EP1998550A1 - Morphological security feature - Google Patents

Abstract

The feature has a carrier substrate on which a continuous-tone image is printed. The continuous-tone image is transferred into a high-resolution bar element by a morphologic transition, which takes place in predetermined portions e.g. depth or interior, in the image. The bar element is mathematically defined by a guilloche function or a polygonal function. A carrier foil e.g. flexible plastic foils, is utilized as the carrier substrate and has a thickness of 5-700 micrometers preferably 12-50 micrometers.

Description

The invention relates to a morphological security feature that can be used as a security element in value documents and packaging.

Morphological image processing is a method of computer-aided image manipulation.

It uses proximity finding methods to consider the environment of a pixel and, as a result, perform operations. For example, Attempts to put a (relatively small image) bitmask (the struct element) on an image such that all the underlying pixels of the source image have the same value as those defined in the mask. You delete all points that are not the center of the somewhere suitably placed mask ("erosion").

By suitable choice of the structural element, certain image areas can be emphasized over others, e.g. horizontal or vertical structures.

Another morphological method in graphic image processing is the dilation, which requires that at least one pixel of the structure element touch a pixel of the original image. In this case, in the original image, the pixels around the center are expanded by the pixels defined in the mask.

Digital image editing is the computer-aided processing of digital images, usually raster graphics, usually photos or scanned documents. These pictures become retrospective changed in order to optimize, alienate, modify, or manipulate, with the aim of achieving a desired result.

While greyscale images are usually represented as a representation of coordinate pairs in terms of color values, binary images are a representation of the set pixels. Operations can then be described as a set operation between the raster dot sets of the binary image and a feature. From the basic operations of morphological imaging, erosion and dilatation, the operations of opening, closing, and finally morphological smoothing can be defined.

• Mittelwertfilter (box filter): Ein Mittelwertfilter der Größe n X m wird durch eine n X m -Impulsantwortmatrix g_nm = (nm)^-1 beschrieben. Die Bildpunkte des transformierten Bildes sind somit die Mittelwerte ihrer nxm Nachbarn. Box-Filter sind nicht isotrop und nicht abklingend und stellen entgegen einer naiven Einschätzung keinen eigentlichen Tiefpassfilter dar.
• Gaußfilter: Gaußfilter der Größe n x m und einer Varianz σ² werden durch eine Impulsantwortmatrix $g_{\mathit{nm}}=\frac{1}{\sigma \sqrt{2\pi }}{e}^{-\frac{n^2+m^2}{2{\sigma }^2}}$
  
  beschrieben. Die Nachbarpunkte des Ausgangsbildes gehen damit nicht wie beim box-filter gleichwertig ein, sondern werden entsprechend einer zweidimensionalen Gaußglocke gewichtet. Gaußfilter sind damit isotrop und abklingend. Sie werden als reales optisches Bauteil als Gauß-Filterung der Fourier-Transformierten in der Apodisation eingesetzt. (Beispiel).
• Nichtlineare Glättungsfilter, der Grauwert des aktuellen Pixels wird dabei ersetzt durch
  • o Medianfilter
    • den Median der Grauwerte der aktuellen Umgebung (Beispiel)
  • o k-zentriertes Mittel
    • den Durchschnitt der k-mittleren Grauwerte
  • o k-NN-Filter
    • der Median der k-nächsten Grauwerte der Umgebung
  • o δ-Nachbarschaftsfilter
    • den Durchschnitt aller Grauwerte deren Abstand kleiner als d zum aktuellen Grauwert ist.
  • o Lee's Filter $$h_{\mathit{nm}}={\mu }_{\mathit{nm}}+\left(f_{\mathit{nm}}-{\mu }_{\mathit{nm}}\right)\cdot \frac{S_{\mathit{nm}}^2-{\sigma }^2}{S_{\mathit{nm}}^2}$$
    
    • µ_nm ist Grauwertmittel, $S_{\mathit{nm}}^2$
      
      Grauwertvarianz und σ² Rauschenergie der aktuellen Umgebung
  • o Minimum-Varianz-Filter
    • es werden die Grauwertmittlel µ_i und Varianzen $S_i^2$
      
      einiger Teilfenster berechnet
    • $${\mathrm{h}}_{\mathrm{nm}}={\mathrm{\mu }}_{\mathrm{j}}\mathrm{wird\; gewählt\; mit}{S}_i^2={\mathit{min}}_i\left\{S_i^2\right\}$$
      

Smoothing can reduce the image noise, but coarser structures are retained. Here is an example of an (artificially) noisy picture. With regard to the frequency spectrum of an image, smoothing equals a low-pass filter. Typical FIR smoothing filters are

• Mean filter (box filter): A mean value filter of size n X m is described by an n X m impulse response matrix g _nm = (nm) ^-1 . The pixels of the transformed image are thus the average values of their n × m neighbors. Box filters are not isotropic and not decaying and, contrary to a naive estimation, are not an actual low-pass filter.
• Gaussian filter: Gaussian filters of size n x m and a variance σ ² are given by an impulse response matrix $G_{\mathit{nm}}=\frac{1}{\sigma \sqrt{2\pi }}{e}^{-\frac{n^2+m^2}{2{\sigma }^2}}$
  
  described. The neighboring points of the output image are thus not equivalent to the box filter, but are weighted according to a two-dimensional Gaussian bell. Gaussian filters are therefore isotropic and decaying. They are used as a real optical component as Gaussian filtering of the Fourier transforms in the apodization. (Example).
• Nonlinear smoothing filters, the gray value of the current pixel is replaced by
  • o median filter
    • the median of the gray values of the current environment (example)
  • o k-centered agent
    • the average of the k-mean gray values
  • o k-NN filter
    • the median of the k nearest grayscale values of the environment
  • o δ neighborhood filter
    • the average of all gray values whose distance is smaller than d to the current gray value.
  • o Lee's filter $$H_{\mathit{nm}}={\mu }_{\mathit{nm}}+\left(f_{\mathit{nm}}-{\mu }_{\mathit{nm}}\right)\cdot \frac{S_{\mathit{nm}}^2-{\sigma }^2}{S_{\mathit{nm}}^2}$$
    
    • μ _nm is gray scale average, $S_{\mathit{nm}}^2$
      
      Gray value variance and σ ² noise energy of the current environment
  • o Minimum variance filter
    • it becomes the gray value average μ _i and variances $S_i^2$
      
      calculated in some subwindows
    • $${\mathrm{H}}_{\mathrm{nm}}={\mathrm{\mu }}_{\mathrm{j}}\mathrm{is\; chosen\; with}{S}_i^2={\mathit{min}}_i\left\{S_i^2\right\}$$
      

If the deviation from the neighboring points is signaled instead of the absolute color values, then the object contours, i. hard color transitions, highlighting an image while softening smooth transitions. This process is called edge enhancement.

• Ableitungsfilter
• Laplacefilter

Often the following FIR filters are used:

• derivative filters
• Laplace

• Varianzfilter
• Extremalspannenfilter
• Roberts-Kreuz
• Kirsch-Filter
• Gradientenfilter
  • o Prewitt-Filter
  • o Sobel-Filter
  • o Canny-Filter

Nonlinear edge detectors:

• variance filter
• Extremalspannenfilter
• Roberts Cross
• Cherry Filter
• gradient
  • o Prewitt filter
  • o Sobel filter
  • o Canny filter

If images are represented as a sequence of color values, one speaks of a representation in time, momentum or place space. From this, images can be injectively converted into the so-called frequency space in which not the color values themselves but the frequency and phase components of the underlying point sequence are stored. Large jumps within the color value sequence then stand for high frequencies, soft color transitions for low frequencies. The Fourier transform finally indicates how the original color value course can be reconstructed by the sole superimposition of sine functions of different frequency and phase angle.

The advantage of this representation lies in its higher efficiency in the application of linear filters. In the pulse space, the convolution of the color value sequence f _i describing the image with the pulse response g _μ (the transformation image) of the filter means a summation (the weighting of the pulses with the impulse response) over the entire filter length. In the frequency domain, on the other hand, the convolution is calculated as the product of the Fourier transform with the frequency response of the transformation. This advantage with respect to the computation time is so great that even the cost of the back and forth transformation into the frequency domain can be accepted. This effort can be significantly reduced by means of the fast Fourier transform (FFT).

Another advantage is the simplification of being able to design filters directly in the frequency domain instead of having to construct impulse responses and calculate their effect on the spectrum. As a result, the transformation of the impulse response g _k into the corresponding frequency response G (z) is unnecessary.

The object of the invention was to provide a security feature that is forgery-proof, technically feasible, with standard colors can be used in addition to special security colors.

The invention is therefore a security feature having a carrier substrate on which a halftone image is printed, characterized in that the halftone image is converted by a morphological transition into a highly finely resolved bar element.

Another object of the invention are film materials having a security feature according to the invention.

In this case, the morphological image processing between two pixel images is not performed as in the prior art, but a morphological processing between pixel image and vector image.

For example, carrier foils are preferably flexible plastic foils, for example, PI, PP, MOPP, PE, PPS, PEEK, PEK, PEI, PSU, PAEK, LCP, PEN, PBT, PET, PA, PC, COC, POM, ABS, PVC in question. The carrier films preferably have a thickness of 5 to 700 .mu.m, preferably 8 to 200 .mu.m, more preferably 12 to 50 .mu.m.

Furthermore, metal foils, for example Al, Cu, Sn, Ni, Fe or stainless steel foils having a thickness of 5 to 200 μm, preferably 5 to 80 μm, particularly preferably 5 to 50 μm, may serve as the carrier substrate. The films can also be surface-treated, coated or laminated, for example with plastics or varnished.

Further, as carrier substrates also paper or composites with paper, for example, composites with plastics with a basis weight of 20 - 500 g / m ^2, preferably 40 - ² are used 200 g / m.

Furthermore, woven or nonwovens, such as continuous fiber webs, staple fiber webs and the like, which may optionally be needled or calendered, may be used as the carrier substrates. Preferably such fabrics or webs of plastics, such as PP, PET, PA, PPS and the like, but it can also be woven or nonwovens of natural, optionally treated fibers, such as viscose fiber webs are used. The fabrics or nonwovens used have a basis weight of about 20 g / m ² to 500 g / m ² . Optionally, these fabrics or nonwovens may be surface treated.

A printed halftone image is transformed by a morphological transition into a highly finely-primed stroke element, which may be mathematically defined.

One thus obtains a transition between a rasterized image (printable pixel image) and a vector graphic (line work) and vice versa.

The use of a print raster is a process of pre-press within the printing technology. Printing presses usually have a certain, very limited selection of colors, which they can only print purely. Thus, most printing methods can not render halftones (except, for example, deep-depth or depth-of-field variable gravure and NIP techniques). The problem is that either one color point can be printed in one place or not. It can not be controlled whether little or much color is printed on a pixel. As a solution, text, images and graphics are rasterized to represent different shades of gray or hues, provided they are not made entirely of pure printing inks. In order to be able to represent mixtures between the printing inks or different brightnesses of these, are doing very fine Pressure points of the colors side by side / on top of each other to achieve the desired resulting color impression.

When rasterizing, image data is converted to specified print data. In this case, binary information (print / do not print) is calculated from halftone templates. The impression of hue values and gray levels is achieved by a suitable arrangement of solid dots (halftone dots).

Roughly 30 years ago, raster effects were achieved by means of glass engraving screens or special raster film films. Today, special image setters (laser printers) are used which work with specially developed software, the "RIP" (Raster Image Processor). In the RIP process, generated data (PS or PDF files etc.) are converted into printable pixel (images) or vector data (line work).

• Amplitudenmodulierte Raster, die für alle Tonwerte die gleiche Anzahl von Punkten verwenden und dunklere Farben durch größere Punkte erzeugen.
• Frequenzmodulierte Raster, die immer gleich große Rasterpunkte verwenden, aber ihre Anzahl je nach Tonwert ändern, dunklere Farben also durch mehr Punkte erzeugen.

The screen frequency (also called screen ruling) is a measure of the resolution of printing screens. The raster frequency indicates how many pressure points per unit of length occur. One differentiates thereby:

• Amplitude modulated rasters that use the same number of dots for all tonal values and produce darker colors through larger dots.
• Frequency-modulated rasters, which use equally large halftone dots, but change their number depending on the tonal value, ie produce darker colors through more dots.

Usual screen frequencies (also called raster width) are in newspaper printing 30-54 points per cm, in offset printing 54-80 points per cm, in high-quality printed products up to 120 points per cm.

Depending on the technical conditions, it is possible or necessary to apply different types of screening.

On a surface of, for example, 16x16 exposure points, a halftone pixel is composed. The more exposure pixels are on this 16-square, the darker the spot. However, if there are only a few imagesetters in the square, the spot appears bright from a distance.

The grid width is the distance between the individual grid cells. From this, the screen frequency is calculated as the Kehnivert of the screen ruling. The number of grid cells per unit of track is given in lines per centimeter or lines per inch (lpi).

Common values in offset printing are 54 to 80 lines / cm (about 135 to 200 ipi) and in newspaper printing 30 to 54 lines / cm.

For an imagesetter with 2540 dpi or 1000 ppcm: 1000/16 = 62.5 => 60s raster.

During exposure, the individual halftone dots are composed of several laser spots.

Since there are technical limits to the size of the halftone dots which can be reproduced in print, it is above all the lowest tonal values in the standard screen widths that can not be reproduced. Another problem is the unwanted joining of adjacent halftone dots (point closure), which can lead to a sudden increase in tone density, where actually a uniform increase is desired.

In the case of four-color printing, in which a plurality of screen angles must be printed on one another to display a color image, this effect is attempted by controlling the angle of the color separations. Common screen angles in four-color offset printing are according to DIN 16 547, yellow = 0 °, cyan = 75 °, black = 135 °, magenta = 15 ° or yellow = 0 °, cyan = 15 °, black = 45 °, magenta = 75 °.

In principle, amplitude-modulated (autotypical raster) and frequency-modulated (stochastic raster) methods can be distinguished.

In the case of the amplitude-modulated screening, the size is varied to produce different tone values, and the number of screen dots per screen cell is varied in the case of frequency-modulated screening (first-order non-periodic method).

The frequency-modulated screening has been implemented in practice only with the introduction of the computer-to-plate technique, ie the direct imaging of the digital data on the offset printing plate. Previously, due to the fineness of the halftone dots (between 10 and 30 microns), transfer from the film to the printing plate was not reliably possible. This raster technique enables a greater fineness of the resolution, prevents moire and enables a color space in offset printing that is extended by about seven percent. In addition, the amount of ink can be reduced with the same color effect, which brings advantages in the drying process and color consumption.

In addition, a combination of the two methods (2nd order non-periodic method) and a change of the halftone dot shape (3rd order non-periodic method) is possible.

Screens can be created using photographic (analog) or electronic (digital) methods.

Analogous procedures are distance rastering as well as the rastering with the help of a contact raster in the repro camera. Both methods, however, are rarely encountered today.

The electronic grid generation is done with a so-called raster image processor (RIP). The data transfer to the RIP is usually done as Postscript file.

A Raster Image Processor is a special piece of hardware or software that converts specific data from a higher page description language, such as PostScript, PDF (including VPS, AFP), or PCL, into a raster image, typically to output to a printer ,

1. 1. Umrechnung von Vektorgrafiken in Rastergrafiken bestimmter Auflösungen
2. 2. Neuberechnung von Rastergrafiken zur Umwandlung in andere Auflösungen bzw. Größen

A raster image processor essentially implements two functions:

1. 1. Conversion of vector graphics into raster graphics of certain resolutions
2. 2. Recalculation of raster graphics for conversion to other resolutions or sizes

• Farbmanagement
• OPI-Bildersetzung
• Separation
• Trapping

The term Raster Image Processor is explicitly used in prepress or reproduction technology only and means a special component in the printing process, which, in addition to the functions mentioned above, performs further tasks:

• color management
• OPI Image Replacement
• separation
• trapping

The most widely used print grid is the postscript grid. It is used by almost all printing presses as well as laser printers. In this color gradations of individual colors are created by the color is decomposed into a grid of mutually perpendicular lines and lines of individual points. The desired brightness of the color is thereby determined by the diameter of the individual, circular pressure points: the brighter the single color is to be printed, the smaller the diameter of the respective pressure point. This is based on the photographic grid. In the usual four-color printing, the rasters of the individual colors are printed one above the other at an angle offset of 30 °.

Halftone pictures or halftone pictures show colors running into each other in every possible tonal gradation, ie from the deepest black to the brightest gray, from the brightest color tone to the darkest. Photographs are usually halftone pictures.

In the case of a halftone image, in contrast to the line image or the solid tone image, there are brightness gradations of the motif in a tone scale from white to black (grayscale image). The (logarithmic) measure of the brightness difference of the brightest and the darkest (blackest) tone value (lights to shadows) is called gradation, measured in gamma. If the gradation is low, the picture is called soft, in the extreme case it is flabby, if it is high, the picture looks hard (ie very rich in contrast).

Reducing the tonal values to a few results in a poster effect (posterized image). If only two tone or color values remain, it is a line image.

If halftone images are reproduced in print, the original must be broken down (rastered) into halftone dots during image reproduction by means of a grid. H. a rasterized image is basically a line image that simulates a halftone image.

A vector graphic is a computer image made up of graphic primitives such as lines, circles, and polygons.

For example, to save the image of a circle, a vector graphic needs at least two values: the location of the center of the circle and the circle diameter. In addition to the intrinsic parameters (shape and position) of the primitives, the color, line width, various fill patterns and other appearance-determining data may also be indicated.

In contrast to raster graphics, vector graphics can be infinitely scaled and distorted without loss of quality, for example by means of homogeneous coordinates. In addition, the properties of individual lines, curves or surfaces are retained in vector graphics and can also be subsequently changed.

For a long time, pure vector graphics were considered unsuitable for the representation of photorealistic graphics, since at first they could hardly be mathematically modeled. However, popular vector graphics programs now offer features that allow you to save vector drawings along with anti-aliasing effects, fine color gradients, and transparencies to create photorealistic results. The generated graphics still remain scalable and changeable.

The creation of vector graphics is usually done by means of a vector graphics program or directly with a markup language. Raster graphics can be converted into vector graphics with certain restrictions (vectorization).

Vector-based drawing programs can be used to create illustrations. For technical drawings, CAD programs are used. 3D modeling tools generate 3D vector graphics for 3D computer graphics scenes.

Vector graphics allow documents to be written independently of the resolution of the output device. With the help of a vector graphics capable Page description language such as PostScript or the resulting Portable Document Format (PDF) documents can be displayed or printed at the highest possible resolution on screens.

Today's popular computer systems mostly use so-called outline fonts, which describe the outlines of each character as vector graphics. Important formats are TrueType, PostScript and OpenType.

A halftone image (scan, digital photo, drawing, 3-D object, etc.) is thus converted into a vector graphic or vice versa. From a halftone image (program Adobe Photoshop), the required footprint is created in the form of a contour in which a morphological feature is incorporated as a vector graphic (TIFF, EPS format). This creates TIFF or EPS format in the program Adobe Photoshop, is transferred in the program Barco Fortuna as a vector graphic.

The created basic shape is mathematically processed with guilloches variably in line width, number of lines, shape and color and returned to the halftone image (as EPS or PDF format).

When editing in the halftone image (program Adobe Photoshop), the vector graphic is adjusted as a whole image.

The transition can be represented linear, radial, frequency-modulated, depth-modulated, etc.

The processing of the halftone image is carried out with, for example, the program Adobe Photoshop. The processing is usually done in TIFF format but can also be done in native Photoshop format. However, the export of the halftone file always refers to open filet structures, for example: TIFF, JPEG, RAW etc.

The editing of the vector graphics is done with eg: Barco Fortuna, Adobe Illustrator or Artwork Artpro. However, other vector-based graphics programs can also be used.

The halftone image can be displayed with different raster shapes (dots, squares, crosses, etc.). The raster width is in particular preferably 1 lpi to 10,000 lpi, particularly preferably 10 lpi to 1000 lpi, very particularly preferably 100 lpi to 500 lpi. The screen angle is preferably 0 ° -90 ° (CMYK).

The vector graphics are adapted to the size, shape, modulation etc. of the halftone image.

This vector graphics adaption, which may also be described by a mathematical formula such as a guilloche function or a polygon function, is determined by parameters such as e.g. Amplitude, frequency, modulation or number of vertices performed.

If the vector graphic is described by a line grid, the adjustment to the halftone image can be made via, for example, the screen ruling, screen angle, screen frequency, or line weight.

Conversely, the halftone image can also be adapted to the vector graphics. To do this, for example: the halftone image can be decomposed by tone levels, for example: for lights,% tones, ½ tones, ¾ tones and depths. An adaptation can then take place, for example, via different screen shapes, screen angles, screen frequencies.

The morphological transition can also occur several times or only in certain subregions such as, for example: only in the depths or only in the lights of a halftone image.

Suitable printing methods are known printing methods, such as gravure, flexographic, screen, offset or digital printing. In this case, conventional printing inks, but also special colors, such as IR or UV inks can be used.

The security features according to the invention are suitable for use in data carriers, in particular value documents such as identity cards, cards, banknotes or labels, seals and the like, but also as Packaging material, for example, in the pharmaceutical, cosmetic, electronics and / or food industry, for example in the form of blister films, cartons, covers, film packaging and the like suitable.

For use as security features, the substrates or the film materials are preferably cut into strips or threads or patches, wherein the width of the strips or threads may preferably be 0.05-10 mm and the patches preferably have average widths or lengths of 2-30 mm have.

For use in or on packaging, the film material is preferably cut into strips, tapes, threads or patches, wherein the width of the threads, strips or bands is 0.05-600 mm, preferably 50-150 mm, and the patches are preferably medium widths and Have lengths of 2-100 mm.

However, it is also possible to print the described security features directly on primary packaging, such as cardboard, foils, aluminum, blisters and the like, or directly on value documents.

Claims (3)

A security feature comprising a carrier substrate on which a halftone image is printed, characterized in that the halftone image is converted by a morphological transition in a highly finely resolved bar element. Security feature according to claim 1, characterized in that the bar element is defined mathematically. Security element according to claim 2, characterized in that the bar element is defined by a guilloche function or a polygon function.