EP0794056B1 - Method and printing machine for carrying out the method and support so printed - Google Patents

Description

The present invention relates to a printing method in color and more particularly but not exclusively a process and machine for dry offset printing.

It is known to implement, to print the outer surface of a cylindrical object of revolution such as a can used to hold a drink, an aerosol can or a tube flexible, a dry offset printing process comprising the steps consisting of depositing several inks of different colors on a blanket, then to transfer by contact of the blanket with the object the inks on the latter.

In practice, a plurality of blankets are used mounted on a rotary cylinder and loaded successively with the different inks.

The inks are deposited on each blanket using respective application rollers with raised portions formed on plates by photoengraving from a negative.

Various methods can be used to carry out these negative.

One known method is to decompose the original into color that we want to reproduce in an overlay of images monochromatic halftone, the density variations of each monochromatic image being restored by varying the size of the points of the frame, known as amplitude modulation.

For each monochromatic raster image, a corresponding negative.

The frames of the different monochromatic images are inclined to each other to reduce the moire phenomenon on printing.

Such a process is not entirely satisfactory, in particular because it does not prevent, when the image to reproduce presents close details and colors different, any superimposition of ink dots on the blankets and mutual ink pollution, resulting in progressive denaturing of the printed image and requires renew the inks, which may even require sending a discarded part of the production.

The subject of the present invention is a new process printing which in particular makes it possible to remedy the problem of ink pollution posed by offset printing processes dry of the prior art.

The invention also relates, more generally, to ability to print image rich in color nuances without having to create colors or shades of colors by mixing inks different colors when printing.

The invention also aims to eliminate the phenomena of moiré when many shades are printed.

• décomposer l'image à reproduire en une composition de surfaces élémentaires colorées, lesdites surfaces élémentaires étant disposées selon une répartition prédéterminée, la couleur de chaque surface élémentaire étant choisie parmi un ensemble prédéterminé de couleurs de base et un ensemble prédéterminé de nuances de ces couleurs de base, le nombre de nuances pour au moins une couleur de base étant supérieur ou égal à trois, le choix de la couleur de base et de la nuance attribuée à chacune desdites surfaces élémentaires étant effectué de manière à ce que ladite composition de surfaces élémentaires colorées reconstitue visuellement l'image à reproduire,
• imprimer ladite composition de surfaces élémentaires colorées sur ledit support au moyen d'encres dont les teintes sont choisies de manière à pouvoir reproduire, compte-tenu de la couleur dudit support, lesdites couleurs de base et leurs nuances lors de l'impression, la nuance d'une couleur de base attribuée à une surface élémentaire donnée étant obtenue lors de l'impression en jouant sur la proportion de chaque encre au sein de la surface élémentaire correspondante sur le support.

The invention achieves this by a method for printing a color image on a medium, characterized in that it comprises the steps consisting in:

• decomposing the image to be reproduced into a composition of colored elementary surfaces, said elementary surfaces being arranged according to a predetermined distribution, the color of each elementary surface being chosen from a predetermined set of basic colors and a predetermined set of shades of these colors of base, the number of shades for at least one base color being greater than or equal to three, the choice of the base color and of the shade assigned to each of said elementary surfaces being carried out so that said composition of colored elementary surfaces visually reconstruct the image to be reproduced,
• printing said composition of elementary colored surfaces on said support by means of inks the hues of which are chosen so as to be able to reproduce, taking into account the color of said support, said basic colors and their nuances during printing, the nuance a basic color assigned to a given elementary surface being obtained during printing by varying the proportion of each ink within the corresponding elementary surface on the support.

Thanks to the invention, it is possible to print a image without overlapping ink dots when printing, which has multiple advantages.

First, the problem of ink pollution, in the particular case of dry offset printing, is considerably limited or even eliminated.

Substantial savings can thus be made when printing of a large number of packaging devices.

Then it is possible thanks to the invention of visually reconstruct an image without involving the mixing printing inks, so without having to determine colors, by calculation or by carrying out tests, resulting from mixing the inks, which is not always easy taking into account in particular the complexity of the mechanism of color perception by humans.

The invention thus finds general application to all printing devices and goes beyond printing dry offset.

While the classic four-color printing uses black, magenta, cyan and yellow inks and restores non-primary colors by subtractive mixing of magenta, cyan and yellow, it is possible in the invention to use only non-primary color inks, for example inks whose colors are chosen according to the subject of the image to print.

So when the image represents for example a face, preferably choose at least one ink whose color is close to the subject's skin color, and the printed image will have a visual rendering very close to reality.

It is also advantageous to be able to print thanks to inventing texts with a single ink without having to generate the corresponding color by mixing printing inks.

It is also interesting to be able to choose the color of the inks used for printing because you can then have colors that it is not possible or that it is difficult to create by subtractive synthesis of primary colors.

It will be understood that the invention is advantageously used to print consumer goods, in the measurement of the visual perception of the image printed on objects exposed to the sale, by a potential buyer, may be decisive.

The invention can also be used to produce high quality reproductions of paintings or photographs.

In a particular implementation of the invention, the shades of at least one base color have the same saturation but different brightnesses.

In this case, the shades of a basic color that have same saturation but different brightnesses can be restored during printing thanks to a more or less less black ink within the elementary surfaces of the support which are concerned with obtaining said nuances during of printing.

In the case of dry offset, to avoid mixing inks, when the same elementary surface of the support is concerned both by a deposit of black ink and by a deposit ink other than black ink, depositing black ink and of said ink other than black ink is effected with patterns predetermined for each of the inks, chosen to avoid superposition of the two inks.

In the case of other printing processes for which the superposition of inks on printing does not constitute no problem, we can deposit the black ink with overlay on at least one other ink already deposited on the support.

In a particular implementation of the method according to the invention, each elementary surface is printed on the support an ink point whose shape and size depend on a predetermined relationship of the shade of the base color to reproduce.

When printing is done by bringing the media to contact of a surface charged with ink, with a direction of displacement substantially tangential to said loaded surface in ink, the ink dots advantageously exhibit, when they do not do not fill the entire extent of the associated elementary surface, a form adapted to reduce the fattening phenomenon, and preferably a substantially elongated and tapered shape in the direction of movement of the support.

So the fattening phenomenon, resulting in a change in the size of the dots on printing, may be reduced, resulting in a printed image closer to the original to be reproduced.

In addition, it was found that the number of surfaces elementary stained entirely tended to decrease so important when the number of shades chosen for each base color was increasing.

Thus, the elementary surfaces are in majority only partially printed and choosing a layout adapted ink dots inside these surfaces elementary we can get a set of ink dots disjoint, which avoids the phenomenon called "contact" which consists of spreading the ink in the contact region of two contiguous ink dots, due to the phenomena of capillarity. Such a suitable arrangement of ink dots at the within the corresponding elementary surfaces on the support can consist in centering each ink point in the surface associated elementary.

In addition, although one might fear a priori a depletion of the image, by the initial choice in the invention of a limited number of basic colors in which the original is broken down into colors (which can have a much larger number of colors than the number of basic colors chosen), it was surprisingly found that the visual rendering of the printed image could be better than with the printing methods of the prior art using a subtractive mixture of primary colors.

On the one hand, the differences in contrast, for a observer, can be made thanks to the invention more important than in the prior art, due to the choice of colors basic non-primary.

On the other hand, it is easier to get an impression of softness restored by a set of high-luminosity shades using non-primary colors than using a blend primary colors as in the prior art.

In a particular implementation of the invention, at less shade is obtained, for at least one of the colors of base, by printing a predetermined pattern such as a logo.

Preferably, several shades of at least one of the basic colors are obtained by printing, in a greater or lesser proportion of a predetermined pattern such as logo.

The pattern is difficult to reproduce because invisible to the naked eye, we thus have a means likely to contribute very advantageously to the authentication of the printed medium.

The invention also relates to a printing machine having ink application surfaces respectively charged by ink dots respectively located within elementary surfaces arranged in a distribution predetermined, so as to visually reconstruct, during printing on a medium, the image to be reproduced, the color of each ink point being chosen from a predetermined set basic colors, each elementary surface given being covered with ink in a proportion chosen according to the shade to be reproduced, the number of shades being at least three for at least one of the basic colors.

The invention also relates to a printing machine dry offset comprising at least one blanket and rollers application bearing reliefs arranged so as to deposit on said blanket a set of ink dots respectively located within elementary surfaces arranged in a distribution predetermined so as to visually reconstruct during printing on a medium the image to be reproduced, the color of each ink point being chosen from a predetermined set basic colors, each elementary surface given being covered with ink in a proportion chosen according to the shade to be reproduced, the number of shades being at least three for at least one of the basic colors.

Preferably, each relief intended to deposit a point ink within an elementary surface to reproduce a intermediate shade has a substantially elongated shape and tapering in the direction of movement of the support when printing so as to reduce the dot gain at the impression.

The invention also relates to a printing medium. having an outer surface on which a image consisting of a set of ink dots respectively located within elementary surfaces arranged in a predetermined distribution, the colors of said ink dots being chosen from a predetermined set of basic colors, each dot of ink of a given base color filling a greater or lesser proportion of the elementary surface corresponding, depending on the shade of said base color to reproduce, the number of shades being at least three for at least minus one of the basic colors.

Preferably, at least one shade of the printed medium is obtained, for one of the basic colors, by a pattern predetermined such as a logo, more or less printed depending on the shade to reproduce.

• la figure 1 est une vue schématique en perspective d'une machine d'impression offset à sec,
• la figure 2 est un schéma illustrant les différentes étapes d'un procédé décrit pour mieux faire comprendre l'invention,
• la figure 3 est un schéma illustrant la décomposition d'une image constituée par un ensemble de pixels juxtaposés en une superposition d'images monochromatiques,
• la figure 4 est un schéma illustrant les différentes étapes d'un exemple de procédé conforme à l'invention,
• la figure 5 est un schéma illustrant la décomposition d'une composition de surfaces élémentaires colorées en une superposition d'images monochromatiques,
• la figure 6 illustre différentes formes de points d'encre disposés au sein de surfaces élémentaires correspondantes,
• la figure 7 illustre la restitution d'une nuance par l'impression, dans une proportion plus ou moins grande, d'un motif prédéterminé,
• la figure 8 est un exemple d'une forme de point d'encre adaptée à limiter le phénomène d'engraissement lors de l'impression,
• les figures 9 à 11 illustrent différentes répartitions de surfaces élémentaires,
• la figure 12 est une vue schématique à échelle agrandie d'une image imprimée conformément à un procédé de l'art antérieur,
• la figure 13 est une vue schématique à échelle agrandie d'une image imprimée conformément au procédé décrit en référence à la figure 2,
• la figure 14 est une vue schématique à échelle agrandie d'une image imprimée conformément au procédé selon l'invention décrit en référence à la figure 4,
• les figures 15 à 17 illustrent la décomposition en images monochromatiques de l'image représentée sur la figure 14,
• la figure 18 illustre la restitution de nuances d'une couleur de base au moyen de l'impression, dans une proportion plus ou moins grande, d'un motif déterminé,
• la figure 19 est un exemple de représentation schématique de l'espace colorimétrique,
• la figure 20 illustre la restitution, lors de l'impression, de diverses nuances d'une même couleur de base, conformément à un autre exemple de procédé selon l'invention,
• la figure 21 illustre l'obtention, lors de l'impresion, de nuances de différentes couleurs de base, avec dépôt d'encre noire, conformément à l'exemple de procédé selon l'invention décrit en référence à la figure 20,
• la figure 22 est un schéma illustrant les différentes étapes du procédé selon l'invention décrit en référence aux figures 20 et 21, et
• les figures 23 et 24 illustrent deux façons de réaliser le dépôt d'encre noire au sein d'une surface élémentaire sur le support.

Other characteristics and advantages of the present invention will emerge on reading the detailed description which follows, of non-limiting examples of implementation of the method according to the invention, and on examining the appended drawing in which:

• FIG. 1 is a schematic perspective view of a dry offset printing machine,
• FIG. 2 is a diagram illustrating the different stages of a process described to better understand the invention,
• FIG. 3 is a diagram illustrating the decomposition of an image constituted by a set of pixels juxtaposed in a superposition of monochromatic images,
• FIG. 4 is a diagram illustrating the different stages of an example of a process according to the invention,
• FIG. 5 is a diagram illustrating the decomposition of a composition of elementary colored surfaces into a superposition of monochromatic images,
• FIG. 6 illustrates different shapes of ink dots arranged within corresponding elementary surfaces,
• FIG. 7 illustrates the restitution of a shade by printing, in a greater or lesser proportion, a predetermined pattern,
• FIG. 8 is an example of an ink dot shape adapted to limit the phenomenon of fattening during printing,
• FIGS. 9 to 11 illustrate different distributions of elementary surfaces,
• FIG. 12 is a schematic view on an enlarged scale of an image printed according to a method of the prior art,
• FIG. 13 is a diagrammatic view on an enlarged scale of an image printed according to the method described with reference to FIG. 2,
• FIG. 14 is a diagrammatic view on an enlarged scale of an image printed according to the method according to the invention described with reference to FIG. 4,
• FIGS. 15 to 17 illustrate the decomposition into monochromatic images of the image represented in FIG. 14,
• FIG. 18 illustrates the restitution of shades of a basic color by means of the printing, in a greater or lesser proportion, of a determined pattern,
• FIG. 19 is an example of a schematic representation of the color space,
• FIG. 20 illustrates the restitution, during printing, of various shades of the same basic color, in accordance with another example of a method according to the invention,
• FIG. 21 illustrates the obtaining, during printing, of shades of different basic colors, with deposit of black ink, in accordance with the example of the method according to the invention described with reference to FIG. 20,
• FIG. 22 is a diagram illustrating the different stages of the method according to the invention described with reference to FIGS. 20 and 21, and
• Figures 23 and 24 illustrate two ways of achieving the deposit of black ink within an elementary surface on the support.

There is shown in FIG. 1, schematically, a dry offset printing machine 1 of a type known per se, comprising a plurality of blankets 2 placed on the periphery of a rotary cylinder 3 to come successively, during the rotation of the latter, in contact with application rollers 4 intended to deposit on these blankets 2 ink dots of different colors.

In the example described, only four have been represented. application rollers 4 for clarity of design, but in practice machine 1 most often has six rollers of application.

These application rollers 4 rotate around axes of rotation parallel to each other and to the axis of rotation of the cylinder 3. Their external surface is constituted for each by a plate curved with reliefs projecting radially towards the exterior and on which an ink is deposited by means of a associated coating roller 5, regularly supplied with ink.

The O objects to be printed are brought by means of a carousel not shown one after the other each in contact a blanket 2 ready for ink transfer.

Each object O rolls on the associated blanket 2 so that the inks of different colors deposited on it are transferred.

We will now describe with reference to Figure 2 a first example of a process, used to make the negatives from which the plates of the application rollers are engraved and which is preferably carried out using a computer equipped with a display means such as a video screen.

We start in the example described from an original 6 in color, stored in the computer, which can be either an image synthesis, i.e. an image stored in digital form and obtained from a silver photograph, again one image delivered by a video camera.

By way of nonlimiting example, the original 6 can be an RGB coded image usable by software version 2.5 "Photoshop", marketed by the company "ADOBE SYSTEMS FRANCE "in Noisy-Le-Grand.

In a step 7 of the method, a number n of basic colors corresponding substantially, in the example particular described, in the different shades of the inks which will be used in the dry offset printing machine 1.

Given the number of application rollers usually encountered on this type of machine, n is chosen the more often equal to 6.

If necessary, a subsequent step 8 is imposed p particular colors, including for example the orange shade used to printing warning logos to bomb users aerosol, or a color chosen to print text.

It can still be one or more shades majority in the original 6 in color, for example a color close to the color of the subject's skin if the image is a portrait.

If the total number n of base colors in which we want to perform the decomposition of the original 6 is higher to the number of colors imposed p, you have to choose, manually or automatically, in the next step 9 of the process, the n-p colors remaining, for example taking shades statistically majority in the original 6 in color and not already selected.

One then constitutes, in a step 10 of the method, starting from n basic colors, a composition of elementary surfaces which are visualized by means of a video screen connected to the computer.

Elementary surfaces are formed in the example described by squares regularly juxtaposed.

The image obtained in step 10 visually reconstructs original 6 to be reproduced.

In the particular example described, the number of shades for each base color is limited to two, i.e. each elementary surface is either completely colored by a base color, either is not colored at all and then leaves appear the background color, which is assumed here to be uniformly white.

If the visual rendering of the surface composition is not entirely satisfactory, we come back to the previous step 9 of determining the n-p colors not imposed, to modify one or more colors and we examine when returning to step 10 the new image obtained.

We can thus proceed by successive iterations as illustrated in the diagram of Figure 2 by the arrow 11 in dotted.

In the example described, the original 6 in color is a RGB coded image where the color (including hue) of each pixel is coded on 24 bits and we use to constitute the composition of colored elementary surfaces the function of creation of an indexed image of the aforementioned "Photoshop" software.

This function allows you to switch from an RGB coded image where the color (including hue) of each pixel is coded on 24 bit to an indexed image where the color (including hue) of each pixel is coded with a limited number of bits referring to reference colors whose color (including the nuance) is coded on 24 bits.

Such a function is originally intended to decrease clutter in the computer memory of images to which it is applied.

When the choice of the n basic colors is satisfactory, the composition of colored elementary surfaces is broken down in step 12, into n monochromatic images 12 ₁ , ..., 12 _n to be superimposed.

By way of example, FIG. 3 shows a fragment 12 _o of the composition of colored elementary surfaces.

This fragment 12 _o is constituted by a set of elementary surfaces of constant size and regularly juxtaposed, the color of which is chosen from a predetermined number of basic colors, equal to five in the example considered.

After decomposition, five monochromatic images 12 ₁ to 12 _{5 are} obtained, the superposition of which returns the image 12 _o .

Negatives 13 ₁ to 13 _{n are} produced in the following step 13 respectively from the n monochromatic images 12 ₁ to 12 _n , which will be used for the subsequent etching, in step 14, of the n plates of the application rollers 4.

When using machine 1, all of the application rollers 4 deposit in step 15 on each blanket 2 ink dots of the same size, square in the example considered, regularly juxtaposed, and whose distribution in color function is linked to that of the different pixels on the image obtained in step 10 of the process.

The ink dots deposited on each blanket 2 are transferred in step 16 to the object to be printed.

FIG. 12 shows an image obtained by a method according to the prior art.

As you will notice on examining this figure, the image is constituted by the superposition of images monochromatic halftone, with different weft angles, density variations on monochromatic images being translated by a variation in the size of the points constituting the frame.

An example is shown in Figure 13 a image obtained by implementing the first example process.

This image is made up of a composition of colored elementary surfaces, the elementary surfaces being squares of the same size and regularly juxtaposed.

In the particular example described, the surface of the object on which the image is printed is assumed to be of a tint uniform, white and we will notice on the image shown in the figure 13 the absence, in places, of ink dots within said composition of colored elementary surfaces, leaving thus appear the background color.

Of course, we can use a different color background than white or not uniform and taken into consideration when generation of the monochromatic images in step 12 of the process.

The first example of a process according to the invention which just described considerably diminishes the problem of pollution of inks during dry offset printing.

However, we still improve the visual rendering of the image printed by choosing, for at least one of the basic colors, a number of shades greater than 2, in accordance with the invention.

In an example of the method according to the invention, described in reference to figures 4 to 11, the original 6 in color is broken down in a composition of elementary surfaces colored with shades selected from n basic colors and m shades by base color.

We indicate in step 7 'the number of basic colors n and shades m by basic colors, and in step 8 'we impose the where appropriate certain basic colors and / or shades.

If the total number of shades you want perform the decomposition of the original 6 in color is higher to the number p of imposed colors, one manually chooses or automatically in step 9 'the remaining n-p shades, taking for example the statistically predominant shades in the original in color and not already selected.

At step 10 ′ of the process, a composition of colored elementary surfaces, said elementary surfaces being arranged in a predetermined distribution, the color of each elementary surface being chosen from among the n colors of base and their nuances, the choice of base color and nuance attributed to each of said elementary surfaces being carried out in such a way that said surface composition colored elementals visually reconstruct the image to be reproduced.

Decomposition of the original in color into n colors base with m shades per base color can be done in using the aforementioned "Photoshop" software, thanks to the function of transition from an RGB coded image to an indexed image.

The user can view in step 10 'of the process the composition of elementary colored surfaces by means of a video screen connected to the computer.

Preferably, the elementary surfaces are formed by squares regularly juxtaposed, as illustrated by figure 9.

However, it is not beyond the scope of this invention by imposing other forms of elementary surfaces and other arrangements of these before performing the decomposition of the original in color, so before assigning a tint at each elementary surface. We can choose for example a juxtaposition of hexagons as shown in Figure 10 or a juxtaposition of triangles as shown in Figure 11.

We can still propose, without departing from the framework of the invention, an irregular arrangement of surfaces elementary, with variable spacing between them.

The use of regularly juxtaposed squares nevertheless has the advantage of corresponding to the structure display on the video screen of the indexed image of the software "Photoshop" above.

As described above, we can return to step 7 'of choosing the basic colors and shades if the image obtained in step 10 'of the process is not satisfactory, by example due to the choice of certain basic colors imposed.

Note that the user can easily perform several tests, without the need to carry out calculations to determine the colors resulting from the mixture subtractive primary colors, unlike printing classic in four colors.

The composition of colored elementary surfaces is then broken down in step 12 'of the process into a set of images monochromatic.

For each monochromatic image corresponding to a base color, we translate the shade assigned to each surface elementary by filling in a predetermined proportion of this one with this base color.

Assuming the shades of a base color are coded on 8 bits, which corresponds to 256 possible nuances by base color, the binary value 0 which results in the absence of base color being considered to correspond to a shade of this basic color, we associate, for the same basic color, with each binary value has a respective point size and shape, this point filling in a predetermined proportion the surface corresponding elementary.

Thus, a slightly saturated shade corresponding to a low binary value will be restored, in the example described, by filling only a small portion of the surface total of the associated elementary surface while a very nuance saturated corresponding to a high binary value will be translated by filling a large proportion of the total area of the elementary surface.

By assigning to each non-zero binary value a point size, we get 255 different point sizes, and we translate the binary null value by the absence of point.

It is also possible, without departing from the scope of the invention, assign a predetermined range of binary values a size single point.

As an example, when the binary value translating the shade of the basic color considered is between 0 and 100, we can translate the nuance by a point filling 30% by example of the total area of the associated elementary area and when said binary value is between 100 and 200, we can translate the shade by a bigger point filling 60% by example of the elementary surface, etc ...

By way of example, FIG. 5 shows a fragment 12 _o 'of a composition of elementary colored surfaces and the rendering of the nuances of the basic colors is illustrated by a more or less large filling of each surface. elementary.

This figure has also illustrated the decomposition of the composition of colored elementary surfaces into monochromatic images 12 ₁ ′ to 12 ₅ ′.

Those skilled in the art will note that the images monochromatic obtained from the decomposition of the image indexed obtained in step 10 'of the process, can have the same aptitude for the restitution of details as this. So the contrast can be located thanks to the invention on the image printed exactly like the indexed image used to take monochromatic images. It is therefore possible avoid, thanks to the invention, the loss of visual definition encountered in the processes of the prior art and due to a difference in resolution between the original image and the resolution of the frame of the monochromatic images used for the engraving of plates and obtained from this starting image.

Once we have obtained the monochromatic images in which the different nuances are translated by points filling in a proportion depending on the shade to reproduce the elementary surfaces, we proceed to the realization negatives in step 13 and then engraving the plates in step 14.

Then, in step 15, the ink dots, which are transferred in step 16 to the object to be printed.

To prevent two ink dots associated respectively to two adjacent elementary surfaces and does not not fully covering the latter do not touch each other during printing, we prefer that each ink point is centered in the corresponding elementary surface.

FIG. 6 shows by way of example three ink dot shapes corresponding respectively to a increasing occupation of the associated elementary surface, for reproduce three shades of a basic color corresponding to increasing binary values representative of one more nuance more saturated.

The choice of ink dot shape allowing restore a shade by covering in a chosen proportion the associated elementary surface is advantageously carried out by taking account of problems related to fattening.

The shape of the ink dots can also be adapted the nature of each ink used.

Preferably, we choose a form of ink dots tapered and elongated in the direction of movement of the support to print when printing.

By way of example, FIG. 8 shows a ink point elongated and tapered in the direction of scrolling of the support to be printed, materialized by an arrow on this figure.

In a particular implementation of the method according to the invention, we choose to restore several nuances of a basic color of the ink dots formed by the drawing more or less complete with a predetermined pattern.

By way of example, FIG. 7 shows three ink dot shapes corresponding to three different shades of a basic color, these ink dots corresponding to the drawing progressive, and preferably according to a random filling, of a predetermined pattern representing an O.

By random filling, it is necessary to understand the fact of draw the pattern by elementary lines or points distributed in chance within the pattern envelope.

The complete printing of the motif inside a elementary surface corresponds to a predetermined shade having a certain degree of saturation.

Any more saturated shade corresponding to a proportion greater colored area within the elementary area is obtained by filling in a given proportion the surface of said elementary surface which is external to the envelope of the pattern, preferably randomly.

So when the shade matches the filling of a most of the associated elementary surface, the pattern becomes more difficult to discern.

From a certain degree of filling, it is not even no longer possible to recognize the pattern.

To restore a less saturated shade corresponding to the filling only a small proportion of the surface elementary, the motif is only partially represented.

Below a certain proportion of surface elementary fulfilled, it is no longer possible to recognize the pattern.

FIG. 14 shows a composition of colored elementary surfaces obtained by the process according to the invention, these elementary surfaces being constituted by squares regularly juxtaposed.

Each elementary surface is either uncoloured, to which case it lets appear the background color, either colored by one of the three colors A, B and C, the nuances being translated by respective points of different sizes as illustrated in this figure.

The three images are shown in Figures 15 to 17 monochromatic obtained by decomposing in each of colors A, B and C the composition of colored elementary surfaces represented on figure 14 and which will be used for the realization of negatives used for printing.

There is shown in Figure 18 an image monochromatic in which each ink point, arranged at the within an elementary surface constituted by a square, represents the same pattern, more or less drawn, depending on the shade to be restored.

As an indication, when the elementary surfaces are squares regularly juxtaposed, each square can have on the printed medium 127 µm side for example, which is equivalent at a resolution of 200 lines per inch (79 lines per cm).

It is possible thanks to the invention to isolate a point for example completely filling an elementary square surface 127 µm side by not filling the elementary surfaces adjacent.

As an indication, in a modulation frame of classic amplitude at 133 lines per inch (52 lines per cm), the point size when separated by a distance equal to their diameter is 95 µm. This dimension is considered to be the dimension for which the visibility of an isolated point is optimal, because when the points are less close together they are smaller and harder to spot and when the dots are closer, they are bigger and it is more difficult to discern each point individually.

So in the example above, despite the fact that a isolated point is larger in the invention than an isolated point of a amplitude modulation frame, it is possible thanks to the invention higher resolution which is quite surprising at first on board.

The best resolution provided by the invention can give the viewer an impression of finesse of the image observed.

Furthermore, when the number of shades is at least minus three per base color, the dots completely filling an associated elementary surface on the support are in the minority and we can tolerate that two points overlap slightly during of printing due to the fattening phenomenon, without fear a major problem of ink pollution with printing in the case of dry offset.

We will now describe another example of a process according to the invention with reference to Figures 19 to 24.

Generally, we can define a color by its hue, its luminosity and its saturation.

An example of space has been represented in FIG. 19 colorimetric.

It is conventional to materialize by projection on an L axis the degree of luminosity of a color, by the position angular around this axis, the hue of the color and by the distance from this axis, the saturation S of the color.

So the colors located in this space on a same circle centered on the L axis have different colors but identical brightness and saturation.

In the example of implementation of the invention, which has been described above, the shades of the same basic color are obtained by filling more or less an elementary surface, this which basically amounts to moving in direction D on the figure 19.

Indeed, by reducing the portion of the surface which is colored, we increase the brightness and we decreasing saturation and increasing the portion of the surface elementary which is colored, we decrease the brightness and we approaches maximum saturation (background is assumed uniformly white).

The example of a method according to the invention which is now described aims to further expand the number of shades can be used to reproduce the original in color, making vary the brightness parameter downwards, i.e. depending on the direction E in FIG. 19, from a shade corresponding to given degrees of brightness and saturation.

To do this, black ink is deposited in a greater or lesser proportion within the elementary surface which is also colored in a proportion chosen according to the saturation of the shade to be reproduced.

FIG. 20 shows a first row of five elementary surfaces 20 ₍₁₎ to 20 ₍₅₎ which are colored in increasing proportion by a given ink.

The surface 20 ₍₁₎ is not colored at all and the surface 20 ₍₅₎ is completely colored.

The surfaces 20 ₍₄₎ and 20 ₍₅₎ which each correspond to given degrees of brightness and saturation are used to generate, by progressive addition of black, second and third families of shades, referenced respectively 20 ₍₄₎ ⁽ⁱ⁾ to 20 ₍₄₎ ^(v) and 20 ₍₅₎ ⁽ⁱ⁾ to 20 ₍₅₎ ^(v) .

We therefore understand that the use of black ink allows increase the number of nuances used to decompose the original into color, since it allows to reproduce variations of brightness at substantially constant saturation.

For each basic color, we are thus led to restore certain nuances using black ink when the impression, which is deposited on certain elementary surfaces of support.

Adding black ink can be done in one operation after depositing all other inks, such as illustrated in figure 21.

At the top of this figure, a fragment of support printed by deposition within elementary surfaces of different colored inks, filling more or less each elementary surface according to the saturation of the shade to be restored.

To obtain certain shades, a deposit of black ink has been done.

This deposit of black ink can be carried out after printing of the other colors used.

We will now describe in detail with reference to the Figure 22 the different stages of the process.

We start with the acquisition of the image to reproduce in the form of a digital data file 21, for example a RGB encoded file.

We choose the distribution 22 of the elementary surfaces, for example one of the distributions illustrated in FIGS. 9 to 11 and we choose the set 23 of the basic colors and shades of these basic colors in which we want to perform the decomposition of the original to be reproduced.

We then create an indexed image 24, corresponding to the coloring by the colors and shades of the set 23 of elementary surfaces defined above, the composition of colored elementary surfaces thus obtained restoring visually the image to be reproduced.

We then decompose in step 25 the indexed image obtained in step 24 in a plurality of monochromatic images, each image corresponding to a given basic color A, B, C, etc ....

Then, in step 26, the monochromatic images A, B, C, ... are broken down into respective images AN _A , BN _B , CN _C , ... grouping the shades which do not involve the color black and by respective images N _A , N _B , N _C , ... using only the color black.

The different images N _A , N _B , N _C , ... are superimposed in step 27 to obtain a monochromatic image N in black and white.

For the plurality of monochromatic images AN _A , BN _B , CN _C , ... and N, at step 28, the size of the ink point to be deposited on the support to restore this shade is determined for each shade and a pattern occupying the elementary surface of the support is assigned in step 29 to each stitch size in the desired proportion.

A plurality of monochromatic images M _A , M _B , M _C , ... and M _N are obtained in which the shades are restored by the presence within each elementary surface of an ink point of shape and size predetermined and negatives are produced if necessary from these monochromatic images used to produce the printing rollers.

We print the support in step 31 with the different basic colors other than black.

We print the black ink in step 32 and we get a reproduction 33 of the original image.

When the printing process used is not a dry offset printing process, black ink can be applied directly on the ink previously printed on the support as illustrated in figure 23.

When the printing process is an offset process dry, to avoid overlapping inks and problems of mixing, we can when determining the shape of the ink point which will be deposited on the support to restore a shade of color given, take into account the shape and size of the ink dot black which will be filed later to change the brightness of the shade considered.

More particularly, it is possible not to deposit ink from color in the region of the elementary surface which will then be covered with black ink, as shown in Figure 24.

The invention is not limited to the example methods which have just been described.

The invention thus advantageously applies to other printing processes as dry offset printing for example printing by screen printing, sublimation or other, the shape of printed dots that can be adapted to each print mode so as to compensate for the defects.

By the structure of the final image on the support printed, the invention is particularly well suited to printing coded images in digital form.

Those skilled in the art will understand that the invention can allow you to benefit from the advantages specific to frames conventional so-called frequency modulation and modulation of amplitude, namely on the one hand the sharpness of the printed image (of by the localization of the contrast identical to the image of departure) and on the other hand the impression of softness (due to the spacing constant of the grid points), without the disadvantages of these grid conventional, namely a noise effect for the weft frequency modulation and blurring for the frame at amplitude modulation.

Claims (34)

1. Method for printing a coloured image on a carrier, characterized in that it comprises the steps involving:

   decomposing the image to be reproduced into a composition of coloured elementary surfaces (24), the said elementary surfaces being arranged in a predetermined distribution (22), the colour of each elementary surface being selected from a predetermined set of basic colours and a predetermined set of shades of these basic colours (23), the number of shades for at least one basic colour being greater than or equal to three, the selection of the basic colour and of the shade assigned to each of the said elementary surfaces being carried out in such a way that the said composition of coloured elementary surfaces (24) visually reconstitutes the image to be reproduced,

   printing the said composition of coloured elementary surfaces on the said carrier by means of inks, the tints of which are selected in such a way as to make it possible, in view of the colour of the said carrier, to reproduce the said basic colours and their shades during printing, the shade of a basic colour assigned to a given elementary surface being obtained, during printing, by action on the proportion of each ink within the corresponding elementary surface on the carrier.
2. Method according to Claim 1, characterized in that at least one of the said basic colours is a non-primary colour other than black.
3. Method according to Claim 1 or 2, characterized in that the shades of at least one basic colour have the same saturation, but different brightnesses.
4. Method according to Claim 3, characterized in that the shades of a basic colour which have the same saturation, but different brightnesses, are reconstituted, during printing, by means of a greater or lesser proportion of black ink within the elementary surfaces of the carrier which are affected by obtaining the said shades during printing.
5. Method according to Claim 4, characterized in that, when the same elementary surface of the carrier is affected both by a deposit of black ink and by a deposit of an ink other than black ink, the deposit of the black ink and of the said ink other than black ink is carried out with patterns which are predetermined for each of the inks and are selected so as to prevent the two inks from being superposed.
6. Method according to Claim 4, characterized in that the deposit of the black ink takes place in such a way that the latter is superposed on at least one other ink already deposited on the carrier.
7. Method according to any one of Claims 1 to 5, characterized in that it is used in dry offset printing.
8. Method according to any one of Claims 1 to 7, characterized in that the said basic colours are non-primary colours.
9. Method according to any one of Claims 1 to 8, characterized in that at least all the inks less one have tints selected from the said basic colours.
10. Method according to any one of Claims 1 to 9, characterized in that the said elementary surfaces have the same shape.
11. Method according to Claim 10, characterized in that the said elementary surfaces are all derived one from the other by translation.
12. Method according to any one of Claims 1 to 11, characterized in that the said elementary surfaces are juxtaposed.
13. Method according to any one of Claims 1 to 12, characterized in that the said elementary surfaces are squares.
14. Method according to any one of Claims 1 to 13, characterized in that each ink spot is centred in the corresponding elementary surface.
15. Method according to any one of Claims 1 to 14, characterized in that each elementary surface on the carrier has printed on it an ink spot, the shape and size of which depend, according to a predetermined relation, on the shade of the basic colour to be reproduced.
16. Method according to Claim 15, printing being carried out by the carrier to be printed being brought into contact with an ink-charged surface, with a direction of displacement substantially tangential to the said ink-charged surface, characterized in that the said ink spots, when they do not fill the entire extent of the associated elementary surface, have a shape designed to reduce the fattening phenomenon, preferably a shape substantially elongated and tapered in the direction of displacement of the carrier.
17. Method according to any one of Claims 1 to 16, characterized in that, during printing, a shade is obtained, for at least one of the basic colours, as a result of the printing of a predetermined pattern, such as a logo.
18. Method according to Claim 17, characterized in that a plurality of shades of at least one of the basic colours are obtained as a result of the printing of a predetermined pattern, such as a logo, in a greater or lesser proportion.
19. Method according to any one of Claims 1 to 18, characterized in that the inks are deposited in the form of ink spots, the geometry of which is different depending on the ink used.
20. Method according to any one of Claims 1 to 19, characterized in that the said carrier is white before printing.
21. Printing machine (1) comprising ink application surfaces (4) respectively charged with ink spots respectively located within elementary surfaces arranged in a predetermined distribution, so as, during printing on a carrier (0), visually to reconstitute an image to be reproduced, the colour of each ink spot being selected from a predetermined set of basic colours, each given elementary surface being covered with ink in a proportion selected as a function of the shade to be reproduced, the number of shades being at least three for at least one basic colour.
22. Machine according to Claim 21, characterized in that at least one of the said basic colours is a non-primary colour other than black.
23. Printing machine according to Claim 21 or 22, characterized in that the colours of the ink spots are non-primary.
24. Dry offset printing machine (1) comprising at least one blanket (2) and applicator rollers (4) bearing reliefs arranged so as to deposit on the said blanket a set of ink spots respectively located within elementary surfaces arranged in a predetermined distribution, so as, during printing on a carrier (0), visually to reconstitute an image to be reproduced, the colour of each ink spot being selected from a predetermined set of basic colours, each given elementary surface being covered with ink in a proportion selected as a function of the shade to be reproduced, the number of shades being at least three for at least one basic colour.
25. Dry offset printing machine according to Claim 24, characterized in that each relief intended for depositing ink within an elementary surface for reproducing a shade has a shape substantially elongated and tapered in the direction of displacement of the carrier to be printed.
26. Print carrier (0) having an outer surface, on which has been printed an image consisting of a set of ink spots respectively located within elementary surfaces arranged in a predetermined distribution, the colours of the said ink spots being selected from a predetermined set of basic colours, each ink spot of a given basic colour filling a greater or lesser proportion of the corresponding elementary surface, depending on the shade of the said basic colour to be reproduced, the number of shades being at least three for at least one of the basic colours.
27. Print carrier according to Claim 26, characterized in that at least one of the said basic colours is a non-primary colour other than black.
28. Print carrier according to Claim 26 or 27, characterized in that the said elementary surfaces have the same shape.
29. Print carrier according to any one of Claims 26 to 28, characterized in that the said elementary surfaces are juxtaposed.
30. Print carrier according to any one of Claims 26 to 29, characterized in that each elementary surface consists of a square.
31. Print carrier according to any one of Claims 26 to 30, the said outer surface being cylindrical, characterized in that the said ink spots, when they do not completely fill the corresponding elementary surface, have a shape substantially elongated and tapered in the direction of displacement of the carrier during printing.
32. Print carrier according to any one of Claims 26 to 31, characterized in that each ink spot is centred within the corresponding elementary surface.
33. Print carrier according to any one of Claims 26 to 32, characterized in that a plurality of shades of at least one basic colour are reconstituted as a result of the printing of a predetermined pattern, such as a logo, in a greater or lesser proportion.
34. Print carrier according to any one of Claims 26 to 33, characterized in that at least one elementary surface has received a spot of black ink and a spot of an ink other than black, the size of the spot of black ink being a function of the brightness of the shade to be reproduced.

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