I expected image pixelation to mean per-pixel math — this tool mostly just redraws the same rectangle twice

The first time I looked at a "blur or mosaic just part of a photo" tool, I assumed the hard part would be the image-processing math. In this implementation, the more interesting part is actually not a giant manual pixel loop. The mosaic effect is mostly a neat canvas trick: draw the selected region smaller, then scale it back up with smoothing disabled. The blur path is different again — it leans on the browser's built-in canvas filter, but only inside a clipped rectangle. Pixelation