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Browse Prior Art Database

DYNAMIC WHITE AND BLACK LEVEL CONTROL FOR DOCUMENT IMAGING

IP.com Disclosure Number: IPCOM000007908D
Original Publication Date: 1996-Nov-01
Included in the Prior Art Database: 2002-May-03
Document File: 2 page(s) / 105K

Publishing Venue

Motorola

Related People

Paul Guidarini: AUTHOR

Abstract

In document copying applications, it is very important that the subject is evenly illuminated. If the illumination of the subject is not even, or if the lens design is such that more light is attenuated at the fringes of the field than at the center, the resulting copy quality will be very poor. This is especially true if the image is 1 bit thresholded to black or white.

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Technical Developments

DYNAMIC WHITE AND BLACK LEVEL CONTROL FOR DOCUMENT IMAGING

by Paul Guidarini

  In document copying applications, it is very important that the subject is evenly illuminated. If the illumination of the subject is not even, or if the lens design is such that more light is attenuated at the fringes of the field than at the center, the resulting copy quality will be very poor. This is especially true if the image is 1 bit thresholded to black or white.

  The invention disclosed here performs dynamic white level and black level adjustments to compen- sate for uneven lighting conditions in document imag- ing applications. There was a specific need to com- pensate for the loss of light at the fringes of a wide angle lens. The approach described here compen- sates not only for uneven illumination due to the optics but also for uneven ambient lighting of the subject. The invention is suitable for use with 1 bit compression techniques such as CCITT Group 3 and 4 Fax as well as byte-wide compression tech- niques such as JPEG or TIFE

  The invention utilizes the fact that the majority of the image in most circumstances will be white, or a color that is lighter than the ink. Because ofthe light fall-off of the lens and CCD array, if a solid white subject were used, the video signal for each line would be hump shaped. The video signal for each line is proportional to the light incident on each pixel in that line. As a line of the CCD is read out in a shift-register-like format, left first and right last, the amplitude ofthe signal will start low at the left side of the image, increase towards the center of the line, and decrease towards the right side of the image (Figure 1). Also, the overall peak amplitude for lines near the center of the CCD array image will have greater amplitude than lines near the top and bottom of the image since the illumination is

best for the center. Once text, graphics, or other dark areas are placed on the subject, negative excursions will be pr...