JP2000261679A - Image processing method and image processor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce an interpolation error without sharply increasing the scale of a color conversion table that is used in common for the interpolation. SOLUTION: At conversion of L* data into a K (black) dot area rate of a printer of an area modulation type, contents of a table such as a negative dot area rate or the dot area rate in excess of 100% that are theoretically impossible are assumed, a range of about -50 to 150% of the K dot area rate is quantized by using 9 bits each in 0-511 bits of the K data. The 128th bit of the K data is assigned to 0% of the K dot area rate, the 383th bit of the K data is assigned to 100% of the K dot area rate. A maximum value 255 of the input L* data is related to, e.g. -5% of the K dot area rate as shown at a point S3, and a minimum value 0 of the input L* data is related to, e.g. 110% of the K dot area rate as shown at a point T3. Since the dot area rate can actually take the range of 0-100%, when a result of the interpolation arithmetic operation indicates a negative K dot area rate, it is clipped to 0% and when the result indicates a value in excess of 100%, it is clipped to 100%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for processing images obtained by an image input device such as a scanner or a digital camera on a computer, or processing these images or images generated on the computer for a printer or a display. The present invention relates to a method and an apparatus for color-converting one color signal into another color signal for output by an image output device, or a method and apparatus for generating a table for the color conversion.

[0002]

2. Description of the Related Art Conventionally, linear or higher-order polynomials have often been used for color conversion. However, recently, due to a reduction in memory cost and an increase in computer operation capability, a table reference type using interpolation has been used. The scheme is rapidly spreading.

As a color conversion of an interpolation combined table reference type, a method using tetrahedral interpolation disclosed in Japanese Patent Publication No. 58-16180 or a method using prism interpolation using the 23rd Image Engineering Conference Paper Collection, p. A method using prism interpolation described in "RGB-Lab conversion of color image" or a method using known cubic interpolation is known.

Recently, ICC (Intern), a private organization,
a. The National Color Consortium) has established a standard for color conversion systems that includes color conversion based on interpolation and table reference.
published as fileFormat (http: www.color.or
g) There are also many color management system (CMS) related products based on this standard, and this standard has become a de facto standard.

According to the ICC Profile Format, a color signal independent of a device is represented by an XYZ tristimulus value or an L * a * b * color system (in this specification and the drawings, * is written right beside for convenience). , And XYZ tristimulus values, respectively.
9997, in the L * a * b * color system, 0 ≦ L * ≦ 10
0, -128≤a * ≤127, -128≤b * ≤127
It has been established. Further, in order to conform to the image processing system, the XYZ tristimulus value is quantized to 16 bits, and the L * a * b * color system is quantized to 8 bits or 16 bits. Is configured.

On the other hand, a color signal as image data is RG
There are various types such as B, L * a * b *, and CMYK, but in many cases, each color is handled after being quantized to 8 bits. For example, in the CMYK data, the color material amount of each color of CMYK represented by 0 to 100% (called a dot area ratio in an area modulation type printer) is usually 0 to 2%.
It is quantized to 55 8-bit / 256 gradations. Therefore, when the color conversion table is composed of table values of each of CMYK, the table value is 0 to 100%
Is often used. However, in order to improve the accuracy of color conversion, 0 to 100%
In the case where the table value is quantized to 65535 16 bits, the data length of the table value may be increased in a real number format such as fixed point or floating point.

In the color conversion of the interpolation-combination table reference type, in order to handle the image data quantized in this way, the range or range of the color signal is determined in advance. For example, in an image format called TIFF, the type and range of a color signal are embedded in a part of image data.
In the ICC Profile Format, a color signal range is fixed in advance, parameters such as table values are created based on the rules, and color conversion is performed. That is, the range of the color signal is determined in some form.

[0008]

However, if the table values of the color conversion table of the combined use type are created based on these rules, no matter how the table values are created correctly, a correct color conversion result can be obtained due to the interpolation error. Some areas cannot be created. 6A and 6B show a case where a color signal dependent on an image input device is converted into a color signal independent of a device and a case where a color signal independent of a device is converted into a color signal dependent on an image output device. 7 is indicated.

FIG. 6 shows an example in which a color signal dependent on an image input device is converted into a color signal independent of the device. In order to facilitate understanding, G (green) data from a scanner is converted to an L * value. However, there is essentially no change in the case of multidimensional input such as conversion from RGB data to L * a * b * values.

In the color conversion table in this case, the input G data values are 0, 32, 64, 96, 128, 160, and 19.
As table values for nine representative points of 2,224,255, each has a corresponding L * value and is plotted as shown. For the input G data values other than the representative point, the output L * value is obtained by approximating a polygonal line as shown in the figure by interpolation. The L * value is defined in the range of 0 to 100. As the L * data, the range of 0 to 100 of the L * value is quantized to 8 bits of 0 to 255.

It should be noted that the input G data value is 224 to
This is a section of 255. As a document read by the scanner, L * = 100 (reflectance is 1) does not actually exist, but L * = 100 is associated with the maximum value 255 of the input G data as at point P1. . This is to deal with paper with high whiteness, such as coated paper, or paper coated with a phosphor.

However, in this case, the input G data value is 22
As a line approximated by interpolation in the section from 4 to 255, the point P0 where the G data value is 224 is plotted from
A line passing through a point P2 having an L * value of 100 between G data values of 224 and 255 has a smaller interpolation error, and is natural and preferable.

FIG. 7 shows an example in which a color signal independent of a device is converted into a color signal dependent on an image output device.
(Black) Although shown as one-dimensional conversion to halftone dot area ratio, there is essentially no change in the case of multidimensional input such as conversion from L * a * b * data to halftone dot area ratio of each of CMYK colors. .

In the color conversion table in this case, the input L * data values are 0, 32, 64, 96, 128, 160, 19
As table values for nine representative points of 2,224,255, each has a corresponding K halftone dot area ratio, and is plotted as shown in the figure. For the input L * data values other than the representative point, the K-dot area ratio of the output is obtained by performing a polygonal line approximation by interpolation as shown in the figure. The K halftone dot area ratio is limited to the range of 0 to 100%, but as K data, the range of 0 to 100% of the K halftone dot area ratio is quantized to 8 bits of 0 to 255. .

As shown in the figure, when converting a color signal independent of a device to a color signal dependent on an image output device, the TR
Starting point of reproduction of C (Tone Reproduction Curve) and 1
If the point where the gradation should be 00% is the point S1 of the maximum value 255 and the point T1 of the minimum value 0 of the input L * data, the interpolation error becomes large.

This is further illustrated with reference to FIG. FIG.
Is an enlarged view of the section of FIG. 7 where the L * data value is 224 to 255, and indicates positive or negative as the K halftone dot area ratio. In theory and in practice, the K halftone dot area ratio is positive. 0-1
It is in the range of 00%. Lines A1 and A2 in the figure are correct values based on the actually measured values, and line A3 is a case where the K halftone dot area ratio of 0% corresponds to the maximum value 255 of the input L * data. Therefore, the interpolation error in this case is represented by the length of arrow E. The same applies to the section in which the L * data value is 0 to 32 in FIG. 7 and the section in which the G data value is 224 to 255 in FIG.

As described above, in the conventional method, the interpolation error generally increases at both ends of the output color signal range. FIGS. 6 and 7 illustrate a one-dimensional conversion, which is a phenomenon that occurs in a local area to be reproduced in white or black, but is actually close to a color gamut outline in the color gamut of the image output device. This phenomenon occurs in an area, and is a phenomenon that occurs in a fairly wide area.

Needless to say, in order to reduce such an interpolation error, the interval between the grid points in the color conversion table should be reduced so that the polygonal line approaches an ideal curve. However, in this method, the size of the color conversion table is significantly increased. For example, in the case of three-dimensional input such as conversion from RGB data to L * value, the number of grid points is 9 × 9 × 9.
= 729, for example, when the output L * data is 8 bits, a color conversion table of 729 bytes is obtained. However, the grid point interval is halved in each axis direction, and the number of grid points is 17 × 17 ×
If 17 = 4913, the output, for example, L * data is 8
When it is a bit, it becomes a color conversion table of 4913 bytes.

Therefore, according to the present invention, it is possible to reduce an interpolation error and to perform a natural color conversion without greatly increasing the scale of a color conversion table of the combined use type. .

[0020]

An image processing method according to the present invention is a method for converting an input color signal of one or more dimensions in a predetermined range into an output color signal in a predetermined value range. The output color signal values calculated in advance or separately for a plurality of representative points are stored as table values in the storage means,
Among the table values stored in the storage means, a method of calculating the value of the output color signal after conversion by interpolation from a plurality of table values corresponding to the value of the input color signal, The value exceeding the predetermined value range is held as a table value, and when the result of the interpolation operation exceeds the predetermined value range, the result of the interpolation operation is clipped within the predetermined value range. is there.

[0021]

According to the image processing method of the present invention, the interpolation error is reduced by increasing the data length of the table value by one bit or compressing the input color signal without increasing the data length. Can be
Natural color conversion can be performed.

[0022]

DESCRIPTION OF THE PREFERRED EMBODIMENTS [Embodiment as Method] (First Embodiment) In a first embodiment, the data length of a table value is increased by one bit to extend the range of the table value. Color conversion is performed by calculating a table value of a value exceeding a predetermined value range, such as a negative L * value or an L * value exceeding 100, or a negative halftone dot area ratio or a halftone dot area ratio exceeding 100%. It is stored in the storage means as a table.

As shown in FIG. 1, when converting G data into L * values, instead of quantizing the range of 0 to 100 in which the L * values are determined to 8 bits of 0 to 255, for example, , L * value in the range of approximately −50 to 150 to 0 to 511
, And the L * data value of 128 is assigned to the L * value of 0, and the L * data value of 383 is assigned to the L * value of 100. However, the L * value is 0 to 1
Since only the range of 00 is available, as will be described later, when the L * data value of the interpolation operation exceeds the above range of 128 to 383 (127 or less or 384 or more), the L * value of the interpolation operation results Clip the data value to 128 or 383.

Then, the point P1 (G data value is 255, L
* Instead of the value 100, the point P3 (G data value 25
5, L * value is, for example, 120) as a grid point, and its L *
A value (for example, 120) is used as a table value for 255 input G data values to form a color conversion table, and the input G data is converted to an output L * value by interpolation.

Accordingly, in this example, in the section where the input G data value is 224 to 255, the point P0 (G data value is 2
24, an L * value smaller than 100) and the above-described point P3, an interpolation operation is performed, and the interpolation error is reduced even in the section where the input G data value is 224 to 255.

However, since the L * value can only take the range of 0 to 100, the L * value of the interpolation operation is 10
0 (L * data value exceeds 383),
* Clip the value to 100 (L * data value is 383).

As shown in FIG. 2, when the L * data is converted to the K dot area ratio, the theoretically possible range of 0 to 100% of the K dot area ratio is changed to 8 bits of 0 to 255. Instead of quantizing, for example, the K dot area ratio is approximately -50 to
The range of 150% is quantized to 9 bits of 0 to 511, the K data value of 128 is assigned to 0% of the K dot area ratio, and the 383 of the K data value is 100% of the K dot area ratio.
Assign to However, the K dot area ratio is 0 to 10
Since only the 0% range can be obtained, as will be described later, when the K data value of the interpolation operation exceeds the above range of 128 to 383 (127 or less or 384 or more), the K data of the interpolation operation becomes Value of 128 or 38
Clip to 3.

Then, the point S1 (L * data value is 255,
Instead of the K dot area ratio of 0% and the point T1 (L * data value is 0 and the K dot area ratio is 100%), a point S3 (L
* Data values are 255, K halftone dot area ratio is, for example, -5%) and point T3 (L * data value is 0, K halftone dot area ratio is, for example, 110%) are grid points, and respective K halftone dot area ratios are used. (Eg, -5% and 110%) as table values for input L * data values of 255 and 0, respectively:
A color conversion table is formed, and input L * data is converted into an output K halftone dot area ratio by interpolation.

Therefore, in this example, in the section where the input L * data value is 224 to 255, the point S0 (L * data value is 224 and the K dot area ratio is larger than 0% as shown in FIG. 8) ) And the above point S3 are interpolated along the straight lines A1 and F. In the section where the input L * data value is 0 to 32, the above point T3 and the point T0 (L * data value is 32, K
Interpolation is performed along a straight line connecting the halftone dot area ratio with a value smaller than 100%, and the input L * data value is 224 to 25.
The interpolation error is also reduced in the section 5 and the section 0 to 32.

However, the dot percentage is 0 to 100%.
When the K halftone dot area ratio as a result of the interpolation operation is negative (K data value is 127 or less), the K halftone dot area ratio is set to 0% as shown by the straight line A2 in FIG.
(K data value is 128) and when the K halftone dot area ratio as a result of the interpolation operation exceeds 100% (K data value is 383), the K halftone dot area ratio is 100% (K data value is 383). ).

According to the first embodiment, the interpolation error can be reduced only by increasing the data length of the table value by one bit. For example, in the case of three-dimensional input such as conversion from RGB data to L * value, when the number of grid points is 9 × 9 × 9 = 729, even if the output L * data is 9 bits, for example, about 820 bytes The color conversion table may be used, and the scale of the color conversion table can be significantly reduced as compared with the case where the grid point interval is reduced as described above.

Generally, when a color signal dependent on an image input device is converted into a color signal independent of the device, the tristimulus value is 1
Is calculated as a table value of a value exceeding a predetermined value range, and a value exceeding a predetermined value, a value lower than 0, or a value theoretically impossible in a color space derived from tristimulus values is calculated, and a color conversion table is configured. What is necessary is just to store it in a storage means.

When the color signal independent of the device is converted into a color signal dependent on the image output device, when the image output device is a printer, a value higher than the highest density reproducible by the printer or the lowest density can be obtained. When the image output device is an area modulation type printer as in the example of FIG. 1, the halftone dot area ratio exceeds 100% or 0%.
When the image output device is a display using a luminous body such as a CRT display, a value exceeding the maximum brightness or a value below the minimum brightness of each luminescent color of the display is determined by the image output device as a reflection type. When it is a display, a value above the maximum luminance or a value below the minimum luminance under the lighting conditions that determine the color conversion coefficient for the reflective display is set as a table value of a value exceeding a predetermined value range, respectively. What is necessary is just to calculate and store it in the storage means constituting the color conversion table.

The above-described color conversion table generation method or image processing method can be realized on a computer by a processing program described on a recording medium such as a disk, but some of the steps can also be performed by hardware. .

(Second Embodiment) In the second embodiment,
By compressing the input chrominance signal without changing the size of the table, the range of the table values is expanded to provide a negative L *
Value or a table value of an L * value exceeding 100 or a value exceeding a predetermined value range, such as a negative halftone dot area ratio or a halftone dot area ratio exceeding 100%, is stored as a color conversion table. To be stored.

Assuming that the data length of the table value is 8 bits, in this embodiment, the input color data Din of 8 bits and 0 to 255 is in the range of 64 to 192 by a one-dimensional table as shown in FIG. Compress to However, between the representative points marked with a circle, the output value Da is converted into an integer by round-off interpolation and calculated.

As described above, the input color data Din is converted into the output value D
a is compressed to the range of 64 to 192, and the output value Da is in the range of 0 to 63 and 193 to 255, the negative L * value or the L * value exceeding 100, or the negative halftone dot area ratio or 100% A value exceeding a predetermined value range, such as a dot area ratio exceeding?

The output value Da is converted into an output value Db as an input value of another one-dimensional table as shown in FIG. As a result, a value exceeding a predetermined range, such as a negative L * value or an L * value exceeding 100, or a negative halftone dot area ratio or a halftone dot area ratio exceeding 100%, is output to the output value Db. Assigned. Also in this case, the output value Db is calculated by interpolation between the representative points.

The output value Db is converted into output color data Dout as an input value of another one-dimensional table as shown in FIG. This table contains the input value D
When b is 0 to 63 or 193 to 255, the output color data Dout is clipped to 0 or 255. Regarding the range of the input value Db from 64 to 192,
Output color data Dout is calculated by interpolation.

The above is the case of a one-dimensional input color signal. However, in the case of a multi-dimensional input color signal, FIGS. 3A and 3B
One-dimensional tables as shown in (C) and (C) are required for the number of dimensions.

The example shown in FIG. 3 is a case where a value exceeding a predetermined value range is assigned to half of 8 bits for 4 bits, but the ratio is arbitrary. However, as this ratio is smaller, there is an advantage that the information contained in the input color signal does not need to be reduced.

[Embodiment as Apparatus] (Example of Color Conversion Table Generating Apparatus) FIG. 4 shows an example of a color conversion table generating apparatus according to the present invention. It comprises a data storage unit 12, a table value calculation unit 13 and a table output unit 14.

In this example, a color conversion table for converting L * a * b * data into CMYK data for a printer is generated.
A plurality of halftone dot area data given from the outside, and L * a * b * of a color chart collected from a printer based on the data.
The pair with the colorimetric value is fetched and written into the data storage unit 12.

The data storage means 12 stores the halftone dot area ratio data and L
* A * b * colorimetric values are sent to the table value calculating means 13,
The table value calculating means 13 calculates the halftone dot area ratio data and the L * a * b * colorimetric values and the externally supplied L * a *
From b * representative point data, CMYK as a table value
The data is calculated in a floating-point format and sent to the table output means 14. The table output means 14 converts the CMYK data as table values into integers based on output format information given from the outside, , And converts the data into a predetermined table format and outputs the converted data.

From the RGB data from the scanner, L * a *
When a color conversion table for conversion to b * data is generated, the data acquisition unit 11 converts the L * a * b * colorimetric values of the color chart for scanner reading and the color chart into actual data obtained by the scanner. The pair with the RGB data obtained by being read is taken in, and the table value calculating means 13
L * a * b * data as a table value may be calculated from the L * a * b * colorimetric values and the RGB data and the externally provided RGB representative point data.

(Example of Image Processing Apparatus) FIG. 5 shows an example of an image processing apparatus according to the present invention.
It is composed of a color conversion table storage unit 21, an interpolation operation unit 22, and a clip unit 23.

The color conversion table storage means 21 stores a color conversion table generated by the color conversion table generating device as shown in FIG. The interpolation calculation means 22
The color conversion table stored in the color conversion table storage means 21 is indexed by the upper bits of the input image data, and the read table value is interpolated by the lower bits of the input image data to obtain output image data. Calculate,
Send it to the clip means 22. The clipping unit 22 clips the output image data based on clip data supplied from the outside and outputs it. Interpolation means 22
May be any of tetrahedral interpolation, prism interpolation, cubic interpolation, and the like.

[0048]

As described above, according to the present invention, the interpolation error can be reduced without greatly increasing the scale of the color conversion table of the combined use type, and the color gamut can be represented more accurately. A good output image signal without fogging or flying can be obtained.

[Brief description of the drawings]

FIG. 1 is a diagram showing an example of the method of the present invention.

FIG. 2 is a diagram showing another example of the method of the present invention.

FIG. 3 is a diagram showing still another example of the method of the present invention.

FIG. 4 is a diagram showing an example of a color conversion table generation device according to the present invention.

FIG. 5 is a diagram illustrating an example of an image processing apparatus according to the present invention.

FIG. 6 is a diagram showing an example of a conventional method.

FIG. 7 is a diagram showing another example of the conventional method.

FIG. 8 is an enlarged view showing a part of FIGS. 2 and 7;

[Explanation of symbols]

 10: Color conversion table generation device 20: Image processing device

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Ryosuke Touhou 430 Nakaicho, Ashigarakami-gun, Kanagawa Prefecture Green Tech Nakai Fuji Xerox Co., Ltd. F-term (reference) 5B057 AA11 BA28 CA01 CA08 CA12 CA16 CB01 CB08 CB12 CB16 CC01 CE18 CH07 CH11 CH18 5C076 AA26 BA06 BA07 5C077 LL17 MP08 NN03 PP14 PP31 PP32 PP33 PP36 PP38 PQ08 PQ22 PQ23 RR19 SS07 TT02 TT06 5C079 HB01 HB03 HB08 HB11 LA31 LB02 MA01 MA05 NA10 PA02 PA03 PA05

Claims (10)

[Claims] 1. A method for calculating a table value of a color conversion table for converting an input color signal of one or more dimensions in a predetermined range into an output color signal in a predetermined value range, wherein the predetermined value range is A color conversion table generation method, wherein a value exceeding the value is calculated as a table value. 2. An apparatus for calculating a table value of a color conversion table for converting an input color signal of one or more dimensions in a predetermined range into an output color signal in a predetermined value range, wherein the predetermined value range is A color conversion table generation device, wherein a value exceeding the value is calculated as a table value. 3. The color conversion table generation device according to claim 2, wherein the input color signal is a color signal dependent on an image input device and the output color signal is a color signal independent of the device. As a table value of a value exceeding the value range, a value in which the tristimulus value is greater than 1 or less than 0, or a value that is theoretically impossible in a color space derived from the tristimulus value is calculated. Characteristic color conversion table generation device. 4. The color conversion table generation device according to claim 2, wherein the input color signal is a color signal independent of a device, and the output color signal is a color signal dependent on an image output device. When the image output device is a printer, a value exceeding the maximum density or a value lower than the minimum density reproducible by the printer is a table value of a value exceeding a predetermined value range. When the image output device is a display using a luminous body, the value where the halftone dot area ratio is more than 100% or less than 0% exceeds the maximum luminance of each luminescent color of the display. When the image output device is a reflective display, the value of which is lower than the value or the minimum luminance is the illumination for determining the color conversion coefficient for the reflective display. A color conversion table generation device, wherein a value exceeding a maximum luminance or a value falling below a minimum luminance under a condition is calculated. 5. A method for converting an input color signal of one or more dimensions in a predetermined range into an output color signal of a predetermined value range, wherein the input color signal is calculated in advance or separately for a plurality of representative points of the input color signal. The value of the output color signal is stored in the storage means as a table value, and among the table values stored in the storage means, from a plurality of table values corresponding to the value of the input color signal,
In a method of calculating a value of a converted output color signal by interpolation, a value exceeding the predetermined value range is stored in the storage unit as a table value, and a result of the interpolation operation exceeds the predetermined value range. Wherein the result of the interpolation operation is clipped within the predetermined value range. 6. An apparatus for converting an input color signal of one or more dimensions in a predetermined range into an output color signal of a predetermined value range, wherein said apparatus is calculated in advance or separately for a plurality of representative points of the input color signal. The output color signal value is stored as a table value, or storage means for storing the output color signal value is provided, and among the table values stored in the storage means, interpolation is performed from a plurality of table values corresponding to the input color signal value. In an apparatus for calculating a value of an output color signal after conversion by calculation, a value exceeding the predetermined value range is held in the storage unit as a table value, and a result of the interpolation calculation exceeds the predetermined value range. Wherein the result of the interpolation operation is clipped within the predetermined value range. 7. The image processing apparatus according to claim 6, wherein when the input color signal is a color signal dependent on an image input device and the output color signal is a color signal independent of the device, the predetermined value range is set. Are stored in the storage means as table values of values exceeding the values of tristimulus values that are greater than 1 or less than 0, or values that are theoretically impossible in a color space derived from the tristimulus values. An image processing apparatus characterized by the above-mentioned. 8. The image processing apparatus according to claim 6, wherein the input color signal is a color signal independent of a device, and the output color signal is a color signal dependent on an image output device. When the image output device is a printer as a table value of a value exceeding the range of values, a value higher than the maximum density or lower than the lowest density that the printer can reproduce is an area modulation type printer. When the halftone area ratio exceeds 100% or falls below 0%, when the image output device is a display using a luminous body, the value exceeds the maximum luminance of each luminescent color of the display or When the value lower than the minimum luminance is a reflection type display, the image output device is under a lighting condition that determines a color conversion coefficient for the reflection type display. The image processing apparatus according to claim 1, wherein a value higher than the maximum luminance or a value lower than the minimum luminance is stored in the storage unit. 9. An apparatus for converting an input color signal of one or more dimensions in a predetermined range into an output color signal in a predetermined value range, wherein the output color signal is calculated in advance or separately for a plurality of representative points of the input color signal. The output color signal value is stored as a table value, or storage means for storing the output color signal value is provided, and among the table values stored in the storage means, interpolation is performed from a plurality of table values corresponding to the input color signal value. An apparatus for calculating a value of an output color signal after conversion by calculation, wherein the input color signal is compressed so that a value exceeding the predetermined value range is held in the storage unit as a table value, and the interpolation is performed. When the result of the operation exceeds the predetermined value range, the result of the interpolation operation is clipped within the predetermined value range. 10. A recording medium on which a processing program for executing the color conversion table generation method according to claim 1 or the image processing method according to claim 5 is described.