WO1996012251A1

WO1996012251A1 - Anti-aliasing printer

Info

Publication number: WO1996012251A1
Application number: PCT/GB1995/002419
Authority: WO
Inventors: Richard Antony Kirk
Original assignee: Canon Research Centre Europe Ltd.
Priority date: 1994-10-12
Filing date: 1995-10-12
Publication date: 1996-04-25
Also published as: JPH10507137A; EP0787335A1

Abstract

The present invention concerns a method of and apparatus for printing characters in which anti-aliasing is carried out by printing at the edges of the characters discrete pixels of ink of the same colour as the characters but of lower density.

Description

ANTI-ALIASING PRINTER

The present invention concerns printers. It is particularly, though not especially, concerned with bubble jet printers. These bubble jet printers are well- known. They print by ejecting small drops of ink on demand from a jet on a print head that is moved over a print medium such as paper.

Bubble jet printers often have an array of jets on the print head. Colour bubble jet printers have several sets of jets that eject different colours of ink; typically the printing primaries of cyan, magenta, yellow, and black. Both these forms of bubble jet printer are well-known.

In a bubble jet printer the size of the ejected drops is usually fixed so halftones are produced by varying the number of drops per unit area. This approach gives problems with tints with a very low proportion of any one ink because the pattern of drops is seen as a texture rather than a flat tint. For example in a light orange (10% yellow, 2% magenta) will look to the eye like a uniform yellow with magenta 'grit' in it. The same problem occurs with many light colours such as flesh tones, light skies, and metallic shades; and light tints on monochrome printers. One problem which is common to all picture displays which are generated by discrete pixels is known as aliasing. The visible results of this problem are jagged edges in lines which are curved or which are transverse to the main and sub-scan axes of a raster generated image. This undesirable effect is particularly noticeable in the generation of text. Most text is printed in black on a white background and the following discussion will be directed to this normal situation. However, it will be appreciated that the problem is not limited to black and white images . The jagged edges produced by aliasing are the result of the scan conversion of the original data signal in which each pixel is either wholly black or white. The size of the jagged edges is a direct result of the resolution of the printer. Thus in a text printing environment one approach has been to increase resolution by decreasing the pixel size. As a result of this it is now possible to buy printers with widely ranging print densities with the general rule being that the greater the resolution the greater the cost of the printer. It will be readily appreciated that doubling the resolution of a printer in this way involves substantially more sophistication and miniaturisation both in the control circuitry and in the actual print head. Additionally, there is a size limitation on the orifices for a printer such as a bubble jet ink printer and their respective spacings which makes increasing resolution increasingly expensive and eventually very difficult to achieve.

The present application is particularly concerned with alleviating this problem.

In accordance with one aspect of the present invention there is provided a method of printing characters in which anti-aliasing is carried out by printing at the edges of the characters discrete pixels of ink of the same colour as the characters but of lower density.

In accordance with a second aspect of the present invention there is provided printing apparatus comprising means for receiving data signals to print text, a print head for printing the text in ink of a first colour, and a print head for printing anti-aliasing pixels adjacent the edges of the pixels of the first colour with pixels of the same colour but of lesser density.

In order that the present application may be more readily understood, an embodiment thereof will now be described by way of example and with reference to the accompanying drawings in which:

Figure 1 is a perspective view of a host computer and a printer;

Figure 2 is a perspective view of the main components of a bubble jet printer;

Figures 3A and 3B are diagrams showing comparative results obtained by printing with a conventional printer and with a printer in accordance with the present invention;

Figure 4 is a diagram showing decisions made during processing of an image for printing;

Figure 5 is a block diagram of a printer which can be used in accordance with the present invention;

Figures 6A to 6D are diagrams showing different forms of printing;

Figure 7 is a Bayer diagram; and

Figure 8 is another flow chart showing another anti¬ aliasing algorith .

Referring now to Figure 1 of the drawings, this shows a conventional computer having a monitor screen 1, a conventional keyboard 2 and a processing unit 3 connected to a bubble jet printer 4. It will be appreciated that this is only one arrangement possible amongst a wide range of applications. Printers are used in many other applications and it is entirely feasible for the printer to be capable of receiving colour data from an external source and carrying out all the necessary signal processing in the same unit which carries the printer head.

Figure 2 of the accompanying drawings is a perspective view of a printer unit. This unit comprises a head cartridge 10 mounted on a carriage 11 which can be reciprocated in the direction S. The carriage 11 is guided in the direction S along a guide shaft 12 and is reciprocated by a timing belt 13 passing over pulleys 14 and 15 with pulley 15 being driven through a suitable gear train by a carriage motor 16. A transport roller 17 is driven by a transport motor 18 to feed a recording medium such as paper, transparent films for overhead projectors or the like, the recording medium being guided by a paper pan 19 from a feed tray (not shown) to a printing position. When the recording medium is being transported it is biased against the transport roller 17 by feed rollers 20.

The printing apparatus so far described is entirely conventional and in a conventional black and white printer the carriage 11 would carry a single print head for black ink. Again, such print heads are well-known and normally include a plurality of outlet orifices each associated with an ink passage which in operation is supplied with ink. The ink is ejected in the well-known manner by the application of short bursts of thermal energy to selected ink passages so as to discharge drops of ink through the orifices onto the recording medium, the discharge being caused by the generation of bubbles in the ink by the applied thermal energy.

In a conventional colour bubble jet printer the head cartridge would include four print heads for respectively printing black, cyan, magenta and yellow. In the embodiment being described the printer is a monochrome printer and has two print heads, identical save for the inks that they contain in operation. The first of these print heads contains conventional black ink. The second contains a diluted black or grey ink. It will be appreciated that what is described in the following is equally applicable for colours other than black.

As is well-known in the art, printers such as bubble jet printers are essentially bi-level devices, that is a dot of ink is either deposited or not. In bi-level hard copy devices a range of level intensities is made available by what is known as half-toning. As a simple example, a 2 x 2 pixel area of a bi-level display can be used to produce five different density levels at the cost of halving the spatial resolution along each axis. In general an n x n group of bi-level pixels can provide n² + 1 density levels whilst bearing in mind there will always be a trade-off between spatial resolution and density resolution. The trade-off choices made are to some extent determined by visual acuity, which for a normal person under normal lighting conditions is approximately one minute of arc, the distance from which the image is viewed, and the dots per inch of resolution of the graphics device. It will also be appreciated that when an n x n pixel pattern is used to approximate half¬ tones it should be designed not to introduce large scale visual phenomena which would be visible to an observer in an area of similar density values.

However, as discussed in the preamble of the specification, when a bi-level printer such as a bubble jet printer is used to print characters such as text, the finite size of the pixels and the on-off nature of the printing causes jagged edges, namely the problem known as "aliasing" .

Figure 3A of the accompanying drawings shows a simple monochromatic figure as printed by a monochromatic printer with the "aliasing" effect clearly visible. It will be appreciated that this figure is greatly enlarged and could represent a full stop at normal magnification.

Figure 3B shows the same figure printed with the two-tone printer described with regard to Figure 3B of the accompanying drawings. The added grey pixels surrounding the original shape will make the resulting visual appearance smoother. It will be appreciated that the "lack of focus" given by the added grey pixels would not be visible when the pixel size was very large.

Referring now to Figure 4, this is a flow diagram of the computations required to produce anti-aliasing when using a printer which in accordance with the present invention has a multiple print head capable of printing as desired either normal black ink or dilute black ink. In this flow diagram it is assumed black < grey < white. As will be described later the printer could be a colour printer in which the inks for the various colour components also include dilute inks as well as normal inks.

In operation the printing and signal processing apparatus receives 1-bit image data at a resolution higher than that of the printer, averages this data and generates 8-bit dither data. The averaging can be carried out either on lines or specified areas of n x n pixels. In the decision boxes pre-set levels are used to determine whether a pixel of interest is either to be printed as black, white or grey, the grey being printed with dilute ink.

This algorithm can be used in conjunction with known algorithms in which larger areas of black and white are detected so that the anti-aliasing algorithm can be by¬ passed.

The result of the anti-aliasing technique just described is, as is common with such techniques, a trade¬ off between spatial resolution for tone resolution. However, utilising grey ink as described with a 400 dpi printer with appropriately added grey pixels at the edges of the characters, the actual appearance of printed text generated by this method would be equivalent to the appearance of 800 dpi text. This is a substantial increase in quality equivalent to moving from a lesser- quality printer to a professional phototype setter.

It will be appreciated also that additional levels of grey could be used. As four-colour printers are common, existing four-colour printers could be loaded with up to four shades of- grey. It is also conceivable that other shades could be obtained by overprinting different greys. It will be appreciated that the foregoing description has basically been directed to describing a bubble jet printer. Such printers utilise inks which are liquid at normal temperature levels. Another form of printer to which the present invention is applicable is the hot wax printer in which the inks are in the form of waxes which are solid at room temperature. In use the solid wax is heated and is supplied to ejection chambers in molten form. Rather than expelling globules of ink by the application of additional thermal energy so as to generate rapidly expanding bubbles as in the bubble jet printer, in hot wax printers globules of ink are ejected from their appropriate orifices by small piezo-electric crystals. These crystals are driven by firing pulses in a manner entirely analogous to the pulses applied to the thermal elements of a bubble jet print head. It will be appreciated that the problems faced by hot wax printers are also entirely analogous to the problems faced by bubble jet printers and that all the procedures described in this specification are equally applicable to hot wax printers. Thus it is entirely possible to provide a hot wax printer in which the four normal inks are complemented by the addition of inks of lesser density, i.e. effectively diluted inks. Other printers in which the present invention could be utilised include piezo- electric ink jet printers and impart printers in which ink density is dependent on the force of the impulse. Figure 5 of the drawings is a simple block diagram of printing apparatus in accordance with one embodiment of the present invention which can implement the anti¬ aliasing techniques just described. Analogue colour signals RGB are converted into digital form in an ADC 20. These digital signals are halftoned in an appropriate manner in circuit 21 where gamma correction and under colour removal can also be carried out if necessary. The resultant Bb, C, Y and M signals for driving the eight print heads B¹ to Y². Heads B¹ - Y¹ print inks of normal density and the remaining heads print inks at a suitably diluted density so that they can be used in the anti¬ aliasing procedure.

A technique to improve the visual appearance of halftoned hard copy is known as error diffusion and in this technique the difference between the exact pixel value and the approximate value actually displayed is added to the values of image array pixels adjacent to the pixel in question. This has the effect of spreading, or diffusing, the error over several pixels in an image array.

Over the past years there has been substantial literature generated relating to the problems of generating faithful halftones using bi-level reproduction. Examples of such literature are articles by Roger Ulichney (1987, ISBN 0-262-21004-6) entitled "Digital Halftoning" and by J C Stoffel and J F Moreland entitled "A Survey of Electronic Techniques for Pictorial Image Reproduction" and published in IEEE Trans. Commun. Vol. COM-29, Dec 1981, S. 1898...1925. The techniques disclosed in these articles are applicable to the present invention.

However no raster bi-level halftoning technique is capable of generating halftones for very light tints beyond without introducing substantial errors in the lower frequencies. These give the problems of 'grain' or 'grit' in light tones as discussed in the preamble of the specification. What is needed is not a new halftoning process, but a new way of distributing the ink more evenly for the light tints.

Figure 6A of the accompanying drawings shows what a patch of 1% dot (99% reflectance) might look like using conventional (1% reflectance) black ink. It will be appreciated that the problems associated with printing light grey scales, i.e. considerable expanse of white with a very low black component poses exactly the same problem as a light tone where there is a substantial quantity of one hue and a much smaller quantity of a second hue. Figure 6B shows a patch having a similar reflectance to that of Figure 6A, but generated using a dilute ink, namely grey ink having 90% reflectance. Viewed simplistically it will be seen that the grey ink requires ten times as many dots to produce the same reflectance as the normal ink and as a result of this considerably increased number of dots tints at the light end of the print range will be much smoother looking.

Figures 6C and 6D show how tints can be obtained beyond the grey level given by the grey ink by mixing grey and black or white, grey and black. Both of the patches shown in Figures 6C and 6D give 11% reflectance. Figure 6C shows using grey with black whilst Figure 6D shows using black, grey and white. Which of these ways of rendering grey would be chosen in practice would depend on a number of factors which would include the printer, the viewing conditions and the subject to be printed.

Considering now Figure 6C of the drawings, this figure has been generated using conventional bi-level dithering. In conventional dithering it can be supposed that there is an image I, which is a 2-D array of values I(X,Y) for an integer set of intervals X = 0,1...NX and Y = 0,1...NY. For conventional screening there is also a dithering function D(X,Y) defined over the same region. Often the screening function is made from a repeating pattern such as a clustered-dot screen or a dispersed dither pattern, such as the Bayer matrix. Figure 7 is an example of a 4*4 Bayer matrix. Dithering the image to produce a bi-level image B(X,Y) is done as follows. For each X and Y there is set independently...

if {(I(X-Y)-White)/(Black-White) > D(X,Y) ]B(X,Y) = Black else B(X,Y) = White

Black and White are the values that correspond to a solid black and solid white. In a large patch of value 0.5, where Black = 0.0 and White = 1.0, dithering would give a bi-level patch with half the pixels black and half the pixels white. For a small pattern like the 4*4 Bayer dither, the rendition of grey levels is only approximate but improves as the pattern gets larger.

A typical Bayer matrix is shown in Figure 7

Figure 6D shows the effect of using a grey ink (dilute black) as well as the normal black ink. Here the same dithering patter is used but with three printing levels: Black, White and Grey. The pixels with values between White and Grey ^re halftoned using White and Grey... if(I(X,Y)-White)/(Grey-White)<1.0){ if {(I(X,Y)-White)/(Grey-White)>D(X,Y) ]B(X,Y) = Grey else B(X,Y) = White

}else{ if {(I(X,Y)-Grey)/(Black-Grey)>D(X,Y)]B(X,Y) = Black else B(X,Y) = Grey

It will be appreciated that the diagram of Figure 6D gives a much smoother transition between total black and total white. It will also be appreciated that exactly the same procedures are available throughout the whole gamut of colours so that light tones can be reproduced with a much greater degree of realism.

Figure 8 is a flow chart showing another anti¬ aliasing algorithm.

Claims

1. A method of printing characters in which anti¬ aliasing is carried out by printing at the edges of the characters discrete pixels of ink of the same colour as the characters but of lower density.

2. A method as claimed in claim 1 in which the two inks are printed from separate ink heads.

3. A method as claimed in claim 2 wherein one ink is black and the other ink is a less dense black.

4. A method as claimed in claim 3 wherein one ink is black and there are at least two other grey levels of ink available for anti-aliasing.

5. A method according to any one of the preceding claims wherein 1-bit data of a resolution higher that the printing resolution is processed to obtain dither data, the dither data is processed by comparison with preset levels to divide it into white, grey or black data for final output.

6. Printing apparatus comprising means for receiving data signals to print text, a print head for printing the text in ink of a first colour, and a print head for printing anti-aliasing pixels adjacent the edges of the pixels of the first colour with pixels of the same colour but of lesser density.

7. Apparatus as claimed in claim 6 and including means for receiving 1-bit pixel data of higher resolution than the resolution of the printer, means for generating dither data from the pixel data, and means for comparing the dither data so generated with preset levels so as to divide the dither data into white, grey or black data for final output by the printing apparatus.

8. Apparatus according to claim 6 or claim 7 wherein the printer has a print head for printing black ink, and a print head for printing dilute black ink.