Heads-up displays analysed
Heads-up displays (HUDs) are semi-transparent dialogs. That allows them to be less obstructive than old skool dialogs and more part of the content one is working on. These properties make it possible to move towards a general interaction where the image window is the preview.
Previews are an area with limited number of pixels that make predictrable what the effeect of a certain operation will be on all the image pixels. The number of screen pixels of an image windows is also limited, by the hardware.