EP1812887A2

EP1812887A2 - Computer-implemented color adjustment method and program using multi-dimensional vector analysis

Info

Publication number: EP1812887A2
Application number: EP05815553A
Authority: EP
Inventors: Allan Blase Joseph Rodrigues
Original assignee: EI Du Pont de Nemours and Co
Current assignee: EIDP Inc
Priority date: 2004-11-05
Filing date: 2005-11-01
Publication date: 2007-08-01
Also published as: WO2006052561A2; WO2006052561A3; JP2008519296A

Abstract

An improved computer-implemented method and program for guiding a refinisher user through adjustment(s) of a candidate refinishing paint color (26B) formed from a combination of a predetermined number of tints T1 through Tn toward a predetermined target paint color (26A) includes the steps of displaying a rendering of a predetermined target paint color and an initial candidate refinishing paint color in respective first and second fields of a video monitor. The predetermined target paint color corresponds to a predetermined target color point in a multidimensional color space. Each tint is also displayed in a respective field of the video monitor. In response to the selection of one of the tints, vector analysis (10) is used to calculate an updated candidate color point in the multidimensional color space.

Description

TITLE

COMPUTER-IMPLEMENTED COLOR ADJUSTMENT METHOD AND PROGRAM USING MULTI-DIMENSIONAL VECTOR ANALYSIS

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority Under 35 U. S. C. §119 from U.S. Provisional Application Serial No. 60/625,559, filed November 5, 2004.

BACKGROUND OF THE INVENTION Field of the Invention This invention relates to an improved computer-implemented method and program for guiding a refinisher through one or more adjustment(s) of a candidate refinishing paint color toward the goal of a color match with a predetermined target paint color.

Description of Related Art When a vehicle body (e.g., an automobile or a truck) is repaired the repair area must be repainted. For a completed repair to be acceptable the color of the repaired area must match that of the rest of the vehicle so that the repaired area is not distinguishable. Obtaining such a color match is often difficult to accomplish since paint color can vary from one vehicle to the next or even on different portions of the same vehicle. To obtain an acceptable match the refinisher must adjust the color of the refinishing paint by adding small amounts of colored tints. Tints used for color adjustment are generally limited to the same tints that are combined to form the paint being refinished. Commonly, four to eight, or more, tints are used to form the original color. Due to the complexity of such tint combinations a refinisher cannot always visualize the color change caused by addition of a specific tint.

To assist refinishers in the art of color matching "shading tips" can be useful. "Shading tips" describe to a refinisher how a given tint affects the color of a refinish paint. For example, when a white tint is added to a blue paint it is intuitive that the blue color will become lighter. However, the resulting blue may also be more or less saturated ("deeper" or "less " upon" the pigments and the ratio of those pigments used in the blue paint.

Verbal descriptions explaining the effect the addition of a given tint will have on the resulting paint color are often ineffective because the color effects being explained are difficult to visualize. A computer-based training tool has been developed and is available to help a refinish trainee visualize the effect of adding specific tints to a paint.

The tool includes a video monitor that displays in a first field a rendering of a predetermined target paint color. A second field of the video monitor displays a rendering of an initial candidate refinishing paint color formed from a combination of a predetermined quantity of each of a predetermined number of tints Ti through T_n. Also displayed in respective fields of the video monitor is each of the tints T-i through T_n. In response to the selection by a trainee of one of the tints the display in the second field is modified to display a rendering of an updated candidate refinishing paint. The set of updated candidate colors is produced by qualitatively determining the effect on the initial candidate color produced by the addition of each component tint. The updated colors are selected from a color palette offered in a commercially available software package, such as the program Microsoft^® PowerPoint^®, sold by Microsoft Corporation. Once heuristically determined the updated candidate colors are stored and displayed when one of the tints is selected by the trainee.

As a further aid to refinishers, the assignee of the present invention has developed a computer-based system known as Chromavision^® color retrieval and management system. With this system a spectrophotometer or a colorimeter is used to measure the color of the car being painted. A database holding the spectral reflectance curves or the CIE L*a*b* system coordinates of all car colors and the paint formula associated therewith is searched. The stored color most closely matching the measured color is selected as an initial candidate color. 'Cd'mMerdiaif re^'firϊisfi color matching tools vary in functionality. Some may test the closest color for other properties such as metamerism and only provide matches low in metamerism. Some may determine whether the color found in the database is an acceptably close match and, if necessary, adjust the formula to improve the match. Once an acceptable color match is determined the computer program provides the refinisher its formula. The refinisher produces a paint in accordance with this formula and proceeds to spray the vehicle. An effective program of the type eliminates the need for further color adjustment by the refinisher. Although the system of this type eliminates the need for further color adjustment by the refinisher, the costs associated with the spectrophotometer or a colorimeter make it expensive to implement. Accordingly it is believed desirable to provide a less expensive computer-implemented tool that can be used for color matching by a refinisher to match a target color.

It is also believed advantageous to provide a computer- implemented tool that permits a refinisher to visually observe color movements of a candidate to toward a target color. This instills confidence in the accuracy of the color adjustment, permitting the refinishing job to be performed more efficiently.

SUMMARY OF THE INVENTION

This invention is directed to an improved computer-implemented method and program for guiding a refinisher user through adjustment(s) of a candidate refinishing paint color formed from a combination of a predetermined number of tints Ti through T_n toward a predetermined target paint color.

In accordance with the improved method and program of the present invention a rendering of the predetermined target paint color and a rendering of an initial candidate refinishing paint color are displayed in respective first and second fields of a video monitor. The predetermined target paint color corresponds to a predetermined target color point in a ^' Ihri'ύMirhMiSlόnSi'dδlOϊ space while the initial candidate refinishing paint color corresponds to an initial candidate color point in the multidimensional color space. The initial candidate refinishing paint color is formed from a combination of a predetermined quantity of each of a predetermined number of tints Ti through T_n. Each tint is also displayed in a respective field of the video monitor.

The method and program are improved in that, in response to the selection of one of the tints, vector analysis in the multidimensional color space is used to calculate an updated candidate color point in the multidimensional color space. The updated candidate color point is defined by the addition of a color adjustment vector to a vector terminating at the position of the candidate color in the multidimensional color space. The color adjustment vector represents the magnitude and direction a predetermined adjustment quantity of the selected tint moves the candidate color point in the multidimensional color space. Upon selection of a tint a rendering of an updated candidate refinishing paint color corresponding to the updated candidate color point in the multidimensional color space is displayed in the second field. In a more detailed implementation the magnitude of the movement effected by an adjustment quantity of a selected tint may vary.

In the case of color matching with solid (non-flake-containing) colors the multidimensional color space contains three dimensions. However, the present invention may be utilized for color matching metallic or pearlescent hues, in which event a vector space of nine or fifteen dimensions is utilized.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be more fully understood from the following detailed description, taken in connection with the accompanying drawings, which form a part of this application and in which: Figures 1A through 1C are a series of stylized vector representations in a three-dimensional color space illustrating the Vθlaflonsf1ip^'"drri^'(5^'nø a¹" vector representing a predetermined target paint color, a vector representing an initial candidate paint color, and adjustments of the candidate paint color toward the target color using adjustment vectors based upon the tint colors constituting the initial candidate paint color; and

Figure 2 is stylized diagram of a computing system for executing a program implementing the method of the present invention, the stylized diagram including a representation of a screen display generated by a computer when implementing the method and program of the present invention.

DETAILED DESCRIPTION OF THE INVENTION Throughout the following detailed description similar reference numerals refer to similar elements in all figures of the drawings. The present invention is directed to an improved computer- implemented method and program for guiding a refinisher through one or more adjustment(s) of a candidate refinishing paint color toward the goal of a color match with a predetermined target paint color. The predetermined target paint color is that color that the refinisher is desirous of reproducing on a given vehicle under repair. The candidate refinishing paint color is formed from a combination of a predetermined number of tints Ti through T_n. Each tint is typically comprised of a single pigment colorant adjusted with one or more selected pigments, resins, solvents or dispersions. The present invention may be used for color matching metallic or pearlescent hues. The principles underlying the present invention may be understood from Figures 1A through 1C. These figures are a series of stylized vector representations in a three-dimensional color space illustrating the relationship between a predetermined target paint color and an initial candidate refinishing paint color, as adjusted by adjustment vectors based upon the tints constituting the initial candidate paint color. "TVdjα'stmeWVSctbrs as herein discussed are discussed in U.S. Patent 3,690,771 (Armstrong et al.), assigned to the assignee of the present invention. Vector representations of color are discussed in Allen, "Basic Equations used in computer color matching", Journal of the Optical Society of America, Volume 56, Number 9, Pages 1256-1259 (September 1966); Allen, "Basic Equations used in computer color matching, II. Tristimulus match, two constant theory", Journal of the Optical Society of America, Volume 64, Number 7, Pages 991-993 (July 1974).

Color can be specified by a three-coordinate system. In A. H. Munsell, "A Color Notation" First Edition, Munsell Color Company (1905) [now Munsell Color GretagMacbeth, 617 Little Britain Road, New Winddsor, New York] the terms "Value" (Lightness), "Hue", and "Chroma" (Saturation) are used for the three attributes of color. A common system used today is the CIE L*a^*b* system because the values for L*, a*, b* can mathematically be calculated from a spectral measurement of the color. A complete description of the CIE L*a*b* system is found in CIE Publication 15.2 or in various standards collections such as ASTM E-308.

In the CIE L*a*b* system, L* quantifies lightness, having values from 0 for the darkest black to 100 for the lightest white. The a* value is negative for greens and positive for reds; the b* value is negative for blues and positive for yellows. These values of L^*, a*, b* can also be mathematically converted to lightness, hue and chroma.

The representation of a particular color in the CIE L*a*b* system can be envisioned as a point in the three-dimensional color space. If the L*, a*, b* values are known for two colors, the differences between each of these values, designated ΔL*, Δa*, Δb* quantify the color difference between those two colors.

When a known amount of a colorant is added to a color, the color shift, ΔL*, Δa*, Δb* caused by that addition is a color vector. It indicates the magnitude and direction of color movement caused by that amount of that colorant in that color. Over short distances in color space [typically lesslfϊarTfive (5.O)^' units in each of the L^*, a^*, b* directions], these vectors are reasonably linear; i.e., twice the colorant addition will cause twice the movement in color space.

Solid colors, those that appear the same at all angles of illumination and view, can be described well by a single value for each of the three coordinates, L*, a*, b*. Gonioapparent colors, those that change color depending on angle of illumination and view, can be created by addition of metallic or pearlescent flakes. These colors must be described by L*, a*, b* determined at more than one angle. It has been shown [e.g., U.S. Patent 4,479,718 (Alman); Rodrigues, Die Farbe, Band 37, 65-78, 1990] that three angles of measurement are sufficient to characterize commercial gonioapparent colors. Such angles are one very near the angle of specular (surface) reflectance, called the near-specular angle, one far from the specular direction, called the flop angle, and one in between called the face angle (ASTM E 284). Thus, gonioapparent colors require an L*, a*, b* value at each of these three angles.

Addition of a colorant to a gonioapparent color can cause a very different color movement at each of these angles. For example, addition of more aluminum flake to a gray metallic color typically causes a large movement in L* at the near specular angle, less at the face angle and still less at the flop angle. Addition of white to a light silver metallic causes the lightness to increase at the flop angle but decrease at the near specular angle and may have a small movement in either direction at the face angle. Thus, color vectors must be determined at each of these three angles, which result in nine dimensions.

Current commercial spectrophotometers allow up to five (5) angle measurements that result, analogously, in fifteen (15) dimensions. There is no theoretical limit to the number of possible dimensions used. The teachings of the present invention are applicable to any multi-dimensional color space. AS' seen'trorriTigure 1A the initial candidate refinishing paint color is represented by point I in the multidimensional color space. The initial candidate color point I has coordinates I_L*, l_a*, and l_b*. The initial candidate refinishing paint color I is formed by combining predetermined quantities of each of a predetermined number of constituent tints Ti through T_n. The color positions of two such tints, i.e., Ti (a yellow tint) and T2 (a blue tint), are illustrated in Figure 1 A. Because it is formed on a combination of a yellow and a blue tint the initial candidate refinishing paint color I point lies in the -a or green direction. Figure 1 B illustrates the target paint color P along with the initial candidate color I. The target refinishing paint color P may be envisioned as the color having the coordinates PL*, P₃*, and Pb* in the multidimensional color space.

The lines drawn provide an aid to visualization of these points in three-dimensional space. However, the line from the origin to the point representing the initial candidate paint color may also represent a vector terminating at the position of the candidate color.

The distance D in the multidimensional color space between the initial candidate color point I and the target color point P manifests itself to a viewer as a perceived difference in color between the candidate color and the target color. The goal for a refinisher is to minimize this perceived difference. This is envisioned in the multidimensional color space as minimizing the distance D. The present invention provides a computer- implemented method and program for assisting the refinisher to accomplish this goal.

Modifications to the initial candidate color I may be envisioned as change(s) in the location in the color space of the candidate color point I from its initial location to an updated candidate color point U. The location of the updated candidate color point is envisioned as the change in location in the multidimensional color space produced by the addition of an aliquot ^'(r.e:"ra"πri^'ea'surecl amount), or predetermined adjustment quantity, of one of the tints forming the initial candidate color.

As seen from Figure 1C adding an adjustment quantity of the constituent blue tint T₂ to the initial candidate color relocates the candidate color to an updated candidate color point U₁ having coordinates (U_1L*, Ui_a*, U-ib*) in the multidimensional color space. Adding an adjustment quantity of tint T₂ moves the color position in the direction indicated by the vector from point I to U-|. The arrow from point I to Ui is the color vector that represents that change. The updated color point U₁ represents a color rendition that appears closer in color to the desired target color.

Since the color adjustment by adding a predetermined adjustment quantity of tint T₂ to the initial candidate color results in an updated color position Ui which is closer to the target color position P the addition of the adjustment quantity of the constituent blue tint has the desired beneficial effect. The color corresponding to the updated candidate color point U₁ would be perceived by a viewer as closer to the color represented by the target color point P. A second color adjustment attained by adding a second aliquot of tint T₂ would then achieve the desired color match. Note that not all such color adjustments would result in bringing the candidate color closer to the target color. For example, the addition of tint Ti would move the color toward the yellow region and thus farther away from the target point P.

It may be appreciated that by iteratively adjusting the position of the updated candidate color point(s) through the addition of adjustment quantities of the same or different adjustment tints the candidate color may be brought as close as possible to the target color. One skilled in the art would understand that limits of visual acuity allow some deviation from an exact match. (The refinish painter's skill can also be utilized to "blend" the sprayout, gradually decreasing the thickness of the repair paint from the fe^'βai'r'afe'a to the'"utirepaired area of the vehicle. This allows for a gradual change in color that is not perceived by the eye.)

Both white and black tints should normally be made available to ensure the ability to attain the correct lightness for an appropriate color match. Note that white and black tints also have some effect in the "a^*" and "b*" directions which is taken into account by their corresponding vectors. For example, if the initial paint has the same hue as the target paint but is more saturated a combination of black and white will decrease saturation, while not appreciably affecting the hue. The ratio of black to white would depend on whether the initial paint color is also lighter or darker than the target color.

The present invention provides a visualization tool that assists the refinisher in determining the best color match between the candidate color and the target color while minimizing a trial and error approach of mixing paints which is time consuming and expensive. In most cases it is desirable to also include the capability of adding a black and/or a white tint to adjust for changes in lightness and darkness that can occur on addition of a tint.

With reference to Figure 2 shown is a stylized block diagram of a computing system generally indicated by the reference character 10 for executing a program implementing the method of the present invention. The computing system 10 may be configured using any standard microprocessor-based computing system having a memory 12 that communicates over a bus 14 with a central processing unit (CPU) 16. The memory 12 is partitioned into a data memory 12D, a program memory (not expressly illustrated), and a tabulation register 12T. The program memory stores the program instructions that cause the computing system 10 to implement the method of the present invention. The program instructions may be encoded onto and carried by any suitable computer readable medium, such as a magnetic or optical disk, semiconductor memory or tape. Tfte^efinϊs'h'eFus^'W'may provide inputs to the system 10 using any input device, such as a mouse 18 and/or keyboard 20 over a line 21. Output from the system 10 to a refinisher-user is provided through a color video monitor generally indicated by the reference character 22. The video monitor 22 includes a cathode ray gun 24 that directs three beams of electrons (for each of the primary colors red, green and blue) toward an array of locations lining the inner surface of the screen 26 of the video monitor 22. As is well understood by those skilled in the art each location has a set of phosphor dots excitable by the incidence of electrons thereon to produce visible light corresponding to the colors red (R), green (G) and blue (B). Appropriate video signals driving the electron gun 24 to excite the appropriate phosphor dots at each given location are provided by a color video driver 28 operating under the control of the CPU 16. The data memory stores information regarding the color coordinates of a predetermined target color P, the color coordinates of an initial candidate refinishing color I, the color coordinates of a predetermined number of tints Ti through T_n that form the initial candidate color, and the magnitude and direction of adjustment vectors corresponding to a predetermined adjustment quantity of each tint. The initial information is loaded into the data memory over a line 29. The preferred manner by which such information is provided to the data memory is discussed in more detail herein.

The display generated on the screen 26 of the color video monitor 22 when implementing the improved method and program of the present invention is illustrated in Figure 2. Under control of the CPU 16 operating in accordance with a program of the present invention a rendering of the predetermined target paint color P is displayed in a first field 26A of the screen 26 of the video monitor 22. Displayed in a second field 26B of the screen 26 of the video monitor 22 is a rendering of an initial candidate refinishing paint color I believed by the refinisher to match the predetermined target paint color. As discussed earlier the candidate fefι^'ni^'sh^'ihcj'^'colo'r I is Tormed from a combination of a predetermined quantity of each of a predetermined number of tints Ti through T_n. A rendering of each of the tints T₁ through T_n is displayed in a respective dedicated display field 26Ti through 26T_n. In the preferred implementation the first field 26A (the target color) and the second field 26B (the candidate color) are closely adjacent to but spaced from [a spacing that appears to a viewer to be on the order of about one-eighth of an inch (0.125 inch)]. However, the first field 26A (the target color) and the second field 26B (the candidate color) may be juxtaposed in abutting relationship against each other, as suggested by the dashed lines in Figure 2. The tint fields 26Ti through 26T_n are juxtaposed in abutting relationship with the second field 26B, but may likewise be slightly spaced therefrom, if desired. The screen 26 should be rendered in a neutral surrounding background color (e.g., gray) for chromatic adaptation.

With a rendering of both the predetermined target paint color P and the candidate refinishing paint color I on the screen 26 of the monitor 22 the refinisher is able to make an assessment as to the acceptability (in the sense of a color match) between the two. If it were felt that the initial candidate refinishing paint color I is sufficiently close to the predetermined target color P the color selection process would be complete.

However, in the event there is a perceived difference between the target paint color and the candidate paint color the method and program of the present invention assist the refinisher through the adjustment of the candidate color to effect a closer color match. In accordance with the present invention the refinisher makes a determination as to which of the constituent tints should be added to drive the candidate color closer to the target color. The refinisher selects the tint to be added by asserting ("clicking") the mouse in the dedicated tint field 26Ti through 26T_n of the selected tint. Information regarding the tint selected is applied over the line 21 to the memory and to the tabulation register 12T. Trt'Veisporfsd^'fo" thVselection of one of the tints the CPU 16, operating as a vector calculator 32, uses vector analysis in the multidimensional color space to calculate an updated candidate color point (e.g., the point Ui in Figure 1C). As discussed in connection with Figure 1 C the updated candidate color point is defined by the addition of a color adjustment vector to the candidate color vector (i.e., the vector terminating in the candidate color point). The color adjustment vector represents the addition of an aliquot (i.e., a predetermined adjustment quantity or measured amount) of the selected tint to the candidate color. Based upon the results of the vector analysis the CPU 16, operating as a R-G-B generator 34, outputs an appropriate video drive signal to color video driver 28. The video driver 28 provides the appropriate video excitation signals to the electron gun 24. A rendering of an updated candidate refinishing color corresponding to the updated candidate color point Ui in the multidimensional color space is displayed in the second field 26B on the screen of the monitor 26.

The candidate refinishing color may be further updated by the iterative selection (e.g., via the use of the mouse) of an adjustment quantity of the same or a different tint until the candidate refinishing color is deemed by the refinisher to be sufficiently close to the target color. As with the initial adjustment, each color adjustment represents the addition a color adjustment vector representing the magnitude and direction a predetermined adjustment quantity of the selected tint to the vector representing the current updated candidate color. The interaction between the refinisher-user and the system may be arranged such that the user is offered a "preview" of an updated candidate color in the field 26B, as by "hovering" the mouse 18 over various tint fields 26Ti through 26T_n. After the "preview" the user effects the actual selection of one of the tints by undertaking the assertion action (e.g., "clicking" of the mouse), with the results as discussed earlier. THe iόtaT'hϋϊnber of aliquot(s) of each tint(s) necessary to oDtain the color match is tabulated in the tabulation register 12T. This tabulation represents the total adjustment quantity of each tint that must be added to the formula of the paint having the initial candidate color to produce a paint having a color deemed by the refinisher to be sufficiently close to the target color.

It lies within the contemplation of the present invention that the predetermined adjustment quantity of each selected tint may be made adjustably selectable. As a result the magnitude of the movement effected by an adjustment quantity of a selected tint could vary.

Modification of the adjustment quantity of a selected tint (and the resulting modification in the magnitude of the adjustment vector corresponding thereto) could be accomplished in the vector calculator 32. For example, upon a predetermined action or signal by the operator (e.g., the assertion of a predetermined key such as the "shift" key) the adjustment vector may be multiplied by the ratio of the distance D divided by the original vector length. Another option would be the implementation of fractional steps upon assertion of the predetermined action. Steps in a "negative" direction, that is, in the direction opposite the direction of an adjustment vector, are also envisioned.

Although the description heretofore has been given in the context of a three-dimensional color space it should be understood that the invention is not limited thereto. The CPU, acting as the vector calculator 32 and the R-G-B generator 34, is able to manipulate any N-dimensional vector quantity. Thus, metallic or pearlescent paints may be color matched using the nine-dimensional vector representation of three angle measurements or the fifteen-dimensional representation for five angle measurements. In such multi-angle implementations the fields 26A, 26B on the screen 26 would display the target and candidate colors at each of the three or five measurement angles, as the case may be. Alternatively, a rendering of a curved panel could be produced on the screen. Respective ^"pόrtlδfis of ^"i\ϊe parieTWbufd have a rendering of the candidate color as observed at each of viewing angle of the multi-angle system. Portions of the panel intermediate the angle positions would have renderings of interpolated candidate colors. PREPARATORY STEPS As noted earlier various inputs are provided to the system 10 over the line 29 and stored in the data memory. These inputs include: the color coordinates P_L*, P_a*, P_b* of a predetermined target color P; the color coordinates IL*, l_a*_> Ib* of an initial candidate refinishing color I; the color coordinates Tu*, Tu*, T-ib*; T2L*, T_2a*, T₂b*; ■ ■ ■ T_nL* , T_na* , Tn_b*of each predetermined number of tints Ti through T_n that form the initial candidate color; and the adjustment vectors corresponding to each tint Ti through T_n.

These inputs may be derived in a variety of ways.

For example, the coordinates of the predetermined target color point and/or the initial candidate refinishing color in the multidimensional color space may be determined using a spectrophotometer, if such a device is available to the refinisher. The target color may be obtained by spectrophotometeric analysis of the vehicle under repair. The candidate color may be obtained by spectrophotometeric analysis of a test panel sprayed with the initial candidate color.

A spectrophotometer measures the percentage of light reflected at each wavelength over the visible region of the electromagnetic spectrum. Typically these readings are taken at ten nanometer (10 nm) intervals from four hundred to seven hundred nanometers ((400-700 nm). A plot of the percent reflectance as a function of wavelength is referred to as a

"spectral curve". Viewing a spectral curve one can determine the hue of a color represented from the peak of the curve, e.g., the spectral curve of a blue color would peak in blue wavelengths. A light color would reflect more light across the spectrum, a darker color reflects less light. A high chroma color would have a reasonably sharper peak and reflect considerably less light at other wavelengths. A low chroma color would h a vfe^a ^""cb'ry'e" wrtti Tittl fe δfϊffe re nee between peak and trough. Grays would tend to be very flat. Thus, a qualitative assessment of the color is possible from a spectral curve.

However, color as seen by a human observer is dependent not only on the spectral curve of the color but also the spectral characteristics of the light source under which it is viewed and the spectral sensitivity of the observer. The human eye has three sensors for color vision-a blue sensor, a green sensor and a red sensor. In 1931 , the International Committee on Illumination (CIE) standardized the mapping of color in a three-dimensional X, Y, Z space, allowing for the spectral characteristics of the color, the light source and the observer. However it is still difficult to visualize a color from its tristimulus values X, Y, Z. Also, these values do not provide a visually uniform three-dimensional mapping of color.

The foregoing difficulties are addressed by using mathematical transformations to "uniform color space" known today as L^*, a*, b* data, which are described in COLOR VISION IN INSTRUMENTAL COLOR MATCHING OF SOLID AND METALLIC COLORS by A. B. J. Rodrigues (Proceedings of the Sixteenth International Conference in Organic Coatings Science and Technology, Athens Greece, 1990) and in ASTM E 308.

As previously discussed the L*, a*, b* values of the color describes the position of the color. The L*, a*, b* data of each color is a three- dimensional rendering of color space in Cartesian coordinates in which a lightness axis (L*), a red-green axis (a*), and a yellow-blue axis (b*), are described by the following equations:

L* = 116 (Y/Yo)^1/3- 16 (1) a* = 500 [(X/Xo)^1/3 - (Y/Yo)¹'³ ] (2) b* = 200 [(Y/Yo)^1/3 - (Z/Zo)^1/3 ] (3)

In the foregoing equations X₀, Yo and Z₀ are the tristimulus values of a perfect white color for a given illuminant; and X, Y and Z are the tristimulus values for the color to be evaluated. Additional information is alsό'-pfό'vetiiή art Article' Entitled "Theory and Implementation of Modern Techniques of Color Conception, Matching and Control" by A. B. J. Rodrigues, which is described in the Fifth International Conference in Organic Coatings Science and Technology Proceedings, Vol. 3, Advances in Organic Coatings Science and Technology Series, p. 272-282, (1979), U.S. Patent 4,403,866, and ASTM E 308.

Once the color characteristics, such as the L*, a*, b* data, are obtained, the color characteristics are converted into video display by determining the corresponding R-G-B data for the color characteristics obtained. The algorithms and procedures for this conversion are described in ASTM E 1682, "Standard Guide for Modeling the Colorimetric Properties of a Visual Display Unit", ASTM International, 100 Bar Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959.

Alternatively, the initial candidate paint color may be selected by the refinisher-user from a suitable color selection chart, or by experience, or by color formulas available from paint suppliers in computer databases that can be searched by color code available on the vehicle or by name. The target color may also be obtained in this manner. These supplier databases normally contain the color coordinates for each of these candidate paints. The color vectors for each of the tints that are used to form these candidate paints may also be stored by the supplier.

Color adjustment vectors corresponding to each tint may be determined empirically. For example, a sample of a candidate color including the tint may be prepared and the color properties measured. After the addition of a predetermined amount of a tint the color properties are again measured. The effect that the addition of the predetermined amount of the tint had on color properties provides an indication of the adjustment vector corresponding to that tint.

Adjustment vectors may also be computed using pigment mixture models relating reflectance at a given wavelength to absorption and " ^'""^{* "}scaft^'ef^'iri^'g cOetficients (e.g., Kubelka-Munk model). Models for flake must also include the multi-angle reflection characteristics of the flakes.

Renderings of the target color, candidate color and constituent tints may then be made in the appropriate display fields 26A, 26B, 26T₁ 5 through 26T_n (Figure 2).

From the foregoing description it may be appreciated that the present invention provides a rapid, cost effective and efficient tool that a refinisher can use to match a color for repair of a damaged vehicle. This present invention overcomes the shortcomings of the use of "shading tips" 10 which use verbal means to describe color changes, rather than the much more effective visual means used in this invention. It also overcomes the limitation of the Chromavision^® system that requires expensive equipment to be used effectively.

Those skilled in the art, having the benefit of the teachings of the 15 present invention as hereinabove set forth may effect numerous modifications thereto. Such modifications are to be construed as lying within the contemplation of the present invention, as defined by the appended claims.

Claims

WHAT IS CLAIMED IS:

1. A computer-implemented method for matching a candidate paint color formed from a combination of a predetermined number of tints Ti through T_n to a target paint color, the method including the steps of: (a) displaying in a first field of a video monitor a rendering of a predetermined target paint color, the predetermined target paint color corresponding to a predetermined target color point in a multidimensional color space; (b) displaying in a second field of the video monitor a rendering of an initial candidate refinishing paint color, the initial candidate refinishing paint color being formed from a combination of a predetermined quantity of each of a predetermined number of tints T₁ through T_n, the initial candidate refinishing paint color corresponding to an initial candidate color point in the multidimensional color space,

(c) displaying in a respective field of the video monitor each of the tints Ti through T_n; and

(d) selecting of one of the tints, wherein the improvement comprising the steps of: (e) in response to the selection of one of the tints, using vector analysis in the multidimensional color space to calculate an updated candidate color point in the multidimensional color space, the updated candidate color point being defined by the addition of a color adjustment vector to a vector terminating at the position of the candidate color in the multidimensional color space, the color adjustment vector representing the magnitude and direction a predetermined adjustment quantity of the selected tint moves the candidate color point in the multidimensional color space; and

(f) displaying in the second field a rendering of an updated candidate refinishing paint color corresponding to the updated candidate color point in the multidimensional color space.

2. The improved method of claim 1 wherein the vector analysis used to calculate the updated candidate color point uses a pigment mixture model relating reflectance at a given wavelength to absorption and scattering coefficients.

3. The improved method of claim 2 wherein the pigment mixture model includes multi-angle reflection characteristics of metallic flakes.

4. The improved method of claim 1 further comprising the steps of:

(g) comparing the updated candidate paint color rendered in the second field to the target paint color rendered in the first field; and

(h) based upon the results of the comparison, iteratively repeating steps (d) through (f) by selection of any of the tints such that, on each iteration, a rendering of a subsequent updated candidate color produced by the addition of a predetermined adjustment quantity of the tint selected during that iteration to the immediately preceding updated candidate color is displayed in the second field, the repetition continuing until an acceptable color match is achieved between the renderings of the subsequent updated candidate color and the target color.

5. The improved method of claim 4 further comprising the steps of: (i) upon achievement of an acceptable color match, tabulating the number of adjustment quantities of each tint added to the initial candidate color to produce a final subsequent updated candidate color.

6. The improved method of claim 5 further comprising the step of: G) mixing a paint by adding the tabulated adjustment quantities of each tint to the predetermined quantity of a paint of the initial candidate paint color. ^'

7. Trie'lrriproveα memod of claim 1 wherein the video monitor is a cathode ray video monitor having a screen with a plurality of locations defined therein, each location including a red, green and blue phosphor respectively excitable by a corresponding red, green and blue excitation signal, and wherein the display of each of the predetermined target color, the initial candidate refinishing color, the updated candidate refinishing color and each tint Ti through T_n in their respective fields is accomplished by converting the respective predetermined target color point, the initial candidate color point, the updated candidate color point, and each tint Ti through T_n into its corresponding red, green and blue color excitation signals.

8. The improved method of claim 1 wherein the second field is juxtaposed in an abutting relationship against the first field on the display.

9. The improved method of claim 1 wherein the first field is closely adjacent to but spaced from the second field on the display.

10. The improved method of claim 1 wherein the field for each of the tints is juxtaposed in an abutting relationship against the second field on the display.

11. The improved method of claim 1 wherein the field of the video screen is rendered in a neutral surrounding background color for chromatic adaptation.

12. The improved method of claim 1 wherein the selection of one of the tints is performed by selecting the respective field of the display corresponding to the selected tint.

T3^"."" The irhprovea method of claim 1 wherein the multidimensional color space is a three-dimensional color space, and wherein each of the predetermined target color point, the initial candidate refinishing color point, the updated candidate refinishing color point and each tint Ti through T_n is defined in the three-dimensional color space by a set of three coordinate values respectively representing lightness, chroma and hue of the color or tint.

14. The improved method of claim 1 wherein the multidimensional color space is a nine dimensional color space, and wherein each of the predetermined target color point, the initial candidate refinishing color point, the updated candidate refinishing color point and each tint Ti through T_n are defined in the nine dimensional color space by a set of nine coordinate values respectively representing lightness, chroma and hue of the color or tint at each of three predetermined viewing angles.

15. The improved method of claim 1 wherein the multidimensional color space is a fifteen dimensional color space, and wherein each of the predetermined target color point, the initial candidate refinishing color point, the updated candidate refinishing color point and each tint T₁ through T_n are defined in the fifteen dimensional color space by a set of fifteen coordinate values respectively representing lightness, chroma and hue of the color or tint at each of five predetermined viewing angles.

16. The improved method of claim 1 wherein the coordinates of the predetermined target color point in the multidimensional color space are determined using a spectrophotometer.

TT^Theimproveα method of claim 1 wherein the predetermined adjustment quantity of each selected tint is adjustably selectable.

18. A machine readable storage medium containing program instructions for causing a digital computer to perform a computer- implemented method for adjusting a candidate refinishing paint color formed from a combination of a predetermined number of tints Ti through T_n, the method including the steps of:

(a) displaying in a first field of a video monitor a rendering of a predetermined target paint color, the predetermined target paint color corresponding to a predetermined target color point in a multidimensional color space;

(b) displaying in a second field of the video monitor a rendering of an initial candidate refinishing paint color, the initial candidate refinishing paint color being formed from a combination of a predetermined quantity of each of a predetermined number of tints T₁ through T_n, the initial candidate refinishing paint color corresponding to an initial candidate color point in the multidimensional color space,

(c) displaying in a respective field of the video monitor each of the tints Ti through T_n; the method being improved in that it further includes the steps of:

(d) in response to the selection of one of the tints, using vector analysis in the multidimensional color space to calculate an updated candidate color point in the multidimensional color space, the updated candidate color point being defined by the addition of a color adjustment vector to a vector terminating at the position of the candidate color in the multidimensional color space, the color adjustment vector representing the magnitude and direction a predetermined adjustment quantity of the selected tint moves the candidate color point in the multidimensional color space; and '(eydis^'plέfyiή'g Tn 'the second field a rendering of an updated candidate refinishing paint color corresponding to the updated candidate color point in the multidimensional color space.

19. The machine readable storage medium of claim 18 wherein the improved method further comprises the steps of:

(f) comparing the updated candidate paint color rendered in the second field to the target paint color rendered in the first field; and

(g) based upon the results of the comparison, iteratively repeating steps (d) through (f) by selection of any of the tints such that, on each iteration, a rendering of a subsequent updated candidate color produced by the addition of a predetermined adjustment quantity of the tint selected during that iteration to the immediately preceding updated candidate color is displayed in the second field, the repetition continuing until an acceptable color match is achieved between the renderings of the subsequent updated candidate color and the target color.

20. The machine readable storage medium of claim 19 wherein the improved method further comprises the steps of: (h) upon achievement of an acceptable color match, tabulating the number of adjustment quantities of each tint added to the initial candidate color to produce a final subsequent updated candidate color.