Pierre Wellner
Xerox Technical Report - Part of PhD thesis
The image produced by a video camera pointing at black printing on a white sheet of paper is not truly black
and white. This image is grey-scale (or colour) no matter where the camera is pointed. Unless lighting on the
desk is carefully controlled, video images of paper lying on the desk are poor representations of the originals.
Unlike the inside of a scanner or photocopier, it is very difficult to guarantee even lighting across the surface
of a desk. This open space may be exposed to desk lamps, overhead lights, windows or moving shadows, all
of which can cause varying brightness across the page. Human vision compensates for this, but when the
image of paper on the desk is manipulated by machine without taking these variations into account, the results
can be very bad. This report describes the various techniques that were developed and tested for thresholding
on the DigitalDesk, and it ends with the description of an algorithm that was found to be suitable.
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