Patent US 4,068,263

This invention provides a method for obtaining a video signal corresponding to selected parts of an image by separating from the video signal relating to the whole image that portion relating to the selected parts. The method includes a comparison of a boundary signal derived from the separated video signal with that relating to the whole image and adjustment of the separation to correct for inaccuracies in the separated signal in respect of the boundaries of the selected parts. The invention also provides apparatus for deriving the boundary signal and displaying the two signals for visual comparison.