COLOR MANAGED EMBEDDING SYSTEM FOR EMBEDDING SIGNALS IN COLOR ARTWORK

1 . A method of encoding a signal in a color image comprising original Cyan (C), Magenta (M), Yellow (Y) by making signal changes in CIELAB a* and b* while maintaining L* within a predetermined tolerance, said method comprising:

at a color image location being changed, determining gradients of L*, a*, b* with respect to changes in the CMY;

using determined L* gradient information to find a direction in CMY-space which moves towards a target a*, b* value while keeping L* unchanged within a predetermined tolerance;

using determined a*, b* gradient information to find a scale value for a*, b* in a determined L* gradient direction to reach the target a*, b* value;

applying the found scale value to CMY, and using the scaled CMY as replacement values for the original CMY values, the scaled CMY carrying the encoded signal.

2 . The method of claim 1 , further comprising, if L* has changed above the predetermined tolerance, use the a*, b* gradient information to find a direction that changes L* while keeping a*, b* unchanged within a predetermined tolerance, and use the L* gradient information to find a scale value of this change to reduce L*.

3 . The method of claim 1 in which the target a*, b* value represents an a*, b* value which includes the encoded signal value for an image pixel or group of image pixels.

4 . The method of claim 1 in which the color image location being changed comprises an image pixel or group of image pixels.

5 . A method of encoding a signal in a color image comprising original Cyan (C), Magenta (M), Yellow (Y) by making signal changes in CIELAB a* and b* while maintaining L* within a predetermined tolerance, said method comprising:

obtaining information represent a contour representing a constant L* for different values of CMY;

at a starting location on the contour represented by an original a*, b*, determining L* gradient information to find a direction in CMY-space which moves towards a target a*, b*, the target a*, b* residing on the contour;

determining a scalar representing the distance between the original a*, b* and the target a*, b*; and

applying the determined scalar to the original CMY, the resulting scaled CMY carrying the encoded signal.

6 . The method of claim 5 further comprising correcting the scaled CMY values when L* is located off the contour.

7 . A method of encoding a signal in a color image comprising original Cyan (C), Magenta (M), Yellow (Y) ink values by making signal changes in CIELAB a* and b* while maintaining L* within a predetermined tolerance, said method comprising:

for an image pixel or group of image pixels, obtaining a target a*, b* value representing the original CMY ink values modulated to include an encoded signal;

estimating gradients of L*, a*, and b* with respect to the original CMY ink values;

determining a plane in CMY-space that orthogonal to the L* gradient, and then determining a direction vector v within this plane that points toward the target a*, b*;

determining a scalar h_target to scale v to reach the target a*, b* by estimating the change in a*, b*;

determining a minimum scalar h_max to cause any of the original CMY ink values be either 0% or 100% values;

determining a scalar h, by the requirements: if h_target<h_max, h=h_target or if h_target>=h_max, h=h_max;

determining new ink values as original CMY ink values+h*v;

using the new ink values to represent the original CMY ink values plus an encoded signal at the image pixel or group of image pixels.

8 . A non-transitory computer readable medium comprising instructions stored thereon to cause one or more multi-core processors to perform the acts of claim 7 .

9 . A non-transitory computer readable medium comprising instructions stored thereon to cause one or more multi-core processors to perform the acts of claim 1 .

10 . A non-transitory computer readable medium comprising instructions stored thereon to cause one or more multi-core processors to perform the acts of claim 5 .

11 . An image processing method comprising:

obtaining a reflectance value representing an image pixel or group of image pixels at or around 660 nm, the image pixel representing artwork including cyan, magenta, yellow and black channels;

altering the reflectance value according to an information carrying signal, said altering yielding a target reflectance value at or around 660 nm;

changing in a first direction the cyan value of the image pixel or group of image pixels to achieve the target reflectance value, said changing yielding changed pixel color values;

evaluating the luminance (L*) of the changed pixel color values relative to the luminance (L*) of the image pixel or group of image pixels;

changing in a second direction opposite of the first direction the magenta value of the changed pixel color to minimize a change in luminance (*L) while maintaining the target reflectance value within 0-5%.

12 . A method of encoding a signal in a color image comprising:

obtaining CIELAB L*, a*, b* values representing a color image comprising CMY channels;

generating a local approximation of a L* contour;

determining a direction along the L* contour to introduce a change to carry a signal in the color image;

determining an amount of change using a rate of change in the determined direction;

applying the determined amount to achieve new L*, a*, b* values, the new L*, a*, b* values carrying the signal.