Why reductions go wrong
Shrinking a picture 4× means every output pixel stands for sixteen input pixels. A resizer that samples only a few of them — nearest neighbour, or a fixed-size bilinear filter — skips detail, and repeating patterns alias into moiré. Thin lines appear and disappear along their length.
The fix is old and well understood: stretch the filter so it covers the whole footprint of each output pixel. Every method here does that automatically when reducing, except Nearest, which you can pick to see the problem for yourself.
Box or Lanczos?
Box (area averaging) is exactly fair: each output pixel is the mean of what it covers. It never rings and suits screenshots and charts. Lanczos-3 keeps a little more crispness for photographs. Both run in linear light by default, so a one-pixel black-and-white pattern becomes the grey you actually perceive rather than a darker one.
Questions
How do I downscale without losing quality?
Use a widened filter in linear light — Auto does this — and save as PNG, or JPEG/WebP at quality 85 or higher. At the new size, the result shows everything the smaller pixel grid can hold.
Should I sharpen after downscaling?
Usually not with Lanczos-3, which is already crisp. If you prefer a softer, artefact-free look, choose Box or Mitchell.
What does “linear light” mean, and why is it on by default?
Pixel values in a JPEG or PNG are gamma-encoded: value 128 is not half as bright as 255, it is about a fifth. Averaging those numbers directly — what most resizers do — darkens fine bright detail and muddies colour edges. With linear light on, every pixel is converted to physical light before it is weighted and converted back afterwards. The test chart’s one-pixel checkerboard shows the difference: reduced in linear light it matches the light grey patch beside it; reduced the naive way it matches the dark one.
Is my picture uploaded anywhere?
No. Decoding, scaling and encoding all run in this browser tab, and the scaling engine runs in a Web Worker with no network code in it. The only download the site can make for you is an AI model file, and only after you press the button that names its size — that request contains no picture.