Downsize an image

Moiré is a false pattern — waves or bands — that appears when fine repeating detail such as fabric, brickwork or a screen door is sampled too sparsely. A reduction samples the picture at a lower rate, and below half size an ordinary kernel samples too sparsely to see the fine pattern correctly, so it folds into a coarser one that was never there.

It averages whole blocks of source pixels first — an integer area-average pass — and only then runs the chosen kernel for the remainder. The averaging removes exactly the detail that would fold, so what is left can be resampled cleanly.

Lanczos keeps the most sharpness and is the automatic choice for reductions. On line art and interface screenshots it can leave a thin bright rim beside hard edges; bicubic gives up a little acuity and does not ring. The kernel strip under the stage shows every kernel on your own picture.

Almost always, because there are fewer pixels to store, but the exact length depends on the format and quality you write. The written length is read back from the saved file and shown with its SHA-256. To hit a limit in kilobytes, the size-limit page searches quality first and pixels second, and says which moved.

Often a little, yes. A reduction averages away some edge contrast that was genuinely in the picture, and the downscale sharpening control restores it with a small unsharp mask after the resample. Leave it at zero if the result is going to be scaled again by a page.