HP Labs Technical Reports

Dynamic Exposure Control In Color Scanners

Beretta, Giordano

HPL-97-30

Keyword(s): image processing; color facsimile; scanning; sheet-fed; shading correction; read data normalization; exposure enhancement

Abstract: The lightness values of white papers cover and approximate range of fifteen jnd. The tone range of scanners is adjusted for the lightness possible substrate. Therefore, a scan is usually preceded by a preview operation in which the image is subsampled and the data is analyzed to determine the actual tone range. In color facsimile and sheet-fed scanners that do not buffer the entire image, such an operation is not possible. We present a technique in which statistical methods are used to estimate the tone level of the paper. This estimate is used to set the parameters for a tone reproduction curve. The technique is incremental, the statistical data is gathered during the scan. While the scan progresses, the estimate is refined based on the increased amount of data available from the accumulated histogram. This has also the advantage that artifacts due to lamp warming during slow scans (typical for color facsimile) are automatically compensated.

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