[Interest] QImage detect full transparent

[maitai]

Hello,
Seems quite basic, but I need a **fast** way to know if a QImage is just 
full transparent. I know the size in advance (256x256). If it were a 
QPixmap I would use cacheKey() and compare with the cacheKey of a full 
transparent image. For a QImage I can compare it with a preset 
transparent image, or extract the alpha channel, or etc of course. But I 
need a really fast way.

Any idea?

Thanks
Philippe.