Pixelate / blur region

That depends on the mode. Pixelate overwrites the region with flat mosaic blocks, and at the default block size of 10 that destroys the covered detail for good, so there is no hidden layer underneath, unlike a black rectangle drawn on top of a PDF. Blur only averages nearby pixels together, and at the default radius of 4 that averaging is light enough that the covered detail is still reconstructable from the file. Your own device keeps a separate copy regardless, since the input file stays untouched.

No. There is no detection of any kind. On the page you drag the rectangle over the picture; the CLI and MCP take it as four numbers instead, Left edge and Top edge of the region, both default 0, plus Region width and Region height, both blank by default, in pixels or, with Unit set to percent, as a share of the image. Leaving the size blank stops with "give both a width and a height for the region".

Pixelate for anything that must not be recoverable: it defaults to averaging each Mosaic block size block, 10 pixels, into one flat colour, and a block that size destroys the covered detail for good. Blur only obscures: it box-blurs each pixel over Box-blur radius, default 4, smoothing the region without erasing it, so the original is still reconstructable from the file. Reach for pixelate, not blur, whenever the redaction has to hold up.

Because the rectangle runs past the edge of the picture. The left edge plus the width must fit the image width, and the top edge plus the height must fit the image height, and the message repeats your numbers and the real image size. All four values must be whole numbers of 0 or more.

One rectangle per run, so run the tool again on the output for a second area. The rest of the image is left exactly as it was, except that the whole file is re-encoded, at the Quality option, default 80, when the format is lossy. The result is named with a -redacted suffix in the original format.

No. The image is decoded, edited and encoded in a Web Worker in this tab with the jSquash codecs. Nothing is uploaded, there is no account, and the page keeps working offline after the first visit, which matters when the thing you are hiding is the reason you came.