NL8701148A - APPARATUS FOR COLOR CORRECTION C.Q. -CALIBRATION OF THE COLOR IMAGE ON A MONITOR. - Google Patents

Description

N.0. 34,492 1 ^ ί

Device for the color correction or calibration of the color image on a monitor.

The applicant mentions as inventor: Ir. Ph. Bierlaire.

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The invention relates to a device for correcting or calibrating colors of a color image to be displayed on a monitor, for instance for producing a so-called hard copy, starting from a stored or received video image. Hard copy 10 means the final result of various processes, such as a photographic process, a printing process, and the like.

In practice, such a device is known in which data of an image original, such as a drawing or design, is obtained and stored by means of opto-electronic scanning and subsequent color filtering and further processed in the color calculator in accordance with the desired hard copy. Depending on the color image required on the final hard copy, corrections can be made in this color calculator. Such a device can for instance be used in printing processes for maps, posters, textile fabrics, carpets, and the like. All these processes are based on the color image obtained by scanning or the video color image available in a memory or provided by another source. In the execution of such a printing process, the said color image will usually be displayed on a monitor for checking.

A major problem here is that the colors of the image displayed on the monitor are not representative and therefore cannot be reproduced correctly. Large differences may occur depending on the type of monitor as well as the quality of the supplied analog image signals. This is very disadvantageous, for example, if one wants to make a hard copy of a video image. The color impression or tonality then obtained often differs considerably from that observed on the screen. The reasons for this are: 1) that the human eye reacts differently to that of a hardcopy on a visual stimulation from a cathode ray tube of a monitor. This is because the image display on the monitor is a direct stimulus (namely the light source and the origin of the information are in the same place) and the hard copy is a direct stimulus, because the colors observed here result of reflection or transmission of certain frequencies present in the incident or 8701 148 * 2

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continuous light; 2) that the more transformations, for example scanning, processing, etc., the video data undergoes, the more they change. That is, change of color without loss of quality having to occur. This is the case, for example, when an image treated by a digital system is to be printed or displayed on a display. Each stage separating the image visualized on the screen from the final printout produces inaccuracies that are ultimately visible to the eye. This is very annoying, for example when writing out or reproducing hard copies. An empirical adjustment of the parameters occurring in the different stages can be carried out to overcome the inaccuracies, which however is a long and unreliable execution; 3) that it has further been proven that the physiological reactions of the eye can differ greatly from person to person. This can occur eg.

when perceived by a person with eye abnormalities of a color image on monitor or television screen. In this case, an accurate adjustment of the colors of the color image on the monitor or the television screen is certainly desirable. With the current devices, a selective adjustment of the additive and subtractive colors - each separately - is not possible; in general, only brightness and contrast control are possible, but always in a substantially linear manner for all three basic colors at the same time; 4) that the different materials used in the printing, such as paper, film, ink, and the like, have a great influence on the definition of the colors themselves. It is clear, for example, that the color of a pure magenta ink is different from the pure magenta shown on the monitor. The ink will contain small amounts of cyan and yellow. This will not be displayed on a monitor 30, which can - by construction - only manipulate pure colors (after all, the colors are defined here according to their spectral content). There is therefore a substantial difference in the definition of the used color axis system of the color impression on the monitor and that of the color impression on the hard copy (see Figure 35 1).

There are, however, some analog image processing systems with which it is possible to manipulate the various spectral components accurately. However, these systems have the disadvantage that they are expensive and complex in design and take up a lot of space. Because of this, they are not suitable for applications, such as adjustment of 8701 1 48% 3 a television screen or monitor. In addition, they cannot be directly linked to a digital image processing system to interactively correct and calibrate colors.

The object of the invention is to overcome the above-mentioned problems and to provide a device with which, via a host processor via a data line or via an independent unit, such as a keyboard, or in more simple embodiments such as a television screen, via a programmable readout memory (PROM) this correction can be made.

This is achieved in a device of the type mentioned in the opening paragraph according to the invention in that the device can be connected in series to the input lines of the monitor, in that the three color signals intended for the monitor (red, green, blue) are digitally each color signal being passed over a number of parallel lines 15, the device containing three parallel processors - one for each additive color - each having at least three adjustable color transforming matrices and a subsequent output command processor, the three additive color signals - in each parallel processor - are each applied to the corresponding color transformation matrices and changed in value therein for correction or calibration, and the outputs of the three matrices are applied to the associated output summation processor to obtain the corresponding summed output color signal .

With this embodiment according to the invention, the colors of the image on the monitor can be adjusted as desired for a range of applications by changing the color information of the picture elements or pixels on the screen without changing the original image information obtained or stored from scanning . In a possible application, this image information is further processed and used for the production of a hard copy.

Other applications can be influencing the color information in household use of television screens, photogrammetric map techniques, air, water and environmental pollution monitoring, remote sensing, etc. An application can be the correction of the color information from, for example, an analog input device, such as a color camera. . It is hereby possible to treat the original image data between their origin and the monitor in real time at a level at which the signals are in analog form.

The invention will be further elucidated on the basis of some exemplary embodiments with reference to the drawings, in which: 8701 148% 4 Figure 1 gives an example of two color axis systems and an axis system of the image on a monitor (defined by theory ) and one coordinate system of the color reproduction of a hard copy influenced by the operating parameters; Figure 2 shows a general diagram of the connection of the device according to the invention in an image processing system for the production of a hard copy; Figure 3 shows an example of a look-up table memory (LUT) as used in the device according to the invention; Figure 4 shows the diagram of the conversion of the video sampling frequency to the internal operating frequency of the device and back again; Figure 5 shows the block diagram of the parallel processor for each of the three colors; Figures 6a and b show an example of the modification of the function stored in a look-up table memory, carried out in a color correction for a hard copy; and Figures 7a to 7d show examples of the application of the device according to the invention in an image processing system.

In Figure 1, C ^, C> 2 * ^ 3 shows the pure color coordinate system, as defined by theory, of a color image visible on a display screen such as a cathode ray tube. In the same figure, C1 ^, C1 ^ C ^ shows a color axis system of the colors of a hard copy as determined by the different operating parameters that have affected the production of this hard copy, such as the ink, the paper, etc. Each color element in the color axis system of the hard copy (C1 · ^, C ^ j C '^) has components in the three pure colors. In practice, these components have the same names in both axle systems. This schematic representation of both 30 axis systems also shows the color difference between the color image on the monitor and the color image of the hard copy.

In the color correction or calibration according to the invention, use is made of color transformation matrices, with the advantage of memories that function as a look-up table (Look-Up Table). A conversion function is loaded into such a memory, whereby an unambiguous and discrete conversion of the input value to the output value of a picture element is obtained: output ^ = f (input ^) with i = number of the discrete level. In practical terms, the input value is used as the memory address.

In addition, a real-time conversion must first take place, since the sampling frequency of, for example, 14 MHz of the video signal used is too high for such a memory. To this end, a conversion takes place to an internal lower operating frequency and after processing a conversion back to the above higher video sampling frequency. Memories can also be used where conversion to a lower operating frequency is not required.

The said look-up table memory has a great flexibility since it uses RAM memories in which arbitrary values and also functions such as linear, logarithmic, exponential and other functions can be stored. In practice, the circuit will be more complicated since it is desired to determine the output colors as a combination of the input colors in order to define any arbitrary color axis system. To this end, a number of look-up table memories are connected in parallel and the outputs thereof are fed to a summation processor.

Figure 2 shows how a device 10 according to the invention is included in a system for producing a hard copy. 0 indicates the operator of the device. 1 denotes an image processing system or source of image data. From the system 1, those data signals are then fed to the production device in direction 8 to produce a hard copy, such as a slide, a drawing, a map, a decor, or carpet, etc. by known techniques, such as offset printing, gravure printing, flexographic printing, screen printing or screen printing, etc. C indicates the three analog image signals, which are each supplied via an A / D converter 2 to a data processing unit 4, as with reference to Figures 4 and 5 will be explained. From the data processing units, the three digital output signals are each converted again into an analog video image signal via a D / A converter 3 and fed to the monitor 7. 5 denotes a synchronization unit in the device 10 corresponding to 30 synchronizes the operations in the device 10 with the synchronizing signal of the system 1. 6 denotes an interface unit for the operator or user 0; such as a keyboard, a ROM memory, a connection line to a host computer, etc.

In Figure 3, a indicates the table of memory that constitutes the output value 35 which has a predetermined fixed correspondence to the input value. B indicates the addresses of the memory that are equal to the input values. As an example, the input value x ^ of the input signal is given. The function loaded in the look-up table memory is indicated by c. In this example, the function equals 40 to f (x) «255-x. After conversion, the input value x ^ results in an 870 1 14 8 6 output value fix. ^). This function is only an example to indicate the principle of this type of memory. Any other unambiguous and adjustable function can be loaded into such a memory, see also figure 6 for this purpose. Figure 5 shows the use of this memory. Any such memory can process a digital word supplied via, for example, N parallel input lines and output it after conversion via, for example, K parallel output lines.

In figure 4 the data processing unit 4 of figure 2 is indicated in more detail. The three digital image or 10 color signals C ^ n (i.e. R ^ n »G ^ n> B. ^) obtained after A / D conversion are each supplied to such a digital image processing unit 4. These color signals C ^ n are each digitally indicated by N bits which are applied to N parallel input lines. Thus, the single input lines shown in Figure 4 (C. ^) and Figure 5 (Rj> Gj> Dj) are essentially N-fold parallel input lines. Also, the single output line shown in Figure 4 (C ^ t) and Figure 5 (C ^ duitj) is in fact an M-fold parallel output line (where M may be equal to or not N) over which M bits are fed.

First, for each color signal, a conversion of the vi-20 sampling frequency to the lower internal operating frequency takes place. For this purpose, a frequency division shift register 11 is used, i.e. in N-fold, a separate shift register for each input line. The exemplary eight output lines of each shift register 11 transmit the input signal at various times of the shift 25 in the register. That is, the pixels or pixels 1, 9, 17, ... 1 + ix8 pass through the output 1 of the register, and pixels j + ix8 pass through the output j (j = 1 ... 8).

Reference numeral 12 denotes a parallel processor at the input of which, in addition to the Nx8 output signals of the N shift registers of, for example, the color signal R ^ n, also the Nx8 output signals of the N shift registers of the color signal Gin and of the N shift registers of the color signal B ^ n are supplied. This also applies to the other two parallel processors 12 of the color signals and Bnn, which are thus each supplied with 3xNx8 input signals. Figure 5 shows the schematic of a parallel processor 12, which is N-shaped.

The three additive color signals R ^, G ^, Β ^ shown in Figure 5 are applied to the three look-up table memories (LUT) R, G, B of the colors red, green, blue. With a further improvement of this 40 parallel processor, three additional look-up table memories C, Μ, Y have been provided for the subtractive 8701 1A8 i 7 color signals C ^ derived from the additive color signals R ^, Gj, B ^ and fed via the input sum processors 21. , M ^, Y of the subtractive colors cyan, magenta, yellow. The lookup table memories are adjustable from the outside. 22 denotes the output summation processor for summing the intensity sword, and in the respective color C output line of each LUT may be K-fold, where K may be equal to or not equal to N.

The summation processor is for each output color C ^ ie R, »G. »B. can be K-fold again. Each out of out processor 22 has M parallel output lines.

Cul (. “R (Rin> + G <Gln) + B (Bln> + 10 Gln‘ Bin »+ M <VRin> Gin’ hnY> + T (flï (Rln ‘Gln’ Bin ^)

Figure 6a shows an example of a function loaded into a single call table memory, which may be, for example, a memory of the memories R, G, B, C, M or Y of Figure 5. At the points 0%, 25%, 50%, 75% and 100%, the function can be changed from the outside in accordance with the arrows. Figure 6b shows how the function can be changed after correction or calibration.

It is clear that with this device according to the invention, a subjective color correction can be performed according to the user's requirements as a comparative calibration, compared to eg an existing hard copy, of the image on the screen of the monitor. . A comparative calibration can be obtained using a standardized test. A test hard copy with a small window in it is placed at a certain distance in front of the screen of the monitor in order to be able to expose the test hard copy without the incident light rays influencing the color of the image on the screen.

Then the color of the area of the screen of the monitor observed through the window of the test hardcopy is compared with the color of the test hardcopy observed around the window. It is then possible to act on the adjusting members of the various look-up table memories in accordance with the manner shown in Fig. 6a, b.

Figures 7a to 7d show some more applications of the device according to the invention.

Figure 7a shows the recording of the device 10 between an image processing system 1 and a monitor 7, as already explained with reference to Figure 2.

Figure 7b shows the recording of the device 10 between an antenna and, if necessary, an antenna amplifier 30 and a television screen 31. Here, the colors of the image on the screen 31 can be adapted to the requirements of the user, eg 8701148 8 r 4. due to eye defects, are adjusted with the device 10.

Figure 7c shows the recording of the device 10 between an input device 32, such as, for example, a camera, and an image processing system or a monitor 33. Here, the colors of the color image output by the device 32 are changed according to the requirements of the user. adapted with the device 10.

Figure 7d shows the recording of the device 10 between an image processing system or monitor 34 and an analog output device 35, such as, for example, a Polaroid print camera. The colors of the color image to be printed can herein be adjusted again with the device 10.

For programming the different look-up table memories, the device is provided with a microprocessor which manages the externally supplied instruction signals. These command signals can be output from a host computer over a data line, through an independent unit, such as a keyboard, or in simpler configurations through a programmable readout memory or PROM.

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Claims (6)

1. Device for correcting or calibrating the colors of a color image to be displayed on a monitor, characterized in that the device is connectable in series to the input lines of the monitor, that the three, for the monitor destined color signals (red, green, blue) are processed on a digital basis, each color signal being passed over a number of parallel lines, the device containing three parallel processors (12), one for each additive color, 10 each provided of at least three adjustable color transformation matrices (R, G, B) and a subsequent summation processor (22), the three additive color signals (R ^, G .. ^, B ^ n) - in each parallel processor - being applied to the corresponding color transformation matrices (R, G, B) and changed in value therein for correction or calibration, and wherein the output signals of the three matrices (R, G, B) are associated with the corresponding output sum rocessor (22) are supplied to obtain the corresponding summed output color signal R Guit ° f Buit * 20 Device according to claim 1, characterized in that each parallel processor (12) additionally comprises three adjustable color transformation matrices (C, Μ, Y) - one for each subtractive color - each of which is preceded by an input summation processor (22) to which in each case the three additive color signals (R ^ n> G ^ n, Bin) are supplied * to obtain a subtractive color signal (C ^ n, Y ^ n), the output signals of the three matrices (C, Μ, Y) are also fed to the output summation processor (22). 30 Device according to claim 1 or 2, characterized in that the device - for each additive color (R, G, B) - has a frequency division shift register (11) at the input and a frequency multiplier shift register (13) at the output, concerning additive color signal (Rin »Gin or B ^ n) in the shift register (11) is converted into a number of parallel data signals to be fed to the summation processor (12), wherein the summation processor (12) is additionally provided, for each additive color, with so many adjustable color transformation matrices (R, G, 40 B) followed each time by an output summation processor (22), and for every 8701148 'i subtractive color, of as many adjustable color transformation matrices (C, Μ, Y) preceded each time by an input summation processor (21) as necessary to the said number of parallel data signals ( R ^, G ^, Β ^), wherein the output color signals (R ^ t> 5. or B) of the output summation processors (22) are applied in parallel out to the respective frequency multiplication register (13) to obtain the output color signal Or 5UJ 'uitj uitj The device according to any one of the preceding claims, characterized in that said color transformation matrix is a look-up table memory (LUT). 5. Device according to claim 1, characterized in that the device is provided at the input of three A / D converters and at the output of three D / A converters for the three analog color signals intended for the monitor. 6. Device as claimed in claim 1, characterized in that a synchronization circuit is arranged to synchronize all operations in the device with the synchronization associated with the three color signals. 870 1 148