Facsimile and static presentation processing – Natural color facsimile – Measuring – testing – and calibrating
Patent
1992-10-23
1995-03-14
Ro, Bentsu
Facsimile and static presentation processing
Natural color facsimile
Measuring, testing, and calibrating
358523, H04N 146
Patent
active
053981219
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to a method for generating a digital lookup table for printing inks in image reproduction equipment using color scan values obtained by photoelectric scanning of an original.
The basis for generation of a transformation table for the primary colors "RGB" into the complementary colors "CMYK" is spectrophotometric measurement of a color scale generated with that process (e.g. Offset, Euroskala, coated paper) for which this table is later to apply. With unlimited expenditure, it would be possible to thereby obtain by measurement the right field for every required color stimulus from a scale with 1% gradation of the printing inks, i.e. 100*100*100 color fields. A method of this type would provide, if the color range of the scale is not always exceeded, the formula for the complementary colors "CMYK" accurate to 1%. Highly saturated hues have no corresponding field. In addition, one million measurements are impracticable for generating a table.
A color correction device for image reproduction equipment is known with which digitalized color density values of individual color separations obtained by photoelectric scanning are converted into complementary color density values. With the complementary color density values it is possible to obtain color separation negatives. The complementary color density values are color-corrected using correction data filed in a memory and addressable and outputtable by the color density values. A gray component is determined from the color density values. This is achieved by comparing the color density values. The gray component is processed by comparison with a predetermined white light value to obtain an equivalent gray value with which a memory is addressed that contains a specific gray value for each complementary color density value. By subtracting the gray components from the color density values, complementary color density values are generated. The various complementary color density values are each combined with an associated specific gray value. Corrections to the chromatic component and to the gray component independently of one another permit a saving of memory capacity for the conversion table (German patent DE-PS 30 15 396).
One of the objects underlying the invention is to develop a method for generation of a digital lookup table (memory table) of complementary printing inks for image reproduction equipment using standardized color scan values obtained by photoelectric scanning of an original, e.g. EBU red, green, blue. With the method of the invention a lookup table can be produced from a few measurement fields that is sufficiently accurate for determining the proportions of printing inks. In addition, non-reproducible color stimuli must be adapted to useful substitute values.
The object is substantially achieved in accordance with the invention in that the full colors of the three printing inks and white are measured by scanning of corresponding originals, in that model colors are determined which are in as linear as possible a relationship to the printing inks by means of a position of the chromaticity range of the print scale such that the primary stimuli of the model colors in the chromaticity diagram are each on one beam of the trajectories of the scale colors through the white point while the triangle set up between the primary stimuli encloses the chromaticity range of the print scale. The scale division of the primary stimuli is adjusted with the cube root to a linear gradation of the printing inks. The space diagonals of the printing inks as a function of the model colors are used to determine the number of fields to be scanned, scan values of which are further processed by cubic spline interpolation with a higher number of support points in order to generate the lookup table. The values of the model colors are each stored as a function of the printing ink values.
A lookup table of the printing inks as a function of the model colors is preferably generated from the stored lookup table. The latter table is
REFERENCES:
patent: 4992861 (1991-02-01), D'Errico
patent: 5003299 (1991-03-01), Batson et al.
patent: 5195043 (1993-03-01), Varner
Kowalewski Joern
Vorhagen Friedrich W.
Linotype-Hell AG
Ro Bentsu
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