I see most projectors with keystone correction state that it is 'digital'. This sounds to me like fudging a rectangular image into a trapesium shape so it looks square on a crooked screen. Is this a resampling of the image, and does it therefore compromise image quality? Is optical correction possible? Is it better? This would involve some sort of tilt/shift lens, but would be preferable, if more expensive. Anyone know what really happens?