Multilevel screening mapping tone values to control signal values for greater color or shade fidelity and reduced print aberrations

A method of producing a printed output on a substrate is provided using a drop emitting device which is adapted to emit a plurality of ink quantities in response to a control signal. The method comprises the steps of receiving image data corresponding to a pixel of an image to be printed, the received image data comprising a tone value; generating a threshold value for use in a screening operation based on the location of the pixel within the image to be printed; mapping the received tone value to a mapped tone value using a first tone mapping, the mapped tone value being generated based on the received tone value; performing the screening operation by comparing the mapped tone value with the threshold value; mapping the received tone value to a control signal value using a first control mapping if the output of the comparison performed in the previous step indicates a first relationship, the control signal value being generated based on the received tone value; and repeating the previous three steps if the output of the comparison performed in the previous step indicates a second relationship, wherein the first mapping step comprises using an alternate tone mapping and the second mapping step comprises using an alternate control mapping.

FIELD OF THE INVENTION

The present invention relates to a method and system for use in producing a printed output on a substrate using a printing device. In particular the present invention relates to a method and a system for use in generating control signals for a drop emitting device from received image data.

BACKGROUND OF THE INVENTION

An inkjet printing system typically comprises an image source and inkjet printhead. Such a system is designed to print a specific output on a substrate. The inkjet printhead typically further comprises a plurality of drop emitting devices, each drop emitting device being configured to emit a quantity of ink onto the substrate below. In a colour printing system a plurality of inkjet printheads are used, wherein each inkjet printhead is adapted to print a different colour component of the complete colour image.

Typically, an image source provides image data in the form of a two-dimensional image file. When an image is to be printed, print drivers upon the image source and control systems upon the inkjet printing device convert the image data into a series of control signals which are used to actuate the drop emitting devices. In a typical colour printing system, image data is first converted into four colour components: each component being one of cyan, magenta, yellow and black (CMYK). The printing system then comprises four sets of drop emitting devices, one on each of a plurality of inkjet printheads, wherein each device is adapted to print a different colour component. In most printing systems, each drop emitting device will be a binary device configured either to print a drop of ink upon receipt of a positive signal or not to print the drop of ink on receipt of a null signal. Hence the image data, which is typically within a colour space, such as RGB or CIE L*a*b* colour space, must be converted into a binary control signal with which to control each drop emitting device.

This conversion process is further complicated by the process of screening. As stated previously, an image is typically decomposed into four colour component images, one for each of the CMYK components. Each of these CMYK images is then printed by a separate set of drop emitting devices. By printing each component on top of each other, a colour image is produced. However, it is difficult to represent different shades of colour with this system, as the amount of ink emitted by each drop emitting device is limited and as such the printing process cannot vary the amount of ink applied to the substrate. Screening solves this problem by representing lighter shades as a series of dots or patterns rather than solid areas of ink. By configuring the screening pattern, a variety of shades can be produced which appear to the eye to form a continuous colour image.

A prior art system for applying a screening process is shown inFIG. 11. The screening operation is performed pixel by pixel, as each pixel of image data is typically printed by a single drop emitting device. The prior art binary screening system1100comprises a threshold array1130. The threshold array1130is typically a two-dimensional array that contains data configured so as to generate a screening pattern on the substrate. The system1100receives a pixel1110of image data and also receives the pixel's X1160and Y1170position within an image to be printed. The X1160and Y1170positional information provides a set of indices with which to retrieve a threshold value T from the two-dimensional threshold array1130. The system1100also extracts a colour tone value Crfrom the image data. This colour tone value Cris typically the intensity level of a particular colour component. The colour tone value Cris then compared with the threshold value T using a binary comparator1140. The output of the binary comparator1140is a control signal CS which is used to control the drop emitting device1150.

For example, if the colour tone value Cris greater than the threshold value T, the comparator1140will output a control signal CS with a value of one. This will instruct the drop emitting device1150to print a drop of ink. If the colour tone value Cris less than the threshold value T, then the comparator1140will output a control signal CS with a value of zero which will instruct the drop emitting device1150not to print a drop of ink. By repeating this process for each pixel in a given scan line, and then repeating for the number of scan lines within the final image, a suitable screening pattern can be printed on the substrate. As the X1160and Y1170positional data for each pixel will vary, a number of different threshold values will be generated. It is this variation that allows a screening pattern to be printed.

The method of printing an output using the system ofFIG. 11is illustrated inFIG. 1. At step110image data Iris received. This image data Iris converted to a control signal value CS by the system ofFIG. 11in step120. The control signal is then sent to a drop emitting device in step130and the drop emitting device prints an output onto the substrate at step140. At step150, the printing system1100checks whether more image data remains: if there are remaining pixels then the printing system1100retrieves the next pixel at step160and the process begins again. If there are no more remaining pixels then the process ends at step170.

In recent times, inkjet printing devices have been invented that can emit a plurality of ink quantities. For example, these inkjet printing devices may comprise adapted drop emitting devices that are able to emit two or more drops for a given pixel or pixel colour component value. These devices enable printed images with greater shade fidelity to be generated. However, the use of these devices also generates two additional problems. The first of these is how to adapt the binary screening system such as that shown inFIG. 11which is designed for binary drop emitting devices. The second of these is how to reduce the appearance of undesirable print artefacts that have been created because a range of ink quantities can now be printed. These undesirable print artefacts arise due to the larger range of the adapted drop emitting devices, which means it becomes possible to print a reduced quantity of ink upon the substrate when compared to a binary drop emitting devices. This then becomes a problem when the resolution of the printing system in a direction perpendicular to the feed direction of a substrate is fixed by the number of drop emitting devices that form the inkjet printhead.

For example, an inkjet printhead may comprise a row of drop emitting devices that extends laterally in a direction perpendicular to the feed direction of a substrate. Using the latest technology this row may contain approximately 140 drop emitting devices per centimeter, fixing the resolution at 360 dots per square inch. At this resolution a binary drop emitting device is able to emit enough ink to cover a corresponding pixel area on the substrate below. However, if adapted drop emitting devices are used that can emit a reduced quantity of ink, then it is possible that the emitted ink will not cover the corresponding pixel area on the substrate below. In this case a viewer of the printed image may be aware of an area of unprinted substrate around the reduced quantity of ink present on the substrate. If two neighbouring adapted drop emitting devices emit a reduced quantity of ink then the problem is further compounded as this action produces an even larger area of unprinted substrate and can produce a “speckling” or “streaking” effect visible to a viewer. One such example is what is referred to as “rain”. This is where a series of vertical lines or streaks are visible on a printed image because a group of neighbouring adapted drop emitting devices have repeatedly been instructed to print reduced ink quantities.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the present invention there is provided a method of producing a printed output on a substrate using a drop emitting device adapted to emit a plurality of ink quantities in response to a control signal, the method comprising the steps of:a. receiving image data corresponding to a pixel of an image to be printed, the received image data comprising a tone value;b. generating a threshold value for use in a screening operation based on the location of the pixel within the image to be printed;c. mapping the received tone value to a mapped tone value using a first tone mapping, the mapped tone value being generated based on the received tone value;d. performing the screening operation by comparing the mapped tone value with the threshold value;e. mapping the received tone value to a control signal value using a first control mapping if the output of the comparison performed in previous step (d) indicates a first relationship, the control signal value being generated based on the received tone value; andf. repeating steps (c) to (f) if the output of the comparison performed in previous step (d) indicates a second relationship, wherein step (c) comprises using an alternate tone mapping and step (e) comprises using an alternate control mapping.

By using a tone mapping and a control mapping a control signal value can be selected from a group control signal values based on a screening operation. This provides greater variation in the printed output and reduces the appearance of visible print aberrations. By using the output of a screening operation to decide whether to iterate the method steps conventional screening systems can be integrated with multilevel drop emitting devices. This results in a high quality printed image with a greater colour or shade fidelity.

By mapping the received tone value to a mapped tone value, the method is able to use standard binary thresholding processes without change. The repetition of the last four steps if the output of the comparison indicates a second relationship allows a control signal value to be selected from a group of possible control signal values based on the number of repetitions. Furthermore, the selection of the control signal value based on the above method may control the number of drops of ink emitted from the drop emitting device. The control signal value may also be inversely proportional to the number of iterations of the last four steps that have been performed. Hence the quantity of ink emitted by the drop emitting device will reduce as more iterations are performed. This then further allows steps common to standard binary thresholding methods to be used with an adapted drop emitting device capable of emitting a plurality of quantities of ink.

The tone mapping and control mapping used on each iteration of the above method will typically use a different mapping strategy. This may then produce a different mapped tone value and possibly a different control signal value for each iteration. By varying the mapping method rather than the method steps standard to binary screening, the method can easily apply a binary screening method to a multi-level system and furthermore is able to reduce the appearance of print aberrations by varying the control signal value.

In certain embodiments using one of the tone mappings corresponding look-up table, each tone look-up table being indexed by the received tone value. Using one of the tone mappings may also comprise using a two-dimensional array that uses the received tone value and the number of repetitions of steps (c) to (f) as indices. Using a control mapping may also further comprise using a corresponding look-up table or using a two-dimensional array. Using a plurality of look-up tables to provide the mapping provides a quick and simple solution to a problem of how to produce a mapped tone value and a control signal value. Using a two-dimensional array further enhances these advantages.

In a preferred embodiment of the present invention, the method further comprises setting an iteration counter to an initial value before performing the mapping between the received tone value and the mapped tone value and the screening operation further comprises, if the output of the comparison indicates a second relationship, incrementing the iteration counter. Each mapping operation can also comprise looking up a mapped output in a two dimensional array using the received tone value and the iteration counter as indices. The use of an iteration counter and the received tone value as indices allow a value to be quickly retrieved from either one of the above two-dimensional arrays. The use of an iteration counter may also allow the control signal value to be inversely proportional to the number of iterations of the screening operation, however this will depend upon the contents of the look-up tables or two-dimensional arrays.

In accordance with a second aspect of the present invention there is provided a method of producing a printed output on a substrate using a drop emitting device adapted to emit a plurality of ink quantities in response to a control signal, the method comprising the steps of:

a. receiving image data corresponding to a pixel of an image to be printed, the received image data comprising a tone value;

b. generating a threshold value for use in a screening operation based on the location of the pixel within the image to be printed;

c. mapping the received tone value to a plurality of mapped tone values using a corresponding plurality of tone mappings, the plurality of mapped tone values being generated based on the received tone value;

d. performing the screening operation by comparing each of the plurality of mapped tone values with the threshold value; and

e. mapping the received tone value to a control signal value using a control mapping, the control mapping being selected from a plurality of control mappings based on the output of step (d), the control signal value being generated based on the received tone value.

This method provides an alternative set of steps wherein, instead of iterating the screening operation, the various steps of the screening operation are performed in parallel, i.e. producing a plurality of mapped tone values and performing a parallel comparison of the threshold value and the plurality of mapped tone values. In certain situations this can provide a faster method but its implementation depends on the hardware and/or software available.

In a preferred embodiment, the control mapping comprises selecting a control signal value from a group of two or more control signal values representative of two or more ink quantities. By using an increased number of control signal values, the tone fidelity can be increased and the appearance of print aberrations reduced. This group may be a non sequential group and may comprise any combination of ink quantity levels.

In certain embodiments, the method further comprises: measuring the characteristics of a test printed output; generating a measured dot gain characteristic curve based on the measured characteristics; and calibrating one or more tone mappings and/or one or more control mappings so as to minimise the difference between the measured dot gain characteristic curve and a reference dot gain characteristic curve. These calibration steps then correct for any alterations from a reference dot gain characteristic curve that may be introduced by the screening operation or the method of the first aspect of the present invention.

The method may also comprise configuring one or more tone mappings and/or one or more control mappings so as to limit the maximum quantity of ink emitted from the drop emitting device or to vary the dot percentage range for a plurality of tone values. This may be required when using a variety of substrates as each substrate may only be able to hold a set quantity of ink.

According to a third aspect of the invention there is provided a system for producing a printed output upon a substrate the system comprising: a drop emitting device for emitting a plurality of ink quantities in response to a control signal; an image receiver for receiving image data corresponding to a pixel of an image to be printed, the received image data comprising a tone value; a threshold generator for producing a threshold value for use in a screening operation; a mapping generator for generating a mapped tone value based on one of a plurality of tone mappings, each tone mapping operating on the received tone value; a comparator for performing a screening operation, wherein the comparator is adapted to compare the mapped tone value with the threshold value; and a controller for processing the comparator output and generating a control signal value based on one of a plurality of control mappings, each control mapping operating on the received tone value, wherein: if the output of the comparator indicates a first relationship, the controller is adapted to select one of the plurality of control mappings to generate control signal value; if the output of the comparator indicates a second relationship the controller is further adapted to instruct the mapping generator to produce a mapping tone value using an alternate one of the plurality of tone mappings and to select an alternate one of the plurality of control mappings with which to generate control signal value.

The third aspect of the present invention then provides a series of technical integers to implement the method of the first aspect of the present invention. The second aspect is then able to provide the advantages previously discussed.

In a preferred embodiment of the present invention the mapping generator comprises a first two-dimensional mapping array for providing a mapped tone value based on the received tone value and an iteration counter; the threshold generator comprises a second two-dimensional threshold array for providing a threshold value based on the location of the pixel within the image to be printed; the controller comprises a third two-dimensional control array for generating a control signal value based on the received tone value and the iteration counter; and the controller is further adapted to perform the following steps in response to receiving a tone value: generating a threshold value using the second two-dimensional threshold array; resetting the iteration counter; generating a mapped tone value using the first two dimensional mapping array; and generating an output using the comparator, wherein, if the output of the comparison indicates a first relationship, the controller is adapted to generate a control signal value using the third two-dimensional control array, and, if the output of the comparison indicates a second relationship, the controller is adapted to increment the iteration counter and repeat steps (iii) and (iv).

The use of three two-dimensional arrays allow an efficient solution to the mapping of a tone value to a mapped tone value and the problem of generating a control signal value based on received tone value. These three two-dimensional arrays can be implemented either in software or in hardware. The controller is then adapted to route signals between various components and to control the sequence of the screening operation. In certain embodiments of the present invention, the mapping array and the comparator are adapted to produce a plurality of outputs. The controller can then perform its control of the system in parallel. This can speed up the operation of the system.

In certain embodiments, the size of each of the first and third two-dimensional arrays in one dimension is determined by the maximum number of ink quantities that can be emitted by the drop-emitting device. In certain embodiments the control signal value is selected from a group of two or more control signal values representative of two or more ink quantities based on a screening operation. This group may also be non-sequential. The drop-emitting device may be adapted to output zero or more drops of ink based on the three or more control signal values.

In other embodiments, the system further comprises a sensor for measuring a printed output upon a substrate, the printed output comprising a test pattern; and a calibration module for calibrating one or more tone mappings and/or one or more control mappings based on one or more measurements received from the sensor. The calibration module may be adapted to generate a measured dot gain characteristic curve based on the one or more measurements received from the sensor and to calibrate the one or more tone mappings and/or one or more control mappings based on a comparison of the measured dot gain characteristic curve and a reference dot gain characteristic curve.

The calibration module then allows the screening operation to be calibrated, based on actual printed output and also allows the mapping operation to be configured so as to produce tonal characteristics that are preferred by the human eye. The mapping generator and controller may also be adapted to configure the control signal so as to limit the maximum quantity of ink emitted from the drop emitting device or the screening system may be adapted to configure the control system based on the type of substrate to be printed upon. The dot percentage range for a group of tone values may also be changed along the range of tone values by adapting the mapping generator.

According to a fourth aspect of the present invention there is provided a colour printing apparatus for producing a coloured printed output on a substrate, the printing apparatus comprising a plurality of systems according to the third aspect of the present invention, wherein each system is adapted to produce a printed output of a different colour component on the substrate. Hence, a plurality of systems according to the third aspect of the present invention can be used to implement a CMYK printing system.

DETAILED DESCRIPTION OF EMBODIMENTS

A first embodiment of the present invention is illustrated inFIGS. 2 and 12.FIG. 12illustrates a printing system1200for applying multilevel screening. The printing system1200shares certain components with the prior art binary screening system shown inFIG. 11. These include threshold array1130, comparator1140, pixel location generators1160and1170and pixel receiving means1110. In addition to the components shared with the binary screening system the printing system1200further comprises a mapping generator1210and a controller1220. The printing system ofFIG. 12also includes a drop emitting device1250that differs from the device1150shown inFIG. 11. The printing system1200can by a combination of software and hardware components.

Drop emitting device1250differs from drop emitting device1150in that it is able to emit a plurality of ink quantities in response to a control signal CS sent from a controller1220. For example, the drop emitting device1250may comprise a piezoelectric actuator adapted to control the device to emit between zero and seven drops of ink per pixel. In this case, the control signal CS will be a three bit data value coding for a respective number of drops. The drop emitting device1250is then adapted to receive the control signal CS and operate the piezoelectric actuator accordingly.

The mapping generator1210is operably connected to pixel receiving means1110, controller1220and comparator1140. The mapping generator1210is adapted to receive a coloured tone value Crthat has been extracted from image data Ircorresponding to a pixel of an image to be printed and to convert said tone value into a mapped tone value Cm. The mapping generator1210is also adapted to supply a mapped tone value Cmto the comparator1140and to receive a mapping signal MS from the controller1220.

The comparator1140is operably connected to the mapping generator1210, the threshold array1130and the controller1220. The comparator1140is adapted to receive the mapped tone value Cmfrom the mapping generator1210together with a threshold value T generated using the two-dimensional threshold array1130. Like the system illustrated inFIG. 11, the two-dimensional threshold array1130is adapted to receive an X, Y co-ordinate of the present pixel to be printed and to use the co-ordinate values to look up a suitable threshold value T within the threshold array1130. The comparator is adapted to compare the two input values Cmand T and produce a comparator output CO that is dependent on the result of the comparison. The comparator is then further adapted to pass the comparator output CO to the controller1220.

FIG. 2shows a multilevel screening method to be implemented using the printing system ofFIG. 12. The method ofFIG. 2presents a new alternate method for implementing step120as illustrated inFIG. 1. Returning toFIG. 1, a method according to a first embodiment of the present invention will begin by receiving image data Iras is illustrated in step110. This image data Irtypically comprises a pixel of a contone CMYK image or an image scanline comprising a row of such pixels. This contone CMYK image can optionally be pre-processed to colour managed CMYK values using a colour management system (CMS). A typical CMS will produce a pixel with four data elements, each element typically being 16 bits in length and coding for one of the CMYK colour components. If a CMS system has not been used then the pixel data may comprise four data elements, each element typically being 8 bits in length. The methods and systems of the present invention may also equally be applied to grayscale image data, in which case the pixel data will only comprise a single data element corresponding to the pixel's gray level. After a pixel has been received and pre-processed at step110, the method proceeds to step120, wherein the steps illustrated inFIG. 2are implemented.

At step210, a tone value Crcorresponding to a one of the data elements of the pixel is extracted from the received image data Ir. The tone or image value is typically an n bit data value coding for 2npossible tone values ranging from light to dark for each colour component. The extracted tone value Crmay optionally be converted to a set number of bits for ease of processing. For example, the print system1200may be adapted to use 10 bit data values throughout; and so, if this is the case, received 8 bit data elements will be shifted up by 2 bits whereas 16 bit data elements will be shifted down by 6 bits; typically by using an 8 or 16 bit to 10 bit converter comprising part of pixel receiving means1110. If the system uses a 10 bit tone value then the tone range will be from 0 to 1023. In such a case, a value of 0 will typically code for the darkest tone and a value of 1023 will typically code for the lightest tone. Pixel receiving means1110then passes the extracted tone value Crto the mapping generator1210. At the same time, the X and Y values for the received pixel are passed to the two-dimensional threshold array1130. At step220, this then enables the threshold array1130to produce a threshold value T.

At step230, the received tone value Cris mapped to a map tone value Cmby mapping generator1210. Mapping generator1210comprises a series of software routines or hardware adapted to map a received tone value Cr, with a value within a first range of tone values, to a mapped tone value Cm, with a value within a second range of tone values. Typically, the first range comprises a subset of the full range tone values and the second range comprises the full range of tone values. For example the mapping generator1310may be adapted to map a received tone value Crwithin a first range of 0 to 127 to a range of mapped tone values Cmwithin a second range of 0 to 1023. The mapping generator1210may be adapted to use a variety of mapping functions to perform this mapping depending on various image processing considerations.

At step250the mapped tone value Cmis compared with the threshold value T generated by the threshold array1130. This comparison is performed by comparator1140. In the example shown inFIG. 2, the comparator checks to see whether the threshold value T is greater than or equal to the mapped tone value Cm. The comparator1140then outputs a value CO representative of the results of the comparison. In the present example binary comparator is used, however it is also possible to use non-binary comparators, for example comparators that output the difference of the two compared values. In the present case, if the threshold value T is greater than the mapped tone value Cm, the comparator output CO will have a binary value of 1 and if it is not the case, the output of the comparator CO will have a binary value of 0. The comparator output CO is then passed to controller1220.

In the present example, if the comparator output CO is positive (i.e. binary 1), then the controller1220is adapted to generate a control signal CS as illustrated in step260. This control signal CS is dependent on the comparator output CO. The form of the comparator output is set by convention and so it is possible to alternatively use different comparator outputs (e.g. the inverse of the above) and configure the controller1220accordingly. The control signal CS will be of a form adapted for the multilevel drop emitting device1250; for example, the control signal CS may be a three bit value and the controller1220may be adapted to set this control signal CS to a value of 7 if the comparator output1255is positive. The method will then return to step130inFIG. 1and the control signal will be sent to the drop emitting device in order to print an output onto the substrate as shown in step140.

In the present case, if the output of the comparator CO is binary 0 then the controller1220is further adapted to send a mapping signal MS to mapping generator1210. This mapping signal MS is designed to tell the mapping generator1210to alter the mapping method as is shown in step240inFIG. 2. The method then returns to step230inFIG. 2and the received tone value Cris again mapped to a map tone value Cm; however, this time the mapping generator1210is adapted to use a different mapping method from that used the first time step230was initiated. Thus it may be the case that a different mapped tone value Cmis generated. The newly created map tone value Cmis then again compared with the same threshold T by a comparator1140, shown in step250. The comparator output CO is then again communicated to the controller1220which is adapted to repeat steps240,230and250until the comparator output CO is again positive. When the comparator output does become positive the controller is further adapted to make a note of the number of times the loop comprising steps230,250and240has been repeated and to set the control signal CS accordingly. For example, if the comparator output CO is set to positive on the third iteration of steps230,250and240then the controller1220may be adapted to a set the control signal CS to a medium drop level, e.g. a value of 4 for a 3 bit system. However, if the iteration has been performed twice, then the controller1220may be adapted to set the control signal CS to a higher level, e.g. a value of 5 in a 3 bit system, upon receipt of a positive output from a comparator1140. The control signal CS is then again passed to drop emitting device1250and the flow of the method returns to step130inFIG. 1.

The above method is repeated for all the pixels within one image scanline and then is again repeated for each scanline within the image. Thus, at step150, the method checks to see whether there are any image data remaining and if there are, the next pixel in the scanline, or the first pixel in a subsequent scanline, is fetched at step160and the method repeats with step110.

The method described above thus allows a multilevel drop emitting device1250to be used and the control signal CS to be selected from a group of control signals based on a screening operation. Furthermore, the method uses a mapping process and a comparison operation to determine which control signal value within a group of control signal values is used to control the drop emitting device1250.

A second embodiment of the present invention is shown inFIGS. 3 and 13. The printing system1300ofFIG. 13comprises pixel location generators1160and1170, pixel receiving means1110, threshold array1130, comparator1140and drop emitting device1250. These components are implemented in a similar manner to those shown inFIG. 12. Printing system1300then further comprises a two-dimensional tone mapping array1310, an extended controller1320and a two-dimensional control array1330.

The two-dimensional tone mapping array1310is adapted to retrieve a mapped tone value Cmbased on a received tone value Crand an index i received from controller1320. Two examples of this two-dimensional tone mapping array1310are shown inFIGS. 5 and 6.FIG. 5shows a two-dimensional tone mapping array500wherein the array is configured to be used with a printing system1300capable of three iterations. The array comprises a plurality of columns520and a series of rows510. Each column510is indexed by one or more received tone values Cr. For example, in the example shown inFIG. 4, the first column is used to map received tone values within a range 0 to 127.

The rows of the array500are indexed by an iteration counter i. Each element of the array500is indexed by a row value510and a column value520. In certain embodiments, the array500may comprise a column for every received tone value Cr, e.g. the array500could have 1024×3 elements, wherein each element comprises a stored value which is then returned as the mapped tone value Cm. In this case the elements of the array500may be generated beforehand using functions F1, F2and F3as illustrated inFIG. 9.FIG. 9shows each function as a different line type (thick, thin, and dashed), which have been respectively labelled “Blue”, “Green” and “Red” for convenience. The colours and line types used to label the functions inFIG. 9are arbitrary and are unrelated to the function of the invention. In other embodiments, as partly illustrated inFIGS. 5 and 6, each element of the array may comprise or link to a function of the received tone value Cr. For example,FIG. 9illustrates three functions that divide the received tone value scale into seven equal segments wherein each segment has a range of 14.28 percent of the possible received tone values Cr. In either case, each of the functions F1, F2and F3will be used in some manner to produce a mapped tone value Cm.

FIG. 6shows a different example in which up to five iterations may be performed. Tone mapping array600comprises five rows610and a series of columns620. In a similar manner to the array500ofFIG. 5the functions F1to F5to be used to create a mapped tone value Crare shown inFIG. 10.FIG. 10shows each function as a different line type, wherein each function or line type is also labelled with a different “colour” for convenience. The colours and line types used to label the functions inFIG. 10are arbitrary and are unrelated to the function of the invention. In certain embodiments, the array600may comprise a column for every received tone value Cr, e.g. the array600could have 1024×5 elements, wherein each element comprises a stored value which is then returned as the mapped tone value Cm. Functions F1to F5(630to670) may then be used to populate the array. The maximum number of rows510,610within the tone mapping array is typically set as the maximum number of drop levels capable of being emitted by the drop emitting device1250, for example eight rows if the drop emitting device is responsive to a three bit control value. The tone mapping array may also be implemented as a plurality of look-up tables, wherein each row or column comprises a separate table, or may be implemented as one or more software functions adapted to receive an iteration index i and a received tone value Crand output a mapped tone value Cm.

Printing system1300also comprises a control array1330that is implemented in a similar manner to the tone mapping array1310. The control array1330is configured to retrieve and output a control signal value CS based on a received tone value Crand an index i. This index i may be an iteration counter.FIG. 7illustrates a control array to be used with a maximum of three iterations andFIG. 8illustrates a control array to be used with a maximum of five iterations, in a similar manner to respectiveFIGS. 5 and 6. Each array700,800comprises a number of rows710,810and a number of columns720,820. The columns are indexed by one or more received tone values Crand the rows are indexed by the index i, which can be an iteration counter. In the present example, each element730,830of the control array1330contains a three bit control signal value CS which will instruct the drop emitting device1250to emit a set quantity of ink: with a range from 0, which will instruct the drop emitting device not to emit any drops of ink, to 7, which will instruct the drop emitting device to emit 7 drops of ink. The form and range of this control signal value CS is dependent on the type of drop emitting device used and so may vary from the example described herein.

A method of creating a suitable control signal using the printing system1300is illustrated inFIG. 3. The printing method begins with step110ofFIG. 1, wherein image data Iris received. The method then moves to step120which, in this case, is implemented by the method step illustrated inFIG. 3. The tone value Cris first extracted from received image data Irat step310. This is performed by the pixel receiving means1110as was described with relation toFIG. 12. At the same time or subsequently, at step320, a threshold value T is generated using the threshold array1130which takes as input an X co-ordinate1160and a Y co-ordinate1170of the pixel. At step330, the controller1320sets the index or iteration counter i to an initial value. In the present example, the iteration counter is incremented from 0 to a maximum value. In other embodiments the iteration counter can alternatively be decremented from a maximum value to a minimum value, although this may require altering the threshold comparison or the operation of the controller1320. The controller1320then passes the iteration counter i to the tone mapping array1310. At step340, the tone mapping array1310uses the received tone value Crand the iteration counter i as indices to select an appropriate mapped tone value Cm. At step350, the threshold value T is compared with the mapped tone value Cm. The comparison is performed by comparator1140which examines whether the threshold value T is greater than or equal to the mapped tone value Cm. If the threshold value T is not greater than or equal to the mapped tone value Cmthen the comparator1140produces a comparator output CO with a first value. If the comparator1140finds that the threshold value T is greater than the mapped tone value Cmthen the comparator1140produces a comparator output CO with a second value. The comparator output CO is then passed to the controller1320.

If the comparator1140is a binary comparator then the second value is typically 0. On receipt of the second value the controller1320is adapted to increment the iteration counter i at step370and repeat step340a further time. This produces a first iteration of the method. In this case, a new mapped tone value Cmis again generated using the tone mapping array1310, the received tone value Crand the incremented index or iteration counter value i. The new mapped tone value Cmis then again supplied to the comparator1140to be compared with the threshold value T. A new comparator output CO is then generated. If this comparator output CO is also negative the controller1320will again increment the iteration counter i and a second iteration will begin.

If the comparator output is a first value, e.g. 1, the controller1320is further adapted to halt the iteration of steps340,350and370and pass the current iteration counter i to the control array1330. The control array1330is adapted to receive the iteration counter i, together with a received tone value Cr, and to use these variables as indices to generate a control signal value CS as shown in step360. This generation is performed using arrays such as those illustrated inFIG. 7andFIG. 8. The resultant control signal value CS is then sent to the drop emitting device1250, as in step130, and an output is printed upon the substrate140. As in the first embodiment, the above process is then repeated for further pixels within an image to be printed.

Steps340and350,370are repeated as long as the comparator output CO is a second value, e.g. 0. In certain embodiments, the controller1320is further adapted to stop the iteration of steps340,350and370when the iteration counter reaches a maximum value, e.g. imax=2 as for the example ofFIGS. 5 and 7or imax=4 as for the example ofFIGS. 6 and 8. When imax=2, the printing system1300and method as described above ensure that no more than fifty percent of any particular drop size or level, other than the largest drop size or level, is used. This method thus has improved linearity over a method using a binary drop emitting device. When imax=4, the printing system1300and method as described above ensure that no more than twenty percent of any particular drop size or level, other than the largest drop size or level, is used. The number of iterations effectively sets the maximum number of different drop levels or sizes that may be printed for a set tonal range. For example, if imax=4 then a maximum of five different ink quantities, printed using five different control signals, can be produced for a given tonal band. The ink quantities can form a non sequential group, e.g. each control signal may be selected from a group representing 1, 2, 5, 6, or 7 drops to be emitted, and the control mapping may select a control signal value from a group of values containing any combination of ink quantity levels. By reducing the likelihood that any particular drop size or level is repeatedly printed, e.g. repeatedly printed within a tonal band, the method prevents the appearance of “rain” and other print aberrations. The method and system do not need to vary the occurrence of the largest drop size or level, as this quantity of ink typically covers an allotted pixel area upon the substrate and thus does not lead to print aberrations

FIGS. 4 and 14illustrate a third embodiment of the present invention. In this embodiment, a multiple value comparison is performed using matrix algebra. As with the first and second embodiments, printing system1400comprises X1160and Y1170pixel location generators, threshold array1130, pixel receiving means1110and drop emitting device1250. However, in the third embodiment the printing system1400comprises a modified tone mapping array1410, a modified comparator1440, a modified controller1420and a modified control array1430. The modified tone mapping array1410is substantially similar to the two dimensional tone mapping array of the second embodiment, however, the array1420is further adapted to use a plurality of indices IR to generate a plurality of mapped tone valuesCm. The modified comparator1440is adapted to receive a plurality of mapped tone valuesCmand to generate a plurality of comparator outputsCO. The modified controller1420is adapted to generate the plurality of indices IR and to generate an ink quantity index IQI based on an analysis of the comparator outputCO. The ink quantity index IQI is used to lookup a value within the control array1430, in a similar manner to index i in the second embodiment.

The method of the third embodiment is shown inFIG. 4. As before, the method steps illustrated inFIG. 4replace step120inFIG. 1. At step410a tone value Cris extracted from received pixel data as in the previous two embodiments. A threshold value T is then generated at step420using threshold array1130and the X and Y pixel co-ordinate values. At step430, the controller1420generates a range of indices IR, typically in the form of a one-dimensional array or vector, e.g. [0,1,2] or [0,1,2,3,4]. The printing system1400is then adapted to lookup a plurality of mapped tone valuesCmusing the range of indices IR and the received tone value Cr1115to index the tone mapping array1410. This can be performed using matrix software libraries or adapted hardware known in the art. The plurality of map tone valuesCmare then provided in the form of a vector or a one-dimensional array to the modified comparator1440, e.g. [560, 230, 110]. The modified comparator1440is adapted to compare each of the plurality of mapped tone valuesCmwith the threshold value T. This can also be performed using known matrix algebra. The result of this calculation in step450is a comparator outputCOcomprising a one-dimensional array or vector, e.g. [0,1,1] or [0,0,1,1,1]. Each entry in the comparator outputCOtypically corresponds to a comparison of a respective one of a plurality of mapped tone valuesCmwith threshold T. The comparator outputCOis then passed to the modified controller1420. At step460, the modified controller1420is adapted to process the comparator outputCOto produce an ink quantity index IQI. This can be performed using a truth table implemented in software and/or hardware. The ink quantity index IQI is a single value that is used to lookup an element of the controller array, together with the received tone value Cr, at step470. The element of the array corresponds to a control signal value CS coding for the number of drops of ink that need to be emitted by the drop emitting device1250. The control signal value CS is sent to the drop emitting device at step130and an output is printed on the substrate at step140. The method is then repeated for additional pixels as required.

A series of method steps to be used in a fifth embodiment of the present invention are illustrated inFIG. 22. These steps represent an alternate method of performing steps340to370inFIG. 3or steps450to470inFIG. 4. The method begins by calculating a plurality of mapped tone values Cr1to CrN, where in this case N=6. These can be calculated using a step similar to step440inFIG. 4or by an alternate method. A threshold value T, generated as in steps320or420, is then compared with each of the mapped tone values in series in steps2250A to2250N. If the comparison returns an indication of a first relationship, e.g. 1 representing that T is greater than or equal to the mapped tone value, then a value of index i is set in one of steps2270A to2270L. If the comparison returns an indication of a second relationship, e.g. 0 representing that T is less than the mapped tone value, then another comparison is performed using a new mapped tone value, e.g. from step2250A the process moves to step2250B. If the last comparison in the chain returns an indication of a second relationship then index i is set to a maximum value, in this case 7, at step2270M. After a value of i has been set at any of steps2270A to2270M the method then proceeds to step2260, wherein a control signal value CS is calculated in a similar manner to steps360and470. This control signal value CS is then sent to the drop emitting device1150.

When implementing the present invention, the screening methods and the use of variable drop sizes may result in a non-ideal printed tone scale. This non-ideal printed tone scale is typically highly non-linear and has a high dot gain when compared to a reference case. Therefore, to achieve a consistent and pleasing tone scale, it may be necessary to calibrate the mapping process and/or the values in two-dimensional arrays1310and1330. Such a calibration is optional and does not form an essential part of the present invention.

The calibration is performed using the system ofFIG. 15. In order to perform the calibration a test chart must first be printed using one of the systems shown inFIGS. 11 to 14. The form of the test chart will depend on the specific configuration of the printing system. An example of a test pattern for a printing system with one iteration (imax=0) is illustrated inFIG. 16and an example of a test pattern for a printing system with five possible iterations (imax=5) is illustrated inFIG. 17. Each test pattern comprises a number of tonal blocks1710covering the full range of possible tone values (e.g. 0 to 1024), wherein each tonal block corresponds to a set range of received tone values Cr(e.g. 128 to 255) and the blocks become darker towards the bottom of each test print. Each tonal block1710will be printed using a plurality of drop emitting devices, each drop emitting device operating according to the present invention. The tonal blocks1710are also repeated in a horizontal direction across the substrate to give a good range of tonal test blocks to be measured, for example if one or more malfunctioning drop emitting devices were present in a particular tonal block with a row of blocks then the measurements for this particular block would be discounted. Typically, only one of the columns of the test print will be measured and the column with the best jetting and no malfunctioning drop emitting devices will be selected. The number of ink quantity levels used to produce each tonal block1710is displayed to the left of the test print. For a colour printing system a test print such as those shown inFIGS. 16 and 17will be printed for each individual colour component, e.g. cyan, magenta, yellow and black.

The system ofFIG. 15is used to measure the test print and update the mapping processes or the mapping arrays1310and1330accordingly. The present example illustrates a case where the calibration process updates tone mapping array1310and the control mapping array1330, however, the present invention is not limited to such an update. The test prints are first of all measured using a tone sensor1510such as a spectrophotometer. A particular column of tonal blocks will be chosen as described above. The tone sensor1510will take a measurement of the plurality of tonal test blocks1710, for example measurements in C.I.E. X, Y, Z colour space. These measurements will then be used to compute the effective drop area based on the Murray-Davies equation shown below:

EDA⁢⁢%=100*(1-msmwhite)(1-msolidInkmwhite)wherein: mwhiteis a measurement of the tone or colour of the substrate; msolidinkis a reference measurement of 100% tint (i.e. a patch of solid ink) and msis a measurement of the sample in question.

In the equation above the m (measurement) variable above will depend on the colour component: for cyan ink m equals the X measurement; for magenta ink m equals the Y measurement; for yellow ink m equals the Z measurement and for black ink m equals the Y measurement.

The above calculation is performed by the calibration system1520based on the measurementsItestsent from the tone sensor1510and is used to generate a measured dot gain characteristic curve as shown inFIG. 18. The calibration system1520is adapted to compare the measured dot gain characteristic curve1820(fromFIG. 16) or1830(fromFIG. 17), respectively representing one drop per band (“1 DPB Cyn”) and five drops per band (“5 DPB Cyn”), with a reference dot gain characteristic curve1810(labelled as “TR004 Cyn”) stored within, or looked up by, the calibration system1520.FIG. 18shows the respective curves using the colour cyan (“Cyn”) as an example. The calibration system1520will effectively map the measured dot gain characteristic curve1820or1830onto the reference dot gain characteristic curve1810using a piecewise linear transform. This process is illustrated by arrows1840inFIG. 18. This mapping of the measured curve onto a reference curve is then used to generate a set of mapping configurations or a set of updated array elements. In the latter case, as shown inFIG. 15, the updated array elements are sent from the calibration system1520to tone mapping array1310and control array1330and the arrays are updated accordingly. A plurality of altered functions that may be used to update the tone mapping array1310are shown inFIG. 19.FIG. 19shows each function as a different line type, wherein each function or line type is also labelled with a different “colour” for convenience. The colours and line types used to label the functions inFIG. 19are arbitrary and are unrelated to the function of the invention. By comparingFIG. 10withFIG. 19, it can be seen that the calibration system has modified the tone mapping array1310in order to produce a non-linear mapping.

In certain embodiments, it may be desirable to limit the maximum amount of ink printed upon a substrate. For example, certain substrates may only be able to hold a certain quantity of ink. In these cases the controller1220to1410may be adapted to limit the control signal value CS or alternatively the functions used to generate the tone mapping array1310or1410may be limited as shown inFIG. 20.FIG. 20shows each function as a different line type, wherein each function or line type is also labelled with a different “colour” for convenience. The colours and line types used to label the functions inFIG. 20are arbitrary and are unrelated to the function of the invention.

In the example mapping arrays ofFIGS. 5 and 6the tone range was divided into a number of equal size segments (e.g. 0-127). In each segment a number of different ink quantity levels were used to generate a given tone value. This resulted in a fixed dot percentage range being covered by each segment, e.g. for a system with 3 different drop levels each drop level in each segment would cover 33% of the area to be printed. However, in some cases it may be required to change the dot percentage for each band, i.e. change the proportion of a certain drop size used for a given tone value within a tone segment. In a preferred case a gradual change in dot percentage is required for each band. For example, if 3 drops are used to print a given range or segment of tone values, then the tone mapping can be calibrated so that the first ink quantity level (e.g. all 3 drops) only covers 27% of the area to be printed, i.e. the tone mapping is applied so that 27% of the pixels printed at a given tone level are of the first ink quantity level. The mapping in further segments can then be calibrated to effect a gradual change in dot percentage across the segments, e.g. so that the first ink quantity level covers 28% in a second segment and 29% in a third segment.

FIG. 21illustrates the case wherein the values within the tone mapping array are calibrated so that the dot percentage change per segment is increased from a low value of 8% to a high value of 20% in a gradual manner. This could alternatively be performed by calibrating a tone mapping process. This results in tone segments at the light end of the tone scale (e.g. towards a tone value of 1023) having a smaller change in dot percentage and the segment width is gradually increased from segment to segment to accommodate the desired increase in dot percentage range for a given segment.FIG. 21shows each function as a different line type, wherein each function or line type is also labelled with a different “colour” for convenience. The colours and line types used to label the functions inFIG. 21are arbitrary and are unrelated to the function of the invention.

The first to fourth embodiments can be implemented using software and/or hardware systems. As typically each of the printing systems1200to1400is adapted to receive a tone value Crfor a given colour component a full colour printing apparatus will typically use a plurality of such systems in parallel to generate multilevel screened values for the plurality of colour components, e.g. in a CMYK system produce four sets of control signal values CS. Each control signal value CS can then be used by a corresponding drop emitting device adapted to emit an ink of a separate colour. Alternatively, each colour component could be sent in series to a single printing system such as1200to1400. Commonly, different colour components will require different mapping operations and/or threshold arrays.