EP0794056A1 - Method and printing machine for carrying out the method and support so printed - Google Patents

Abstract

The image for reproduction is decomposed into elementary coloured surfaces (24) which are arranged in a predetermined manner (22). The colour of each elementary surface is selected from predetermined groups of basic colours and three or more shades for each basic colour. The composition of elementary coloured surfaces is printed on to a support using inks whose colours reproduce the selected base colours and shades after taking account of the support colour. The shade of base colour allocated to a given elementary surface is obtained by varying the proportion of each ink on the support elementary surface.

Description

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

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

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

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

Various methods can be used to make these negatives.

A known method consists in decomposing the original in color which it is desired to reproduce into a superposition of raster monochromatic images, the variations in density of each monochromatic image being restored by varying the size of the points of the frame, called modulation amplitude.

A corresponding negative is produced for each monochromatic raster image.

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

Such a method is not entirely satisfactory, in particular because it does not prevent, when the image to be reproduced presents close details and different colors, any superposition of ink dots on the blankets and pollution mutual inks, which leads to a gradual denaturation of the printed image and requires renewing the inks, which may even require sending part of the production to scrap.

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

The invention also aims, more generally, to be able to print an image rich in shades of color without having to create colors or shades of colors by mixing inks of different colors during printing.

The invention also aims to eliminate the moiré phenomena when many colors 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 of 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 an image without superimposing the ink dots on printing, which has multiple advantages.

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

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

Then, it is possible thanks to the invention to visually reconstruct an image without involving the mixing of inks in printing, therefore without having to determine beforehand, by calculation or by carrying out tests, the colors resulting from the mixing of inks , which is not always easy, especially given the complexity of the human perception of colors.

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

While conventional four-color printing uses inks of black, magenta, cyan and yellow colors and restores the 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, the hues of which are chosen according to the subject of the image to be printed.

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

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

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

It will be understood that the invention is advantageously used for printing consumer goods, to the extent that the visual perception of the image printed on the objects displayed for sale, by a potential buyer, can be decisive.

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

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

In this case, the nuances of a basic color which have the same saturation but different luminosities can be restored during printing thanks to a greater or lesser proportion of black ink within the elementary surfaces of the support which are concerned. by obtaining said nuances during printing.

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

In the case of other printing methods for which the superposition of inks on printing is not a problem, it is possible to deposit the black ink with superposition on at least one other ink already deposited on the support.

In a particular implementation of the method according to the invention, an ink point is printed in each elementary surface on the support, the shape and size of which depend according to a predetermined relationship on the shade of the basic color to be reproduced.

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

Thus, the fattening phenomenon, resulting in a modification of the size of the dots at printing, can be reduced, which makes it possible to obtain a printed image closer to the original to be reproduced.

In addition, it has been found that the number of elementary surfaces entirely colored tends to decrease significantly when the number of shades chosen for each base color increases.

Thus, the elementary surfaces are mostly printed only partially and by choosing a suitable arrangement of the ink dots inside these elementary surfaces one can obtain a set of disjoint ink dots, which makes it possible to avoid 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 the ink dots within the corresponding elementary surfaces on the support may consist in centering each ink point in the associated elementary surface.

In addition, although one might fear a priori an impoverishment of the image, by the initial choice in the invention of a limited number of basic colors in which the decomposition of the original into colors takes place. (which may include a much larger number of colors than the number of basic colors chosen), it has surprisingly been 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 an observer, can be made thanks to the invention greater than in the prior art, due to the choice of non-primary basic colors.

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

In a particular implementation of the invention, at least one shade is obtained, for at least one of the basic colors, 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 a logo.

The pattern being difficult to reproduce because invisible to the naked eye, there is thus a means capable of contributing very advantageously to the authentication of the printed medium.

The invention also relates to a printing machine comprising ink application surfaces respectively loaded with ink dots respectively located within elementary surfaces arranged in a predetermined distribution, so as to visually reconstitute, during printing on a medium, the image to be reproduced, the color of each ink point being chosen from a predetermined set of basic colors, each given elementary surface 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 dry offset printing machine comprising at least one blanket and application rollers carrying reliefs arranged so as to deposit on said blanket a set of ink dots respectively located within surfaces elements arranged in a predetermined distribution so as to visually reconstruct the image to be reproduced when printing on a medium, the color of each ink dot being chosen from a predetermined set of basic colors, each given elementary surface being covered 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 of ink within an elementary surface to reproduce an intermediate shade has a substantially elongated shape and tapered in the direction of movement of the support during printing so as to reduce the fattening of the point at printing.

The subject of the invention is also a printing medium having an external surface on which has been printed an image constituted by 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 ink dot of a given basic color filling a greater or lesser proportion of the corresponding elementary surface, according to the shade of said basic color to be reproduced, the number of shades being at least three for at least one of the basic colors.

Preferably, at least one shade of the printed support is obtained, for one of the basic colors, by a predetermined pattern such as a logo, more or less printed according to the shade to be reproduced.

• 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 Figure 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 application rollers 4 have been represented for the sake of clarity of the drawing, but in practice the machine 1 most often has six application rollers.

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

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

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

We will now describe with reference to FIG. 2 a first example of a process, used to produce the negatives from which the plates of the application rollers are engraved and which, preferably, is implemented 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 a computer-generated image, or an image stored in digital form and obtained from a silver photograph, or even a image delivered by a video camera.

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

In a step 7 of the method, a number n of basic colors is chosen corresponding substantially, in the particular example described, to 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 most often chosen to be 6.

Where applicable, a specific step 8 is imposed on p particular colors, including for example the orange tint used to printing warning logos to users of aerosol cans, or a color chosen to print text.

It can also be one or more major shades in the original color 6, 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 basic colors in which one wishes to carry out the decomposition of the original 6 is greater than the number of colors imposed p, it is necessary to choose, manually or automatically, in the next step 9 of the process, the np colors remaining, taking for example the statistically predominant shades in the original 6 in color and not already selected.

Then, at a step 10 of the method, a composition of colored basic surfaces is created from the n basic colors, which is visualized by means of a video screen connected to the computer.

The elementary surfaces are constituted in the example described by squares regularly juxtaposed.

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

In the particular example described, the number of shades for each basic color is limited to two, that is to say that each elementary surface is either completely colored by a basic color, or is not colored with everything and then lets appear the background color, which is supposed here to be uniformly white.

If the visual rendering of the composition of elementary surfaces is not entirely satisfactory, we return to the previous step 9 of determining the np non-imposed colors, to modify one or more colors and we examine during the return 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 lines.

In the example described, the original color 6 is an RGB coded image where the color (including the shade) of each pixel is coded on 24 bits and the function of creation of an indexed image of the aforementioned "Photoshop" software.

This function allows to pass from an RGB coded image where the color (including the shade) of each pixel is coded on 24 bits to an indexed image where the color (including the shade) of each pixel is coded with a restricted number q of bits referring to reference colors whose color (including the shade) is coded on 24 bits.

Such a function is originally intended to reduce the size in the computer memory of the 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 the machine 1, all of the application rollers 4 deposit in step 15 on each blanket 2 ink dots of the same size, of square shape in the example considered, regularly juxtaposed, and whose distribution as a function of colors is linked to that of the different pixels on the image obtained in step 10 of the method.

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

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

As can be seen on examining this figure, the image is constituted by the superposition of raster monochromatic images, with different screen angles, the variations in density on the monochromatic images being translated by a variation in size. points constituting the frame.

FIG. 13 shows by way of example an image obtained by implementing the first example of a method.

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 uniform, white shade and it will be noted on the image represented in FIG. 13 the absence, in places , ink dots within said composition of colored elementary surfaces, thus revealing the background color.

Of course, one can use a background color other than white or non-uniform and taken into account when generating the monochromatic images in step 12 of the method.

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

However, the visual rendering of the printed image is further improved 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 with reference to FIGS. 4 to 11, the original color 6 is broken down into a composition of elementary surfaces colored with hues selected from n basic colors and m shades per basic color .

The number of basic colors n and of shades m per basic colors is indicated in step 7 ′, and in step 8 ′ certain basic colors and / or shades are imposed where appropriate.

If the total number of shades in which it is desired to carry out the decomposition of the original 6 in color is greater than the number p of imposed shades, the remaining np shades are chosen manually or automatically in step 9 ', taking for example the statistically predominant shades in the color original and not already selected.

A composition of colored elementary surfaces is formed in step 10 ′ of the method, said elementary surfaces being arranged in a predetermined distribution, the color of each elementary surface being chosen from among the n basic colors and their shades, the choice of color of base and of the nuance assigned to each of said elementary surfaces being effected so that said composition of colored elementary surfaces visually reconstitutes the image to be reproduced.

The decomposition of the original in color into n basic colors with m shades per basic color can be carried out using the above-mentioned "Photoshop" software, thanks to the function of switching from an RGB coded image to an indexed image.

The user can visualize in step 10 ′ of the method the composition of colored elementary surfaces by means of a video screen connected to the computer.

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

However, it is not going beyond the scope of the present invention to impose other forms of elementary surfaces and other arrangements of these before decomposing the original in color, therefore before assigning a shade. to 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.

It is also possible to propose, without departing from the scope of the invention, an irregular arrangement of the elementary surfaces, with a variable spacing between them.

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

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, for example due to the choice of certain basic colors imposed.

It will be noted that the user can easily carry out several tests, without it being necessary to carry out calculations to determine the shades resulting from the subtractive mixture of primary colors, unlike the conventional four-color printing.

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

For each monochromatic image corresponding to a basic color, the shade attributed to each elementary surface is translated by filling a predetermined proportion thereof with this basic color.

Assuming that the shades of a base color are coded on 8 bits, which corresponds to 256 possible shades per 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 a respective size and shape of point, this point filling in a predetermined proportion the corresponding elementary surface.

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 total surface of the associated elementary surface while a very saturated shade corresponding to a high binary value will be translated by the filling of a large proportion of the total surface of the elementary surface.

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

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

By way of example, when the binary value translating the shade of the basic color considered is between 0 and 100, the shade can be translated by a point filling 30% for 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 larger point filling 60% for 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 monochromatic images obtained from the decomposition of the indexed image obtained in step 10 ′ of the method, may have the same ability to restore the details as the latter. Thus, the contrast can be located thanks to the invention on the printed image precisely the same as the indexed image used to produce the monochromatic images. It is therefore possible, thanks to the invention, to avoid the loss of visual definition encountered in the methods of the prior art and due to a difference in resolution between the starting image and the resolution of the frame of the monochromatic images used to the etching of the plates and obtained from this initial image.

Once we have obtained the monochromatic images in which the different shades are translated by dots filling in a proportion depending on the shade to reproduce the elementary surfaces, we proceed to the production of negatives in step 13 and then to etching the plates in step 14.

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

To prevent two ink dots associated respectively with two adjacent elementary surfaces and not covering all of the latter from touching each other during printing, it is preferred that each ink point is centered in the corresponding elementary surface.

FIG. 6 shows, by way of example, three forms of ink dots corresponding respectively to an increasing occupation of the associated elementary surface, for reproduce three shades of a basic color corresponding to increasing binary values representative of an increasingly saturated shade.

The choice of the shape of the ink point making it possible to restore a shade by covering in a chosen proportion the associated elementary surface is advantageously carried out taking into account the problems associated with fattening.

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

Preferably, a shape of dots of tapered and elongated ink is chosen in the direction of movement of the support to be printed during printing.

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

In a particular implementation of the method according to the invention, ink dots formed by the more or less complete drawing of a predetermined pattern are chosen to restore several shades of a basic color.

By way of example, FIG. 7 shows three forms of ink dots corresponding to three different shades of a basic color, these ink dots corresponding to the progressive drawing, 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 drawing the pattern by lines or elementary points distributed at random within the envelope of the pattern.

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

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

Thus, when the shade corresponds to the filling of a major part of the associated elementary surface, the pattern becomes more difficult to discern.

From a certain degree of filling, it is no longer even possible to recognize the pattern.

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

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

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

Each elementary surface is either uncolored, in which case it reveals the background color, or colored by one of the three colors A, B and C, the shades being translated by respective dots of different sizes as illustrated in this figure.

FIGS. 15 to 17 show the three monochromatic images obtained by breaking down in each of the colors A, B and C the composition of colored elementary surfaces represented in FIG. 14 and which will be used to produce the negatives used for printing.

FIG. 18 shows a monochromatic image in which each ink point, disposed within an elementary surface constituted by a square, represents the same more or less drawn pattern, depending on the nuance to be restored.

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

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

As an indication, in a frame with conventional amplitude modulation at 133 lines per inch (52 lines per cm), the size of the points when they are 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 more difficult to see and when the points are closer, they are larger and it is more difficult to discern each point individually.

Thus, in the example above, despite the fact that an isolated point is larger in the invention than an isolated point of an amplitude-modulated frame, it is possible, thanks to the invention, to have a resolution superior which is quite surprising at first.

The better resolution provided by the invention can give the observer an impression of fineness of the image observed.

Furthermore, when the number of shades is at least three per base color, the dots entirely filling an associated elementary surface on the support are in the minority and it can be tolerated that two dots overlap slightly when printing in because of the fattening phenomenon, without fear of a major problem of pollution of printing inks in the case of dry offset.

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

Generally, you can define a color by its hue, its brightness and its saturation.

FIG. 19 shows an example of a color space.

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

Thus, the colors located in this space on the same circle centered on the axis L have different hues 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, which generally amounts to moving in the direction D in Figure 19.

Indeed, by decreasing the portion of the elementary surface which is colored, one increases the luminosity and one decreases the saturation and by increasing the portion of the elementary surface which is colored, one decreases the luminosity and one approaches the maximum saturation ( the background is assumed to be uniformly white).

The example of the method according to the invention which is now described aims to further extend the number of shades that can be used to reproduce the original in color, by varying the brightness parameter downwards, that is to say say in 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 moreover 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) .

It is therefore understood that the use of black ink makes it possible to increase the number of shades used to decompose the original in color, since it makes it possible to restore variations in brightness at substantially constant saturation.

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

The supply of black ink can be carried out in a single operation after the deposition of all the other inks, as illustrated in FIG. 21.

At the top of this figure, there is shown a fragment of support printed by deposition within elementary surfaces of different color 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 was made.

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

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

We start with the acquisition of the image to be reproduced in the form of a digital data file 21, for example an RGB coded file.

The distribution 22 of the elementary surfaces is chosen, for example one of the distributions illustrated in FIGS. 9 to 11 and the assembly 23 of the basic colors and of the shades of these basic colors is chosen in which it is desired to carry out the decomposition of the original to be reproduced.

An indexed image 24 is then created, corresponding to the coloring by the colors and shades of all 23 of the elementary surfaces defined above, the composition of colored elementary surfaces thus obtained visually restoring the image to be reproduced.

The indexed image obtained in step 24 is then broken down in step 25 into 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.

The support is printed in step 31 with the different base colors other than the black color.

The black ink is printed in step 32 and a reproduction 33 of the original image is obtained.

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

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

More particularly, it is possible not to deposit colored ink in the region of the elementary surface which will then be covered with black ink, as illustrated in FIG. 24.

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

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

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

Those skilled in the art will understand that the invention can make it possible to benefit from the advantages specific to conventional frames known as frequency modulation and amplitude modulation, namely on the one hand the sharpness of the printed image (due to the localization of the contrast identical to the initial image) and on the other hand the impression of softness (due to the constant spacing of the weft dots), without the disadvantages of these conventional wefts, namely a noise effect for the frequency modulated frame and a blurring effect for the amplitude modulated frame.

Claims (31)

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 (24), said elementary surfaces being arranged in a predetermined distribution (22), the color of each elementary surface being chosen from a predetermined set of basic colors and a set predetermined shades of these basic colors (23), the number of shades for at least one basic color being greater than or equal to three, the choice of the basic color and the shade assigned to each of said elementary surfaces being made from so that said composition of colored elementary surfaces (24) visually reconstitutes the image to be reproduced, printing said composition of elementary colored surfaces on said support by means of inks, the shades 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 shade of 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. Method according to claim 1, characterized in that the shades of at least one basic color have the same saturation but different brightnesses. Method according to claim 2, characterized in that the shades of a basic color which have the same saturation but different brightnesses are restored during printing by means of a greater or lesser proportion of black ink within the surfaces elementary of the support which are concerned with obtaining said nuances during printing. Method according to claim 3, characterized in that when the same elementary surface of the support 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 said ink other than black ink is effected with patterns predetermined for each of the inks, chosen to avoid overlapping of the two inks. Method according to claim 3, characterized in that the deposition of the black ink takes place with superposition on at least one other ink already deposited on the support. Method according to any one of Claims 1 to 4, characterized in that it is applied to dry offset printing. Method according to any one of Claims 1 to 6, characterized in that the said basic colors are non-primary colors. Method according to any one of Claims 1 to 7, characterized in that at least all the inks minus one have colors chosen from said basic colors. Method according to any one of Claims 1 to 8, characterized in that the said elementary surfaces have the same shape. Method according to claim 9, characterized in that said elementary surfaces are deduced from each other by translation. Method according to any one of Claims 1 to 10, characterized in that the said elementary surfaces are juxtaposed. Method according to any one of Claims 1 to 11, characterized in that the said elementary surfaces are squares. Method according to any one of Claims 1 to 12, characterized in that each ink point is centered in the corresponding elementary surface. Method according to any one of Claims 1 to 13, characterized in that an ink dot is printed in each elementary surface on the support, the shape and size of which depend according to a predetermined relationship on the shade of the color basic to reproduce. Method according to claim 14, the printing being carried out by bringing the support to be printed in contact with a surface loaded with ink with a direction of movement substantially tangential to said surface loaded with ink, characterized by the fact that said ink dots have, when they do not fill the entire extent of the associated elementary surface, a shape adapted to reduce the fattening phenomenon, and preferably a shape that is substantially elongated and tapered in the direction of movement of the support. Method according to any one of Claims 1 to 15, characterized in that a shade is obtained on printing, for at least one of the basic colors, by printing a predetermined pattern such as a logo. Method according to claim 16, characterized in that several shades of at least one of the basic colors are obtained by printing, in a greater or lesser proportion, a predetermined pattern such as a logo. Method according to any one of Claims 1 to 17, characterized in that the inks are deposited in the form of ink dots, the geometry of which is different according to the ink used. Method according to any one of Claims 1 to 18, characterized in that the said support is white before printing. Printing machine comprising ink application surfaces respectively loaded with ink dots respectively located within elementary surfaces arranged in a predetermined distribution, so as to visually reconstitute, when printing on a support, a image to be reproduced, the color of each ink point being chosen from a predetermined set of basic colors, each given elementary surface being covered with ink in a proportion chosen according to the shade to be reproduced, the number of shades being d '' at least three for at least one base color. Printing machine according to claim 20, characterized in that the colors of the ink dots are non-primary. Dry offset printing machine (1) comprising at least one blanket (2) and application rollers (4) carrying reliefs arranged so as to deposit on said blanket a set of ink dots respectively located within elementary surfaces arranged in a predetermined distribution so as to visually reconstruct when printing on a medium an image to be reproduced, the color of each ink dot being chosen from a predetermined set of basic colors, each given elementary surface being covered with ink in a chosen proportion depending on the shade to be reproduced, the number of shades being at least three for at least one basic color. Dry offset printing machine according to claim 22, characterized in that each relief intended to deposit ink within an elementary surface to reproduce a shade has a substantially elongated shape and tapered in the direction of movement of the support to print. Printing medium having an external surface on which an image consisting of a set of ink dots respectively located within elementary surfaces arranged in a predetermined distribution has been printed, the colors of said ink dots being chosen from a predetermined set of base colors, each ink point of a given base color filling a greater or lesser proportion of the corresponding elementary surface, according to the shade of said base color to be reproduced, the number of shades being at least three for at least one of the basic colors. Printing medium according to claim 24, characterized in that the said elementary surfaces have the same shape. Printing medium according to any one of claims 24 and 25, characterized in that said elementary surfaces are juxtaposed. Printing medium according to any one of Claims 24 to 26, characterized in that each elementary surface consists of a square. Printing medium according to any one of Claims 24 to 27, the said external surface being cylindrical, characterized in that the said ink dots have, when they do not completely fill the corresponding elementary surface, a substantially elongated shape and tapered in the direction of media travel during printing. Printing medium according to any one of Claims 24 to 28, characterized in that each ink point is centered within the corresponding elementary surface. Printing medium according to any one of Claims 24 to 29, characterized in that at least several shades of a basic color are restored by printing in a greater or lesser proportion of a predetermined pattern such as 'a logo. Printing medium according to any one of Claims 24 to 30, characterized in that at least one elementary surface has received a dot of black ink and a dot of an ink other than black, the size of the dot d black ink being a function of the brightness of the shade to be reproduced.

Images