Signal encoding for inks with low reflectivity
This disclosure relates to advanced image signal processing technology including encoded signals and digital watermarking. The technology may be applied to retail packages and other printed objects, e.g., such as hang tags, labels and receipts.
1. An image processing apparatus comprising:
memory for storing first color values representing luminance*, color channel ‘a*’ and color channel ‘b*’ for a first color and an overprint of the first color with a background color;
memory for storing second color values representing luminance*, color channel ‘a*’ and color channel ‘b*’ for the background color;
means for determining an encoded signal error that is associated with the first color values, the second color values, and a reflectance difference associated with a first machine-vision wavelength between: i) an average within an image area of the first color and the overprint, and ii) the background color;
means for determining a color error that is associated with the first color values, the second color values, and the reflectance difference; and
means for combining the encoded signal error and the color error to determine whether to use the first color as an encoded signal carrier.
2. The image processing apparatus of claim 1 further comprising means for transforming digital imagery to carry an encoded signal represented by the first color.
3. The image processing apparatus of claim 1 in which the encoded signal error (RWV) comprises:
RWV=Δ 660/((Δ L *){circumflex over ( )}2+(Δ a*/ 8){circumflex over ( )}2+(Δ b*/ 16){circumflex over ( )}2){circumflex over ( )}(½)
in which Δ660 represents a reflectance difference at or around 660 nm for one of a first plurality of colors and a substrate or the background color, ΔL* represents a difference in luminance between the first color values and the second color values, Δa* represents a difference in ‘a’ channel color values between the first color values and the second color values, and Δb* represents a difference in ‘b’ channel color values between the first color values and the second color values.
4. The image processing apparatus of claim 2 in which the color error (RCV) comprises:
RCV=Δ 660/((Δ L *){circumflex over ( )}2+(Δ a *){circumflex over ( )}2+(Δ b {circumflex over ( )}*){circumflex over ( )}2){circumflex over ( )}(½).
5. The image processing apparatus of claim 4 in which ΔL*, Δa* and Δb* comprise CIELAB values.
6. The image processing apparatus of claim 4 in which RCV and RWV are weighted to emphasize or deemphasize the encoded signal error or the color error.
7. The image processing apparatus of claim 2 in which the encoded signal comprises a sparse mark.
8. The image processing apparatus of claim 1 in which the encoded signal comprises a sparse mark.
9. The image processing apparatus of claim 4 in which the encoded signal comprises a sparse mark.