Artifacts in the alpha channel are surprisingly subtle. Well, it makes sense if you think about it: The opacity is low, which makes those little details very ha

Artifacts in the alpha channel are surprisingly subtle. Well, it makes sense if you think about it: The opacity is low, which makes those little details very hard to see. A gray-scale JPEG with medium quality (even if it was written by a somewhat crude encoder) will look perfect if used as alpha channel in the vast majority of cases.