Image analysis – Color image processing
Reexamination Certificate
2000-10-05
2004-07-27
Johns, Andrew W. (Department: 2621)
Image analysis
Color image processing
C358S504000, C358S523000
Reexamination Certificate
active
06768814
ABSTRACT:
The invention pertains to methods applying color measurement by means of an electronic imaging device. More particularly, the invention pertains to a method of determining a color formula for matching a selected color measured with an electronic imaging device. The invention is also directed to a method of determining a color formula for matching a selected color of a textured material measured with an electronic imaging device. Finally, the invention is directed to a method for checking a selected color measured with an electronic imaging device with a standard color sample.
It is well-known to measure selected colors with the aid of color meters, such as spectrophotometers and tri-stimulus meters . The measured signals may be used for the determination of a color formula. Thus U.S. Pat. No. 4,813,000 discloses measuring a selected color with the aid of a tri-stimulus color analyser and using the measured chromaticity data to search for a color formula in a databank. A series of articles by W. R. Cramer published in
Fahrzeug+Karosserie
, 9, 1997, 11-12, 1997, and 1-5, 1998, describes commercial applications of measuring a selected color with the aid of a spectrophotometer and using the measured spectral data to search for a color formula in a databank. Such methods are especially suitable for use at points of sale where paints have to be available in every color.
It is also possible to use the measured signals to check the selected color with a standard color sample. Such a method is currently used in the printing inks industry.
The human eye is highly sensitive to color differences. If a color is to be matched, it is essential that the measurement of the color be as accurate as possible. High measuring accuracy requires calibration. To this end there are fixed standards defining color in terms of standard values, so-called calorimetric data. Most common calorimetric data has been laid down by the Commision International de I'Eclairage (CIE), e.g., CIELab (L*
ab
, a*, b*), CIEXYZ (X, Y, Z), and CIELUV (L*
uv
, u*, v*). Spectral measuring data and tri-stimulus measuring data therefore have to be converted to colorimetric data if a spectrophotometer or a tri-stimulus meter is to be calibrated.
The drawback to spectrophotometers is that they are very delicate instruments. Hence a certain expertise is required on the part of the user which is not always available at the aforementioned points of sale. Moreover, spectrophotometers are expensive. A further drawback to spectrophotometers and tri-stimulus meters is that they cannot be used for measuring color appearance including texture of the material.
The invention pertains to a method of determining a color formula for matching a selected color measured with an electronic imaging device, which method comprises the following steps:
a) an electronic imaging device is calibrated by measuring the color signals of at least two calibration colors, the calorimetric data of each of the calibration colors being known;
b) at the same time or in a next step the selected color is measured with the aid of the electronic imaging device;
c) using a mathematical model, parameters are calculated for converting the measured color signals of the calibration colors to the known calorimetric data;
d) using the mathematical model and the calculated parameters, the color signals of the measured selected color are converted to calorimetric data; and
e) using a databank, the color formula is determined of which the colorimetric data most closely matches the calculated calorimetric data of the measured selected color.
The invention has the advantage that it is possible to make use of inexpensive consumer electronics. Consumer electronics often do not have the accurate settings required for specialist applications. The method according to the invention now makes it possible to utilise an inaccurate device for the determination of a color formula for matching a selected color and yet achieve a high level of measuring accuracy. In addition, the method can be performed easily by a non-specialist without him needing extensive training. The method according to the invention also makes it possible to measure a specific attribute of the color appearance, the so-called texture.
In the method according to the invention the term “electronic imaging device” stands for all devices with which an electronic image can be recorded that can be processed with the aid of a computer. Examples of such electronic imaging devices are digital recording devices. Preferably, the electronic imaging device is a digital video camera, a digital camera, a flatbed scanner, a drum scanner, or a manually operated scanner. However, an analogue video camera coupled to a so-called frame grabber which converts the analogue signal to a digital image is also covered by the term “electronic imaging device.” Finally, the term “electronic imaging device” also covers multi-spectral-imaging equipment and monochrome cameras with multiple color filters. Examples of flatbed scanners are the Hewlett Packard 3C, Hewlett Packard Scanjet lec, Sharp JX450, Agfa Focus Color, and Afga Arcus Plus. Examples of drum scanners are the Howtek D4000, Optronics Color Getter, and LeafScan 45. Examples of digital cameras are the Ricoh RDC 5000, Olympus C-2000Z, and Nikon Coolpix 950. Preferably, a digital camera is employed.
A minimum of two calibration colors is used, i.e. white and black. Optionally, use may be made of grey or neutral colors. For a more accurate conversion of the color signals of the selected color to calorimetric data preference is given to including calibration colors other than the neutral colors. The calibration colors may be selected at random. Preferably, use is made of calibration colors distributed over the entire calorimetric color space. More preferably, use is made of calibration colors distributed in the vicinity of the selected color.
In theory, the physical calibration pattern can comprise as many calibration colors as may be present within the image field of the electronic imaging device. The calibration colors are recorded on the pattern in the form of patches. In theory, the calibration patches may have the size of a single pixel. In that case the size of the measuring surface will be equal to the size of the calibration patch. Depending on the electronic imaging device employed, phenomena may occur which require the calibration patch to be bigger than a single pixel. Such phenomena include stability, non-linearity, distortions, reproducibility of positioning, and cross-talk. Generally speaking, between 2 and 1000 calibration colors may be present, preferably 10-500, more preferably 25-150.
Of course, the calibration patches need not be square. Nor do they have to be rectangular or regularly shaped. There is no need to separate the colors, i.e. the color is allowed to shift gradually.
The support on which the calibration patches are provided may be flat or curved. Preferably, the support is of uniform color, e.g., white or grey. A clear space may be left around a portion or all of the calibration patches so as to leave the support's surface area visible. The uniform color of the support may also serve to measure and correct any spatial non-uniformity of the electronic imaging device.
Depending on the measuring accuracy required, it may be preferred to measure the calibration colors and the selected color simultaneously. In such cases the calibration pattern support may be provided with a recess, e.g., at the centre. Alternatively, a support may be selected which is smaller than the image field, so that the remaining image field can be used to record the selected color.
Also, within the framework of the present invention it is possible to calibrate beforehand in step a) using a calibration pattern with more than 10 colors, then in step b) carry out a black and white calibration and measure the selected color simultaneously. This combination of steps is useful in reducing the variation in brightness due to the influence of the light source.
Processing the recorded image,
Lucassen Marcel Petrus
Spitzer Daniel
Akzo Nobel N.V.
Alavi Amir
Johns Andrew W.
McGillycuddy Joan M.
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