Color halftoning for printing with multiple inks

Details Color halftoning for printing with multiple inks Color halftoning for printing with multiple inks

2002-02-22

2003-10-28

Nguyen, Lamson (Department: 2861)

Incremental printing of symbolic information

Ink jet

Controller

C347S043000, C358S518000

Reexamination Certificate

active

06637851

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a method for halftoning of colour images. In a specific embodiment the invention is related to an improved method for halftoning images using error diffusion.
BACKGROUND OF THE INVENTION
Traditionally halftoning images for printing is done using only two levels. The continuous tone density value of an input pixel to be reproduced is approximated by printing an appropriate percentage of dots within an area representing the pixel. At a particular position of the binary reproduction either a dot or no dot can be placed. It can be considered that a continuous tone image is an image containing multiple grey levels with no perceptible quantization to them.
Error diffusion is a well-established halftoning technique, especially fit for printers that are able to produce dispersed dots, like inkjet printers. Error diffusion algorithms exist for both grey-level images and colour images. The basic error-diffusion algorithm works as follows and is illustrated in FIG.
1
. The first input pixel value
21
of the original image
22
is quantized by the quantizer to the nearest allowed output value (reproducible level) of the output system to obtain the output pixel value
24
. The quantization error
25
, i.e. the difference between the continuous tone input value and the output pixel value
24
, is stored in an error buffer
26
and is later diffused to future pixels. For following pixels the input pixel value
21
is modified by the content of the error buffer
26
resulting in modified input pixel value
27
which is fed to the quantizer.
The simplest way to extend the algorithm from grey scale to colour is to perform the grey-scale algorithm separately on each colour component or component image of the colour-separated image, e.g. Cyan, Magenta, Yellow for a 3-ink device where there are 3 colour components; or,
Cyan, Magenta, Yellow, Black for a 4-ink device where black is also colour component and there are 4 colour components.
Each colour component contains pixels which correspond spatially to pixels in other colour component separations. The result of the extended algorithm for a pixel is formed by the combination of the result obtained from the algorithm for spatially corresponding pixels in each colour component.
Error diffusion performed on an image or image component, where each pixel is represented by a single number, is called a scalar error diffusion.
The alternative is to make explicit use of the three-dimensionality of colour space and to diffuse a three dimensional colour-error vector instead of 3 or 4 scalar ink errors. This approach was described by Sullivan et al. in patent U.S. Pat. No. 5,070,413 for binary colour images. For n inks, there are 2
n
basic (i.e. printable) colours in a binary device. A pixel's colour is mapped to the closest basic colour, and the resulting colour error vector is diffused to the unprocessed neighbouring pixels. See e.g. Ronald S. Gentile, Eric Walowit, and Jan P. Allebach. Quantization and multilevel halftoning of colour images for near original image quality. J. Opt Soc. Am. A Vol. 7, 1019-1026.
The procedure is readily extended to multi-level printing. In contrast to binary printers that can attain only two values per ink (ink
o ink), multi-level printing devices can attain more density values by either varying the ink-drop size or the ink density. For a printer capable to produce k levels for all n inks, the number of basic colours equals k
n
. Again, an input pixel's colour is mapped to the closest basic colour in the multilevel output system, and the resulting colour error vector representing the difference between the input colour and the reproduced output colour is diffused to the unprocessed neighbouring pixels.
The original contone image is typically quantized to 256 values per colour component.
Due to restrictions of the output system, the reproduced halftone image contains much less different colour values, resulting in a visual difference between the original contone image and the reproduced halftone image.
Effort has been made to minimise the visual difference between the original and the reproduced image.
First of all, the method of error diffusion itself does quite a good job because of its “blue-noise” characteristic. This means that the resulting difference contains predominantly high-frequency errors, which are less visible to the eye than lower-frequency errors.
More investigation on this blue-noise characteristic has led to variants of the basic error-diffusion algorithm. See e.g. Robert A. Ulichney. Dithering with blue noise.
Proceedings of the IEEE,
76(1):56-79, January 1988.
Further incorporation of characteristics of the human visual system is described in the above-mentioned Sullivan patent U.S. Pat. No. 5,070,41. Here the error vector is blurred according to a model of the human modulation transfer function. It takes into account human contrast sensitivity data. Contrast in relation to colour images can be defined as a difference in colour which is a combination of brightness, hue and chroma.
More attempts have been made since then, to optimise halftoning algorithms with respect to contrast sensitivity.
A great deal of effort has gone to incorporating the shape of the contrast sensitivity curve into halftoning algorithms. Much less attention has been paid to differences in contrast sensitivity between luminance and chrominance channels. In view of the blue-noise characteristic of error diffusion, the high-frequency side of the contrast sensitivity curves is most important. We refer to a red-green or blue-yellow grating as discussed in: Kathy T. Mullen. The contrast sensitivity of human colour vision to red-green and blue-yellow chromatic gratings. J. Physiol. 359, 381-400, 1985. When comparing the contrast sensitivity curves measured from monochrome gratings with those of coloured gratings, it is immediately clear that high-frequency errors in the luminance channel are more noticeable than those in the chrominance channels. This implies that isolated dots will be far more noticeable when they differ in luminance with their background than when they differ in chrominance and have a similar luminance. A good halftoning method should take that into account. A method that does so is described in U.S. Pat. No. 5,991,438 (Shaked, Arad, Fitzhugh, Sobel, McGuire), where the set of output halftone colours is limited to tetrahedra in colour space to minimise dot-brightness contrast in the output image.
Another issue concerns the visibility of dot patterns of the dots from colours similar in luminance. If e.g. a colour is printed by placing few darker pixels in between lighter ones, the dot pattern of these darker pixels is important. These dots should have an homogeneous appearance. Colour-error diffusion does not fully solve this problem. Moreover, normal colour-error diffusion is not optimal with respect to printer deficiencies such as misalignment. This method picks out the closest possible output colour in a colour space, mostly three-dimensional, but this can yield awkwardly discontinuous separation images, which cause colour shift if the print heads are not perfectly aligned.
An interesting approach to the problem is given in U.S. Pat. No. 5,621,546 (Klassen, Eschbach, Bharat), where a guiding ink-error-diffusion process is used to assist the colour-error diffusion. This ink-error diffusion is a bi-level process. In addition, a distorted colour space is used for the colour-error diffusion. Whereas the Shaked patent U.S. Pat. No. 5,991,438 does not fully takes into account the dot pattern problem, the Klassen patent U.S. Pat. No. 5,621,546 runs short in discriminating multiple levels of luminance. In addition, both patents leave undiscussed the matter of printing with more than 3 inks.
None of the prior art documents takes account of both the dot brightness contrast and the dot pattern visibility problems at the same time resulting in undesirable drawbacks as mentioned herein above.
OBJECTS OF THE INVENTION
It is an object of the present invention to

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