Image analysis – Color image processing – Color correction
Reexamination Certificate
2002-03-13
2004-11-30
Boudreau, Leo (Department: 2621)
Image analysis
Color image processing
Color correction
C382S254000, C345S590000, C358S001900, C358S515000, C358S517000, C358S518000, C358S520000
Reexamination Certificate
active
06826304
ABSTRACT:
TECHNICAL FIELD
The technical field is color image processing using spatially dependent gamut mapping.
BACKGROUND
Gamut mapping is used to modify a representation of a color image to fit into a constrained color space of a given rendering medium. A laser-jet color printer that attempts to reproduce a color image on regular paper would have to map the photographed picture colors in a given color range, also known as the image “color gamut,” into the given printer/page color gamut. Gamut mapping of an original color image may produce unwanted artifacts in the rendered image. One such artifact is referred to as a halo. Halo artifacts occur near edges of objects in the rendered image, and are a consequence of using spatially dependent gamut mapping algorithms to map an original color image onto a constrained color space.
SUMMARY
A method and apparatus for color image processing using gamut mapping reduces halo artifacts by correcting terms in a spatially dependent gamut mapping algorithm. The color image may be represented by f, an in gamut image by g, a general gamut volume by C, and a gamut constraint by c. The method for reducing halo artifacts includes two correction steps. First, a color distance term L
2
in the gamut mapping algorithm is corrected. Second, a distance measure of an image gradient in the gamut mapping algorithm is corrected. The first correcting step comprises computing a function u=min(f,c). The second correcting step comprises computing a monotone increasing function {tilde over (ƒ)}=S(ƒ), such that the derivative of S is smaller than unity for high values of ƒ. Next, a function g(x,y) is determined that minimizes a functional comprising the color distance term and the image gradient-distance term. The solution may be determined by iteration using a gradient descent operation by first initializing g
0
=min(f,c) and then performing one or more iteration steps to compute g(x,y).
The apparatus may comprise a suitably programmed processor or other general purpose computer that receives an input image, processes the input image according to the corrected processing routines, and produces an output electrical signal capable of being rendered as a color image on a suitable rendering device such as a color laser printer, for example.
The method may be embodied on a computer readable medium comprising routines for reducing halo artifacts during color image processing using gamut mapping. The computer readable medium may be one of a hard disk, a floppy disk, or other magnetic storage, an optical storage device, or any other computer readable medium.
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Levy Avraham
Shaked Doron
Boudreau Leo
Edwards Patrick
Hewlett--Packard Development Company, L.P.
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