Black enhancement in perceptual gamut mapping
View Patent ↗The present invention provides for gamut mapping between a source device and a destination device, the destination device having a destination device black point with a hue, and the source device and the destination device being respectively characterized by a source gamut shell and a destination gamut shell. A lightness compression is performed using the source gamut shell, the destination gamut shell and the destination device black point. A constant lightness transformation is performed, wherein the constant lightness transformation modifies the destination gamut shell so that a shell boundary of the destination gamut shell on the hue plane containing the destination device black point is shifted from an original position to align with a neutral axis of the destination gamut shell. A chroma compression is performed using the source gamut shell and the modified destination gamut shell. An inverse constant lightness transformation is performed to obtain mapped colors for the destination device, wherein the inverse constant lightness transformation returns the shell boundary to the original position. As such, dark, almost neutral colors can be produced with virtually the highest possible density for the destination device.
1. A method for gamut mapping between a source device and a destination device, the destination device having a destination device black point with a hue, and the source device and the destination device being respectively characterized by a source gamut shell and a destination gamut shell, the method comprising the steps of:
performing a lightness compression using the source gamut shell, the destination gamut shell and the destination device black point,
performing a constant lightness transformation, wherein the constant lightness transformation modifies the destination gamut shell so that a shell boundary of the destination gamut shell on the hue plane containing the destination device black point is shifted from an original position to align with a neutral axis of the destination gamut shell;
performing a chroma compression using the source gamut shell and the modified destination gamut shell; and
performing an inverse constant lightness transformation to obtain mapped colors for the destination device, wherein the inverse constant lightness transformation returns the shell boundary to the original position.
2. A method according to claim 1 , wherein the neutral axis intersects the source gamut shell and the destination gamut shell.
3. A method according to claim 2 , wherein the neutral axis intersects the source gamut shell at a neutral maximum lightness and at a neutral minimum lightness, and wherein the neutral axis intersects the destination gamut shell at a neutral maximum lightness and at a neutral minimum lightness, and wherein the neutral minimum lightness and neutral maximum lightness of the source gamut shell match those of the destination gamut shell following performance of the lightness compression and the constant lightness transformation.
4. A method according to claim 1 , wherein the shell boundary includes all points between the neutral axis and the destination device black point, inclusive.
5. A method according to claim 1 , wherein the color appearance space is CAM02, and uses Jab/Jch coordinates.
6. A method according to claim 5 , wherein the destination gamut shell is modified using the following transformation:
χ
(
J
,
a
,
b
)
=
{
(
J
,
a
,
b
)
if
J
≥
J
neutralMin
(
J
,
a
-
J
neutralMin
-
J
J
neutralMin
-
J
k
a
k
,
b
-
J
neutralMin
-
J
J
neutralMin
-
J
k
b
k
)
if
J
<
J
neutralMin
where J k a k b k are the Jab coordinates of the destination device black point and J neutralMin is the neutral minimum lightness of the destination gamut shell.
7. A method according to claim 6 , wherein the transformation is applied to Jab as a whole, rather than only the hue plane containing the destination device black point.
8. A method according to claim 5 , wherein the inverse constant lightness transformation is performed as follows:
χ
-
1
(
J
,
a
,
b
)
=
{
(
J
,
a
,
b
)
if
J
≥
J
neutralMin
(
J
,
a
+
J
neutralMin
-
J
J
neutralMin
-
J
k
a
k
,
b
+
J
neutralMin
-
J
J
neutralMin
-
J
k
b
k
)
if
J
<
J
neutralMin
9. A color management module which converts source color image data associated with a source color device to destination color image data associated with a destination color device using gamut mapping,
wherein the gamut mapping is performed by a method according to claim 1 .
10. Computer-executable process steps stored on a computer readable medium, the computer-executable process steps for gamut mapping between a source device and a destination device, the destination device having a destination device black point with a hue, and the source device and the destination device being respectively characterized by a source gamut shell and a destination gamut shell, the computer-executable process steps comprising process steps executable to perform a method according to claim 1 .
11. An apparatus for gamut mapping between a source device and a destination device, the destination device having a destination device black point with a hue, and the source device and the destination device being respectively characterized by a source gamut shell and a destination gamut shell, the apparatus comprising:
a program memory for storing process steps executable to perform a method according to claim 1 ; and
a processor for executing the process steps stored in the program memory.