QRgb QImage::pixel ( const QPoint & position ) const
typedef QRgb
An ARGB quadruplet on the format #AARRGGBB, equivalent to an unsigned int.
The type also holds a value for the alpha-channel. The default alpha channel is ff, i.e opaque. For more information, see the Alpha-Blended Drawing section.
int qAlpha ( QRgb rgba )
Похоже то, что вам нужно.