WO2004015380A9

WO2004015380A9 - Method for the quantitative evaluation of the colorimetric potential of a pigment composition and applications thereof

Info

Publication number: WO2004015380A9
Application number: PCT/FR2002/002662
Authority: WO
Inventors: Jean-Pierre Guillemin
Original assignee: Jean-Pierre Guillemin
Priority date: 2002-07-25
Filing date: 2002-07-25
Publication date: 2004-04-08
Also published as: WO2004015380A1; AU2002341011A1; EP1525441A1

Abstract

The invention relates to an evaluation method consisting in representing the pigment combination (B, W, C1 and C2) in the form of a volume in a standardised colorimetric space (Lab) and voxelising said volume. According to the invention, all of the resulting occupied voxels correspond to all of the hues that can be obtained from said pigment combination and each occupied voxel (V2) corresponds to a determined proportion between the constituent pigments (B, W, Cl and C2).

Description

Method for quantitative evaluation of the colorimetric potential of a pigment composition and applications.

The present invention relates to a method for quantitative evaluation of the colorimetric potential of pigment combination (s), in particular for the production of tinting systems.

It is known that obtaining a color paint on a tinting machine is done in two stages. A first step consists in making a tinting base, that is to say a finished paint which has the desired appearance, texture, chemical class, etc. characteristics. A second step consists in mixing with this tinting base pigment pastes, sometimes called dyes, having a relatively high pigment concentration, so as to add the pigments necessary for achieving the desired shade.

In practice, and in order not to alter the quality of the paints, the quantity of pigment pastes which is added to the base to be tinted must be relatively small, namely of the order of 2 to 24% relative to the total quantity. paint expressed by weight or volume, depending on the bases and pigment pastes.

It follows that, in order to be able to offer a diverse choice of shades, the low rate of "authorized" pigment pastes obliges the manufacturer to pre-condition, for a single quality of paint, several bases to tint generally comprising a single pigment. in various quantity and quality. The manufacturer will then supply the distributor with several types of pigment pastes that the distributor will mix with the base to be tinted so as to obtain the paint color he wishes.

Tinting systems have recently been developed which make it possible to color paints of various qualities, conditioned, by using a tinting machine capable of producing, directly on the sales site, mixtures of colored components. More in particular, a tinting system on the sales site consists of a line of paint products, each of which consists of a set of tinting bases, in different packaging, capable of being tinted by adding pigment pastes. The tinting machine makes it possible to distribute, by generally volume impulses, sometimes by weight, the pigment pastes in the tinting base according to a given operating mode, called a recipe.

The manufacturer must therefore provide the distributor, for each quality of his product line, not only the bases to be tinted, calibrated in color and coloring strength, and the pigment pastes, calibrated in the same way, but also the recipes for the different colors referenced in the majority of color charts on the market, that is to say collections of color illustrations.

Currently, although the capacity of tinting machines can produce several hundreds of thousands of different colors, the capacities of manufacturers are limited to only a few thousand colors, rarely exceeding ten thousand.

Indeed, the paint manufacturer, owner of the tinting system, is required to provide, and therefore to design in the laboratory, the recipes describing the proportions of the components, in order to reproduce a maximum number of colors illustrated on the color charts on the market. Each recipe search requires, to date, human intervention which does not meet the demand quantitatively, nor to make the most of the capacity of the tinting machines. The result is a deep discrepancy between, on the one hand, the ever increasing needs of distributors and, on the other hand, the small number of color recipes available as a result of the processes currently used by manufacturers. At the same time, computerization and the increase in the number of usable machines have increased production capacity for colors. The present invention aims to overcome this lack of the prior art by providing a method for quantitative evaluation of the colorimetric potential of a pigment combination, so as to allow, subsequently, the production of a large number of recipes.

By "colorimetric potential" is meant all of the possible combinations in a given tinting system between the pigment pastes and the tinting bases. Generally, it is accepted, according to Grasman's law, that a color can be characterized by three values, for example clarity, tone and liveliness, which can be compared to three dimensions. As a result, each color can be represented by a point in a three-dimensional space, a point which will hereinafter be called "color point".

Thus, a pigment combination can be represented in a three-dimensional space by the "geometric location" defined by the set of color dots capable of being obtained from the pigments composing said combination. In the case of a combination of two pigments, this place will consist of a curve segment and, in the case of a combination of three pigments, this place will be a surface. In the most frequent case of a combination of at least four pigments, this place will consist of a volume.

On the basis of this principle, the present invention provides a method for quantitative evaluation of the colorimetric potential of a pigment combination made up of at least four pigments, including a black and a white, each of which is defined by three values and can thus be represented. , in a standard color space, by a point called "primary color point", said pigment combination being, for its part, represented by a volume delimited by faces which are contiguous, two by two, at the level of bounded edges, each, by a couple of primary color dots, said method comprising a step of splitting said volume, characterized in that said color space is determined by defining the geometric location of each pair of pigments, to unite said locations in a new geometric location and to voxellize said new geometric location, all of the resulting solid voxels corresponding to all of the shades capable of being obtained from said pigment combination, and each full voxel corresponding to a determined proportion between the pigments, by virtue of which said method is a method of quantitative evaluation of the colorimetric potential of the pigment composition.

It should be noted that, when the support intervenes in the color (inks, textiles, etc.), the white is generally provided by this support so that the pigment composition constituted "of at least four pigments among which a black and a white "may, in practice, include only three pigments.

It is certainly known to split three-dimensional representations of the shades which can be obtained from several basic pigments.

Thus, in patent EP 0 619 876, a process is described based on the representation of a set of colors and the division of the space thus represented, for the production of a color chart. However:

- the fractionated color space according to EP 0 619 876 is not composed in the same way as that concerned by the present invention, - the fractionation according to EP 0 619 876 results in sectors of rings (see FIG. 4) whereas , according to the invention, it results in cubes (voxellization),

- the result obtained is different.

More precisely, according to EP 0 619 876, in order to produce the color space which will be divided, the basic chromatic components, except black and white, are classified according to their increasing shade angle and, from this information, we draw a certain number of polygons of different surfaces, to keep only the components which form the external limits of the polygon of larger surface (Figure 1 of EP 0 619 876); from this larger polygon, volume sectors are obtained, one of which is shown in Figure 2 of EP 0 619 876.

In terms of actual fractionation, the fractionation of the volume sector in EP 0 619 876 in fact involves several successive fractionations, in particular a fractionation in isoluminance planes (one of which is represented in Figure 3 of EP 0 619 876) then a fractionation into ring sectors (Figure 4) of each fraction thus obtained. In the present invention, a single fractionation is carried out which determines voxels (cubes) full or lit, that is to say totally included in the geometrical place, and voxels which are either completely outside the geometrical place, either partially outside, and called empty or extinct voxels. Each full voxel corresponds to a shade and a recipe allowing the corresponding shade to be produced from the chromatic components underlying the definition of the geometric location. Each voxel therefore gives not only qualitative information, but also quantitative information.

EP 0 619 876 only gives qualitative information, however, since each ring sector corresponds to a shade of a color chart, without a corresponding recipe. As appears from what has been written above, the implementation of the process which is the subject of the present invention supposes that the geometrical location corresponding to the given pigment combination has been defined in a standardized color space. To do this, each of the at least four pigments of said pigment combination is represented in said color space by its own color point called "primary", all of these points defining a volume whose shape of edges and faces remains to be defined.

To define the edges, the method preferably used consists, after having previously evaluated the factors K and S of the basic pigments, to virtually produce, for each pair of pigments of said pigment combination, a set of mixtures by varying by one step regular the relative concentrations of each of the two pigments, to calculate the values of K and S of each of said mixtures, to deduce therefrom its spectral factor of reflection, to calculate the coordinates tristimulus X, Y and Z and to transpose them into units of said space normalized colorimetric in order to be able to represent each of said mixtures in the form of a point, called "secondary color point", in said standard colorimetric space, and to draw the segment of curve which joins together the primary color points of the pair of pigments concerned with, between them, all the secondary color points obtained, said curve segment constituting the sought edge.

The above-mentioned K and S values correspond respectively to the spectral absorption factor and the spectral diffusion factor. For K and S obeying the law of Kubel a Munk, we can substitute the Beer-Lambert coefficients when it comes to applications using tinctures.

In practice, it has been demonstrated that the preferred variation step for the production of the various mixtures is between approximately 1/16 and approximately 1/64, and is preferably 1/32.

Once the edges thus defined, it is necessary to define the various surfaces constituting the faces of the volume.

To do this, for each pair of edges having in common the primary color point of a given pigment, the pairs of secondary color points are used which, on each of said edges, correspond to pairs of combinations of pigments having the same concentration relative to this given pigment, and a set of mixtures is made for each of these couples in the same manner as described above.

Each mixture makes it possible to obtain a new point, known as a "tertiary color point", in said standardized color space.

We then trace the curve segments connecting each point of primary color, which constitutes a vertex of the face, with each of the secondary color points of the opposite edge, each of said curve segments passing through those of the tertiary color points which correspond at a relative concentration between said pairs of combinations of pigments equal to the relative concentration, at the level of the secondary color point bounding said curve segment, of the pair of pigments whose primary color points border said opposite edge.

Once the volume thus determined, it remains to voxellize it. The voxellization of a volume, or of a space, consists in expressing said volume, or space, in voxels, that is to say in a matrix of cubes which can be visualized in a three-dimensional image. This matrix makes it possible to define the parts included ("full" voxels) and not included ("empty" voxels) in this volume. The expression in voxels of a three-dimensional image is analogous to the expression in pixels of a two-dimensional image.

This description refers to a "standard color space". By "standard color space" is meant any three-dimensional space containing the shades of single pigment materials constituting a collection of basic chromatic components and all the shades achievable by mixing these materials. We can cite, for example, the CIExyY, CIELUV or CIELab spaces.

In a preferred embodiment, the method according to the invention uses the CIELab space as space normalized colorimetric and voxellise said geometrical place in cubes of side ΔE ₀ , where ÙE ΔE ₀ represents the unit of voxellization.

The CIELab space is defined by the ISO DIS 7724/3 standard, corresponding to the French standard NF X 08-

14. In practice, we prefer to use this space because it is the most homogeneous color space with visual perception.

In a preferred embodiment, the method which is the subject of the present invention consists in circumscribing said geometric location in a rectangular parallelepiped whose vertices are defined by the points (L * maximum, a * maximum), (L * maximum, b * maximum), (L * maximum, a * minimum), (L * maximum, b * minimum), (L * minimum, a * maximum), (L * minimum, b * maximum), (L * minimum, a * minimal), (L * minimal, b * minimal) of said geometrical location and to voxellize said rectangular parallelepiped so that the edges of the cubes are, as the case may be, parallel or perpendicular to the edges of said parallelepiped.

In practice, this step is carried out in two stages, namely the search for the maxima and minima for the set of points carried by the edges of the geometrical place in two of the three landmarks of the CIELab space, the landmarks a and b for example. . The coordinates a * minimum, a * maximum, b * minimum and b * maximum are thus determined which correspond to the vertices of the surface circumscribing the geometrical location in said plane ab.

In a second step, the projection of this surface is carried out according to the third dimension, namely the dimension L, which makes it possible to obtain the correspondence L * minimum and L * maximum of the points determined previously. We thus obtain the coordinates of the vertices of the parallelepiped circumscribing the geometrical place in space Lab.

According to another aspect, the present invention relates to the application of the method defined above for evaluation quantitative of the colorimetric potential to the quantitative evaluation of the colorimetric potential of a tinting system capable of achieving a plurality of pigment combinations, application which consists in defining the geometric places of each of said pigment combinations, in uniting said places in a new geometric place and to voxellise said new geometrical place, thus obtaining the voxellized volume of the system to be tinted, all the resulting solid voxels corresponding to all of the tints, to the dimension of the voxel close, capable of being produced by said tinting system .

The new geometrical place thus defined corresponds, in practice, to the union of the set of all the possible geometrical places between, on the one hand, the bases to tint of the play of bases and, on the other hand, the pigment pastes of the palette that equips the tinting machine.

According to a further aspect, the present invention also relates to the application of the method defined above of quantitative evaluation of the colorimetric potential to the determination of the number of shades separated by a given difference and achievable by a tinting system, which comprises the steps consisting of: α) implementation of the method for evaluating the colorimetric potential of the system to be tinted as described above, β) determining the desired color difference ΔE _X between two neighboring colors, and γ) determining the number of shades distant from this ΔE _X in the standardized space by applying the following formula:

N = L / [ΔE _O / ΔE ³

in which :

N is the number of shades sought, ΔE ₀ represents the voxellization unit L is the geometrical place voxellized in cubes of side ΔE ₀ , in the normalized space, and

ΔE _JL represents the desired difference between two neighboring colors in the standardized space. The fact of being able to evaluate a system to be tinted in number of shades at constant ΔE, allows a comparative analysis by simulation of the various possible adjustments prior to the determination of the parameters of the system. Such an evaluation brings great flexibility of use to the tinting system. Indeed, it is obvious that the quality needs will be different depending on whether the paints are intended for the general public, such as those sold in supermarkets, or whether they are for the professional environment. An individual will favor ease of use, without worrying about the amount of paint used, while a professional will want a paint with strong masking power so as to limit the cost as much as possible. In addition, facade paints do not require the production of bright or dark colors, and therefore will not include the corresponding tinting bases, unlike an interior paint.

The method which is the subject of the present invention makes it possible to take these elements into consideration by using the notion of generic system and of client system.

A generic system is a theoretical system in which all the parameters of the system are taken into account. These parameters are, for example, the nature of the palette of pigment pastes and tinting bases, the volume of the distribution pulse, the intended use for each product, their packaging, the desired opacifying character, etc.

A client system constitutes a practical system, forming a subset of the generic system, which takes up the specific parameters making it possible to obtain the applications or qualities desired by the client. The method according to the invention allows the identification of an optimal recipe corresponding to a given shade, in the sense that, according to one of its aspects, it consists, in a tinting system the quantitative evaluation of which has been made as indicated above, to link each full voxel to a recipe by geometrical place to which said voxel belongs and, in the case where, to the same voxel, correspond several possible recipes, to choose among these recipes the one which is best suited to the field of intended use for said shade.

The process which is the subject of the present invention, by authorizing the creation of a generic system, makes it possible to determine the fundamental parameters of the generic system before having definitively chosen the palette of pigment pastes, or designed the set of bases to be tinted.

It is therefore possible to define a real customer strategy, by studying, by simple simulation, the result of the different possible options, a study that was previously possible only through laboratory experiments and partial evaluations.

According to yet another aspect, the present invention also relates to the application of the method of quantitative evaluation of the colorimetric potential which is the subject of it to the evaluation of the feasibility of a shade by a given tinting system, application which consists to determine the coordinates of the color point corresponding to said color in the CIELab space and to check whether the voxel corresponding to said color point thus obtained is included in the voxellized volume of said system to be tinted.

The method of quantitative evaluation of the colorimetric potential of pigment combination (s) according to the invention can also be applied to the evaluation of the feasibility of a color chart by a given tinting system, and this in two different ways. , depending on the goal. According to a first embodiment, this evaluation of the feasibility of a given color chart by a given tinting system consists in determining the coordinates in the CIELab space of all the color points corresponding to each shade of said color chart, represent the volume corresponding to all the points thus obtained and to voxellize it, and compare the voxellized volume of the color chart with the voxellized volume of said system to be tinted, and, - if the voxellized volume of the color chart is included in the voxellized volume of the system to tint, to declare the feasibility rate equal to 1;

- if the voxellized volume of the color chart is not included in the voxellized volume of the system to be tinted, to determine the intersection between said two volumes, and i) if there is no intersection, to declare the rate of feasibility equal to 0, ii) if there is an intersection, to determine the volume resulting from the difference (voxellized volume of the color chart - voxellized volume of the system), and to declare the feasibility rate equal to (voxellized volume of the color chart - voxellized volume of the system) / voxellized volume of the color chart.

It will be understood that such a process is particularly suitable for a paint manufacturer, who will be interested in having a global vision of all the colors that can be produced from a given tinting system and in determining which supplements to bring to its ranges in order to fill the gaps.

According to a second embodiment, the evaluation of the feasibility of all or part of a color chart given by a given tinting system consists in separately determining the feasibility of each shade or certain shades of said color chart by the shade system of as described above. Compared to the previous process, it is more suited to the needs of distributors who have to determine the feasibility, using a given tinting system, particular shades from a color chart given according to customer requests.

In addition to determining the feasibility of one or more shades or of a color chart in its entirety, by a given tinting system, the method which is the subject of the present invention makes it possible to compare two tinting systems and / or two color charts with each other.

To do this, it plans to determine respectively the voxellized volumes of each of said tinting system and / or color charts, and to check whether one of said voxellized volumes is included in the other, in whole or in part. Such a method makes it possible to identify all the possible duplicates between the two voxellized volumes and, if necessary, to eliminate in one of the systems to tint and / or color chart the pigmentary combinations which make duplication.

Always with a concern for speed and maximum quality assurance, a process has been developed to assess the feasibility of a shade, or a color chart, by a tinting system starting directly from the shade or said color chart. .

There are several ways for both a manufacturer and a distributor to identify a given shade and match it. The first method, which is only qualitative, consists in searching in a color chart offered by a given paint manufacturer for the shade corresponding to that which one wishes to reproduce and, from the corresponding references, in mixing the raw materials supplied by the manufacturer in question and according to the recipe also supplied by him. However, this method of proceeding, although being the most used today, is not satisfactory because the color charts are not completely reliable. Indeed, the same color chart is printed in multiple copies, and this results for the same color differences depending on whether said color chart was printed at the beginning or at the end of a series, depending on the quality of the ink used, etc. In addition, this method does not take into account the wear and tear of the color chart, the colors of which change over time.

A second quantitative approach has been developed, which uses the transfer of color information on the shade to be matched, to the paint manufacturer so that he can study and communicate an appropriate recipe.

A first technique consists in measuring, directly on the sample, the X, Y and Z values characterizing it, using a spectrophotometer for example. This technique is not entirely satisfactory either because it is too dependent on the make and model of the measuring device used.

A second technique consists in transmitting, no longer measured values, but directly the hue via its X, Y and Z values. To do this, we use ICC profiles (International Color Consortium), which allow us to calibrate the 'all of the computer devices and peripherals used for such transmission. Such a calibration certainly makes it possible to reduce the dispersion factor between the original image and the transmitted image, but there remains nonetheless an uncontrollable difference between these two images.

The ICC profile refers to a constant illuminant of the device, or to the white balance which is substituted for it, and therefore does not take into account the drift, always possible over time, of the illuminant of the material, no more than it is not applicable to digital cameras or web cams, the illuminating image varying with each shot.

There is therefore, to date, no satisfactory process allowing the transmission of a shade, ensuring both maximum fidelity of the color transmitted and a quantitative evaluation of the dispersion generated by this transmission, in accordance with the procedures of the 'quality assurance. In yet another aspect, the method which is the subject of the present invention aims to remedy the color differences which may appear between the original colors and those obtained after a succession of representations by different devices or peripherals, and to do this, it plans to transmit said shade accompanied, at each stage, by a reference system of which the variations between all the stages of representation are known. Thus, it is possible to determine a correction factor for the same shade of the reference frame between each stage of the representation and to apply this same factor to the shade that one wishes to transfer.

More particularly, this process consists in:

- define a color chart in X, Y and Z values,

- digitally photograph, on the same photograph, on the one hand, the shade (s) that one wishes to transfer and, on the other hand, the color chart, - transfer the scanned data to a computer screen given,

- calculate the correction vector between the X, Y and Z values of the original charter and the X, Y and Z values of the screen representation of the said charter, from the RGB values known from digital photography and those of the computer screen determined using the ICC profile of said screen,

- apply this correction vector to the X, Y and Z values of the transferred shade (s), and - evaluate, by quantifying the average correction vector, the dispersion generated by the transmission.

The correction vector is evaluated from the so-called mathematical definition of the morphisms of vector spaces with convergent sequence (evs), or by any other algorithm applicable to the topology of spaces. In this the process according to the invention is different and more specifies that the one using the ICC profiles for the transfer of a color between various computer peripherals, and becomes compulsory when said peripheral is of the digital camera type because, in this case, not only the nature of the illuminant is unknown, but also the tristimulus X, Y and Z values of the primary colors of the device, required for the determination of the ICC profile, are also indeterminable. By "color chart", it is necessary to understand a set of colors of which we know, for a reference illuminant, the values X, Y and Z of each of the colors, thus making it possible to correct them from one device to another, and to extrapolate these corrections to all the colors for the reference illuminant. This charter can be assimilated to a working standard within the meaning of quality assurance standards.

Such a method makes it possible to envisage applications in networks, in which a shade, or simply a colored object, is photographed digitally with a color chart, all the data is then transferred, in any manner whatsoever, to a display screen, and due to the presence of a reproduction of the known color chart, it is easy to determine the X, Y and Z coordinates of the tint, or of the color of the object, which one wish to match.

According to yet another embodiment, the method which is the subject of the present invention comprises the additional steps which consist in: transposing said values X, Y and Z into units L, a and b,

- assess the feasibility of said or said tint (s) by a given tinting system as indicated above, and - quantify the dispersion generated by the transmission. In practice, the present invention contemplates carrying out the steps described above by computer means, such as software sold on CD-ROM or floppy disk (s), or downloaded or distributed. Thanks to all the arrangements proposed by the invention, it becomes possible to obtain a colored product, in accordance with quality assurance procedures, of the color characterizing a sample, by transferring the information over a remote network, in particular for the supply of color paints using a tinting system, according to the following steps:

1 - identification of the color of the sample in the customer's color identification system,

2 - export of information identifying the color of the sample from the client station to the supplier station,

3 - receipt of customer information by the supplier station,

4 - transfer of customer information to the supplier's color identification system, under the conditions set out above for simultaneous transmission of a digital photograph of the sample and a color chart with correction of errors related to the photography and transmission, and quantification of these errors,

5 - study of the feasibility of color in the requested product,

6 - if the color is feasible, definition of the means necessary to achieve the color in the product requested, this, as indicated above,

then

6a - production, 6b - remote color submission to the customer for acceptance by implementing steps 1 to 4 above but from the supplier to the customer, 6c - shipment of the product to the customer, and

6d - dispatch of information for the realization of the product by the customer.

Thus, maximum quality assurance is guaranteed. Compared to a conventional measurement of color, the method according to the invention, using an identification of colors from a digital photograph, has the advantage of allowing in particular:

- visualization of color in the environment;

- simultaneous measurement of several colors;

- the measurement of colors on a non-flat substrate;

- taking into account interference reflections (for example, pearly objects) from a number of angles greater than 3;

- measurement of metallic reflection;

- the quantification of visual phenomena other than color, for example the surface texture, the "haze" (veil), surface defects (orange peel), etc. ; - the archiving of the shot used for the control.

A particular and quite interesting application of the method resides in the recovery of a database established with a given device for use in another device, without it being necessary to take photographs. To date, such recovery is impossible.

According to the invention, this retrieval comprises the following steps: - the establishment of a measurement file of a repository for each of a first and a second device, thus obtaining a first and a second repository, in X , Y, Z;

- with the ICC profile of any monitor serving as a reference, the transfer in RGB of the first and second frames of reference; - the establishment of a correction matrix from the first benchmark to the second;

- the correction, using said matrix, of the RGB matches of the data in the database to be transferred from the first device to the second device;

- with the ICC profile of the reference monitor, the transfer in X, Y, Z of the RGB data thus corrected, so that the second device can use the database of the first device without altering the result. We can therefore consider that the proposed process allows color control in artificial vision.

The invention will be better understood, and its advantages will emerge more clearly, in the light of the following detailed description made with reference to the appended drawings in which:

- Figure 1 is a representation, in the CIELab space, of the geometrical location of the possible pigment compositions between four pigments;

- Figure 2 illustrates the voxellization of the geometric location of Figure 1;

- Figure 3 is an explanatory diagram of the definition of the shape of the edges and faces of the geometric location of Figure 1; and

- Figure 4 illustrates the schematic diagram of color measurement using a digital camera.

Figures 1 and 2 illustrate the quantitative evaluation of the colorimetric potential of a pigment combination made up of four pigments, a white W, a black B and two chromatic Cl and C2. These four pigments are represented respectively by the four primary color points W, B, Cl and C2 in the CIELab space, and thus constitute a volume which will be called VCQP (for Volume of a Pigmentary Quaternary Combination). One face, W Cl C2, of said VCQP is shown diagrammatically in FIG. 3 in the form of a triangle flat for simplicity, and for clarity of design and presentation.

To determine the edges of the VCQP, a set of mixtures is made virtually, for each pair of pigments, W and Cl for example, by varying the respective concentrations of each of the two pigments with a regular step, for example 1/32 , each mixture thus obtained can be represented by a point, called "secondary color point" (W, Cl) S _1/32 , (W, Cl) S _{1 / 1S} , (, Cl) S _3/32 , etc. , in said standard color space.

In practice, each variation of 1/32 of the respective concentrations of each of the two pigments makes it possible to determine, respectively according to the laws of Kubelka Munk and Beer-Lambert, the values of the coefficients K and S in the case of pigments (paints , inks, plastics, cosmetics, etc.) and the value of the transmission coefficient in the case of textile dyes. From these coefficients, the spectral reflection factor is calculated and then, from this, the X, Y and Z coordinates corresponding to each mixture. It only remains to transpose said coordinates X, Y and Z into coordinates L, a and b.

The edge, C1 is the curve segment which joins the different secondary color points (W, Cl) Si _{/ 32} , (W, Cl) S _1/16 , (W, Cl) S ₃₃₂ , etc. thus obtained at the primary color points W and Cl. The same operation is then repeated for the other edges, namely, C2, B, C2, B, C1 and C2, C1.

The next step is to define the areas delimited by said edges.

To do this, for each pair of edges, for example, C1 and W, C2, having in common the primary color point of a given pigment, here W, the pairs of secondary color points are used, for example ( W, Cl) S _1/32 and (W, C2) S _1/32 which, on each of said edges, correspond to pairs of combinations of pigments having the same relative concentration of this given pigment W and we realize, for each of these couples, a set of mixtures in the same manner as described above.

Each mixture makes it possible to obtain a new point, known as a "tertiary color point" identified by:

[(W, Cl) S _1/32 , (, C2) S _1/32 ] T _1/32 ,

[(W, Cl) S _1/32 , (W, C2) S _1/32 JT _{1 / 1S} ,

[(W, Cl) S _1/32 , (; C2) S _1/32 ] T _3/32 , etc. in said standard color space.

We then draw the curve segments connecting each point of primary color, which constitutes a vertex of the face, with each of the secondary color points of the opposite edge, such as W connected to (Cl, C2) S _1/32 , (Cl, C2) _{S1 16} (Cl, C2) S _{3 32,} etc., each of said curve segments, for example, N- (Cl, C2) S _{_1/22 l} through those color dots tertiary, namely:

[(W, Cl) S _1/32 , (W, C2) S _1/32 ] T _1/32 ,

[(W, C1) S _1/16 , (, C2) S _{1 / ls} ] T _1/32 ,

[(W, Cl) S _3/32 _/ (W, C2) S _3/32 ] T _1/32 , etc. which correspond to a relative concentration, here 1/32 between said pairs of combinations of pigments equal to the relative concentration, at the level of the secondary color point (C2, Cl) S _1/32 bounding said segment of curve, of the pair of pigments C2 and Cl, the primary colored points of which border said opposite edge.

Once these steps have been completed, it is possible to represent said VCQP in the CIELab space (Figure 1). The next step is to determine the maximum and minimum values for all the points carried by the edges on the axes L, a and b of the CIELab space. The values (L * maximum, a * maximum), (L * maximum, b * maximum),

(L * maximum, a * minimum), (L * maximum, b * minimum), (L * minimum, a * maximum), (L * minimum, b * maximum), (L * minimum, a * minimum), (L * minimum, b * minimum) correspond respectively to the eight vertices of a rectangular parallelepiped P circumscribing the VCQP (Figure 2). Voxellization consists in fragmenting said parallelepiped P into cubes of ΔE tolerated from the side. As can be seen in Figure 2, some voxels, such as VI, are entirely outside of the VCQP and are said to be "empty" or "extinct". Others, such as V2, are fully contained in the VCQP and are said to be "full" or "on". Finally, others, such as V3, are partially outside the VCQP: depending on the operator's choice, for using the VCQP by default or by excess, they may be considered, respectively, as "empty" or as "full". It is the set of full voxels which represents the colorimetric potential of the pigment combination considered and each full voxel has known coordinates in the CIELab space. We understand that, knowing the coordinates in the CIELab space of a particular shade, it will be possible to determine whether these coordinates correspond to a full voxel in the voxellized volume of a given pigment combination, or more probably in a union of volumes voxellized (pigment combinations capable of being made using a given tinting system). If so, the coordinates of said voxel will directly identify the pigments to be used and their relative proportions. If not, it will be immediately clear that the desired shade cannot be obtained by means of this tinting system.

It is also understood that the invention makes it possible to detect and eliminate duplicates within the same tinting system or between two different tinting systems.

Although we have mainly referred above to a combination of quaternary type pigments and to the CIELab space, it is understood that the invention is not limited to one or the other. . In terms of the number of pigments, voxellization which supposes a volume simply requires at least four pigments. As for the reference space, other possibilities than the CIELab space were mentioned above.

In addition, the application of the process which is the subject of the present invention can be envisaged in fields other than that of paint, such as for example those of cosmetics, inks, plastics or else that of textiles.

If we come to Figure 4, box 1 represents a monitor, box 2 a measurement step performed by means of an emission spectrophotometer, belonging to a "profiler" system, on the empty screen of said monitor and box 3 the ICC profile data of the monitor as provided by said profiler. A profiler is a set of devices used to calibrate monitors and output the ICC profile. Box 4 represents a color reference frame, box 5 a measurement step performed on each color of said reference frame, using a reflection spectrophotometer and box 6 the color data of the reference frame in X, Y, Z specific to the device used to measure the colors of the reference frame.

By implementing the method according to the invention (box 7), from the data of the ICC profile of the monitor (box 3) and color data from the repository X, Y, Z (box 6), one obtains (box 8) the color data of the frame of reference in R, G, B of the monitor 1, the frame of reference as displayed on the monitor 1 from the data measured and processed by the method according to the invention, appearing in box 9.

In addition, a digital photograph, side by side, of the product presenting the shade (s) to be matched and of the color reference frame is made (box 10). The resulting photograph is displayed (box 10) on the monitor 1 and, from the photo of the reference system as displayed (box 12) and of the reference system displayed on the monitor 1 from the measurements processed (box 9), a matrix of correction vectors is calculated (one per color). The resulting matrix (box 14) makes it possible to correct (box 15) the colors of the photo as displayed on the screen (11), thus obtaining a corrected photo (box 16). From the ICC profile data of the monitor 1 and the corrected photo 16, using the method 7, the X, Y, Z restitution of all the colors of the photo is obtained (box 17).

Claims

1. Method for evaluating a pigment combination consisting of at least four pigments, one of which is black (B) and one white (W), each of which is defined by three values and can thus be represented, in a standard color space , by a point called "primary color point" (W, B, Cl and C2), said pigment combination being, for its part, represented by a volume

(VCQP) delimited by faces which are contiguous, two by two, at the level of bounded edges, each one, by a pair of primary color points, said method comprising a step of fractioning said volume, characterized in that said color space is determined by defining the geometrical place of each pair of pigments, to unite said places in a new geometrical place and to voxellize said new geometrical place, the set of resulting solid voxels corresponding to the set of colors capable of being obtained from of said pigment combination, and each full voxel (V2) corresponding to a determined proportion between the pigments, whereby said method is a method for quantitative evaluation of the colorimetric potential of the pigment composition.

2. Method according to claim 1, characterized in that it uses the CIELab space as normalized and voxellized color space said geometrical place in cubes of side ΔE ₀ , where Ù ΔE ₀ represents the unit of voxellization.

3. Method according to claim 1 or 2, characterized in that it consists in circumscribing said geometric location in a rectangular parallelepiped (P) whose vertices are defined by the points (L * maximum, a * maximum), (L * maximum, b * maximum), (L * maximum, a * minimum), (L * maximum, b * minimum), (L * minimum, a * maximum), (L * minimum, b * maximum), (L * minimal, a * minimal), (L * minimal, b * minimal) of said geometrical location and to voxellise said rectangular parallelepiped (P) so that the edges of the cubes are, as the case may be, parallel or perpendicular to the edges of said parallelepiped.

4. Method according to any one of the preceding claims, applied to the quantitative evaluation of the colorimetric potential of a tinting system capable of producing a plurality of pigment combinations, characterized in that it consists in defining the geometrical locations of each. said pigment combinations, to unite said places in a new geometrical place and to voxellize said new geometrical place, thus obtaining the voxellized volume of the system to be tinted, the set of resulting solid voxels corresponding to the set of tints, to the dimension of the voxel close, capable of being produced by said tinting system.

5. Method according to any one of claims 1 to 3, applied to the identification of an optimal recipe corresponding to a given shade, in a tinting system whose quantitative evaluation has been made according to claim 4, characterized in what it consists, moreover, in linking each full voxel to a recipe by geometrical place to which said voxel belongs and, in the case where, to the same voxel, correspond several possible recipes, to choose among these recipes the one which is the better suited to the field of use provided for said shade.

6. Method according to any one of claims 1 to 3, applied to the determination of the number of shades separated by a given difference and achievable by a tinting system, the evaluation of the colorimetric potential of which was carried out according to claim 4, as step ce, characterized in that it comprises the steps consisting of: β) determining the desired color difference ΔE _X between two neighboring shades, and γ) determining the number of shades distant from this ΔE- _L in l standard space by applying the following formula:

N = L / [ΔE _Q / ΔE ³ in which :

N is the number of shades sought, ΔE ₀ represents the voxellization unit, L is the geometrical place voxellized in cubes of

ΔE ₀ aside, in the standardized space, and

ΔE- _L represents the desired difference between two neighboring colors in the standardized space.

7. Method according to any one of claims 1 to 3, applied to the evaluation of the feasibility of a tint by a tinting system whose volume has been voxellized according to claim 4, characterized in that it consists, in addition, to determine the coordinates of the color point corresponding to said color in the CIELab space and to check whether the voxel corresponding to said color point thus obtained is included in the voxellized volume of said system to be tinted.

8. Method according to any one of claims 1 to 3, applied to the evaluation of the feasibility of a given color chart by a given tinting system whose volume has been voxellized according to claim 4, characterized in that further consists in determining the coordinates in the CIELab space of the set of color points corresponding to each shade of said color chart, in representing the volume corresponding to all the points thus obtained and in voxellizing it, and in comparing the voxellized volume of the color chart with the voxellized volume of said tinting system, and

- if the voxellized volume of the color chart is included in the voxellized volume of the tinting system, to declare the feasibility rate equal to 1;

- if the voxellized volume of the color chart is not included in the voxellized volume of the system to be tinted, to determine the intersection between said two volumes, and i) if there is no intersection, to declare the rate of feasibility equal to 0, ii) if there is an intersection, to determine the volume resulting from the difference (volume voxellized from the color chart - voxellized volume of the system), and to declare the feasibility rate equal to (voxellized volume from the color chart - voxellized volume from the system) / voxellized volume from the color chart.

9. Method according to claim 7 applied to the evaluation of the feasibility of all or part of a color chart given by a given tinting system, characterized in that it consists in separately determining the feasibility of each shade or of certain shades said color chart by the system to be tinted.

10. Method for transferring a color, or a set of colors, of a given computer hardware to one or more given computer peripheral (s), characterized in that it comprises the steps which consist of

- define a color chart in X, Y and z values,

- photograph digitally, on the same photograph, on the one hand, the shade (s) that one wishes to transfer and, on the other hand, the color chart,

- transfer the digitized data to a given computer screen, - calculate the correction vector between the X, Y and Z values of the original charter and the X, Y and Z values of the screen representation of the said charter, from the known RGB values of digital photography and those of the computer screen determined using the ICC profile of said screen,

- apply this correction vector to the X, Y and Z values of the transferred shade (s), and

- evaluate, by quantifying the average correction vector, the dispersion generated by the transmission.

11. Method according to claim 10, characterized in that it comprises the additional steps which consist in: - transpose said values X, Y and Z into units L, a and b, and

- Evaluate the feasibility of said shade (s) by a given tinting system using any of claims 7 to 9, and

- quantify the dispersion generated by the transmission.

12 - The method of claim 10 or 11, applied to obtaining a colored product, in accordance with the quality assurance procedures for the color characterizing a sample, by transferring information over a remote network, in particular for the supply of color paints using a tinting system, characterized in that it comprises the following stages:

2 - export of information identifying the color of the sample from the customer station to the supplier station, 3 - reception of information from the customer by the supplier station,

4 - transfer of customer information to the supplier's color identification system, with simultaneous transmission of a digital photograph of the sample and a color chart with correction of errors related to photography and transmission, and quantification of these errors,

5 - study of the feasibility of color in the requested product,

6 - if the color is feasible, definition of the means necessary to achieve the color in the product requested,

then

6a - production, 6b - remote color submission to the customer for acceptance by implementing steps 1 to 4 above but from the supplier to the customer,

6c - shipment of the product to the client, and 6d - shipment of information for the completion of the product by the client.

13 - Method according to claim 10, applied to the recovery of a database established with a given device for use in another device, characterized in that it comprises the following steps:

- the establishment of a measurement file of a reference frame for each of a first and a second device, thus obtaining a first and a second reference frame, in X, Y, Z; - with the ICC profile of any monitor serving as a reference, the transfer of the first and second repositories to RGB;

- the establishment of a correction matrix from the first benchmark to the second; - the correction, using said matrix, of the RGB matches of the data in the database to be transferred from the first device to the second device;

- with the ICC profile of the reference monitor, the transfer in X, Y, Z of the RGB data thus corrected, so that the second device can use the database of the first device without altering the result.