A clear answer. Here’s how.
Copies each decoded alpha value into red, green and blue and sets output alpha to 255. White represents opaque, black transparent and grey partial coverage. Source RGB is ignored. An opaque JPEG therefore produces an all-white mask. This extracts existing transparency rather than estimating an object mask. Supports PNG/JPEG/WebP up to 10 MB and 4 million decoded pixels per file. Browser decoding and single-frame PNG re-encoding remove source metadata/colour profiles; no files leave this device.