In this specification, electrographic process means a process for converting a digital image comprising pixels into a latent image comprising dots using light or other exposure means, e.g. from a light source arranged to act on a photoconductive surface, by striking the surface, to form the latent image on the surface by changing the charge distribution on the surface in the regions of the dots, applying a toner/liquid ink to the surface such that the toner/liquid ink adheres to the surface in regions of the latent image and transferring the toner from the surface to a substrate to form a final image.  The latent image corresponds to a digital image which is required to be reproduced.  Some examples of xerographic machines which use xerographic processes are laser printers, digital printing presses, photocopiers, fax machines, plate setters, direct-to-film laser printers and scanned laser displays. 
The term dot is intended to cover any shape which is produced by the light source when forming the latent image, e.g. circles, dashes, lines etc, and could be considered to be “pixel”, and is not limited to any particular shape.  For example in most laser printers these dots would be substantially circular since they are formed by light from a laser striking a photoconductive surface at a point corresponding to a pixel to be reproduced and charge distribution is affected substantially symmetrically outwardly from this point. 
In this specification dot gain means the dot gain associated with an electrophotographic process i.e. it is an expression of the size difference between the dot in the final physical image of the xerographic process (e.g. on paper) compared to the electronic, digital coverage in an original image being copied/printed etc.  For example if the xerographic process is used to reproduce an original digital image comprising a pixel, the area covered by toner forming a dot representing the pixel in the final physical image will be different to the area covered by the pixel in the original electronic digital image. 
Dot gain can be defined in a number of ways.  For example, using the above example, dot gain can be defined as the logarithm of the ratio of the actual dot area (in the final image) and the digital pixel area (in the original image).  Alternatively this dot gain can be expressed as the difference between covered area in the final image (i.e. area covered by dots) and covered area in the original image (i.e. area covered by pixels).  These two definitions are examples of ways in which dot gain can be defined and both of these examples have the same sign (positive/negative) structure.  Using these definitions, if the coverage in the original and final images is the same then the dot gain will be zero.  In most printing processes the dot gain is usually non-zero and positive.  Using the above example to illustrate, the coverage of the dot in the final image is usually greater than the coverage of the pixel in the original image which the dot represents. 
The level of dot gain in an image formed using a xerographic process is dependent on, amongst other things, the way in which the light source acts on the surface to form the latent image.  The extent to which light from the light source changes the charge distribution on the photoconductive surface affects the amount of toner or liquid ink (or other pigmenting material) which will adhere to the surface and therefore affects the level of dot gain.  As an example, a first latent dot (at the photoconductive surface) may be formed using a xerographic process by a light source discharging a region on a charged surface at a first laser intensity for 0.1 seconds and a second latent dot may be formed using the xerographic process by the light source discharging a region on a charged surface at the first laser intensity for 0.2 seconds.  The first and second regions may be discharged to different extents which may cause different amounts of ink or toner to adhere to the surface and thus to form the final image.  This can affect the area covered by the ink or toner in the final image.  Therefore the way in which the light source acts on the surface can affect dot gain. 
In this specification the light source level is used to indicate how much light from the light source acts on the photoconductive surface.  As discussed, this is related to the extent of change in charge distribution on the surface in regions where the light strikes and thus the amount of toner/ink which will adhere to the surface and is thus linked to the level of dot gain.  Some other examples of how to vary the light source level received at the photoconductive surface are by operating the light source in different modes (e.g. power modes or scanning modes) for different periods of time, by operating the light source in bursts, by operating the light source at different intensity/power levels or by causing different amounts of light to act upon the surface in any other suitable way.  If the light source is a laser one way of achieving a variation in the light source level is by laser power modulation or by laser pulse width modulation.  Light acts on the surface by hitting the surface.  Different amounts of light acting on the surface will cause different amounts of ink/toner to adhere to the surface in desired regions.  Light source, in this specification can therefore be used to refer to, for example a laser, optics associated with the laser and scanning means, e.g. a polygon mirror associated with the laser, all in combination. 
Optical Density (OD) is defined as the absorbance of light by a (printed) element and is defined as
  OD  =            log      10        ⁡          (              I        IO            )      where IO is input light amplitude and I is output or reflected light.  The OD of a print is dependent on the toner/ink thickness and on the coverage.  For a solid patch where coverage is, by definition, complete the OD is dependent only on the toner thickness. 
The common situation in Xerographic print that the final toner or ink thickness on the substrate (e.g. paper) and the area covered are determined by the latent image formed by the light source on the photoconductor and the interaction of the various voltage potentials driving the charged toner in the system.  If the overall light amplitude is reduced the horizontal dimension (orthogonal to thickness) of the printed elements, or the coverage, will be largely reduced and the thickness of toner will be somewhat reduced.  On the other hand if the voltage potentials are changed then the toner thickness will be largely changed and the coverage will be somewhat changed.  Thus, normally, the thickness, resulting in Optical Density and the Dot Gain (DG) (which is a measure of the actual cover) are coupled and one may not change one without affecting the other.  The color consistency of solid patches depends mainly on OD, while the width of graphic elements such as text/lines etc, depends mainly on the Dot Gain. 
According to one aspect of the present invention, there is provided a method for decoupling the tuning of Optical Density (OD) from the tuning of Dot Gain (DG), both on a global basis (same dot gain across the page), and on a local basis where the dot gain is adapted to local image characteristics, e.g. to protect sensitive graphical elements such as small dots from print instabilities by locally modifying the dot gain. 
Advantageously, accurate and consistent tuning of both Dot Gain and optical density is obtained in the same image.  However, as FIG.  21 illustrates, the coupling of OD and DG can limit the possibility to obtain a desired Dot Gain value (e.g. zero) for a given OD value that is tuned for solid-patch color consistency. 
The connection between ink thickness and printed object coverage/size is illustrated in FIG.  21, which displays the result of attempting to print a 3 pixel wide line.  The curves 2001, 2002, 2003 are the 3 Gaussian shaped beams which, together, write the line (the height is in arbitrary units).  The dashed 2004 and dotted 2005 curves are the resulting charge distributions on the photo-conductor (in arbitrary units) for two different power levels —the dashed curve 2004 represents a higher power level than the dotted curve 2005.  The horizontal line 2006 depicts for a certain condition the development field indicating separation between foreground and background.  Anything above the brown line will be background and anything below will be printed.  The shaded box 2007 represents the boundary of the common variation in development field needed to compensate consumable variation.  It will be noticed that the width for the lowest condition (represented by the arrow 2008) is much less than the width for the highest condition (represented by the arrow 2009).  This shows that changing the ink thickness by changing the development voltage also changes the dot gain.  The variation induced by changing the laser power is shown by the difference in size between arrow 2010 (dashed line (higher power)) and arrow 2011 (dotted line (lower power)), indicating a change in dot gain.  Towards the bottom of the curves, arrow 2013 illustrates that the two curves 2004, 2005 have different depths indicating a difference in ink thickness when laser power is changed.  Thus the laser power and developer voltage (field induced thereby) together affect and couple dot gain and ink thickness. 
The central arrow 2012 represents the zero dot gain condition for the three pixel-wide line.  Since the arrow 2012 does not touch the dashed (higher power) curve 2005 and in this typical case the curve 2005 represents the lowest allowable laser power before instability sets in, the zero dot gain condition is not accessible. 
Moreover, for some system setting aimed to obtain certain OD values, small graphical elements such as small dots or narrow lines, may suffer from print instabilities if the dot gain is insufficient.  Therefore it can be desirable to increase the dot gain for such elements (“protect” them), without modifying the overall OD which is already tuned for solid-patches. 
According to another aspect of the present invention, there is provided a method to control the dot gain separately from optical density, by irradiating edge (or close to edge) dots differently from internal dots, since the dot gain is defined at the edges of the printed elements, and does not depend on internal dots.  The provided method can bring the dot gain to a desired nominal value, in particular, zero dot gain. 
According to another aspect of the present invention, there is provided a method to improve print stability, by locally adapting the dot gain according to local characteristics of the latent image, so that small graphical elements such as small halftone dots are printed in a stable fashion, i.e. always appear on the final print and preferably with a constant size. 
According to another aspect of the invention, there is provided a dot gain compensation method for taking into account dot gain in a xerographic process which comprises converting a digital image comprising pixels into a latent image comprising dots using light from a controllable light source arranged to strike a photoconductive surface and change charge distribution on the surface to form the dots making the latent image on the surface, the digital and latent images each having an edge and comprising an edge pixel or edge dot respectively, which is at or near the edge, and a non-edge pixel or non-edge dot respectively, which is not at or near the edge, wherein the method comprises the step of identifying whether or not a dot to be formed is an edge dot and using a different light source level incident at the photoconductive surface when forming the edge dot compared to when forming the non-edge dot such that charge distribution is changed to a different extent when forming the edge dot compared to when forming the non-edge dot. 
Preferably each pixel of the digital image has an associated instruction indicating a default light source level which should be used when forming its corresponding dot in the latent image, the method comprising forming the edge dot using a light source level different to the default light source level. 
Preferably the light source acts differently by (i) operating for a different period of time, (ii) operating in different bursts, (iii) operating at a different intensity, (iv) scanning light across the surface at a different rate, or (v) causing a different amount of light to strike the surface when forming a dot in any other suitable way, or (vi) any combination of (i) to (v). 
The edge dot identifying step may comprise the step of comparing a selected pixel and its neighbouring pixels to templates known to be indicative of an edge pixel to determine whether or not the selected pixel is an edge pixel. 
Alternatively the pixel may have a tag identifying it as an edge pixel or as a non-edge pixel, the method comprising reading the tag to determine whether or not the pixel is an edge pixel. 
Preferably the method includes the step of calibrating the action of the light source on the surface so that the light source forms the edge dot so as to provide a desired level of dot gain for an edge dot in a physical image produced by the xerographic process.  Preferably the desired level of dot gain is substantially zero. 
The method may comprise using a lower light source level when forming the edge dot than when forming the non-edge dot. 
The edge dot may comprise a protected edge dot and the method comprises the step of identifying whether or not an edge dot to be formed is a protected edge dot and using the same light source level when forming the protected edge dot compared to if it were a non-edge dot or using a light source level which is not reduced to the same extent compared to if it were an edge dot which is not a protected edge dot. 
The protected dot identifying step may comprise the step of comparing a selected pixel and its neighbouring pixels to templates known to be indicative of a protected edge pixel to determine whether or not the selected pixel is a protected edge pixel. 
The edge pixel may have a tag identifying it as a protected edge pixel or as a non-protected edge pixel, the method comprising reading the tag to determine whether or not the edge pixel is a protected edge pixel. 
The method may comprise the further steps of controlling the light source used in the xerographic process, the process comprising converting the digital image comprising pixels into a physical image comprising corresponding dots, the method arranged to achieve a desired light source level when forming edge dots such that charge distribution on the photoconductive surface is changed to a desired extent and achieve a desired level of dot gain in edge dots of physical images produced by the process, the light source being operable in a plurality of modes to produce differing levels of dot gain;                an optical ratio between two xerographically produced physical images being defined as a ratio of mean average optical densities of each image;        the method comprising using the xerographic process to produce a first physical image having a first mean average optical density and a first attribute which influences the mean average optical density of the image for a given level of dot gain and a second physical image having a second average optical density and a second attribute which influences the average optical density of the image for a given level of dot gain, the first and second physical images with their associated first and second attributes being such that at a particular optical ratio between the first and second physical images, the level of dot gain in the second physical image will be at the desired level,the method comprising adjusting the light source level to produce first and second physical images until they substantially provide the desired optical ratio between the xerographically produced physical images, and thereby establishing the desired light source level.         
According to another aspect of the present invention there is provided a computer program product encoded with software code which when run on a processor of a xerographic machine causes a processor of the machine to instruct a light source of the machine to operate to cause a different light source level when forming edge dots of an image than when forming non-edge dots of the image in order to control dot gain in the xerographic process such that a desired final line width/dot size results in the image. 
According to another aspect of the present invention there is provided a computer program product encoded with software code which when run on a processor of a xerographic machine causes a processor of the machine to control a light source of the machine to provide a desired light source level such that charge distribution on a photoconductive surface of the machine is changed to a desired extent and a desired level of dot gain in final images produced by the process is achieved, wherein the light source is operable in a plurality of modes to produce differing levels of dot gain;                an optical ratio between two xerographically produced final images is defined as a ratio of mean average optical densities of each image;        the processor is arranged to use the xerographic process to produce a first final image having a first mean average optical density and a first attribute which influences the mean average optical density of the image for a given level of dot gain and a second final image having a second average optical density and a second attribute which influences the average optical density of the image for a given level of dot gain, the first and second final images with their associated first and second attributes being such that at a particular optical ratio between the first and second final images, the level of dot gain in the second final image will be at the desired level,the processor further being arranged to adjust the light source level to produce first and second final images until they substantially provide the desired optical ratio between the xerographically produced final images, and thereby establishing the desired light source level.         
According to a further aspect of the present invention there is provided a computer program product encoded with software code which when run on a processor of a xerographic machine is arranged to perform the steps caused by the computer program products of the above two defined aspects of the present invention. 
According to another aspect of the present invention there is provided a method of making a xerographic machine, such as a printer or photocopier, comprising installing software code encoded on a computer program product according to any of the previously defined aspects of the invention on a control processor of an existing xerographic machine arranged to be able to run the software. 
According to another aspect of the present invention there is provided a method of printing an image using a xerographic printer having a photoconductive substrate and a xerographic light source arranged to irradiate the photoconductive substrate in pixels, the amount of xerographic light falling on a pixel being influenced by a digital image representation of an image, the digital image having for each pixel at least one light control parameter which is used to control the amount of xerographic light which falls onto each pixel during the formation of a latent image on the substrate, the method comprising determining whether a pixel of the digital image is an edge pixel at the edge of a feature in the image and, pursuant to that determination, altering the amount of light that falls on the equivalent edge pixel of the latent image on the photoconductive substrate in comparison to the amount that would otherwise fall on a non-edge pixel which had the same pixel light control parameter(s) associated with it. 
According to another aspect of the present invention there is provided a method of xerographically printing images comprising setting a desired dot gain for a xerographic printer or photocopier by comparing two print images having a different ratio of number of edge pixels to total number of pixels so as to establish what print control settings that influence the amount of light falling on pixels of a latent xerographic image are used to achieve a desired result of said comparing, and printing images using print control settings so established. 
According to another aspect of the present invention there is provided a xerographic image producing machine comprising:                a photoconductive substrate adapted to produce a latent image to be produced;        a control processor capable of accessing a memory containing a digital reproduction of an image to be printed;        a light source;the control processor being programmed to perform an evaluation of the digital representation to differentiate between pixels of a latent image to be produced on the substrate that are edge pixels at the edge of a feature in the latent image and pixels that are more central in features of the latent image than edge pixels and to modify an amount of light falling on pixels of the latent image pursuant to said evaluation.         
According to another aspect of the present invention there is provided a method of forming a xerographic image from a digital image comprising pixels and associated light level control values adapted to control the amount of light incident upon latent image pixels, on a photoconductive substrate, associated with the digital image pixels, the method comprising illuminating latent image pixels which correspond to edge pixels at the edge of a feature in the image with a lower amount of light per unit area than is used for pixels with equivalent light level control values that are non-edge pixels. 
According to another aspect of the present invention there is provided a method of xerographic printing using a xerographic printer having a photoconductive substrate upon which a latent image is formed from a xerographic light source and a digital image to be printed having colour intensity levels associated with pixels of the digital image, the method comprising determining if a pixel in the digital image is an edge pixel at the edge of a feature in the digital image, and pursuant to such a determination differentially modifying the exposure of latent image pixels on the photoconductive substrate dependent upon whether or not the latent image pixels correspond to edge pixels of the digital image. 
According to another aspect of the present invention there is provided a digital image with edge pixels flagged as such with an “edge pixel” flag. 
Further aspects of the invention are defined in the claims. 
It should be appreciated that when an aspect of an invention is claimed or described as a particular category (e.g. as a method, system, data carrier, xerographic machine etc.)  then protection is also sought for that aspect but expressed as a different category of the claim.  For example the first aspect of the invention may also be expressed as a system, a xerographic machine, a method etc.  For example a claim to a method may also be expressed as a xerographic machine capable of carrying out the method or a data carrier having software on it which instructs a processor to carry out the method.