Information coding and decoding in spectral differences
Information is encoded in an image signal by exploiting spectral differences between colors that appear the same when rendered. These spectral differences are detected using image sensing that discerns the spectral differences. Spectral difference detection methods include using sensor-synchronized spectrally-structured-light imaging, 3D sensors, imaging spectrophotometers, and higher resolution Bayer pattern capture relative to resolution of patches used to convey a spectral difference signal.
1. A method of encoding a machine-readable data signal in an image, the method comprising, with a programmed processor:
generating an information signal comprising message symbols;
mapping message symbol elements of the information signal to insertion locations within a host image signal; and
at the insertion locations, inserting the information signal by computing values for controlling application of a spot color ink and a combination of process color inks, the values for controlling application of process color inks forming a spectral difference signal between process color ink applied at a first pixel location and spot color ink applied at a second pixel location to encode a message symbol element in relative spectral difference between the process color ink at the first pixel location and spot color ink at the second pixel location, the computing encoding a relative spectral difference at the first and second pixel locations that conveys the message symbol element within the host image, the spectral difference signal representing a distinguishable spectra between a spot color and a combination of process colors that appear similar in the human visual system;
wherein the relative spectral differences at the insertion locations comprise differences between spectral distributions of spot and process color inks at corresponding pairs or sets of locations.
2. The method of claim 1 wherein the mapping of message symbol elements comprises mapping discrete elements of the information signal to spatial locations within the host image signal.
3. The method of claim 2 wherein the mapping comprises mapping the discrete elements to plural dimensions of colors at the spatial locations.
4. The method of claim 2 wherein the spatial locations comprise regions of plural pixels in the host image signal.
5. The method of claim 1 wherein the process color inks comprise Cyan, Magenta and Yellow inks.
6. The method of claim 1 wherein values for controlling application of process color inks at a first insertion location are adjusted toward a first color combination to encode a first binary value of the information signal, and values for controlling application of process color inks at a second insertion location are adjusted toward a second color combination to encode a second binary value of the information signal.
7. The method of claim 1 wherein values for controlling application of process color inks at a first insertion location are adjusted toward a first color combination to encode a first M-ary value of the information signal, and values for controlling application of process color inks at a second insertion location are adjusted toward a second color combination to encode a second M-ary value of the information signal, and the M-ary value comprises more than two different data values.
8. A non-transitory computer readable medium, on which is stored instructions, which configure a processor to perform a method of encoding a machine-readable data signal in an image, the method comprising:
generating an information signal comprising message symbols;
mapping message symbol elements of the information signal to insertion locations within a host image signal; and
at the insertion locations, inserting the information signal by computing values for controlling application of a spot color ink and a combination of process color inks, the values for controlling application of process color inks forming a spectral difference signal between process color ink applied at a first pixel location and spot color ink applied at a second pixel location to encode a message symbol element in relative spectral difference between the process color ink at the first pixel location and spot color ink at the second pixel location, the computing encoding a relative spectral difference at the first and second pixel locations that conveys the message symbol element within the host image, the spectral difference signal representing a distinguishable spectra between a spot color and a combination of process colors that appear similar in the human visual system;
wherein the relative spectral differences at the insertion locations comprise differences between spectral distributions of spot and process color inks at corresponding pairs or sets of locations.
9. The computer readable medium of claim 8 wherein the mapping of message symbol elements comprises mapping discrete elements of the information signal to spatial locations within the host image signal.
10. The computer readable medium of claim 9 wherein the mapping comprises mapping the discrete elements to plural dimensions of colors at the spatial locations.
11. The computer readable medium of claim 9 wherein the spatial locations comprise regions of plural pixels in the host image signal.
12. The computer readable medium of claim 8 wherein the process color inks comprise Cyan, Magenta and Yellow inks.
13. The computer readable medium of claim 8 wherein values for controlling application of process color inks at a first insertion location are adjusted toward a first color combination to encode a first binary value of the information signal, and values for controlling application of process color inks at a second insertion location are adjusted toward a second color combination to encode a second binary value of the information signal.
14. The computer readable medium of claim 8 wherein values for controlling application of process color inks at a first insertion location are adjusted toward a first color combination to encode a first M-ary value of the information signal, and values for controlling application of process color inks at a second insertion location are adjusted toward a second color combination to encode a second M-ary value of the information signal, and the M- ary value comprises more than two different data values.
15. A method of decoding an information signal from an image comprising:
obtaining spectra of an image signal at locations within the image signal;
detecting a spectral difference at the locations by determining, at the locations, a relative difference between a spectral distribution of a combination of process inks and a spot color ink; the process color inks forming a spectral difference signal between process color ink applied at a first pixel location and spot color ink applied at a second pixel location that encodes a message symbol element in relative spectral difference between the process color ink at the first pixel location and spot color ink at the second pixel location, the detecting comprising determining a relative spectral difference at the first and second pixel locations to extract the message symbol element from the host image, the spectral difference signal representing a distinguishable spectra between a spot color and a combination of process colors that appear similar in the human visual system; wherein the relative spectral differences at the locations comprise differences between spectral distributions of spot and process color inks at corresponding pairs or sets of locations; and
decoding an information signal from the spectral difference by extracting message symbols from the message symbol elements extracted from relative differences between spectral distributions of the process and spot color inks, and error correction decoding a message from the message symbols.
16. The method of claim 15 wherein the detecting the spectral difference comprises determining whether a spectra at a location is one of a particular set of distinguishable spectra of a combination of process inks.
17. The method of claim 16 wherein the particular set of distinguishable spectra include a first spectra with a first shape, and a second spectra with a second shape.
18. The method of claim 17 wherein the first shape corresponds to a combination of process inks, and the second shape corresponds to a spot color ink.
19. The method of claim 15 wherein the combination of process color inks and the spot color appear similar to human vision yet have spectral differences, and detecting the spectral difference includes computing ratio of a measurement of reflectance of a combination of process color inks and reflectance of a spot color ink at plural different spectral bands.
20. The method of claim 15 wherein the spot color and combination of process color inks form a spatial arrangement of first or second visible inks of a metameric pair, which are similar as determined by a human vision model, yet have spectral differences that are detectable in the spectra of the image signal; and obtaining the spectra of the image signal comprises capturing distinct image frames of an object, each being illuminated with different narrow band LED illumination.