Rapid spectral color measurement method
阅读说明：本技术 快速光谱颜色测量方法 (Rapid spectral color measurement method ) 是由 J·范加尔瑟 于 2019-10-16 设计创作，主要内容包括：一种通过用于在多个光谱范围()中进行颜色测量的颜色测量设备(3000)对数字图像的渲染副本的颜色进行颜色测量的方法；其中所述方法包括步骤：在所述多个光谱范围()的子集(；-(3305))中测量所述颜色；并且其中所述子集()由以下各项确定：选择所述数字图像(1200)的色域边界的颜色；和确定所述多个光谱范围()的所述颜色对应的光谱反射率因子()；和将具有最小和最大光谱反射率因子的光谱范围()添加到所述子集(),其中其对应的光谱反射率值()大于所述最大光谱反射率因子减去所述最小光谱反射率因子的10%。(A method for applying a light beam in a plurality of spectral ranges ( ) A method of color measuring a color of a rendered copy of a digital image by a color measuring device (3000) of color measuring; wherein the method comprises the steps of: in the plurality of spectral ranges ( ) A subset of (a) ； 3305 ) Measuring the color; and wherein said subset: ( ) Is determined by: selecting a color of a gamut boundary of the digital image (1200); and determining the plurality of spectral ranges ( ) The spectral reflectance factor corresponding to the color of (a), (b), (c), (d ) (ii) a And the spectral range that will have the minimum and maximum spectral reflectance factors ( ) Is added to the subset ( ) Wherein its corresponding spectral reflectance value ( ) Greater than 10% of the maximum spectral reflectance factor minus the minimum spectral reflectance factor.)
1. A pass through can be in a plurality of spectral ranges () A method of color measuring a color of a rendered copy of a digital image having a color gamut boundary by a color measuring device;
wherein in the plurality of spectral ranges () A subset of (a)) Measuring the color;
the subset of () Is determined by:
a) selecting a color within the gamut boundary; and
b) determining the plurality of spectral ranges () The color within the color boundary of (a) a spectral reflectance factor corresponding to the color of (b) (ii) a And
c) will utensilSpectral range with minimum and maximum spectral reflectance factors: () Is added to the subset () Wherein its corresponding spectral reflectance value () Greater than 10% of the maximum spectral reflectance factor minus the minimum spectral reflectance factor.
2. The color measurement method of claim 1, wherein the selected color within the gamut boundary is determined by:
a) determining an image gamut of the digital image in an N-dimensional device independent color system (N > 1); wherein the image gamut comprises color values of the digital image; and
b) determining a skeleton of the image gamut, wherein the skeleton comprises a plurality of points determined by:
-selecting a luminance range for color values in the N-dimensional device independent color system; and
-determining a sub-image gamut of the image gamut; wherein the sub-image gamut corresponds to the luminance range and comprises color values having luminances in the corresponding luminance range; and
-determining a color value in the sub-image gamut as a point of the skeleton; and is
Wherein the selected color has a color value, wherein a minimum color difference between the color value and the skeleton is less than 3。
3. The color measurement method of claim 1, wherein the selected color is determined by:
a) determining an image gamut of the digital image in an N-dimensional device independent color system (N > 1); wherein the image gamut comprises color values of the digital image; and
b) determining a skeleton of the image gamut, wherein the skeleton comprises a plurality of points determined by:
-selecting a range along a determined axis in the N-dimensional device independent color system; and
-determining a sub-image gamut of the image gamut; wherein the sub-image gamut comprises color values belonging to the range with respect to a projection towards the determined axis; and
-determining a color value in the sub-image gamut as a point of the skeleton; and is
Wherein the selected color has a color value, wherein a minimum color difference between the color value and the skeleton is less than 3。
4. The color measurement method according to claim 1, wherein the selected color is determined by
a) Determining an image gamut of the digital image in an N-dimensional device independent color system (N > 1); wherein the image gamut comprises color values of the digital image; and
b) refining the image color gamut into a skeleton; and is
Wherein the selected color has a color difference of less than 3 relative to the skeletonThe color value of (a).
5. The color measurement method according to any one of claims 2 to 4,
wherein the skeleton has endpoints; and is
Wherein the selected color has a color difference of less than 3 relative to the endpointThe color value of (a).
6. Wherein a minimum color difference between the color value and the endpoint of the skeleton is less than 3。
7. The color measurement method according to any one of claims 2 to 4,
wherein the skeleton has intersections; and is
Wherein the selected color has a color difference of less than 3 relative to the intersection pointThe color value of (a).
8. The color measurement method according to any one of claims 2 to 4,
wherein the skeleton has an inflection point; and is
Wherein the selected color has a color difference of less than 3The color value of (a).
9. The color measurement method of claim 1, wherein the selected color is determined by:
a) determining an image gamut of the digital image in an N-dimensional device independent color system (N > 1); wherein the image gamut comprises color values of the digital image; and
b) clustering the color values in the image gamut into K clusters by K-means clustering;
c) selecting a cluster of the K-clusters;
and wherein the selected color has a color difference in which the color difference relative to the selected cluster is less than 3The color value of (a).
10. A color measurement method in which a plurality of rendered copies of the digital image of a rendering device are color measured according to any one of claims 1 to 8 for color analysis on the rendering device.
11. The color measurement method according to claim 9, wherein the digital image is a pattern using only a few colors.
12. The color measurement method according to any of claims 9 to 10, wherein the rendered copy is rendered by a single pass inkjet printing apparatus at a speed of more than 70 meters per minute.
13. A color measurement device comprising a sensor for measuring a plurality of spectral ranges () Comprising means for performing the method steps from any of claims 1 to 11.
14. Use of a color measuring device according to claim 12 in the manufacture of a trim panel for color controlling the color acceptance of the trim panel.
Technical Field
The invention relates to a method of performing spectral color measurements on rendered copies of digital images, in particular while rendering said copies.
Background
A color spectrophotometer is a device that captures and evaluates colors in the visible spectrum. They are useful in communications relating to color, such as brand owners and designers, and in monitoring color accuracy throughout production.
The color measurement made by the color spectrophotometer is a spectral reflectance curve within the visible spectrum, which is near the range of 380 nm bluish violet to 780 nm (red). During a certain measurement period, a color measurement is reported to the software via the interface. The spectrophotometer typically includes one or more sensors for measuring color in multiple spectral ranges. Thus, not all spectral reflectance factors for colors in the visible spectrum are measured, and reports of spectral reflectance curves are typically within 20 nm or 10 nm intervals. Thus, the spectral reflectance curve is a look-up table with the spectral range as an input and the spectral reflectance factor as an output.
The color spectrophotometer is used in the graphics field while rendering digital images on a printing device, also known as point-of-care spectral color measurements, wherein measurements of and/or analysis of rendered copies are reported to an operator of the rendering device for ensuring color consistency and for ensuring that the rendered copies meet ISO, G7 or customer specific standards. The guarantees increase productivity by reducing ink and paper consumption, significantly reduce set-up time, and improve communication between prepress operators, quality management, and print procurement.
Disclosure of Invention
Today, the rendering speed of printing devices is becoming faster, so there is a need to speed up the measurement cycle of a color spectrophotometer without losing quality when reporting the measured spectral reflectance curve. To ensure color consistency, more measurements of the rendered digital image are also needed for better analysis; where faster measurement cycles are required.
These problems are solved by a color measurement method of claim 1, wherein the method measures the color of a rendered copy of the digital image by a color measurement device; and wherein the color measurement device comprises means for measuring a plurality of spectral ranges () One or more sensors of the color of (a); and is
WhereinSaid plurality of spectral ranges for said color(s) (() A subset of (a)) To measure; and is
Wherein the subset of (A)) Is determined by:
-selecting colors of gamut boundaries of the digital image; and
-determining the plurality of spectral ranges () The spectral reflectance factor corresponding to the color of (a), (b), (c), (d) (ii) a And
-spectral range with minimum and maximum spectral reflectance factor: () Is added to the subset () Wherein its corresponding spectral reflectance value () Greater than 10% of the maximum spectral reflectance factor minus the minimum spectral reflectance factor.
The color measurement device is, for example, a color spectrophotometer, preferably an inline color measurement device, for measuring the color of a rendered copy of a digital image while rendering the digital image.
By using only said plurality of spectral ranges () A subset of (a)；M <Q), when measuring the color of a digital image and wherein the subset depends on the color of the digital image, the measurement cycle is faster and the memory consumption for reporting is smaller.
Due to the high throughput color measurement method of the present invention, the color measurement device is able to follow the speed of the rendered copy of the digital image by shortening the measurement period.
In the present invention, spectral range () Depending on the particular condition of the selected color of the digital image, i.e. whether there is:
wherein b is 10 in the present invention;is the spectral range of the color () The corresponding spectral reflectance factor of (1); and isIs the spectral range of () Maximum/minimum spectral reflectance factor.
Preferably, more than one colour is selected and a subset is determined for each of the selected colours by the determining step and the adding step. If the subset is still too large, e.g. to follow the rendering speed, the b-value may be enlarged, thus reducing the number of spectral ranges in the subset. In the present invention, b is preferably 20, and more preferably 30. The color of the digital image itself may also be selected because it is part of the gamut boundary of the digital image.
The present invention ensures that redundant or less important information in the spectral reflectance curve report is not used for further analysis of color consistency in the rendering device.
The following three preferred embodiments relate to the selection of said colors within said gamut boundary, wherein a subset can be limited without loss of quality when reporting the measured spectral reflectance curve. With this particular choice, the b values of the previous formula can also be enlarged, so the subset will be limited even more.
In a first preferred embodiment, the selected color is determined by:
a) determining an image gamut of the digital image in an N-dimensional device independent color system (ND-DICS, N > 1); wherein the image gamut comprises color values of the digital image; and
b) determining a skeleton of the image gamut, wherein the skeleton comprises a plurality of points determined by:
b1) selecting a luminance range for color values in the N-dimensional device independent color system; and
b2) determining a sub-image gamut of the image gamut; wherein the sub-image gamut corresponds to the luminance range and comprises color values having luminances in the corresponding luminance range; and
-determining a color value in the sub-image gamut as a point of the skeleton; and is
Wherein the selected color has a color value, wherein a minimum color difference between the color value and the skeleton is less than 3Preferably, the selected color has a color value on the skeleton. Thus, the selection is determined in the vicinity of the skeletonThe selected color. Further preferred embodiments thereof are disclosed further below.
In the present invention, the steps b1 and b2 may further:
-selecting a range along a determined axis in the N-dimensional device independent color system; and
-determining a sub-image gamut of the image gamut; wherein the sub-image gamut comprises color values whose projection towards the determined axis belongs to the range. An axis is thus a straight line determined in the ND-DICS, which may be an axis of the ND-DICS coordinate system. The axes are preferably determined as straight lines which substantially define a line of symmetry of the image gamut (200) or are oriented substantially parallel along the image gamut.
In a second preferred embodiment, said selected color is determined by
a) Determining an image gamut of the digital image in an N-dimensional device independent color system (N > 1); wherein the image gamut comprises color values of the digital image; and
b) refining the image color gamut into a skeleton; and is
Wherein the selected color has a color value, wherein a minimum color difference between the color value and the skeleton is less than 3Preferably, the selected color has a color value on the skeleton. Thus, the selected color is determined in the vicinity of the skeleton. Further preferred embodiments thereof are disclosed further below.
In a third preferred embodiment, the selected color is determined by:
a) determining an image gamut of the digital image in an N-dimensional device independent color system (N > 1); wherein the image gamut comprises color values of the digital image; and
b) clustering the color values in the image gamut into K clusters by K-means clustering;
c) selecting a cluster of the K-clusters;
and wherein the selected color has a color value, wherein a color difference between the color value and a centroid of the selected cluster is less than 3Preferably, said selected color is said centroid. Thus, the selected color is determined in the vicinity of the centroid. k-means clustering is a method of vector quantization, initially a method of signal processing, which is popular for cluster analysis in data mining.
The invention is also a color measurement device comprising means for measuring a plurality of spectral ranges () Comprising means for performing the steps of the method of claim 1.
The invention is useful in measuring the color of patterns with only a few colors, such as wood patterns, and is particularly useful in manufacturing decorative panels for color controlling the color acceptance of decorative panels comprising such patterns with only a few colors.
The present invention is useful, and preferably is a part of, scanning a rendered copy of a digital image by the color measurement device to obtain a color spectrum scan, which is a digital image representing the rendered copy. The color spectrum scan includes a plurality of pixels having a color spectrum defined by the subset () A defined color. The color measurement device is not only suitable as a color spectrum scanner. The scanning requires less memory than when using multiple spectral ranges of the color measurement device, because each pixel stores a finite spectral reflectance factor. Consequently, analysis of the scan and subsequent scans of subsequently rendered copies of the digital image is much faster.
Drawings
The figures from 1 to 6 are illustrations of the present invention; defined in CIEXYZ (950) as ND-DICS (N-dimensional independent color system) having an axis X (900); axis Y (901) and axis Z (902). The Y coordinate in the ND-DICS is the luminance value of the color value.
Fig. 1 illustrates an image gamut (201) with patterns of only a few colors; comprises a plurality of color values (215), the plurality of color values (215) being illustrated as open circles.
Fig. 2 shows a luminance range (400), illustrated as two parallel planes, selected for the pattern of fig. 1 with only a few colors.
Fig. 3 shows the determined sub-image gamut (250) between the two parallel planes (not visible) of fig. 2, and fig. 3 also shows the determination of color values in the sub-image gamut (250); and here is a balance point (300), illustrated as a solid black circle.
FIG. 4 is a view similar to FIG. 1; FIG. 2; the result of selecting 5 luminance ranges in the image gamut (201) of the same pattern with only a few colors as in fig. 3 and determining 5 balance points (300) in the determined 5 sub-image gamuts.
FIG. 5 shows a skeleton (500) of the image gamut (200) with a plurality of skeleton points (305); wherein the skeletal points are the balance points of fig. 4.
Fig. 6 shows a skeleton (500) of an image gamut (200) with another pattern of only a few colors as in the previous figures. The skeleton (500) is determined according to the invention and thus shows the skeleton (500) including the intersection in its skeleton points (305).
Fig. 7 shows how the prior art measures a rendered copy with a spectrophotometer (3000) positioned within the rendering device.
Fig. 8 shows a preferred embodiment of the invention, where the rendered copy is measured with a spectrophotometer (3000) positioned within the rendering device.
Description of the embodiments
The digital image includes a plurality of pixels, wherein the pixels have a color value. The image gamut of the digital image is the set of color values. It may have a boundary, which is the volume to which the color value belongs. The sub-image gamut is a part of the image gamut, which may have a boundary. The image gamut is a specific set of color values in ND-DICS. The boundary is a specific region of ND-DICS.
ND is "N dimension"wherein N is an integer of more than one, and is preferably three in the present invention, followed by 3D representation"Three-dimensional". DICS is an abbreviation for device independent color system such as CIELAB, CIELUV, CIELCH, or CIEXYZ. DICS is also sometimes referred to as color space. DDCS is an abbreviation for device-related color system, such as CMYK or tristimulus values including the three primary colors of light in a TV tube. DDCS is sometimes referred to as colorant space.
Color spectrophotometer
There are several types of color spectrophotometers. The choice depends on the application, the desired functionality and the portability. Color spectrophotometers range in size from portable devices to large bench-top instruments. They include methods for measuring multiple spectral ranges () Of the color of (a). The most common color spectrophotometers measure light under certain illumination conditions reflected at a fixed angle (typically 45 °) relative to the color. The light is preferably from a built-in light source. The light source may be an LED or a light of a bulb. Other types are spherical spectrophotometers and multi-angle spectrophotometers. Examples of such illumination conditions for color measurement are CIE D50 or CIE D65 of the international commission on illumination. The illumination conditions are generally known by their spectral power distribution.
The connection of the color measurement device to the data processing apparatus for collecting color measurements may be performed by a link via a network such as a LAN (= local area network). The color measurements may be stored in a memory accessible by the data processing apparatus:
the invention makes the light splitThe meter is adapted to use only its multiple spectral ranges () A subset of (a)) Measuring a color, wherein the subset is determined from the digital image represented by the rendered copy. The spectrophotometer includes or is linked to a control unit that stores the digital image in memory. From the memory, a gamut boundary of the digital image may be determined for selecting the color, or a color may be selected from the memory. Determining a spectral range according to a condition based on said color(s) (() To become part of the subset. Other spectral ranges not belonging to the subset are not used in the color measurement, for example by switching off the sensors of the color measurement device. The determination of the subset may be performed prior to rendering, such as when a print job including the digital image is input into a print queue for rendering. The print queue is preferably part of a prepress workflow system comprising one or more raster image processors (of the RIP). The determined subset is adapted more than according to the print job as the print job is rendered.
Spectral reflectance factor (v) of the selected color1..vQ) Wherein the factors correspond to the plurality of spectral ranges (a)) If, for example, by means of a color conversion model, a digital image is defined as a spectral reflectance factor in the colorant space RGB (red, green, blue), the corresponding spectral reflectance factor (v |)1..vQ) A color conversion method, such as RGB, may be included. The color conversion model is a mathematical relationship that expresses an ND-DICS color value, such as CIELAB, as input @A colorant gamut of the rendering device, and vice versa. The plurality of spectral ranges is, for example, ND-DICS, where N is equal to Q. The model mainly uses LUT (look-up table) and interpolation techniques. A well-known way of defining the LUT is that defined by International Color Consortium (ICC); in his canonical ICC. 1: 2001-12 "file format for color profile".
For measuring the color, a light source is used on the color at the time of measurement. In a preferred embodiment, the color measurement device comprises one or more light sources for illuminating a color by a plurality of illumination conditions, and wherein the color is measured under said plurality of illumination conditions for reporting a spectral reflectance curve with said illumination conditions. The report under the illuminated conditions is useful for metamerism (metamerism) analysis on the color measurements. By the shortened measurement period in the present invention, the additional color measurement is made possible within the time of the currently most advanced measurement period.
The color measurement device of the present invention may be included in a frame scan camera for scanning a portion or the entire digital image, or may be included in a line scan camera for line-by-line scanning.
For line scanning techniques, high speed image capture is required, especially in continuous network applications. With the present invention of high throughput spectral color measurement, one advantage is that the method is performed by the line scan camera, with or without optics, for capturing a rendered copy of a digital image in ND-DICS defined by the determined subset of spectral ranges. The rendered copy captured is called a scan; which may be compared to previous scans of other rendered copies of the digital image.
Using patterns of only a few colours
The digital image in the present invention is preferably a pattern using only a few colors; which is a solution having several (= a few,) The color (color,) The decorative pattern of (1). Patterns with only a few colors are formed by a combination of color shades (color shades) of a minimum of two and a maximum of eight dominant color values, but mainly a maximum of five dominant color values. And thus is no less than two because it should be more than a monochrome (monochromatic) pattern and no more than eight because it should be a polychromic (polychromic) pattern that also includes multiple objects, rather than a maximum of 8 objects in such a pattern with only a few colors.
In the present invention, such a pattern using only a few colors is preferableWood pattern. Wood patterns have a hue, contrasting color, wood grain lines, and sometimes wood holes and/or wood imperfections such as knots and cracks. The wood grain lines are usually elongated in a dominant direction, called the nerve direction, and the hue of the wood image is determined between these wood grain lines. An elongated grain line is a line that repeats at varying frequencies on top of the hue.
It was found that when skeletonizing such an image gamut with patterns of only a few colors, it is advantageous to select points near or on the skeleton for an optimized subset.
Manufacture of decorative panel
The pattern of only a few colors is preferably used for rendering on a substrate, more preferably for forming a decorative layer, wherein the pattern is rendered on a substrate such as a decorative paper. The decorative layer can be used for manufacturing decorative panels mainly used for decorating floors and walls. The decorative layer comprises a thermosetting resin impregnated decorative paper that is assembled with one or more core layers to form the decorative panel, more than in a hot press. The core layer is preferably a board consisting essentially of wood fibres, but a synthetic core layer may also be used. The decorative paper is preferably a paper having a porosity according to Gurley's method (DIN 53120) of between 8 and 20 seconds. U.S. 6709764 (ARJO WIGGINS) also discloses a suitable sheet having high porosity and a method for making the same. Due to the porosity, color mapping using the skeleton of the present invention has been found to have high accuracy without hue jumps or color jumps after color mapping.
The invention is preferably part of the manufacture of the trim panel for color control of the decorative layer and/or the trim panel or for color acceptance calculation of the trim panel.
Framework
The skeleton is a narrowed version of the ND-object in ND-space. In the present invention, the ND-object is an image gamut of a digital image. The skeleton in ND-space comprises ND-points, preferably 3D-points forming one or more legs with linked or connected ND-points. The skeleton preferably forms a path or a set of paths with side paths between the ND-points of the link. Two of the ND-points are linked or connected together by a subpath. A path is a sequence of at least one such sub-path. If the digital image is a pattern with only a few colors, the skeleton has a maximum of 8 side paths. If it is a wood pattern, the skeleton is found to have a maximum of 2 side paths, but most have no side paths.
The sub-paths may be straight lines, but may also be curves defined as ND-functions between ND-points, such as polygons, Bezier curves, or parametric equations. The links or connected sub-paths forming the skeleton need not be defined by the same ND-function. An ND-point is preferably defined as a point having N coordinate values as used in a cartesian coordinate system. A polar coordinate system may also be used. The skeleton may be an intermediate axis of said ND-object.
In preferred embodiments using a scaffold; the skeleton has an end point; and the selected color has a color value, wherein a minimum color difference between the color value and the endpoint of the skeleton is less than 3Preferably, the selected color is the endpoint. Thus, the selected color is determined in the vicinity of the endpoint.
In another preferred embodiment using a scaffold; the skeleton has a cross-point; and is
Wherein the selected color has a color value, wherein a minimum color difference between the color value and the intersection of the skeleton is less than 3Preferably, the selected color is the intersection point. Thus, the selected color is determined in the vicinity of the intersection.
In another preferred embodiment using a scaffold; the skeleton is provided with an inflection point; and is
Wherein the selected color has a color value, wherein a minimum color difference between the color value and the inflection point of the skeleton is less than 3Preferably, the selected color is the inflection point. Thus, the selected color is determined in the vicinity of the inflection point.
When reporting measured spectral reflectance curves, using the selected color near the end point or intersection or inflection point determines a good subset without loss of quality, but results in a fast report with a short measurement period.
In a preferred embodiment, said selected color of the invention is determined by:
a) determining an image gamut of the digital image in an N-dimensional device independent color system (ND-DICS, N > 1); wherein the image gamut comprises color values of the digital image; and
b) determining a skeleton of the image gamut, wherein the skeleton comprises a plurality of points determined by:
-selecting a luminance range for color values in the N-dimensional device independent color system; and
-determining a sub-image gamut of the image gamut; wherein the sub-image gamut corresponds to the luminance range and comprises color values having luminances in the corresponding luminance range; and
-determining a color value in the sub-image gamut as a point of the skeleton; and is
Wherein the selected color has a color value, wherein a minimum color difference between the color value and the skeleton is less than 3Preferably, the selected color has a color value on the skeleton. Thus, the selected color is determined in the vicinity of the skeleton.
The determined color value is preferably a balance point of the sub-image gamut and more preferably a centroid of the sub-image gamut.
In another preferred embodiment, said selected color is determined by
a) Determining an image gamut of the digital image in an N-dimensional device independent color system (N > 1); wherein the image gamut comprises color values of the digital image; and
b) refining the image color gamut into a skeleton; and is
Wherein the selected color has a color value, wherein a minimum color difference between the color value and the skeleton is less than 3Preferably, the selected color has a color value on the skeleton. Thus, the selected color is determined in the vicinity of the skeleton.
The image gamut of the digital pattern may be refined into connected N-dimensional points, also referred to as ND-points. Refinement algorithms are well known. Some of them are disclosed in chapter 9 of "Algorithms for Graphics and Image Processing" (ISBN 0-914864-65-X, published by the computer science Press, 1982) of the o Pavlidis.
Balance points of ND-Point set
The balance point of the ND-point set is a point that satisfies a predetermined condition based on the ND-point set. For example, a balance point may be a centroid of the set of ND-points; it may be an average of the set of ND-points or it may be a weighted average of the set of ND-points.
Thus, as an example, for determining a set of w 3D-points ((ii))) Of the equilibrium point of (1), whereinHaving three coordinatesWherein i =1.. w:
wherein i =1.. w; or
Wherein i =1.. w andand is andas a weighting factor.
The set of ND-points form a boundary in ND-space. Of the boundary; it is an ND-object; the centroid can be determined (as a predetermined condition); it may be more than the balance point of the ND-point set. In mathematics, the determination of a boundary or centroid of a set of ND-points is well known. The centroid is sometimes referred to as the geometric center.
Luminance factor/brightness
Luminance factor — a photometric measure of luminous intensity. It describes the amount of light that passes, is emitted or is reflected from a particular area.
In the present invention, the luminance range is composed of a minimum luminance factor and a maximum luminance factor; wherein the minimum luminance factor and the maximum luminance factor may be equal to each other.
In a preferred embodiment, a plurality of luminance ranges is selected, said luminance ranges being consecutive to each other, preferably equidistantly consecutive, to have an accurate skeleton (500) of the image gamut (200).
Determination of the luminance factor for the color values in ND-DICS is well known to those skilled in the art. As ND-DICS, L for CIELAB*The value is a measure of brightness and thus a luminance factor of the color value. The skilled person also knows the transformation model and/or mathematical functions between ND-DICS.
Among color values in ND-DICS, chroma (chroma), hue and saturation may also be calculated by a conversion model and/or a mathematical function.
Rendering apparatus
The rendering device is thus a device for rendering digital images and/or text, such as a display or an inkjet printer. Examples of rendering devices for rendering images are CRT's, LCD's, Plasma Display Panels (PDP), electroluminescent displays (ELD), carbon nanotubes, quantum dot displays, laser TV, electronic paper, E-ink, projection displays, conventional photography, electrophotography, dot matrix printers, thermal transfer printers, dye sublimation printers, and inkjet systems, to name a few.
A rendering device has a specific colorant gamut, such as RGB (red, green, blue) or CMYK (cyan, magenta, yellow, black), which determines the colorants that can be used to reproduce an image on the rendering device. It was found that for rendering wood patterns, the rendering device is preferably CRYK (cyan, red, yellow and black) or CRY (cyan, red and yellow) because the colorant gamut generates a color space that is wide enough for rendering wood patterns.
The rendering device is capable of rendering a plurality of color values defined in ND-DICS. The color values are collected in an output device gamut of the rendering device. The output device gamut may have a boundary, which is the volume to which the color value belongs. The larger the output device gamut, the more colors can be reproduced.
In a preferred embodiment, the rendering apparatus is a single pass inkjet printing apparatus, which preferably renders at a speed above 70 meters per minute. The present invention is an advantage of the printing apparatus as it enables high throughput color measurements while rendering.
Examples of the invention
Fig. 7 shows an example in which a spectrophotometer (3000) is used in a rendering device for measuring rendered copies as known to those skilled in the art. A digital image (1200) comprising two color channels (1201, 1202) is rendered on a rendering device comprising two inkjet heads (1301, 1302), wherein a first color channel (1201) is transmitted to the first inkjet head (1302) via a controller (1300) and the second color channel (1202) is transmitted to the second inkjet print head via the controller (1300). The droplets form first and second ink layers (1102) by the transporting while passing (1000) through a substrate (1100) under the inkjet print heads (1301, 1302).
The rendered copy is measured by the spectrophotometer (3000), the spectrophotometer (3000) having a light source (3100) and a sensor (3150) for measuring color. The spectrophotometer (3000) reports the measured spectral reflectance curve (3305). The spectral reflectance curve (3305) includes thirteen spectral reflectance factors in the visible spectrum.
Fig. 8 shows an example of the invention, wherein the spectrophotometer of fig. 7 is adapted by having another controller (3300). The further controller (3300) also receives the digital image (1200) for analyzing the digital image to select a subset of spectral ranges. The adapted spectrophotometer (3000) reports a measured spectral reflectance curve (3300) according to the selected subset, wherein the spectral reflectance curve (3305) includes six spectral reflectance factors in the visible spectrum based on the analysis of the digital image (1200). Said analysis of said digital image is explained in detail in a preferred embodiment of the invention.
REFERENCE SIGNS LIST
200
Image gamut
215
Colour values
250
Sub-image gamut
300
Balance point
305
Points of the skeleton
400
Range of brightness
500
Framework
900
X axis
901
Y-axis
902
Z axis
950
CIEXYZ as ND-DICS
1000
Passing under the print head
1100
Substrate
1101
Printing ink layer
1102
Printing ink layer
1200
Digital image
1201
Color channel
1202
Color channel
1300
Controller
1301
Ink jet print head
1302
Ink jet print head
3000
Spectrophotometer
3100
Light source
3150
Sensor with a sensor element
3300
Controller
3305
Spectral reflectance curve
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