METHOD TO IMPROVE PRINTING QUALITY OF GREY SCALES AND COLOUR FOR PRINTERS

Abstract: Method to improve the printing quality at reproduction of half-tone originals by means of printers of the type, in which a number of dots (7) are brought together in a cell (pixel) (8). A number of - minimum two - dots (7) form a cell (8) where each dot in the cell is controlled individually with respect to the size and/or the colour value. The dots form a pattern in each cell, which is variable with respect to the number of dots, the dot size and/or the dot colour value.

Description: METHOD TO IMPROVE PRINTING QUALITY OF GREY SCALES AND COLOUR FOR PRINTERS The invention refers to a method to improve the printing quality at reproduction of half-tone originals by means of printers of the type, in which a number of dots are brought together in a cell (pixel). Background of the invention Digital reproduction of grey scale or half-tone pictures demands that the grey-tones are converted to screen dots the size and division of which is proportional to the percieved blacking impression. That is, sparsely distributed small dots are percieved as a light shade while large dots lying closely together are interpreted by the eye as a dark shade. This technique is used today in all forms of reproduction of half-tone pictures where the printing method is not able to vary the blackness in every single screen dot or other unit area of the printed picture. However in certain printing methods the blackness in the screen dots of the picture can be varied. In most of the methods used today for printing from compu¬ ters there are on possibilities to change the size and/or the blackness of the dots of the printers, without simulta¬ neously reducing the resolution of the print and thereby reduce the quality of the print. The method which thereby is most common, consists of bring¬ ing together the increments of the printer, in the following called dots, into cells (or pixels)in which cells the number of written dots is directly related to the size of the cell. This technique is called dithering. A great deal of research has been devoted to quantify the percieved quality of grey scale pictures as a function of the resolution (cells/inch) of the printed picture and the number of grey tones. A rule of thumb in this case can be that the product of resolution and the number of grey-tones hare called Q, should be some thousands, within certain limits for the cell resolution and the number of grey-tones contained. In Fig 14 to 16 this dependence is illustrated, whereby Fig. 14 shows how conventional photographic technique is able to reproduce grey-tones. The resolution (cells/inch) in this picture is defined by the photographic contact screen that has been used. eg. 120 cells/inch, (or 120 lines per inch as is recommended by the nomenclature in this connection). The number of grey-tones in fig. 14 can be regarded as large and only limited by the grain size in the photgraphic film. The rectangles 9 in fig. 14-16 are shown enlarged in fig.l4A, 15A and 16A. The conventional photographic technique gives a very high Q-value. In fig. 15 is shown how the same picture has been reproduced by means of dithering technique in a laser printer with the resolution 300 dots/inch, which technique cannot change neither the size of the dot or the dot blackness. This picture is produced with a cell resolution of 75 cells / inch, that is every cell contains 4x4 dots which in fig. 15 gives 16 grey-tones in addition to the purely white area in the grey-scale. Hereby the Q-value will be 17 x 75 = 1275, which should be regarded as optimal capability of a laser printer, what concerns reproduction of half-tone pictures. In the Swedish patent no. 8704883-1 (= PCT-SE88-00653) is shown a method of developing pictures and text with pigment particles on an information carrier, directly from computer generated electrical signals, whithout need for these sig¬ nals to be intermediately stored at a temporary conversion to light energy, which is the case in photo conductive printers, eg. laser printers. This has been solved by bring¬ ing the information carrier into electrical cooperation with at least a screen or gate formed matrix, preferably an electrode matrix, which through control in accordance to desired configuration of the pattern, at least partly opens and closes passages through the matrix by galvanic connec¬ tion of this to at least a voltage source and by thus opened passages an electrical field is set free for attraction of the pigment particles towards the information carrier. This method (henceforth called the EMS-concept), as it is descri¬ bed in the above patent, makes it possible to individually control the size and /or blackness of the dots. Another method which makes the control of the size and/ or blackness of the dots possible is contained in photoconduc- tive printers the light source of which consists of an array of light emitting diodes, so called LED-printers, in which the emitted amount of light and/or light energy of the light emitting diodes is individually controllable. Large efforts in research and development are made today on printer concepts which allow this type of dot-size or dot- density control. Common to all these methods is that the printig speed is related to the number of sizes and/or degrees of blacking. That is, if a picture with Q-value is to be pinted, the printing speed of the printer has to be substantially reduced. Another problem for this type of printing method constitutes in the repetition accuracy for the different dot sizes and degrees of dot blacking respectively. A grey-scale picture should contain at least a number of ten grey-scales top be recognized as lifelike. In those cases where pigment par¬ ticles are used to generate the blackness the number of well limited and repeatable dot sizes and/ or degrees of blacking of the dots can be limited. To sum up it can be mentioned that even if printing methods with control of the size of the dots and/ or blackness are developed in order to improve grey-scale reproduction this technique will not substantially influence the customers benefit since the printers will be slow and /or limited to a too small number of grey-tones. The same technique which has been described here, can be applied att reproduction of colour scales, where the colours magenta, cyan and yellow together can be mixed by means of dots of different sizes to cells with new colours. Even here the customers benefit is limited by speed performance and the number of colours in accordance to the above. The object of the invention and most important features The object of the invention is to create a method which improves the printing quality for grey-tones and color in printers which can control and change the size and/or the degree of blacking of the dots of the printer. The object here is to make printing possible of a large number of grey- tones or colours with good repetition accuracy and uniform quality. Moreover the invention implies that the printing speed of the printer has not to be reduced in corresponding degree as would be the case when using only control of the size of the dots and /or blackness, for many types of prin- ters. These problems have been solved by a number - minimum two - dots form a cell where each dot in the cell is controlled individually with respect to the size and/or the colour value and that the dots form a pattern in each cell, which is variable with respect to the number of dots, the dot size and/ or the dot colour value. By creating cells consisting of at least two individually controllable dots as for size and/or blackness, the number of percieved grey- tones can be substantially increased in comparison to what can be repro¬ duced each by dot control and dithering technique respec¬ tively. The need for a number of well limited and repeatable dot sizes and/or blacking degrees of the dots hereby can be made smaller, whereby the speed of the printer has not to be reduced as much. This technique, in the following called DDC (Dot & dithering Control) , gives EMS, LED and other printer concepts improved performance when printing grey-scales and at reproduction of colour. In fig. 16 the quality is exemplified of a grey scale picture with a Q-value over 3000, which can be achie- ved with a printer with the resolution 300 dots/inch and four dot sizes. Description of the drawings The invention will be described below in greater detail with reference to accompanying drawings, which show some embodi¬ ments. Fig. 1 shows four sections, eg. in the form of a mesh of an electrode matrix and diagrammatically the control of the dot size according to the EMS- concept. Fig. 2 shows in a diagram how the potential of the elec¬ trodes in the Fig. 1 affects the dot size. Fig. 3 shows four sections, eg. in the form of a mesh of an electrode matrix and diagrammatically the control of the dot blacking according to the EMS-concept. Fig. 4 shows in a diagram how the potential of the elec¬ trodes in Fig. 3 influences the dot blackness. Fig. 5 shows four sections, eg. in the form of a mesh of an electrode matrix and diagramatically the control of dot size and blackness according to the EMS-concept. Fig. 6 shows in a diagram how the potential of the elec¬ trodes in Fig. 5 influences the dot size and the blackness. Fig. 7 shows four dots of different sizes in addition to 0- level which represents the absence of a dot. Fig. 8 shows diagrammatically how the dithering technique can be combined with control of the dot size at five levels. Fig. 9 shows four different blacking degrees of dots of equal size plus a 0-level which represents the absence of a dot. Fig. 10 shows diagrammatically how the dithering technique can be combined with control of the degree of blacking at five levels. Fig. 11 shows four different degrees of blacking of four dots of different size plus a o-level which represents the absence of a dot. Fig. 12 shows diagrammatically how the dithering technique can be combined control of both thwe dot size and the dot blackness at five levels. Fig. 13A och 13B illustrates examples of picture qualities, which can be achieved with the DDC-technique in a 300 dots /inch printer. Fig. 14-16 shows the difference between different reproduc¬ tion methods. By influencing geometries and other parameters in the EMS - concept the dependence of the dots to the control voltage of the electrodes can be changed. The dots illustrated in the figures shown represent a grey- scale but instead of grey-tones, the dots can have any tone of colour Fig. 1 and 2 diagrammatically an in enlargement show how the size of the dot 7 only can be changed by varying the con¬ trast voltage of the electrodes 5a and 6a, which are in¬ cluded in an electrode matrix. The values on the axes in the diagrams in fig. 2, 4 and 6 only constitute examples. The shaded line in the diagrams in fig. 2, 4 and 6 diagrammatically show how the parameter- dependence is somewhat uncertain. In fig. 3 and 6 is shown diagrammatically how only the blacking degree of dot 7 (in % ; of which 100% represents completely black and 0% represents completely white) is influenced by the potential at the electrodes 5a och 6a. In fig. 5 and 6 is shown diagrammatically how both the size and the blacking degree of the dot 7 is influenced by the potential at the electrode 5a and 6a. By changing eg. the intensity of field between the information carrier and the developing roller ( see the Swedish patent 8704883-1), the coupling between dot size and blackness can be changed, whereby fig. 6 only constitutes an example of such a coup¬ ling. In fig. 8 is shown diagrammatically and enlarged an embodi¬ ment of the method according to the invention where four dots together form a cell 8. Each cell is on the top in¬ dicated by a percent number (eg. 62,5%) which aims at sho¬ wing the percieved degree of blacking for the whole cell 8 observed in natural size. Every dot can be printed in four different sizes 1,2,3 and 4, which is shown in fig. 7. The absence of a dot, the 0- level in fig.7 also represents a state, which with advantage can be used to increase the number of percieved grey-tones of the cell 8. Conventional dithering technique in this example only should offer four grey-tone levels of the cell 8 (100,75,50 and 25%) in addition to the completely white state (0%). By combining the control of the size of the dots in five levels in the embodiment shown the number of grey-tones percieved from the cell 8 can be increased to 16 ( 100%, 93,75%, 87, 5%, .- ..,6,25%) plus the completely white state (0%). This is carried out by the first state 100% of the cell 8 contains of four dots 7 in size 4. The first grey- tone 93,75% after the completely black cell 100% is represented by one of the dots 7 being reduced in size at level 3. In the subsequently following grey-tone 87,5% a further dot 7 has been reduced to size 3. In this case it can be advantageous to choose the diagonally placed dot 7, which is shown in fig. 8, since the eye by this finds it more difficult to dissolve the internal structure of the cell. The pattern according to which the dots are changed in the cell 8 is however according to the invention not fixed to the pattern shown, but can be carried out according to quite different sequences. For pedagogic reasons it has been chosen to present one and the same sequence in the drawings. In the cell 8 which is represen¬ ted by the grey-tone 81,25% three of the dots 7 of the cell 8 have been reduced to size 3 whereby only one dot 7 has maintained its original size 4. In the subsequently following grey-scale 75% all dots 7 in the cell 8 have been reduced to size 3. The principle for changing the grey-scale for the whole cell 8 then follows the same description as the one given above. The absence of a dot "level 0" in fig. 7 is used as a structure element in the lightest grey-tones 18,75, 12,5, 6,25 and 0%. For pedagogic reasons it has been chosen here to show the dimensional relation of the dots as linear. The perception of the eye of the degree of blacking is however not linear either to the size of the cell or to the size of the dots contained in the cell 8. In fig. 9 and 10 is shown how the invention can be applied on a printer the controllability of which on the dots 7 follows the one shown in fig. 3 and 4, that is, only the blacking degree of the dot can be controlled. The descrip¬ tion of this embodiment follows the one given for fig. 7 and 8 above. Even here the blacking degree of the dot 7 has to be adapted to the total perception of the grey-tone of the cell 8, so that the diffrent grey-tones of the print dis- tribute evenly over the scale from 100% to 0%. In fig. 11 and 12 is shown how the invention can be applied on a printer the controllability of which on the dots 7 follows the one shown in fig. 5 and 6, that is, both the blackining degree and the size of the dot can be controlled. The description of this embodiment follows the one given for fig. 7 and 8 above. Since the DDC-technique has to be explained and illustrated on dot level, drawings are made at an enlarged scale it might be difficult to understand the use of the technique. All screen technique uses the limited resolution capability of the eye to distinguish the smallest structure elements of the picture. If the quality of the picture is high enough (Q-value larger than about 1500) the human eye without aid can not determine how the cells are built up. This is illustrated in fig. 13A and 13B where 18 grey-tones plus a white and a black level has been used in a resolution of 80 cells/inch (that is Q=20x80 = 1600). in fig. 14b, which has a resolution of 17 grey-tones (Q = 1275), the eye is disturbed by the unregula- rity of the cells. In fig.14c, which has a more coarse cell resolution than 14b but has substantially more grey-tones or cell sizes, the eye cannot distinguish the internal struc¬ ture of the cell. A printer which uses DDC with the resolution 300 dots/inch and ability to control the dot size and/or the blackness in eg. 4 levels plus the white level, can reproduce grey-tone pictures in eg. a resolution of 75 cells/inch and 75 grey- tones (that is Q= 4875) or eg. 150 cells /inch resolution and 17 grey-tones (that is Q = 2550). The corresponding picture quality reproduced by merely conventional dithering technique demands a printer with the resolution 600 dots /inch. Even if the Q-value is a very coarse quality measure, which has been used here only for pedagogical reasons, the invention will substantially improve printing performance at reproduction of grey-tones and colour. Moreover it is of course possible that dots of different size and blackness are mixed with each other, so that 4 4 tones are obtained, if every cell contains 4 dots. If the cells are divided into other patterns, with eg. 5 or 6 dots in every cell the number of tones in corresponding degree will be larger. The invention is not limited to embodiments of method desc¬ ribed herein. THus it is possible within the scoppe of invention use both fewer and more levels of size and or blackness of the dots in combination with both fewer and more dots in every cell. A printer with 400 dots / inch with possibility to vary the eg. the dot size in 8 levels thus can reproduce eg. 128 grey-tones with the cell resolution 100 dots/inch and a 200 dots/inch printer with possibility to vary the blackness of the dots in two levels plus the purely white level can reproduce 19 grey-tones with the cell resolution 66,7 cell¬ s/inch. In the last mentioned case every cell consists of 3 x 3 =9 dots. Further the pattern according to which the dots are changed in the cell can be freely selected for optimum picture quality within the scope of the invention. The invention is neither restricted to quadratic or circular dots but with advantage can be applied on printer concepts which only can eg. change the length of the dots in the feeding direction of by eg. changing the paper speed and/ or the printing time for every single dot.

Claims: CLAIMS 1. Method to improve the printing quality at reproduction of half-tone originals by means of printers of the type, in which a number of dots (7) are brought together in a cell (pixel) (8), c h a r a c t e r i z e d t h e r e i n, that a number - minimum two - dots (7) form a cell (8) where each dot in the cell is controlled individually with respect to the size and/ or the colour value and that the dots form a pattern in each cell, which is variable with respect to the number of dots, the dot size and/ or the dot colour value. 2. Method according to claim 1, at electrographic printers of the type, in which a latent electrical charge pattern is produced by electrical signals by means of an electrode matrix or the like for attraction of pigment particles towards an information carrier, c h a r a c t e r i z e d t h e r e i n, that the control of the size of the dots and colour value is carried out by changing the voltage level applied to elec¬ trodes of the electrode matrix.

Inventors: LARSON OVE, LARSON, OVE
Applicants: ARRAY PRINTERS AB
Application year: 1990
Main classification: B41J2
IPC codes: B41J2, H04N1