Method and apparatus for printing digital halftone images

Abstract: A method of reproducing images on the basis of digital image data corresponding to an image and containing binary values of basic pixels arranged in a raster, with an electrophotographic printing unit comprising at least one electrically chargeable photo-conductive medium, a selectively activatable light source for image wise discharge of the said medium and a developing unit for developing the said medium with toner powder. The method comprises: processing the said digital image data in order to compensate for properties of the printing unit, wherein for each basic pixel of the digital image data two sub-pixels are generated, namely a first sub-pixel always corresponding in value to the value of the associated basic pixel and a second sub-pixel whose value is determined in dependence upon the value of the associated basic pixel and upon the values of basic pixels in a predetermined area around the associated basic pixel, and the generation of control signals to activate the said light source in accordance with each of the generated sub-pixels. In particular, the said second pixel is in the first instance given the same value as that of the associated basic pixel but if this value specifies the activation of the light source and in addition, the values of basic pixels in the said area around the associated basic pixel indicate that there is a transition in that area between an image portion for exposure and one which is not to be exposed, the value of the second sub-pixel is converted to the value which does not activate the light source. The boundaries between image portions can be positioned more accurately by this step.

Description: The invention relates to a method of reproducing images on the basis of digital image data containing binary values of basic pixels arranged in a raster, with an electrophotographic printing unit comprising an electrically chargeable photo-conductive medium, a selectively activatable light source for image-wise discharge of the said medium and a developing unit for developing the said medium with toner powder, wherein control signals for activating the said light source are generated on the basis of the digital image data, said control signals being adapted to compensate for properties of the printing unit. The invention also relates to apparatus for application of the said method. Methods of reproducing digital binary images (also termed "half-tone images") are generally known. The charged photo-conductive medium is exposed pixel-wise and thus discharged by means of the said light source, e.g. an array of LEDs which is disposed transversely of the direction of transport of the photo-conductive medium. The result is a charge image consisting of charged and discharged areas. The photo-conductive medium is then passed along the developing unit, usually a rotating magnetic roller with a developing powder. In these conditions the charge image is developed with toner. There are two alternative systems with regard to exposure. In a first system, charged areas on the photo-conductive medium are developed while the discharged areas of the photo-conductive medium remain free. A system of this kind is termed a "white-writer". In the other system the converse is the case. This is achieved by applying between the developing unit and the photo-conductive medium an electrical voltage difference equal to the surface potential of charged areas of the photo-conductive medium. A system of this kind is termed a "black-writer". In electrophotographic printers, the light source is usually such that it exposes a somewhat larger area than corresponds to the dimensions of a pixel. The reason for this is to prevent unexposed areas from remaining between the pixels. In the case of white-writers the result of this is that the areas developed with toner powder become smaller than intended, and this becomes particularly visible as a result of narrowing of lines and partial or complete disappearance of fine details. In black-writers, on the other hand, the areas for development are the ones that become somewhat larger, so that narrow lines are conspicuously widened and, for example, small letter characters merge. Another effect that influences the edges of developed Image portions is that in many developing systems, particularly those using developing powders of relatively high resistivity, electrical edge fields occur which cause enhanced development of toner at and just outside the edge of areas for development. In the case of white-writers, this electrical edge effect counteracts the said exposure effect and partly cancels it out, but in black-writing systems the two effects work in the same direction, so that action is required particularly in the case of the latter systems. An obvious remedy to counteract extension of the exposed area would be to make the area exposed per pixel smaller. This can be done, for example, by reducing the intensity of the light source. The intensity of the light spot projected on to the photo-conductive medium for a pixel decreases from the centre, frequently approximately like a Gaussian curve. A reduction of the intensity is therefore first perceptible at the edges of the light spot because a larger proportion of the descending flanks falls below the sensitivity threshold of the photo-conductive medium. A disadvantage of this remedy, however, is that the discharge is locally no longer complete In larger exposed areas. In black-writers, the remaining residues of the charge obstruct the development so that the optical density of a print becomes unacceptably low in larger image areas. In white-writers, the remaining charge residues cause a (slight) development of toner powder in areas which should have remained white. A second remedy to counteract the described undesired extension of the exposed area comprises shifting the time when the light source is switched on when the latter enters and leaves respectively an area for exposure during its relative movement with respect to the photo-conductive medium. In the case of a white-writer a method of this kind is described in American patent 4 387 983. In this way the edge of an exposed and hence discharged area can be so shifted on the photo-conductive medium that the boundary of the developed area is situated exactly at the required location. A disadvantage of this solution is that it requires intervention in the timing of the light source control. Since high-frequency signals are involved here, specialised expensive components must be used for implementation. Moreover, in the case of LED arrays particularly, the control circuits associated with the commercially available products are often not suitable for carrying out small timing changes applicable solely to portions of the array. LED arrays in which this is possible are very expensive. The object of the invention is to provide a solution to the above problems while retaining fixed timing for the light source. To this end, according to the invention, two sub-pixels are generated for each basic pixel of the digital image data, namely, a first sub-pixel always corresponding in value to the value of the associated basic pixel and a second sub-pixel having a value that: is always the same as that of the associated basic pixel, if that basic pixel has a value corresponding to non-activation of the light source, and is determined in dependence on the values of the associated basic pixel and on the values of basic pixels in a predetermined area around the associated basic pixel, else, and in that control signals are generated for selectively activating the light source in accordance with the values of each of the generated sub-pixels so as to compensate for properties of the printing unit. Thus according to the invention each pixel is divided up into two sub-pixels which are exposed independently of one another by the light source. The first sub-pixel is exposed in accordance with the digital image data applicable to the pixel of which it forms part. The same applies to the second sub-pixel unless in the direct surroundings, i.e. in the said predetermined area, there is a transition between an area for exposure and one which is not to be exposed. In the latter case any exposure of the second sub-pixel is switched off so that then there is only the exposure of the associated first sub-pixel. Expressed in conditional terms this means that the second sub-pixel is exposed if the digital image data specify the exposure of the basic pixel and the values of basic pixels in the said area around the associated basic pixel indicate that in that area there is no transition between an image portion for exposure and one which is not to be exposed. In this way the light energy at edges and small structures is so reduced that there is no extension of an exposed area. It is noted that the prior art discloses local fine-tuning of the light energy in an exposure system by introducing sub-pixels for the purpose of anti-aliasing. EP 0 526 738 A discloses a method for "smoothing" the step-like course of a contour between an exposed and an un-exposed image area, when said contour runs under a small angle with the basic raster directions. Sub-pixels positioned between the basic raster lines are used to position the exposure spots more accurately, in order to form an Inclined course. This known method analyzes the basic pixel data and changes a current basic pixel into sub-pixels in dependence of the proximity of a contour step. This method does not solve the problem of excessive toner development along an entire contour, irrespective of the angle thereof. DE 41 06 458 A discloses another anti-aliasing method, in which the basic image data are locally changed when a contour between an exposed and an unexposed image area does not completely include a pixel. An intermediary value is given to such a pixel, which value is then converted into an intermediary exposure time for the light source. This method requires an expensive tunable laser energy control unit. DE 37 29 936 A solves the problem of excessive toner development at a contour between an exposed and an un-exposed image area by regulating the laser energy for pixels positioned at the contour. This method also requires an expensive tunable laser energy control unit. A digital copier/printer is disclosed in EP 0 606 139 A. This document does not address the problem of excessive toner development at a contour between an exposed and an un-exposed image area. An apparatus for reproducing an image, comprising a first generator for generating digital image data in the form of binary values of basic pixels arranged in a raster, an electrophotographic printing unit for reproducing an Image on an image support on the basis of processed digital image data, which printing unit comprises an electrically chargeable photo-conductive medium, a selectively activatable light source for image-wise discharge of the said medium and a developing unit for developing discharged portions of the said medium with toner powder, and a control unit for delivering control signals to selectively activate the light source on the basis of the digital image data, said control signals being adapted to compensate for properties of the printing unit, is characterised according to the invention by a processing unit connected to the first generator for receiving digital image data therefrom and to the control unit for delivering processed digital image data thereto, said processing unit being provided with an analysis module for analysing the values of basic pixels in an analysis area of basic pixels and a second generator adapted to generate a first sub-pixel and a second sub-pixel for each basic pixel and to give the first sub-pixel a value equal to the value of the associated basic pixel and to give the second sub-pixel a value that: is always the same as that of the associated basic pixel, if that basic pixel has a value corresponding to non-activation of the light source, and is dependent upon an analysis by the analysis module (106) of the values of the basic pixels (a-r) in a predetermined area around the associated basic pixel (k), else, and in that the control unit delivers control signals to the light source in accordance with each of the sub-pixels in the processed image data, so as to compensate for properties of the printing unit. The said first generator can be a receiving and interpretation unit for coded image information supplied via a network, or "front end", but also an electro-optical scanner with a half-tone device. In a printing device provided with an array of LEDs for exposure of the photoconductive medium wherein said medium is advanced transversely of the longitudinal direction of the LED array in order to form a two-dimensional charge image, the division of the pixels is achieved by doubling the line frequency with which the LED array is activated. The sub-pixels accordingly lie two-by-two in the transport direction of the photo-conductive medium, i.e. two new image lines of sub-pixels form from an image line of basic pixels. The following description of one embodiment of the invention relates to a printing device of this type. There are also printing devices in which exposure is carried out by a laser, the beam of light of which is moved by means of a rotating polygonal mirror over the surface of the photo-conductive medium in a direction extending transversely of the direction of transport of said medium. In such printing devices, it is simpler to place the sub-pixels two-by-two in the direction of the beam movement since otherwise the speed of rotation of the polygonal mirror would have to be doubled, and this involves engineering difficulties. Of course technical implementation of the invention in the latter printing device differs to some extent from that for an apparatus with an LED array, but it is a self-evident matter for the skilled addressee to adapt the teaching of the invention for this purpose. In a first implementation of the method according to the invention, the position of the first and second sub-pixels relative to one another is the same for all the basic pixels, i.e., for example, in the case of an LED array, all the first sub-pixels are situated in a new image line and all the second sub-pixels in the other new image line. In a second implementation, the position of the first and second sub-pixels relative to one another is determined separately for each basic pixel by reference to the values of the surrounding basic pixels. In this way it is possible, if the two sub-pixels have different values, to link the exposed pixel to any exposure pattern in the neighbouring (basic) pixels. An important additional aspect of the invention is that the light energy with which the first sub-pixels are exposed can differ from that with which the second sub-pixels are exposed. Practical research has shown that the ratio of the light energies of the first and second sub-pixels has a considerable influence on the print quality achieved and that adjustment thereof can be used as an instrument to optimise print quality. Different ratios can also be selected for digital image data from different sources. The invention will now be explained with reference to the accompanying drawings wherein: Fig. 1 is a diagram showing an apparatus according to the invention. Fig. 2 shows a processing unit for making prints on the basis of digital image data. Fig. 3A is a raster of input pixels. Fig. 3B is a raster of reproduction pixels. Fig. 4 is a window for analysis of input pixel values. Fig. 5 is a diagram showing the principle of a processing unit for pixel values. Fig. 1 diagrammatically illustrates apparatus according to the invention. This apparatus (1) comprises a signal source 2, a buffer memory 3, a processing unit 4 and a printing unit 5. The signal source delivers digital image data which comprise binary pixel values which describe an image for printing in accordance with a raster of white or black pixels having a resolution which in the embodiment described here is the same in the horizontal and vertical directions. The signal source 2 is, for example, a unit for receiving coded image information for printing from a workstation and converting said image information into a binary raster image as frequently used in printers. The signal source 2 can also be formed by an electro-optical scanner with a half-tone device which converts the scanner signals to binary pixel values, in which case the apparatus 1 forms a digital copying machine. The apparatus 1 can also comprise both a scanner and a receiving unit, both connected to the processing unit 4 via the buffer memory 3. The processing unit 4 receives digital image data from the memory 3, processes it in the manner to be described hereinafter, and delivers it, possibly via a buffer (not shown), to the printing unit 5, which converts the processed digital image data into a physical print 6 on a sheet of paper. The printing unit 5 is of the "black-writer" type and is shown in greater detail in Fig. 2. A cylinder 10 is coated with a layer of photo-conductive material and is surrounded by process stations 16, 17, 18, 19 and 20. In a charging station 16 the photo-conductive layer is provided with uniform surface charge. The charged photo-conductive layer is then exposed image-wise in accordance with a raster pattern of pixels in the image data as supplied by the processing unit 4, the exposure taking place in the exposure station 17 formed by an LED array with an associated control unit. The surface charge partially flows away in exposed areas. The LED array always exposes a linear area in accordance with an image line of the digital image. Since the cylinder with the photo-conductive layer continues to rotate beneath the LED array and this array always exposes a new image line, there forms on the photo-conductive layer a pattern of charged areas with surface potentials of 800 V (unexposed) and -300 V (exposed) respectively. The photo-conductive layer with the charge pattern thus formed is then taken along a developing station 18 consisting of a roller with a magnetic core and covered with a layer of two-component developer. Here a voltage of -700 V is applied, which substantially compensates for the surface potential of the unexposed areas on the photo-conductive layer, and the exposed areas are developed with negatively charged black powder, which is termed a toner. The resulting toner image is transferred to a sheet of paper in a transfer station 19 formed by a corona unit. The photo-conductive layer 10 then has any residues of non-transferred toner removed in a cleaning unit 20, whereupon it is ready for the next operation in the process. The said sheet of paper is supplied from a feed tray (not shown) and is fed by a pair of rollers 21 to the transfer station 19. From here the paper with the toner image is passed between two heated rubberised rollers 22, where the toner is fused on the paper surface. This process is generally known in electrophotography and will therefore not be explained in greater detail here. The processing of the digital image data in the processing unit 4 will now be explained with reference to Figs. 3A, 3B and 4. An image for printing Is conventionally represented by a raster of image elements or pixels, the grey values of which are given by digital numbers. In many systems, and that is also the case in the system described here, the pixels can have one of two different values, namely white or black. This is specified by a binary number, 1 or 0. The raster comprises rows (often termed image lines) and columns, usually of identical resolution, so that the pixels are square. The resolution, for example, is 400 dots per inch (dpi). According to the invention, each input pixel is converted into two new pixels, hereinafter referred to as pels, which together occupy the same space as an input pixel. All the input pixels are divided into pels in the same way, i.e. either by cutting through in the direction of the row so that an input image line is converted to two parallel new image lines, or by cutting through in the column direction so that the resolution of the pixels is doubled in the row direction. The choice depends on the imaging system: the two new pels are situated next to one another in the scanning direction of the light source. In the system of an LED array described herein, in which the photo-conductive medium is transported along an array of fixed LEDs, the pels are therefore one beneath the other in the direction of transport, and this means that two new lines are formed from one input image line and are exposed successively on the medium. Of the two pels, one is always given the same value as the input pixel had. This pel is hereinafter referred to as the "information pel". The other pel, hereinafter referred to as the "blackening pel", is given a value which is dependent on the values of the surrounding input pixels, in the following manner. If the corresponding pixel is situated in an area for developing but not at an edge thereof, or in an area which is not to be developed, then the blackening pel is given the same value as the input pixel. If the input pixel is in an area for developing but just on or near an edge thereof, or if it forms part of a narrow line for development or some other small detail, then the blackening pel is given the value which is the opposite of that of the input pixel, i.e., it is given a value which corresponds to "not for exposure". The object of this is to prevent edges and narrow lines from being developed too intensively, while nevertheless ensuring good area blackening. It should be noted that the light spot is of dimensions such that the light spots on the two pels overlap one another to a significant extent and have a joint effect on the developed image rather than that supplied by two separately distinguishable dots. If both pels are exposed, the area developed is somewhat larger than the area of the input pixel. This results in good blackening in areas where the developed pixels thus overlap to some extent, but edges and narrow lines are reproduced with too great a width by this effect. In the latter case, therefore, the blackening pel is not exposed, so that development is produced solely by the information pel. In illustration of the foregoing, Fig. 3A shows part of a digital image divided up into pixels. Pixels which have to be exposed (and developed) are shown by hatching. Fig. 3B shows the pattern of information pels and blackening pels as generated according to the invention from the pixel pattern of Fig. 3A. The pixels are divided up vertically into information pels and blackening pels and hatching indicates which of these are exposed. The decision whether a blackening pel of a pixel is to be exposed is taken according to one embodiment of the invention by means of a window operation. A window as shown in Fig. 4 is placed over the pixel raster of the image and the pixel values within the window are evaluated in accordance with a predetermined algorithm. The pixels within the window are denoted by a letter. The pixel with the letter "k" is considered the evaluation centre and the evaluation of the pixel values within the window is the basis for determining whether the blackening pel of pixel "k" is exposed. After each evaluation the window is shifted one raster position for evaluation of the next pixel. The following algorithm has proved satisfactory in practice (the letter "w" hereinafter denotes: not to be exposed, and the letter "z" denotes: to be exposed): If k = w, then do not expose the blackening pel; If k = z , then expose the blackening pel, unless: p=w or f = w and b = w or f = w and b = z and Σ w [a,b,c,e,g,j,l]>3 or I = w and g = w and q = w or I = w and Σ z [g,q] = 1 and Σ w [f,g,h,m,p,q,r]>3 or j = w and e = w and o = w and i = w or j = w and Σ z [e,o,i] = 1 and Σ w [d,e,f,i,n,o,p]>3 Here "Σ w [a,b,c]" denotes: the number of pixels with the value w in the set a, b, c. The term "Σ z " is defined similarly. Fig. 5 is a diagram showing the principle of the relevant parts of the processing unit 4 by means of which exposure or non-exposure of the blackening pels can be determined in accordance with the above-described algorithm, and the printing unit 5. The processing unit comprises four series-connected shift registers 101, 102, 103 and 104. These shift registers have a length (number of cells) equal to an image line (number of pixels). The input of shift register 101 is connected to the buffer memory 2 in order to receive therefrom a series of pixel values of an image for printing. A number of cells of the shift registers is connected to the analysis circuit 106, which is in tum connected to line buffer 107, again of a length of an image line. One of the cells (105) of the shift register 103 is also connected to line buffer 108. The line buffers 107 and 108 are both connected via a multiplexer 109 to a printhead control unit 110. The latter is connected to the printhead 111. The printhead control unit 110 and the printhead 111 together form the exposure station 17 of the printing unit 5 (Fig. 2). A control unit 112 is connected to the line buffers 107 and 108, the multiplexer 109 and the printhead control unit 110 in order to control these elements. In operation, input pixel values of an image for printing are fed to the input of shift register 101 and successively traverse the shift registers 101, 102, 103 and 104. In this way all the pixels of the image pass through the cell 105. At any time, the pixel in this cell is considered the central pixel in an analysis window formed by the pixels in the cells which are connected to the analysis circuit 106. The latter comprises logic circuits for combining the pixel values in the connected cells to give a value which is then given to the blackening pel associated with the pixel in cell 105. The values of the blackening pels as thus determined are successively fed to line buffer 107 and form an image line of blackening pels therein. The values of the input pixels are fed from cell 105 to line buffer 108 where they form an image line of information pels. Elements (not shown) ensure that the value of the blackening pel and the information pel associated with the pixel value present in the cell 105 at any time are fed simultaneously to the line buffers 107 and 108. Immediately after an image line of information pels and of blackening pels have been written into the line buffers 107 and 108, the printhead control unit 109 reads them out, on command from the control unit 112, via the multiplexer 109 and has them printed by the printhead so that first a line of information pixels and then a line of blackening pels are exposed on the photo-conductive medium in synchronisation with its movement. In a first embodiment, the lines of information pels and blackening pels are exposed with equal energy, i.e., the LED's of the printhead are so controlled that they deliver the same light energy for both lines. The amount of light energy can be controlled by changing the duration of electrical energisation of the LEDs per image line, this being referred to as the "duty cycle". In practice, however, it has been found that the effect is often better if different line energies or duty cycles are used for the two types of lines. In a second embodiment, therefore, control of the duty cycle is added to the printhead control unit 109. This is shown in Fig. 3, where the value of the duty cycle for both the information pels and the blackening pels is fed as an external control signal to the printhead control unit 109. The duty cycle setting can be changed by adjusting the control signals. The printhead control unit simultaneously selects with each line the corresponding control signal and adapts the duty cycle thereto. The duty cycle value applies to all the LEDs simultaneously, but may possibly be combined with a duty cycle control for each LED separately for the purpose of individual uniformity control. By means of the duty cycle adjustment it is possible to adapt the exposure to the properties of the image-forming system. In fat-developing systems, for example, the ratio of the duty cycles of the information pels and blackening pels can be selected at 1 2. In addition, tolerances in image-forming systems could be compensated in the same way. In such cases, the adjustment of the duty cycles can be carried out, for example, by a service engineer, the copy quality being an (experimental) parameter. At the same time, different ratios can be selected also for digital image data from different sources. Image data generated by an electro-optical scanner usually have noise pixels, i.e. pixels having the wrong value due to noise in the scanner or other causes. These noise pixels are annoying particularly in white areas. The processing of the image data according to the invention suppresses these pixels to varying degrees, depending on the light energy ratio set. This ratio, of course, will be so optimised that noise pixels are suppressed as far as possible while on the other hand small details (which are of course also suppressed) are still reproduced acceptably. Digital image data generated by a receiving and interpretation unit for print data, e.g. from a workstation, have no noise pixels whatever and, on the contrary, many finer details, such as lines with a thickness of a pixel, and consequently the reproduction of small details will be emphasized on optimisation of the light energy ratio. It will be apparent that the light energy ratio will then be different from that applicable to image data from a scanner. To allow such an adapted choice of light energy ratio, the printhead control unit 109 is connected to the general apparatus control system from which it receives data concerning the origin of the particular image data to be printed. In another embodiment, it is possible to select for each input pixel the location of the information pel and the blackening pel if these pels have different values. In this way it is possible to allow for any variation in blackening in the surroundings of the pixel. The information required for this purpose can be obtained from an analysis of the surrounding pixels. If, however, in such cases it is desired to apply the above-mentioned unequal distribution of duty cycles, then an LED array is required in which the duty cycles can be controlled separately per LED and per pel, and such arrays are very expensive. It has also been found that this does not always result in an improvement in image quality, so that in some systems this embodiment has no advantage over the variant with fixed allocation of the information and blackening pixels. Here again a choice can be made depending on the origin of the image data for printing. Although the invention has been described with reference to the above exemplified embodiments, it is not restricted thereto. The skilled addressee can devise other embodiments within the scope of the following claims.

Claims: A method of reproducing images on the basis of digital image data containing binary values of basic pixels arranged in a raster, with an electrophotographic printing unit (5) comprising an electrically chargeable photo-conductive medium (10), a selectively activatable light source (17) for image-wise discharge of the said medium and a developing unit (18) for developing the said medium with toner powder, wherein control signals for activating the said light source (17) are generated on the basis of the digital image data, said control signals being adapted to compensate for properties of the printing unit (5), characterised in that exactly two sub-pixels (Fig. 3B) are generated for each basic pixel of the digital image data, namely, a first sub-pixel (INFORMATION PEL) always corresponding in value to the value of the associated basic pixel and a second sub-pixel (BLACKENING PEL) having a value that: is always the same as that of the associated basic pixel, if that basic pixel has a value corresponding to non-activation of the light source, and is determined in dependence on the values of the associated basic pixel (k) and on the values of basic pixels (a-r) in a predetermined area around the associated basic pixel (k), else, and in that control signals are generated for selectively activating the light source (17) in accordance with the values of each of the generated sub-pixels so as to compensate for properties of the printing unit (5). A method according to claim 1, characterised in that the said second sub-pixel is given a value corresponding to the activation of the light source if the value of the associated basic pixel specifies the activation of the light source and the values of basic pixels in the said area around the associated basic pixel indicate that in that area there is no transition between an image portion for exposure and one which is not to be exposed. A method according to claim 2, characterised in that a printing unit (5) is used in which exposed portions of the photo-conductive medium are developed with toner powder and in that the said second sub-pixel is given a value corresponding to the activation of the light source if the value of the associated basic pixel specifies development with toner powder and the values of basic pixels in the said area around the associated basic pixel indicate that in that area there is no transition between an image portion for development and one which is not to be developed. A method according to claim 2, characterised in that a printing unit is used in which unexposed portions of the photo-conductive medium are developed with toner powder and in that the said second sub-pixel is given a value corresponding to the activation of the light source if the value of the associated basic pixel specifies non-development with toner powder and the values of basic pixels in the said area around the associated basic pixel indicate that in that area there is no transition between an image portion for development and one which is not to be developed. A method according to any one of the preceding claims, characterised in that the basic pixels are so divided into sub-pixels that an image line of basic pixels is converted to two consecutive image lines of sub-pixels. A method according to any one of claims 1 to 4, characterised in that the basic pixels are so divided up into sub-pixels that an image line of basic pixels is converted to an image line with doubled resolution, formed by the sub-pixels. A method according to claim 5 or 6, characterised in that the position of the first and second sub-pixels relative to one another is the same for all the basic pixels. A method according to claim 5 or 6, characterised in that the position of the first and second sub-pixels relative to one another is determined separately for each basic pixel by reference to the values of the surrounding basic pixels. A method according to any one of the preceding claims, characterised in that the light source is so activated as to deliver a first light energy in response to signals in accordance with a first sub-pixel and a second light energy in response to signals in accordance with a second sub-pixel, said first and second light energies differing from one another. A method according to claim 9, characterised in that a light energy is determined by the duty cycle with which the light source is activated. A method according to claim 1, wherein the step of generating first and second sub-pixels and determining a value for the second sub-pixels uses the following algorithm, where said algorithm is applied to a window arrangement of basic pixels defined as follows: a b c d e f g h i j k l m n o p q r and wherein the letter "w" hereinafter denotes: no light source activation, and the letter "z" denotes: light source activation; the algorithm specifying: if k = w, then do not activate the light source for the second sub-pixel; if k = z, then activate the light source for the second sub-pixel, unless: p=w or f = w and b = w or f = w and b = z and Σ w [a,b,c,e,g,j,l] > 3 or l = w and g = w and q = w or I = w and Σ z [g,q] = l and Σ w [f,g,h,m,p,q,r] > 3 or j = w and e = w and o = w and i = w or j = w and Σ z [e,o,i] = l and Σ w [d,e,f,i,n,o,p] > 3 wherein "Σ w [a,b,c]" denotes: the number of pixels with the value w in the set a, b, c, and the term "Σ z [g,q] "denotes: the number of pixels with the value z in the set g, q. Apparatus for reproducing an image, comprising a first generator (2) for generating digital image data in the form of binary values of pixels arranged in a raster, an electrophotographic printing unit (5) for reproducing an image on an image support on the basis of processed digital image data, which printing unit (5) comprises an electrically chargeable photo-conductive medium (10), a selectively activatable light source (17) for image-wise discharge of the said medium (10) and a developing unit (18) for developing discharged portions of the said medium with toner powder, and a control unit (112) for delivering control signals to selectively activate the light source on the basis of the digital image data, said control signals being adapted to compensate for properties of the printing unit, characterised by a processing unit (4) connected to the first generator (2) for receiving digital image data therefrom and to the control unit for delivering processed digital image data thereto, said processing unit being provided with an analysis module (106) for analysing the values of basic pixels in an analysis area of basic pixels and a second generator (106, 107, 108) adapted to generate a first sub-pixel and a second sub-pixel for each basic pixel (k) and to give the first sub-pixel a value equal to the value of the associated basic pixel and to give the second sub-pixel a value that: is always the same as that of the associated basic pixel, if that basic pixel has a value corresponding to non-activation of the light source, and is dependent upon an analysis by the analysis module (106) of the values of the basic pixels (a-r) in a predetermined area around the associated basic pixel (k), else, and in that the control unit (112) delivers control signals to the light source (17, 110, 111) in accordance with each of the sub-pixels in the processed image data, so as to compensate for properties of the printing unit. Apparatus according to claim 12, characterised in that the second generator (106, 107, 108) allocates the said second sub-pixel a value corresponding to the activation of the light source (17) if the analysis shows that the value of the associated basic pixel specifies the activation of the light source (17) and that in the said area around the associated basic pixel there is no transition between an image portion for exposure and one which is not to be exposed. Apparatus according to claim 13, characterised in that the printing unit (5) is of the type in which exposed portions of the photo-conductive medium (10) are developed with toner powder, and in that the second generator (106, 107, 108) allocates to the said second sub-pixel a value corresponding to the activation of the light source (17) if the analysis shows that the value of the associated basic pixel specifies development with toner powder and that in the said area around the associated basic pixel there is no transition between an image portion for development and one which is not to be developed. Apparatus according to claim 13, characterised in that the printing unit (5) is of the type in which unexposed portions of the photo-conductive medium are developed with toner powder, and in that the second generator (106, 107, 108) allocates to the said second sub-pixel a value corresponding to the activation of the light source (17) if the analysis shows that the value of the associated basic pixel specifies development with toner powder and that in the said area around the associated basic pixel there is no transition between an image portion for development and one which is not to be developed. Apparatus according to any one of claims 12 to 15, characterised in that the basic pixels are so divided up into sub-pixels that an image line of basic pixels is converted to two consecutive image lines of sub-pixels. Apparatus according to any one of claims 12 to 15, characterised in that the basic pixels are so divided up into sub-pixels that an image line of basic pixels is converted into an image line with doubled resolution, formed by the sub-pixels. Apparatus according to claim 16 or 17, characterised in that the position of the first and second sub-pixels relative to one another is the same for all the basic pixels. Apparatus according to claim 16 or 17, characterised in that the position of the first and second sub-pixels relative to one another is determined separately for each basic pixel by the processing unit by reference to the values of the surrounding basic pixels. Apparatus according to any of claims 12 to 19, characterised in that , in accordance with a first sub-pixel, the control unit (112) delivers to the light source (17, 110, 111) a first control signal (1 ○) so that the light source delivers a first light energy and, in accordance with a second sub-pixel, the control unit delivers a second control signal (2 ○) so that the light source delivers a second light energy, said first and second light energies differing from one another. Apparatus according to claim 20, provided with means (113) for adjusting the first and second light energies separately. Apparatus according to claim 20, characterised in that a light energy is determined by a duty cycle with which the light source (17, 110, 111) is activated. Apparatus according to claim 12, wherein the first generator (2) comprises a receiving and interpretation unit for coded image information fed via a network. Apparatus according to claim 12, wherein the first generator (2) comprises an electro-optical scanner with a half-tone device. Apparatus according to claim 12, wherein the first generator (2) comprises both a receiving and interpretation unit for coded image information fed via a network and an electro-optical scanner with a half-tone device, wherein the control unit in accordance with a first sub-pixel delivers a first control signal (1 ○) to the light source (17, 110, 111) so that the latter delivers a predetermined first light energy and, in accordance with a second sub-pixel, the control unit delivers a second control signal (2 ○) as a result of which the light source (17, 110, 111) delivers a predetermined second light energy, which first and second light energies are different from one another, and wherein the predetermined first and second light energies delivered for digital image data from the said receiving and interpretation unit are different from those for digital image data from the said electro-optical scanner with a half-tone device.

Inventors: OYEN JOHANNES PAULUS HUBERTUS, OYEN, JOHANNES PAULUS HUBERTUS
Applicants: OCE TECH BV, OCE-TECHNOLOGIES B.V.
Application year: 1997
Main classification: G06K15
IPC codes: H04N1, B41J2, G06K15