Image analysis – Color image processing – Color correction
Patent
1998-04-06
2000-08-15
Boudreau, Leo H.
Image analysis
Color image processing
Color correction
382162, 382281, 358518, G06K 900, G06K 936, G06F 15332, G03F 308
Patent
active
061048305
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a method of correcting colour casts in digitally represented images and comprising the features disclosed in the introductory part of claim 1, and an apparatus having the features disclosed in the introductory part of claim 16.
The term colour cast is used to designate the colour deviations of a photograph from what is expected. That is, it is experienced that the colours represented in a given photograph deviates from what is considered correct. The phenomenon of colour casts relates exclusively to photographs since they are considered representations of impressions from the surroundings, and during the evaluation of the representation they are compared to our experience of how the world looks.
Colour casts occur either as a result of a mistake on the photographer's part or a mistake in the photographic production of the photograph, or it may be due to the fact that the photograph does not reflect the prevailing visual conditions. In the photographic production of the photograph, colour casts may occur if the development does not correspond to the film used, or if the film or the chemical development process is faulty. Mistakes may also occur in connection with a scanning of the image. The scanning equipment may be operated in a wrong manner or the scanner may be calibrated wrongly. Lastly, the transformation between the colour models of the different apparatuses may be faulty. Images with colour casts are experienced as wrong or poor even if the the photograph is a technically correct representation of the surroundings which has merely been recorded in unusual visual conditions.
Colour casts are the result of a subjective phenomenon which originates in our expectations to a subject, but the expectation has a certain general character thereby allowing guidelines to be established for the correction to be carried out of the photograph in order to make it meet the non-formulated expectations. This is among others discussed in "Color in Business, Science and Industry", by D. B. Judd et al., 3rd edition, John Wiley & Sons Inc., New York, ISBN 0-471-45212-2.
The colour fidelity of a photograph describes the colour authenticity with which a photograph reproduces the original subject. In order to obtain such authenticity, a variety of conditions must be allowed for in the photographic process.
Over the years, this discovery has resulted in a technique which allows a lithographer to correct a photograph. The technique presupposes the presence in the photoghraph of areas which may be considered neutral. They may be completely light or completely dark areas. The correction of the colour planes is divided into three independent areas, the most light areas being designated highlights, the most dark areas being designated shade and the grey shades therebetween being designated intermediate shades. Much experience and insight are required for carrying out adequate correction of a photograph. Even to an experienced lithographer the process will be lengthy. A variety of software packages for the correction of colour casts is already available. Examples include Adobe Photoshop.RTM. and Corel PhotoPaint.RTM. which are colour processing programmes whose function is to adjust colour planes in highlights, shades and intermediate shades, respectively. It requires that the lithographer in charge is capable of evaluating which pixels are neutral and which planes are to be corrected in order to obtain the desired neutrality.
The only programme which comprises a function for the correction of colour casts is the image processing programme LinoColor.RTM. available from the Linotype-Hell AG company. LinoColor.RTM. corrects colour casts by determining a shade and a highlight point in the image, and the operator may subsequently choose to perform corrections in the shade, in the highlight or in both places. The effect of the discolouration correction may be controlled in several steps thereby allowing for a strong, a standard, a weak, or no correction at all.
The grey-balance problem is discussed among others
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Bartleson, et al., "Exposure Determination Methods for Color Printing: the Concept of Optimum Correction Level", Journal of SMPTE, vol. 65, pp. 205-215, 1986.
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Boudreau Leo H.
Novik Dmitry A.
Purup-Eskofot A/S
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