The fringe names the mistake.

Colour and alpha can be stored multiplied together or not. Multiplied is premultiplied; not multiplied is straight. Both are correct, both are common, and almost nothing in a file reliably says which one it is. Reading one as the other does not corrupt the picture — it puts a halo round every soft edge, and the two mistakes have different signs.

The engine is free and needs no account. Premultiplied data read as straight gives an error of −Cf × a × (1−a): always negative, genuinely peaking at half coverage, so it is a dark band sitting inside the outline. Straight data read as premultiplied gives +Cf × (1−a): always positive, growing as the coverage falls, and it exceeds 1 so a display clamps it — a flat bright halo hugging the outside. Three signatures, and a screenshot is enough to tell which bug you have. The page also works out what undoing a premultiply costs at each bit depth, why a logo flattened over white keeps that white forever, and what one bit of alpha does to every partial coverage. Every lane ships a worked example that costs nothing. Writing the report needs credits.