anything with transparency or too few colours for a lossy codec, JPEG for a photograph without transparency.
How does it choose the quality number?
It encodes the image, decodes the result and measures the root-mean-square luminance difference from the original, then binary-searches for the lowest quality whose error stays inside the budget — about 3.2 luminance values for Balanced, 6.5 for Smallest and 1.6 for Best looking. The search runs on a copy scaled to 480 px on its long edge so half a dozen encodes stay quick; the chosen quality is then applied to the full-size export.
Can it handle PDFs and animated GIFs?
No. PDFs are turned away when you drop them, with a pointer to the PDF shrink tool, and an animated GIF comes out as a single still frame — the page warns you when it sees one.

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