Luminance correction for color scanning using a measured and derived luminance value

• US 6,614,471 B1
• Filed: 05/10/1999
• Issued: 09/02/2003
• Est. Priority Date: 05/10/1999
• Status: Expired due to Fees

• Current Assignee: BancTec Incorporated
• Original Assignee: BancTec Incorporated
• Inventors: Ott, Cynthia D.
• Primary Examiner(s): Garber, Wendy R.
• Assistant Examiner(s): MISLEH, JUSTIN P
• Application Number:
  US09/309,024
• Time in Patent Office: 1,576 Days
• Field of Search: 348/234, 348/235, 348/236, 348/237, 348/238, 348/272, 348/273, 348/280, 348/281, 358/505, 358/513, 358/514, 358/518, 358/482, 358/483
• US Class Current: 348/238
• CPC Class Codes: H04N 1/486 with separate detectors, each detector being used for one specific colour component
• CPC Class Codes: H04N 1/60 Colour correction or control H04N1/54 takes precedence
• CPC Class Codes: H04N 1/6002 Corrections within particular colour systems

Abstract

A color scanning technique that maintains a high image quality while permitting improved scanning speed and improved perceived resolution. A color pixel array and a luminance pixel array are generated representative of a target object. The pixel arrays are related in that one or more luminance pixels of the luminance pixel array cover each pixel of the color pixel array. A luminance value is sensed for each luminance pixel, and three primary color values are sensed for each color pixel. A measured luminance value is then associated with each respective color pixel wherein the measured luminance value is a function of the sensed luminance values for the one or more luminance pixels covering the respective color pixel. A derived luminance value is also calculated for each respective color pixel wherein the derived luminance value is a function of the three sensed primary color values for the respective color pixel. A luminance correction factor is then determined for each respective color pixel as a function of the color pixel'"'"'s derived and measured luminance values. The luminance correction factor for each respective color pixel is applied to the sensed primary color values of the color pixel, or to a linear combination thereof, to determine luminance-corrected color values. In this way the overall pixel luminance implied by the aggregate measured color values will show a consistency with the directly measured luminances. The luminance-corrected color values together with the measured luminance values may then be subjected to further appropriate transformation to determine color coordinates for each luminance pixel and/or for each color pixel, which may then be used in further processing. A logarithmic method is used for efficient computation of correction factors and luminance-corrected color values.

Claims

1. A method of providing color values representative of a scanned target object, comprising the steps of:

generating a color pixel array representative of at least a portion of said scanned object;

generating a luminance pixel array representative of said portion of said scanned object,

wherein at lease one luminance pixel of said luminance pixel array covers each

color pixel of said color pixel array;

sensing a luminance value of each luminance pixel of said luminance pixel array;

providing a measured luminance value for each said color pixel as a function of the sensed luminance value of the at least one luminance pixel covering said color pixel;

sensing three primary color values for each said color pixel;

calculating a derived luminance value for each said color pixel as a function of the three sensed primary color values thereof;

determining a luminance correction factor for each said color pixel as a function of the derived luminance value thereof and the measured luminance value thereof; and

applying said luminance correction factor to a desired linear combination of the sensed primary color values of each said color pixel to determine luminance corrected color values thereof.

2. The method of claim 1 wherein a plurality of luminance pixels of said luminance pixel array covers each color pixel of said color pixel array whereby said luminance pixel array provides a higher resolution than said color pixel array.

3. The method of claim 1 wherein said luminance pixel array is generated at a higher speed than said color pixel array.

4. The method of claim 1 wherein said luminance correction factor comprises the ratio of said derived luminance value to said measured luminance value at least for all color pixels with sufficient luminance content.

5. The method of claim 4 wherein the determination of said luminance correction factor includes the steps of:

determining a logarithm of said derived luminance value;

determining a logarithm of said measured luminance value; and

subtracting said logarithm of said measured luminance value from said logarithm of said derived luminance value to determine a logarithm of said luminance correction factor.

6. The method of claim 5 the application of said luminance correction factor to the sensed primary color values includes the steps of:

determining a logarithm of a sensed primary color value; and

adding said logarithm of said luminance correction factor to said logarithm of said sensed primary color value.

7. The method of claim 4 wherein said luminance correction factor is applied only to red and blue primary color values.

8. Apparatus for use in color scanning for correcting the luminance of a target object, comprising:

a color CCD sensor for generating a color pixel array representative of at least a portion of said scanned object and for sensing three primary color values for each color pixel comprising said portion;

a luminance CCD sensor for generating a luminance pixel array representative of said portion and for sensing a luminance value for each luminance pixel comprising said portion,

wherein at least one luminance pixel of said luminance pixel array covers each color pixel of said color pixel array;

means for providing a measured luminance value for each said color pixel as a function of the sensed luminance value of the at least one luminance pixel covering said color pixel;

means for calculating a derived luminance value for each said color pixel as a function of the three sensed primary color values thereof;

means for determining a luminance correction factor for each said color pixel as a function of the derived luminance value thereof and the measured luminance value thereof; and

means for applying said luminance correction factor to a desired linear combination of the sensed primary color values of each said color pixel to determine luminance corrected primary color values thereof.