US 5,450,216 AGranted

Color image gamut-mapping system with chroma enhancement at human-insensitive spatial frequencies

AssigneeInternational Business Machines Corporation

InventorJames M. Kasson

Life of this patent

Estimating the term…

Loading patent…

Abstract

A method and system for gamut-mapping color images from device-independent form to device-dependent gamut in a Cartesian color space. Digital images are mapped to any device-dependent gamut in a manner that minimizes the human visual response both to the luminance and the chrominance changes necessary to force out-of-gamut pixels into the specified device-dependent gamut. A "neighborhood gamut mapping" technique considers the subjective visual effects of nearby pixels on the mapping of each pixel. Image luminance is biased toward the luminance in the device-dependent gamut at which the greatest chromamagnitude is available for a fixed hue angle. The chrominance of the mapped image is thereby imperceptibly adjusted to compensate for the human visual effects of luminance changes. Spatial filtering exploits the differing spatial frequency regions of insensitive human visual response to both luminance and chrominance changes.

Claims22

Dependent claims

A method for mapping an image from a first color space to a bounded display gamut in a second color space, said first and second color spaces each spanning a plurality of points each having one luminance (L) and two chrominance (C.sub.1, C.sub.2) values, wherein said two chrominance (C.sub.1, C.sub.2) values together represent a hue angle (H) value and a chroma magnitude (M) value, said method comprising the steps of:

(a) converting said image into a plurality of signals including an image luminance signal (L.sub.I) and two image chrominance signals (C.sub.I1, C.sub.I2) that specify image hue angle (H.sub.I =arctan C.sub.I2 /C.sub.I1) and image chroma magnitude (M.sub.I =sqrt (C.sup.2.sub.I1 +C.sup.2.sub.I2));
(b) producing a chroma-maximized luminance signal (L.sub.CM) representing the luminance value of the point on said display gamut boundary having the maximum chroma magnitude (M.sub.BX) at said image hue angle (H.sub.I) value in said second color space;
(c) subtracting said chroma maximized luminance signal (L.sub.CM) from said image luminance signal (L.sub.I) to create a color-biased luminance signal (L.sub.CB =L.sub.I -L.sub.CM);
(d) producing a chroma-corrected weight signal (0.ltoreq.W<1) corresponding to the difference between said image chroma magnitude (M.sub.I) and the chroma magnitude (M.sub.B) of a first point on said display gamut boundary in said second color space, wherein said chroma-corrected weight signal (W) is zero-valued when M.sub.B .gtoreq.M.sub.I ;
and
(e) weighting said color-biased luminance signal (L.sub.CB) and said image luminance signal (L.sub.I) according to said chroma-corrected weight signal (W) and combining both to produce a display luminance signal (L.sub.D =WL.sub.CB +(1-W)L.sub.I representing the luminance of said mapped image in said second color space.

3 more elements

An image display system for rendering digital images in visible form by mapping said image from a first color space to a bounded display gamut in a second color space, said first and second color spaces each spanning a plurality of points each having one luminance (L) and two chrominance (C.sub.1, C.sub.2) values, wherein said two chrominance (C.sub.1, C.sub.2) values together represent a hue angle (H) value and a chroma magnitude (M) value, said system comprising:

image signal input means for convening said image into a plurality of signals including an image luminance signal (L.sub.I) and two image chrominance signals (C.sub.I1, C.sub.I2) that specify image hue angle (H.sub.I =arctan C.sub.I2 /C.sub.I1) and image chroma magnitude (M.sub.I =sqrt (C.sup.2.sub.I1 +C.sup.2.sub.I2));
first lookup table means coupled to said image signal input means for producing a chroma-maximized luminance signal (L.sub.CM) representing the luminance value of the point on said display gamut boundary having the maximum chroma magnitude (M.sub.BX) at said image hue angle (H.sub.I) value in said second color space;
subtractor means coupled to said first lookup table means and said image signal input means for subtracting said chroma-maximized luminance signal (L.sub.CM) from said image luminance signal (L.sub.I) to create a color-biased luminance signal (L.sub.CB =L.sub.I L.sub.CM);
chroma correction means coupled to said image signal input means and said first lookup table means for producing a chroma-corrected weight signal (0.ltoreq.W<1) corresponding to the difference between said image chroma magnitude (M.sub.I) and the chroma magnitude (M.sub.B) of a first point on said display gamut boundary in said second color space, wherein said chroma-corrected weight signal (W) is zero-valued when M.sub.B.gtoreq.M.sub.I ;
and
display signal output means coupled to said chroma correction means and said subtractor means for weighting said color-biased luminance signal (L.sub.CB) and said image luminance signal (L.sub.I) according to said chroma-corrected weight signal (W) and combining both to produce a display luminance signal (L.sub.D =WL.sub.CB +(1-W)L.sub.I) representing the luminance of said mapped image in said second color space.

3 more elements

A color image printing system for printing color images by mapping an original digital image from a first color space to a bounded display gamut in a second color space, said first and second color spaces each spanning a plurality of points having one luminance (L) and two chrominance (C.sub.1, C.sub.2) values wherein said chrominance (C.sub.1, C.sub.2) values represent a hue angle (H) value and a chroma magnitude (M) value, said system comprising:

image signal input means for converting said image into a plurality of signals including an image luminance signal (L.sub.I) and two image chrominance signals (C.sub.I1, C.sub.I2) that specify image hue angle (H.sub.I =arctan C.sub.I2 /C.sub.I1) and image chroma magnitude (M.sub.I =sqrt (C.sup.2.sub.I1 +C.sup.2.sub.I2)).
first lookup table means coupled to said image signal input means for producing a chroma-maximized luminance signal (L.sub.CM) representing the luminance value of the point on said display gamut boundary having the maximum chroma magnitude (M.sub.BX) at said image hue angle (H.sub.I) value in said second color space;
subtractor means coupled to said first lookup table means and said image signal input means for subtracting said chroma-maximized luminance signal (L.sub.CM) from said image luminance signal (L.sub.I) to create a color-biased luminance signal (L.sub.CB =L.sub.I -L.sub.CM);
chroma correction means coupled to said image signal input means and said first lookup table means for producing a chroma-corrected weight signal (0.ltoreq.W<1) corresponding to the difference between said image chroma magnitude (M.sub.I) and the chroma magnitude (M.sub.B) of a first point on said display gamut boundary in said second color space, wherein said chroma-corrected weight signal (W) is zero-valued when M.sub.B .gtoreq.M.sub.I ;
and
display signal output means coupled to said chroma correction means and said subtractor means for combining said color-biased luminance signal (L.sub.CB) and said image luminance signal (L.sub.I) according to said chroma-corrected weight signal (W) and combining both to produce a display luminance signal (L.sub.D =WL.sub.CB +(1-W)L.sub.I) representing the luminance of said mapped image in said second color space.

3 more elements

Dependencies are inferred from the claim text.