Patent Description:
In the technical field of printing plates, a digital image is imaged on a plate precursor such as a lithographic printing plate precursor.

The use of plate precursors and the imaging of digital images on plate precursors to form printing plates are well-known as part of conventional printing technologies. "<NPL>) especially under chapter <NUM> "Printing technologies" discloses several types of said conventional printing technologies and Chapter <NUM> "Printing Technologies with Permanent Printing Master". Imaging is sometimes also called imagewise exposure.

An example (by infrared radiation) how a lithographic printing plate precursor is made and how a digital image is imaged on said plate precursor is disclosed in <CIT>.

Another type of plate precursor is a flexographic printing plate precursor. Examples how such precursors are made or how a digital image is imaged on said plate precursor by laser engraving is disclosed in <CIT>.

In flexography, imaged precursors are mainly a photopolymer plate with a raised or recessed pattern that corresponds to the desired halftone. When ink is applied to the plate and transferred to the substrate, the varying depths of the plate produce dots of different sizes, which in turn create the illusion of continuous tone.

A digital image that is imaged on a plate precursor contains the information that need to be printed with ink on a print media. The digital image contains image pixels and non-image pixels to determine where ink must be formed on a print media when using the printing plate. For example, a pixel in the digital image may having the value of <NUM> if it is an image pixel and having the value of <NUM> if it is a non-image pixel.

To simulate a digital continuous-tone image such as photo on print media with a conventional printing technology, the digital continuous-tone image is firstly halftoned by a halftone technology whereby a halftone image is generated comprising dots, varying in size and/or spacing, are formed to simulate the digital continuous-tone image. The formed dots are then image pixels or non-image pixels. This allows the image to be printed using only a limited number of ink colors, as the eye blends the dots together to perceive the desired shade. Examples of halftoning technologies are disclosed in <CIT> wherein spiral-shaped halftone dots with image pixels are formed. Also, in the previous mentioned "Handbook of Print Media" under chapter <NUM>. <NUM> " Halftone Process / Screeningʺʺ of Helmut Kipphan other halftoning technologies are disclosed.

Halftoning technologies are typically implemented in raster image processors (RIP) or prepress workflows such as Prinect Workflow™ from manufacturer Heidelberg Durckmaschinen Aktiengesellschaft™ or Apogee™ from manufacturer ECO3 where files according page description languages are converted to digital continuous-tone images and halftone images, where said halftone images are transferred the halftone images to an imaging system for making a printing plate from a plate precursor, such as an image setter.

To ensure the simulation of the continuous-tone image the continuous-tone image is linearized, also called calibrated, before or during the halftoning. This is normally done by a dot gain (compensation) curve which defines which percentage to use on plate for each desired percentage on printing press. A linearization serves namely to control a conventional printing technology in a way that the printed colours behave linearly to the input colours of the continuous-tone image.

It is found that at the boundary of a printed contiguous area of image pixels of a digital image has a local density difference versus at the centre of said printed contiguous area of image pixels and/or has a printing anomaly such as ghost or halo printing. Said printing artefacts depends on used print media and/or print parameters of the conventional printing technology e.g., more pressure on the printing plate during ink transfer via intermediate press roll or directly to the print media, less use of fountain solution or use of more oily ink. An example of modifying the boundary of contiguous areas of image pixels is disclosed in <CIT>, where a fine texture pattern is applied to edge regions and a coarse texture pattern to interior regions of image features.

Therefor there is need for a solution to have less of said printing artefacts, less dependent of said print parameters and preferably without disturbing the linearization.

It is an object of the present invention to provide a method for making a printing plate which has excellent printing properties and overcomes the above-mentioned print-artefacts. The present invention ensures also that a performed linearization before or during halftoning of the to be printed continuous-tone image remains stable or the same.

The objective has been achieved by a method for making a printing plate defined in claim <NUM>.

Another embodiment of the present invention not explicitly claimed is a printing plate made according said method.

Other features, elements, steps, characteristics, and advantages of the present invention will become more apparent from the drawings and the following detailed description of preferred embodiments of the present invention. Specific embodiments of the invention are also defined in the dependent claims.

An embodiment of the present invention to solve the above-mentioned print-artefacts is a method for making a printing plate comprising the steps:.

Hereby is N a non-zero positive integer. Said steps are illustrated in <FIG>.

The present embodiment moves thus at a first location in the first border area (<NUM>, BA<NUM>) one or more image pixels of the first border area (<NUM>, BA<NUM>) to the second border area (<NUM>, BA<NUM>). At the first location the one or more image pixels become non-image pixels and vice versa one or more non-image pixels become image pixels in the second border area (<NUM>, BA<NUM>).

By the movement at the boundary (<NUM>, B) the contiguous area of image pixels (<NUM>, Aimgpix) is smoothed out towards the contiguous area of non-image pixels (<NUM>, Anonimgpix). Hereby the number of image pixels remain the same after the adaptation of the digital image (<NUM>, IMGadp). Other border areas within the contiguous area of image pixels (<NUM>, Aimgpix) and within the contiguous area of non-image pixels (<NUM>, Anonimgpix) may also be determined.

Further by said movement at the boundary the ink layer thickness is influenced as an advantage because ink- accumulation formed during the printing at the boundary (<NUM>, B) and also at the inside of the contiguous area (<NUM>, Aimgpix) gets the chance to fill the formed non-image pixels in the first border area (<NUM>, BA<NUM>) whereby the ink thickness becomes thinner and depending on the used conventional printing technology whereby less drying capacity is needed.

Preferably the plate precursor is for lithographic printing. More preferably the plate precursor is for flexographic printing wherein ghost printing and halo printing is a bigger challenge to solve it.

The locations of the selection of the one or more contiguous clusters (CLsel,<NUM>. N) are preferably distributed, preferably evenly distributed, in the first border area (<NUM>, BA<NUM>).

In a preferred embodiment the print direction is known when the printing plate is used in a certain conventional printing technology. The plate precursor is mainly rectangular-shaped and a side of said plate precursor is hereby parallel to said print direction when mounted in a system with said conventional printing technology. The selected one or more contiguous clusters (CLsel,<NUM>. N) are then preferably substantially located along a direction which is perpendicular to the print direction especially to avoid halo printing and ghost printing.

To get more uniform print density in a printed contiguous area of image pixels (<NUM>, Aimgpix) especially when lithographic or flexographic printing technology is used, the N contiguous clusters (CLsel,<NUM>. N) are preferably be selected according a predetermined pattern, preferably a regular pattern such as a tiled rectangles, ellipses or rhombuses and/or each of the N contiguous clusters has the same size and shape such as an elliptical shape, rectangular shape or rhombus shape. If more than the first and second border areas (<NUM>, BA<NUM>, <NUM>, BA<NUM>) are determined, another pattern or other shape/size may be used for the other border areas.

In a preferred embodiment the plate precursor is for lithographic printing or for flexographic printing.

A plate precursor is a well-known technical means in the field of flexography and lithography which are imaged with a digital image by chemical and/or mechanical reaction to form a printing plate which is then used in a system capable of lithographic or flexographic printing such as a printing press. The imaging is preferably done directly on the plate precursor with a plate setter, but it may be done indirectly by exposing light on the plate precursor through an intermediate film or a digital mask panel wherein said film or digital mask panel contains a representation of the digital image. Said intermediate film may be made by an image setter. Examples of plate setters are Avalon™ series of manufacturer ECO3 or FLEXCEL NX Wide <NUM> System of manufacturer MIRACLON. An example of an image setter is Avantra™ series of manufacturer ECO3. Examples of said digital mask panel is disclosed in <CIT>. Examples of imaging flexographic plate precursors are disclosed in <CIT>. An example of a process free plate precursor is Eclipse™ of manufacturer ECO3.

Directly on plate is also called Computer-To-Plate (CTP). Here for inkjet technology may be used which is also called inkjet CTP. Examples of inkjet CTP are disclosed in <CIT>.

The imaging may also be performed on the printing press itself, which is called Direct-On-Press (DOP).

A digital image is achieved by suitable commercially available hardware, such as scanning a photograph or taking an image by a digital camera, and/or commercially available software, such as Adobe Photoshop™ to manipulate and create digital images. The digital image may be a logo, text, a photo, a figure, or a combination of logo('s), text, figure(s) and/or photo(s).

The digital image is a two-dimensional digital image having a certain dimension (width and height) and having a plurality of pixels. A pixel is also called a picture element. Most digital images are organized in a square grid, but a rectangular grid or hexagonal grid are also possible. Each pixel of said plurality of pixels has a certain value that expresses an intensity. The resolution of a digital image (horizontal, vertical, hexagonal) defines the number of pixels in <NUM> or <NUM> inch. Preferably the (vertical and horizontal) resolution digital image are between <NUM> and <NUM> pixels per inch. For example, nowadays plate setters capable of outputting printing plates at <NUM> pixels per inch are used for security printing (super fine lines, super small text.

If there are only <NUM> values possible to express said intensity the digital image is also called a binary digital image.

The content of a digital image may be defined in raster graphics format such as Portable Network Graphics (PNG), Tagged Image File Format (TIFF), Adobe Photoshop Document (PSD) or Joint Photographic Experts Group (JPEG) or bitmap (BMP).

A digital image may be stored and/or loaded as one or more files on a memory of a computer.

The digital image may be adapted by moving pixels from a first location to a second location which is a common feature in digital image processing especially in digital image editing. For example, in Adobe Photoshop™ Version: <NUM>. <NUM> one or more pixels can be selected by an operator at a certain location with the "Rectangular Marquee Tool' and with the "Move Tool' moving by drag and drop the selected pixels to an other location. In fact, after the movement the one or more pixels at the certain location become non-image pixels (visible as white) and at the other location the values of pixels are overwritten by the values of the selected pixels.

Preferably the digital image is linearized to control a conventional printing technology in a way that the printed colours behave linearly to the input colours of the continuous-tone image especially when there is an effect of dot gain. Linearization can be achieved by tone mapping a well-known method in digital image processing whereby measurements of printed test-targets are used. One of the advantages of the present embodiment and preferred embodiments is that said linearization of the digital image (<NUM>, IMGsel) remains substantially the same after the adaption of the digital image (<NUM>, IMGadp), because the number of image pixels remains the same.

In a preferred embodiment the value of moved image-pixels are adapted preferably to a higher luminosity but remain an image-pixel. Said adaptation is hereby needed to drive the plate-setter or image-setter. At the location of the moved image-pixels with adapted value the chemical and/or mechanical reaction while forming the printing plate shall be different than the non-moved image-pixels e.g. a less chemical and/or mechanical reaction. A less chemical and/or mechanical reaction may for example result in less height after the imaging of a precursor for flexography.

The digital image is preferably a halftone image, also called a raster image. Said halftone image is suitable for rendering a continuous-tone image, i.e., it creates the illusion of a continuous-tone image on a printed copy.

The halftone image preferably comprises halftone dots more preferably halftone dots selected from the list AM dots (Amplitude Modulated dots) and/or XM dots (Cross Modulated dots) and/or semi-random located dots such as (clustered) FM dots (Frequency Modulated dots) and/or DM dots (Digital Modulated Dots).

The halftone dots may be arranged according a regular tile or semi-random localized.

A contiguous area of image pixels (<NUM>) is an area of connected image pixels to form said contiguous area. Non-image pixels are not part of such a contiguous area of image pixels.

A contiguous area of non-image pixels (<NUM>) is an area of connected non-image pixels to form said contiguous area. Image pixels are not part of such a contiguous area of non-image pixels.

In a preferred embodiment the contiguous area of non-image pixels (<NUM>, Anonimgpix) has more than one image pixel and more preferably minimum <NUM> non-image pixels.

The number of non-image pixels in the contiguous area of non-image pixels (<NUM>, Anonimgpix) is preferably larger than the number of image pixels in the contiguous area of image pixels (<NUM>, Aimgpix).

The total number of image pixels in the selected N contiguous clusters (CLsel,<NUM>. N) is smaller than the number of image pixels in the contiguous area of image pixels (<NUM>, Aimgpix).

If the selected digital image (<NUM>, IMGsel) is a halftone image comprising a plurality of halftone dots then the selected contiguous area of image pixels (<NUM>, Aimgpix) may be one of said plurality halftone dots. The shape of said halftone dot may be round, elliptical or rectangular or diamonds or rhombus or spiral-shaped or one-ring-shaped or two-ring-shaped. Said shape influences tonal transitions in printed digital images and visual artifact such as moiré patterns.

In a preferred embodiment has the contiguous area of image pixels (<NUM>, Aimgpix) more than one image pixel and more preferably minimum <NUM> image pixels. Especially when the contiguous area of image pixels is a halftone dot in the highlights it is found that it is better to preserve said halftone dot instead of moving image pixels towards the second border area (<NUM>, BA<NUM>).

In halftone imaging a boundary refers to the transition between two different tone levels in the halftone image.

There are in image processing several known image boundary detection methods such as edge detection (e.g. Canny Edge Detection); thresholding which involves setting a threshold value and then highlighting all pixels in the image that exceed that value. This can create a binary boundary that separates regions of the image: morphological operations, such as dilation and erosion: contour detection which involves identifying the boundaries of objects in an image by detecting curves that connect points of similar color or intensity. An example of a method how the boundary can be determined is disclosed in <NPL>.

If the digital image (<NUM>, IMGsel) is suitable for imaging on a plate precursor to form a printing plate, it is preferably a binary digital image. For determining a boundary said digital image may be converted to an <NUM>-bit gray-value image and then a convolution filter may be applied with a kernel size equal to predetermined border. The result is an image having pixels which are original <NUM> as value and after filter a value not equal to <NUM>. They defined a boundary of a contiguous area of image pixels.

In a preferred embodiment the convolution filter is using a square or circular convolution mask wherein more preferably the values in the kernel are all <NUM>.

In the context of image processing, the border area of a contiguous area of (non)-image pixels refers to the pixels or region around a boundary of said contiguous area. The shape of the boundary is (more or less) similar as the shape of the border area.

With convolution filtering as described under chapter c) boundary you may also determine border areas at the boundary of a contiguous area of (non)-image pixels e.g. by adapting the kernel size such as requested border size extent times <NUM> + <NUM>. By using <NUM> convolutions with different kernel size a border area with a certain selected thickness, sometimes called width or depth, can be determined in said contiguous area of (non)-image pixels.

An example how to define border area's BA<NUM> and BA<NUM> and close to boundary can be achieved is as follow:
The binary image is converted towards an <NUM> bit gray value image. Depending on the requested border area thickness, a convolution filter will be applied with a kernel size equal to the (requested border extent x <NUM> + <NUM>). Before the filter is applied all pixels will have either value <NUM> (black) or <NUM> (white). After the convolution with the filter, <NUM> types of pixels can be defined:.

When the extent of the close to the contiguous area of image pixels, where we want to move the one or more contiguous clusters (CLsel, <NUM>. N) is different from the requested border extent, <NUM> convolutions are required with a filter with a different kernel size. The convolution mask can either be square or circular. The values inside the kernel are all <NUM>.

The border area has preferably between its edges (more or less) the same distances (= same thickness / same width / same depth) and said edges follow the boundary.

In a preferred embodiment the thickness, also called the width or depth, of the border area (<NUM>, BA<NUM>) is equal or larger than the thickness, also called the width or depth, of the first border area (<NUM>, BA<NUM>) more preferably if the contiguous area of non-image pixels is larger than the contiguous area of image pixels.

In the embodiment a selected contiguous cluster (CLsel,i) contains one or more image-pixels, preferably maximum <NUM> image-pixels, more preferably maximum <NUM> image-pixels, most preferably maximum <NUM> image-pixels. A small selected contiguous cluster is better to avoid print quality issues and loss of detail in the selected digital image (<NUM>, IMGsel).

Each of said one or more selected contiguous cluster (CLsel,i) has a certain size, also called dimensions and a shape. Said size and shape can be defined by a selected pattern e.g. 2x2-pattern. Preferably is the size and shape the same for all the selected one or more contiguous clusters (CLsel,<NUM>. N is here a positive integer larger than zero.

The total number of image-pixels in the selected one or more contiguous clusters is preferably less than <NUM>% of the total number of image-pixels of the first border area (<NUM>, BA<NUM>).

Preferably the selected one or more contiguous clusters (CLsel, <NUM>. N) are spaced away from each other if more than one contiguous clusters is selected and more preferably evenly distributed in the first border area (<NUM>, BA<NUM>). The one or more contiguous clusters (CLsel, <NUM>. N) may be selected by using an image mask which is lay down on the first border area (<NUM>, BA<NUM>) to know the locations of said one or more contiguous clusters (CLsel, <NUM>. Said contiguous clusers (CLsel, <NUM>. N) may be selected according a pattern, such as checkerboard, within the first border area (<NUM>, BA<NUM>) or a pseudo-random way e.g. with a blue-noise mask within said first border area (<NUM>, BA<NUM>).

Preferably the selected one or more contiguous clusters (CLsel, <NUM>. N) are spaced apart from the boundary.

Preferably the moved contiguous clusters (CLmov, <NUM>. N) are spaced away from each other and more preferably evenly distributed in the second border area (<NUM>, BA<NUM>). The moved contiguous clusters (CLmov, <NUM>. N) are preferably pseudo-random spread in the second border area (<NUM>, BA<NUM>).

In a preferred embodiment (<FIG>) minimum one of the selected contiguous clusters (CLsel, i), preferably each selected contiguous cluster, having more than one image pixels, is.

In a preferred embodiment (<FIG>) a central area (<NUM>) between the first border area (<NUM>, BA<NUM>) and the centre of the contiguous area of image pixels (<NUM>, Aimgpix) is determined and a plurality of image pixels according a pre-determined regular pattern in said central area (<NUM>) are changed to non-image pixels. Said change-step makes the uniformity in a print of the large contiguous area of image pixels (<NUM>, Aimgpix) better. Said large contiguous area of image pixels is preferably larger than <NUM> image pixels, more preferably larger than <NUM> image pixels. Said regular pattern may be regularly spaced lines or dots or stars or spirals or any other geometric structures. The regularly spaced lines have preferably a maximum thickness of <NUM> pixels. Said regularly spaced lines may be angled and preferably has an angle of <NUM> or <NUM> degrees versus the printing direction. The regularly spaced dots have preferably a maximum dimension of <NUM> on <NUM> pixels. Also said regularly spaced dots may represent broken lines. Said broken lines have preferably an angle of <NUM> or <NUM> degrees versus the printing direction.

Step d) of the embodiment and its preferred embodiments may comprise an additional step especially for contiguous area of image pixels (<NUM>, Aimgpix) having a long boundary (<NUM>, B):.

In a preferred embodiment each of the contiguous clusters of the selected one or more contiguous clusters (CL<NUM>. N) is moved along the axis from centre of the contiguous area of image-pixels and the location of the selected contiguous cluster.

The moving distance along said axis is preferably maximum <NUM>%, more preferably maximum <NUM>% of the distance along said axis from the location of the selected contiguous cluster and the boundary.

In a preferred embodiment maximum <NUM>%, more preferably maximum <NUM>%, of the image pixels in the first border area (<NUM>, BA<NUM>) is moved to the second border area (<NUM>, BA<NUM>).

Claim 1:
Method for making a printing plate comprising the steps:
- selecting (<NUM>) a digital image (<NUM>, IMGsel) having a contiguous area of image pixels (<NUM>, Aimgpix) surrounded by a contiguous area of non-image pixels (<NUM>, Anonimgpix);
- determining (<NUM>) a boundary (<NUM>, B) of said contiguous area (<NUM>, Aimgpix); a first border area (<NUM>, BA<NUM>) according said boundary (<NUM>, B) and within said contiguous area of image pixels (<NUM>, Aimgpix); and a second border area (<NUM>, BA<NUM>) according said boundary (<NUM>, B) and within said contiguous area of non-image pixels (<NUM>, Anonimgpix);
- selecting one or more contiguous clusters (CLsel,<NUM>..N) from the first border area (<NUM>, BA<NUM>) and adapting (<NUM>) the selected digital image (<NUM>, IMGsel) by moving minimum one contiguous cluster (CLsel, i) of the one or more selected contiguous clusters (CLsel,<NUM>..N) to said second border area (<NUM>, BA<NUM>);
- imaging (<NUM>) the adapted digital image (<NUM>, IMGadp) on a plate precursor to form said printing plate.