Abstract: Statistical methods are used to estimate the reflectance (250) of paper and align the tone gamut to the dynamic range of a digital input device such as a scanner during a single scan. A histogram (230) of the occurrence of each tone level is accumulated during the scan. The background colour of the image, i.e. the tone value of the paper is estimated by analysing the histogram (230). When this value is known, the parameters for a tone reproduction curve (TRC) (270) can be calculated (260). The shape of the TRC (270) is not altered by these parameters, i.e. the exposure can be controlled independently of the tone reproduction. This process is repeated from time to time during the scan. This increases the robustness with respect to unfavourable originals (e.g. originals with a dark border) and at the same time also can take into account such phenomena as the change of the response of the sensor due to thermal effect during slow scans.

Int'l Class: H04N001407

Priority: ;US 724711 19960930

Designated States: AT BE CH DE DK ES FI FR GB GR IE IT LI LU MC NL PT SE