Selective dithering
View Patent ↗Techniques are disclosed for selectively dithering only a subset of a digital image. One or more ranges of digits are selected for dithering. Only those pixels having digits within the selected range(s) in the digital image are dithered. The image is printed after being selectively dithered. Digits may be selected for dithering if they have values within the range(s) of one or more non-monotonic regions of a printer transfer function. Dithering may be performed on the subset of the digital image by applying a nonlinear transformation is applied to the image and adding a dither pattern to the transformed image. The result is quantized, and the inverse of the nonlinear transformation is applied to the quantized image to produce a dithered image. The nonlinear transformation is constructed such that the effects of the dither pattern appear only in that subset of the image having digits in the selected range(s).
1. A method of processing a digital image tangibly stored on a first computer-readable medium, the method performed by a computer processor executing computer program instructions tangibly stored on a second computer-readable medium, the method comprising:
(A) identifying at least one non-monotonic portion of a transfer function of a printer, the transfer function relating input digits to output density, the at least one portion having at least one range of digital values which is less than the full range of digital values; and
(B) dithering only that subset of the digital image having digits within the at least one range of digital values to produce a dithered image tangibly stored on a third computer-readable medium,
wherein the step (B) comprises:
applying a nonlinear transformation to the digital image to produce a transformed image;
selectively adding a dither pattern to the transformed image to produce a dithered pre-image;
quantizing the dithered pre-image to produce a quantized image; and
applying an inverse of the nonlinear transformation to the image to produce the dithered image,
wherein the nonlinear transformation comprises:
dividing a transfer function of a printer into a plurality of portions, each portion defined by a pair of anchor points;
for each portion of the transfer function, selecting a slope less than 1 if the portion corresponds to a first part of the digital image where the dither pattern will be added; and
for each portion of the transfer function, selecting a slope equal to 1 if the portion corresponds to a second part of the digital image where the dither pattern will not be added.
2. The method of claim 1 , wherein the step (A) comprises identifying at least one indeterminate portion of the transfer function.
3. The method of claim 1 , wherein the at least one portion of the transfer function comprises a starting point and an end point, and wherein the value of the transfer function at the end point is greater than the value of the transfer function at the starting point.
4. The method of claim 1 , further comprising:
(C) printing the dithered image.
5. The method of claim 1 , wherein the image, the quantized image, and the dithered image are represented in an n-bit color space, wherein the transformed image and the dithered pre-image are represented in an m-bit color space, and wherein m>n.
6. The method of claim 5 , wherein n=8 and wherein m=16.
7. The method of claim 1 , further comprising:
(C) prior to performing the operation (B), using a first calibration function to calibrate that subset of the digital image having digits within the at least one range of digital values; and
(D) after performing the operation (B), using a second calibration function to calibrate that subset of the dithered image having digits not within the at least one range of digital values, the second calibration function having a higher precision than the first calibration function.
8. The method of claim 1 , wherein for a portion defined by a first anchor point x 0 and a second anchor point x 1 , and the portion corresponding to the first part of the digital image, the selected slope is 1/(x 1 −x 0 ).
9. A method of processing a digital image tangibly stored on a first computer-readable medium, the method performed by a computer processor executing computer program instructions tangibly stored on a second computer-readable medium, the method comprising:
applying a nonlinear transformation to the digital image to produce a transformed image;
selectively adding a dither pattern to the transformed image to produce a dithered pre-image;
quantizing the dithered pre-image to produce a quantized image; and
applying an inverse of the nonlinear transformation to the image to produce the dithered image;
wherein the nonlinear transformation comprises:
dividing a transfer function of a printer into a plurality of portions, each portion defined by a pair of anchor points;
for each portion of the transfer function, selecting a slope less than 1 if the portion corresponds to a first part of the digital image where the dither pattern will be added; and
for each portion of the transfer function, selecting a slope equal to 1 if the portion corresponds to a second part of the digital image where the dither pattern will not be added.
10. The method of claim 9 , wherein for a portion defined by a first anchor point x 0 and a second anchor point x 1 , and the portion corresponding to the first part of the digital image, the selected slope is 1/(x 1 −x 0 ).
11. A device for processing a digital image, the device comprising:
means for identifying at least one non-monotonic portion of a transfer function of a printer, the transfer function relating input digits to output density, the at least one portion having at least one range of digital values which is less than the full range of digital values; and
means for dithering only that subset of the digital image having digits within the at least one range of digital values to produce a dithered image,
wherein the means for dithering comprises:
means for applying a nonlinear transformation to the image to produce a transformed image;
means for selectively adding a dither pattern to the transformed image to produce a dithered pre-image;
means for quantizing the dithered pre-image to produce a quantized image; and
means for applying an inverse of the nonlinear transformation to the image to produce the dithered image,
wherein the means for applying a nonlinear transformation is configured to:
divide a transfer function of a printer into a plurality of portions, each portion defined by a pair of anchor points;
for each portion of the transfer function, select a slope less than 1 if the portion corresponds to a first part of the digital image where the dither pattern will be added; and
for each portion of the transfer function, select a slope equal to 1 if the portion corresponds to a second part of the digital image where the dither pattern will not be added.
12. The device of claim 11 , wherein the means for identifying comprises means for identifying at least one indeterminate portion of the transfer function.
13. The device of claim 11 , wherein the at least one portion of the transfer function comprises a starting point and an end point, and wherein the value of the transfer function at the end point is greater than the value of the transfer function at the starting point.
14. The device of claim 11 , further including:
means for printing the dithered image.
15. The device of claim 11 , wherein the image, the quantized image, and the dithered image are represented in an n-bit color space, wherein the transformed image and the dithered pre-image are represented in an m-bit color space, and wherein m>n.
16. The device of claim 11 , further comprising:
first calibration means for calibrating that subset of the digital image having digits within the at least one range of digital values; and
second calibration means for calibrating that subset of the dithered image having digits not within the at least one range of digital values, the second calibration function having a higher precision than the first calibration function.
17. The device of claim 11 , wherein for a portion defined by a first anchor point x 0 and a second anchor point x 1 , and the portion corresponding to the first part of the digital image, the selected slope is 1/(x 1 −x 0 ).