AU676419B2 - Reduction of luminance noise in colour dithering - Google Patents

Description

-1 Reduction of Luminance Noise in Colour Dithering.

The present invention relates to the field of digital image processing and more pai ricularly, to a method for digital halftoning of colour images using the process of dithering.

The basic methods of dithering used in the prior art will now be described with reference to Fig. 1 to Fig. 3. in which: Fig. 1 illustrates schematically the structure of a normal Cathode Ray Tube type screen; Fig. 2a to Fig. 2e illustrate a number of different dither patterns; and Fig. 3 illustrates the process of dithering.

Colour raster graphic display devices are well known in the art. The display of colour images in these devices is normally achieved by means of a pixel map comprising individual pixels. These pixels in turn are normally represented internally by a collection of bits which represent the colour value of that pixel on the display device. The number of different possible bits in this collection corresponds with the number of different colours which may be displayed by the display device and hence the resolution with which the device can display a given picture. Common colour systems store 8 or 24 bits per pixel, although other variations are possible.

,A display device displays the corresponding colour value of each pixel, often to a high resolution. Common screen displays are capable of displaying a number of different pixels in the range of 1280 by 1024 pixels with each pixel capable of displaying up to 224 different colour values.

Colours are often displayed on a computer display according to one of many particular models. The red, green, blue (RGB) colour model is one that is in common use with Cathode Ray Tubes (CRT) and colour raster display devices. Other colour display models '25 include the cyan, magenta, yellow (CMY) or the cyan, magenta, yellow and black (CMYK) model often used in colour-printing devices. An example of the RGB model is in the National Television Standards Committee (NTSC) picture display standard in common use with computer-displays. In this standard, each pixel element is divided into 3 separate subgroupings. These separate subgroupings represent the Red, Green and Blue portion of a given pixel element respectively.

Referring now to Fig. 1, the viewing surface of a colour CRT, suitable for use with the NTSC standard, often consists of closely spaced pixels 20. Each pixel can be made up of a red 21, green 22 and blue 23 phosphor dot or pixel element. These dots are so small that light emanating from the individual dots is perceived by the viewer as a mixture of the corresponding three colours. A wide range of different colours can thus be produced by a pixel element by variation of the strength with which each phosphor dot is excited. A conversion arrangement (not illustrated) is normally provided so that the strength of each phosphor dot's excitation has some proportionality to the value of each of the above mentioned pixel element subgrouping. By way of example, a 24 bits per pixel colour display system divided into 8 bitsfor each of the three colours red, green and blue will be assumed. This.

[N:\LIBEIMACR011 :rhk I I -2corresponds to 28 or 256 separate intensity levels of each of red, green and blue respectively and 224 different possible colour values. A colour display capable of displaying this many colours can approximate a continuous tone image to such a degree that for all practical purposes the display can be considered to be a continuous tone display.

Many display devices are unable to actually display the full range of colours provided by, for example, a 24 bit input pixel. For example, a black and white raster image display can only display 2 colours, namely black and white and is known as a bi-level device. Other colour display devices can only display a finite number of discrete intensity levels for each colour unit. By way of further example, in a colour bi-level device, such as a bi-level ferroelectric liquid crystal display (FLCD), each illuminated portion on the screen can be at just two intensity levels, either fully on or fully off.

If the display device receives an input which has been generated on the basis that each pixel is able to display a larger nunioer of intensity levels than can actually be displayed, then there will be an error in the colour displayed, being the difference between the exact pixel value required to be displayed and the approximated value actually displayed.

Methods of reducing the effects of limitations of discrete level displays are known to those skilled in the art as halftoning. For an explanation of the different aspects of halftoning, reference is made to the textbook 'Digital Halftoning' by Robert Ulichney, published in 1991 by MIT Press.

For ease of understanding, the discussion of halftoning teclhniques will proceed in relation to a bi-level display capable of displaying only a single primary colour, namely, black or white. Fig. 2 and Fig. 3 show one method of halftoning, called dithering, used for increasing the number of apparent intensity levels. In this example, the pixel arrangements of Fig. 1 of the display screen are grouped together into 2x2 pixel areas 24. The 2x2 pixel area 24 of a bi-level display is used to produce five separate intensity levels for each primary colour by selectively turning on individual primary pixels. These levels range from a lowest level shown in Fig. a first level of brightness shown in Fig. a second level brightness shown in Fig. a third level shown in Fig. 2(d) and a fourth level shown in Fig. giving a total of 5 levels. In general, a grouping of n pixel elements will be able to produce a maximum of 1) separate intensity levels for each primary colour.

In a further refinement of the known halftoning method of dithering an image, the decision to intensify a particular point on the screen O(x,y) is made dependent on the desired intensity S(x,y) at that particular point and on a predetermined dither matrix value To display the required point at it is necessary to generate i x modulo n (EQ 1) j y modulo n (EQ 2) Then, if S(x,y) the point at O(x,y) is intensified, otherwise it is not. An example of this process is shown in Fig. 3. Input matrix 30 containing and dither matrix 31 containing locations are used in conjunction with the above display intensification rule to produce output matrix elements O(x,y) with 0 corresponding to those (N:\LIBEIMACRO11 :rhk -1 L~ points that are not intensified, and 1 corresponding to those points that are intensified. Hence, input element S(0,0) with a value of 0, will be compared with D(0,0) which has a value 0 also, and the comparison determination ia not satisfied, so the point 0(0,0) will not be intensified.

Input element S(2,3) with a value of 1, will be matched with dither matrix element D(0,1) since 2 modulo 2=0 and 3 modulo 2=1. D(0,0) has a value 0, so the output element 0(2,3) is illuminated and so on.

The above example illustrates the operation of a dither matrix of size 2x2 elements, for a black and white display. The choice of particular values of a dither matrix, although somewhat arbitrary, is determined by a number of important factors including: 1. The display device which is to display the image.

Some devices, such as laser printers and film recorders, are poor at reproducing isolated 'on' pixels and hence dither matrix elements must be chosen such that output matrix elements are clustered together as much as possible. Such choices are kncwn by the name 'clustered-dot ordered dither'.

2. The need to avoid the introduction of visual artifacts.

It is common for an output image to display regions of equal or slowly varying intensities of a particular colour. Certain permutations of matrix values will result in artifacts such as lines appearing when displaying these areas. For example, the use of the 3x3 dither matrix 33 illustrated in Fig. 4 results in horizontal lines appearing in any large area of the "2 image which had an intensity equal to 3.

3. Preferably the dither matrix should encompass all the values in the range 0 to so as to provide a maximum range of 'granularity' of displayable values.

Many methods are known for forming larger dither matrices. For example, one classical technique due to Bayer is set out in Bayer, "An Optimum Method for Two- Level Rendition of Continuous-Tone Pictures," in Conference Record of the International Conference on Communications, 1973, 26-11-26-15. Another method for forming large dither matrices is disclosed in Australian Patent Specification No. 53111/94 filed 10 January, 1994 entitled "Dithering Optimization Techniques.

In prior art methods, in order to display images on full colour displays, the same 30 principles are applied to a display having multiple primary colours by treating each primary colour separately, independent of the other primary colours that a given display is using.

Additionally, where the display has multiple intensity levels, the principles are applied by the grouping together of 1) separate intensity levels.

The perception of a colour image usually involves three quantities: hue, saturation and luminance. Hue refers to the dominant wavelength of the colour that is seen by the eye and distinguishes among the colours such as red, green, purple and yellow. Saturation refers to how far the colour is from a gray of equal intensity and luminance is a measure or the eye's perceived intensity of the reflected light.

The prior art methods for full colour dithering are not necessarily optimal as they are likely to produce more 'luminance noise' than is strictly necessary. For example, grey scaled [N:\LIBEMACR01 1:rhk I ~LI11 images are normally formed from pixels having equal amounts c f red, green and blue.

Therefore, when grey scale images are dithered using the same dither matrix for each of the three primary colours Red, Green and Blue, the portions of the pixel will be illuminated or not illuminated at the same time. Therefore, on close examination, the final image will appear to consist of portions of the image containing black and white pixels, which collectively make up the image. Such an image has a high level of 'luminance noise', comprising a speckled pattern which is highly noticeable to the human observer. Luminance is a measure of the intensity of a particular colour. In the NTSC standard, for example, the luminance component, denoted Y, is defined by a luminance equation to be: Y= 0.5 G 0.39 R 0.11 B (EQ 3) where G, R and B being the Green, Red and Blue components of a signal respectively.

It is an object of the present invention to provide an improved form of full 15 colour dithering that takes into account the colour aspects of the image.

In accordance with one aspect of the present invention there is disclosed a method of dithering a colour image, the method comprising the steps of: dividing the colour image into a plurality of primary colour components, dividing the colour components into a plurality of groups, and dithering the colour components of the plurality of groups using a different dither matrix for each of the groups.

*e Preferably the number of groups is two and the dividing step includes dividing the colour components according to luminance content, with the groups having a substantially equal luminance content.

25 Generally, the primary colour components comprise a red component, a green component and a blue component and the dithering step is applied to a first group containing the red and blue components and a second group containing the green component.

in a alternative embodiment, the primary colour components include a cyan component, a magenta component and a yellow component and wherein the dithering step is applied to a first group containing the cyan and yellow components and a second group containing the magenta component.

Typically, the colour components of the first group are didhered with a first dither matrix and the colour components of the second group are dithered with a second dither matrix which is inverted with respect to the first dither matrix.

288459 Malcrol I CF110278AU Io:\cisra\limcro\cnipilete\maicro I 11288459au.doc I-~-s _dCF~_ T~ -4A- In accordance with another aspect of the present invention there is disclosed apparatus for dithering a colour image, the colour image comprising a plurality of primary colour components, the apparatus comprising: a plurality of primary colour input means, a first dither matrix value determination means connected to a first series of the plurality of primary colour input means, and a second dither matrix value determination means connected to a second series of the plurality of primary colour input means, wherein the first dither matrix value means determines a first series of dither matrix values and the second dither matrix value means determines a second series of dither matrix values.

Preferably the first dither matrix value determination means and the second dither matrix value determination means are connected to a dither matrix storage means and the first and second series are determined from a cu'rent dither matrix value.

15 A preferred tmbodiment of the present invention will now be described with reference to the accompanying drawings in which: Fig. 5 illustrates an example of full colour dithering in accordance with prior art methods; Fig. 6 illustrates an example of dithering in accordance with the preferred embodiment; Fig. 7 is a schematic view of an apparatus implementing the preferred embodiment.

9 288459 Macral I CFP0278AUU [o:\cisra\macro\cofflplte\macroI 1288459au.doc rrr The preferred embodiment of the present invention can be used with most dither matrices. Preferably the dither matrix of choice is relatively large so as to avoid unwanted artifcts.

In the preferred embodiment it is desired to use a dither matrix for full colour dithering of an image stored in a RGB colour space. The initial dither matrix is used to dither both the red and blue input colours. The green dither matrix is then formed by 'inverting' the initial dither matrix by subtracting the initial dither value from a maximum possible value.

Referring now to Fig. 5, there is shown a portion of an image dithered according to the prior art methods. The original image comprises a grey area 1 of an image, with each primary colour being dithered with the same dithr matrix. Being a grey image, each pixel of the original image will consist of equal amounts of red, blue and green. Hence, in dithering this image, either a pixel of the image will not be illuminated 2, or the red, green and blue pixels will be illuminated together 3. Hence the final resultant dithered-image will appear, on closer examination, to consist of black portions 2 and white illuminated portions 3. As a result of the differing process the image 1 will comprise a high degree of 'luminance noise' in that there is a large alteration in the luminance values, in accordance with Equation 3, of adjacent pixels.

Referring now to Fig. 6 there is shown a similar example using the method of the preferred embodiment. In the preferred embodiment, the green dither matrix is inverted with 0"'2 respect to the red and the blue dither matrices. This means that the red, green and blue pixels are no longer turned on together. The red and blue pixels 4 will be turned on together, as in the previous case, however the green pixel is unlikely to be turned on at the same pixel position. Instead the green pixel, say 5, of another pixel is turned on. On a larger scale hcwever, approximately the same number of green, red and blue pixels are illuminated, so that "1 5 on the larger scale, the image will appear to be of a similar colour content as that shown in 5 11oO.e S °It can be seen from the luminance equation (Equation that the effect of inverting the green dither matrix is to spread out the luminance of the output over a wider area, thereby reducing the luminance noise in the output image, at the expense of some chrominance noise.

ooee° On a finer scale, the grey areas appear to be made up of magenta portions (formed from the red and blue portions) and green portions. It has been found that the eye is a lot more sensitive to luminance noise than to chrominance noise and so the inversion of the green dither matrix surprisingly provides an improved form of final image.

It has been further found that the final picture is also improved in other areas apart from grey regions and this again is thought due to the reduction in luminance noise in the final picture at the expense of chrominance noise.

Referring now to Fig. 7 there is shown a simple form of a dither apparatus 6 implementing the preferred embodiment. The preferred embodiment can be implemented through the simple addition of one subtracter 10 in the dither apparatus 6. In the apparatus of the preferred embodiment each pixel is represented by 8 bits of red, green and blue colour.

IN:\LIBEIMACRO1 :rlk I I I -6information. Input pixels 12 are fed into the dither apparatus 6 in addition to a input position address value 13. The position value is used to lookup a particular value of a dither matrix 11 using modulo "hmetic, the values of the dither matrix being predetermined by the choice of dither matrix.

The dither output value 14 is then used by comparators 8, 9 to determine whether the current pixel is to be illuminated or not. In order to invert the values of the green matrix, the dither output value 14 is fed to a subtracter 10 to be subtracted from a maximum value (which for an 8 bit input component will be 255) to form an inverted value 15. This value is fed to the comparator 7 with the green pixel value and a determination is made if the green portion of the output pixel should be turned on.

The foregoing describes only one embodiment of the present invention.

Modifications, obvious to those skilled in the art, can be made thereto without departing from the scope of the invention. Additionally, the present invention can be applied to other colour systems. For example, in a CYMK system, the inversion of the magenta dither matrix allows for a reduction in luminance noise in colour output devices which output CYMK data.

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Claims (9)

1. A method of dithering a colour image, said method comprising the steps of: dividing the colour image into a plurality of primary colour components, dividing said colour components into a plurality of groups, and dithering said colour components of said plurality of groups using a different dither matrix for each of said groups.

2. A method of dithering a colour image as claimed in claim 1, wherein the number of groups is two and said dividing step includes dividing die colour components according to luminance content, with said groups having a substantially equal luminance content.

3. A method of dithering a colour image as claimed ii; claim 1 or 2, wherein said primary colour components comprise a red component, a green component and a blue component and said dithering step is applied to a first group containing the red and blue components and a second group containing the green component.

4. A method of dithering the colour image as claimed in claim 1 or 2, wherein said primary colour components include a cyan component, a magenta component and a yellow component and wherein said dithering step is applied to a first group containing the cyan and yellow components and a second group containing the magenta component.

5. A method of dithering a colour image as claimed in claim 2 wherein said colour components of said first group are dithered with a first dither matrix and said colour components of said second group are dithered with a second dither matrix which is inverted with respect to the first dither matrix. 25

6. An apparatus for dithering a colour image, said colour image comprising a plurality of primary colour components, said apparatus comprising: a plurality of primary colour input means, a first dither matrix value determination means connected to a first series of said plurality of primary colour input means, and a second dither matrix value determination means connected to a second series of said plurality of primary colour input means, wherein said first dither matrix value means determines a first series of dither matrix values and said second dither matrix value means determines a second series of dither matrix values.

7. An apparatus for dithering a colour image as claimed in claim 6 wherein said first dither matrix value determination means and said second dither matrix value determination means are connected to a dither matrix storage means and said first and second series are determined from a current dither matrix value. 288459 Malcrol I CFP0278AU 2o:\cisra\macro\complctc\iacro I 1J288459au.doc I III

8. A method of dithering a colour image substantially as hereinbefore described with reference to Figure 6.

9. An apparatus for dithering the colour image substantially as hereinbefore described with reference to Figure 7. DATED this Third Day of January 1997 Canon Information Systems Research Australia Pty Ltd Patent Attorneys for the Applicant SPRUSON FERGIJSON 9 *ee e S 288459 Macroll Cill'0278AUU lo :\cisra\lncro\coilClce\ilicroI 11288459au.doc Abstract Reduction of Luminance Noise in Colour Dithering. In order to produce an improved colour image which has been subjected to dithering, a method is disclosed for reducing the Luminance noise in the dithered image, at the expense of Chrominance noise. As the human eye is substantially more sensitive to Luminance noise, an improved overall dithered image is produced. a e o* o IN:\IBEIMACRO 11:rhk -I I I I I