JPH066611A - Picture converter - Google Patents

Abstract

PURPOSE:To realize the picture converter capable of color correction processing sufficiently effective at a high speed without need of provision of a special hardware or the like when full color picture data in a form of frequency data are used for input data. CONSTITUTION:A DC color correction section 102 applies color correction processing to DC component data 106 in input frequency data to generate color correction DC component data 107. A decoding section 103 generates decoding color data 109 from the color correction DC component data 107 and AC component data 108 and prints out a color picture by a color output device such as a color printer 104.

Description

Detailed Description of the Invention

[0001]

The present invention relates to gradation color image data converted into frequency data by a method such as discrete cosine transform (hereinafter referred to as DCT or DCT transform).
The present invention relates to an image conversion device that decodes and performs color correction processing suitable for an output device.

[0002]

2. Description of the Related Art In recent years, JPEG (Joint Photo
A technique has emerged that allows highly efficient compression of gradation image data, such as the opaque Expert Group), while maintaining high image quality, resulting in an enormous amount of data. It has become easier to handle.

Here, the principle of image data encoding using orthogonal transformation, which is used in the JPEG standard and the like, will be briefly described. The transform coding by orthogonal transform is to realize efficient coding by removing the correlation between sample values by converting sample values into mutually orthogonal coordinate systems. In addition, by converting to a frequency component, allocating a large number of bits to a low-frequency component in which the power is concentrated and also has a great influence on visual characteristics, the high-frequency component is quantized with a small number of bits, Increase data compression. The orthogonal transform for transforming an image into frequency components includes Hadamard transform, DCT and the like.

Next, DCT and its inverse transformation (hereinafter referred to as IDCT
With reference to FIG. 3, an example of encoding 8 × 8 pixels as one block will be described. The original image data is divided into blocks of 8 × 8 pixels and converted in block units. Depending on the relative position of each pixel in one block, the upper left pixel is the zeroth data of the origin,
The data of the i-th pixel in the right direction and the j-th pixel in the downward direction is represented as f (i, j) (0≤i <8, 0≤j <8, i, j is an integer). The origin data in the upper left corner is f (0,
0) and the data in the lower right corner is f (7,7). DC at this time
The T-transformed data F (u, v) (0 ≦ u <8, 0 ≦ v <8, u, v are integers) is

[0005]

[Equation 1]

It becomes like.

F (0,0) represents a value corresponding to average data in the image block, and is called a DC coefficient in the sense of a DC component. In the case of Expression 1, it is actually eight times the average value of the data in the block. The other 63 elements represent AC components and are also called AC coefficients. The larger the values of u and v in Expression 1, the higher the frequency component. When data compression is performed, the fact that the higher the frequency component in a natural image is, the smaller the amount is, and the sensitivity of the human eye is also reduced, the lower bits of the high frequency component are discarded and quantization is performed with a small number of bits. .

The inverse discrete cosine transform (IDCT) for obtaining f from F is

[0009]

[Equation 2]

[0010] However, c (u), c (v) in Equation 2
The definition of is the same as in the case of Expression 1. As I mentioned earlier, J
According to the PEG standard, 1) two-dimensional DCT 2) quantization of DCT coefficients to appropriate precision 3) Huffman coding of quantized DCT coefficients Highly efficient compression with little deterioration in image quality is performed by the above procedure. ing.

On the other hand, when outputting the gradation color image data to the output device, a color correction process suitable for the output device is widely performed to improve the color reproducibility. Often performed is a masking process by a 3 × 3 matrix operation, which is performed on color data represented by three subtractive primary color components of cyan (hereinafter C), magenta (hereinafter M), and yellow (hereinafter Y) In order to correct that the color characteristics of the existing CMY ink deviate from the ideal CMY ink and include unnecessary color components, the input data is C, M, Y, and the actual ink amount actually printed by the printer is Ci. , Mi, Yi,

[0012]

[Equation 3]

The color correction is performed by the method. Also,
If this is still not enough, add more quadratic terms

[0014]

[Equation 4]

There are also cases where color correction is performed using. Equation 3,
Kij (i, j is an integer of 1 to 9) in Equation 4 is an appropriate coefficient for correction.

Further, based on the actual output color measurement result, a three-dimensional conversion color table from C, M, Y to Ci, Mi, Yi is created, and the result is written in the ROM or the like.
Color correction may be performed in some cases.

In addition, even in the three primary color system of additive color mixture of red (hereinafter R), green (hereinafter G), and blue (hereinafter B), similar color correction is performed when the emission characteristics of the CRT phosphor to be output are matched. Processing is performed.

[0018]

The color reproducibility is surely improved by these color correction processes. However, even in the simplest color correction processing by 3 × 3 matrix calculation, nine times of multiplications are required to correct all three colors, which takes a very long processing time, and a method using a three-dimensional ROM table or the like. Then, a huge ROM memory capacity is required.

In particular, when image data converted into frequency data by using a technique such as DCT is handled, it takes time to decode the data, and further color correction processing is required. For this reason, it is not practical unless some speed-up means is taken.

Therefore, an object of the present invention is to solve the above-mentioned problems, and when full color image data in the form of frequency data is used as input data, no special hardware or the like is required, and high speed operation is possible. The object is to realize an image conversion device that enables color correction processing that is sufficiently effective at.

[0021]

The image conversion apparatus of the present invention is an image in which frequency image data obtained by converting gradation color image data composed of color components of at least three primary colors into spatial frequency components for each color component is input image data. A DC color correction unit, which is a conversion device, performs color correction processing on the DC component in the frequency data and outputs corrected DC component data, and decoding processing based on the AC component data and the color corrected DC component data. It is characterized by comprising a decoding unit for performing.

[0022]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the image conversion apparatus of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing an embodiment of the present invention in the case of handling DCT-converted image data with full color CMY data for each color component and 8 × 8 pixels as one block.

The DCT image data 105 of C, M and Y components are input to the data input unit 101. The DCT image data 106 may be obtained directly from a file in the magnetic disk storage device or a communication line, or may be obtained in the decoding process of data compressed according to the JPEG standard or the like.

Of the DCT image data 105, C, M,
Only the DC component data 106 of each Y color is stored in the DC color correction unit 1.
Go to 02.

The DC color correction unit 102 performs the color correction processing by the 3 × 3 matrix calculation of the equation 3 as shown in the conventional example according to the characteristics of the color printer 104 to output, and the colors of CMY colors. Corrected DC component data 1
07 is created. The color-corrected DC component data 107 goes to the decoding unit 103 next. The AC component data 108 in the DCT image data 105 directly from the data input unit 101 is also input to the decoding unit 103. Decoding unit 103
On the basis of the AC component data 108 and the color-corrected DC component data 107, the data is decoded by the IDCT process of Expression 2 to generate decoded color data 109. The decoded color data 109 is the color printer 104.
The color printer 104 outputs a color image according to the CMY ink amounts of the decoded color data 109.

In the above embodiment, the CMY color data is color-corrected and output to the printer, but the RGB data may be color-corrected and output to the CRT.

FIG. 2 is a block diagram showing another embodiment of the present invention, which is an embodiment when the input is RGB data and the output is CMY data. In the present embodiment, in addition to the original color correction processing, there is a pre-processing for converting AC and DC RGB data into CMY data. In the data input section 101,
DCT image data 105 of R, G, and B components are input. Of these, only the DC component data 106 of R, G, and B colors go to the DC color correction unit 102.

In the DC color correction unit 102, first, the DC data of R, G, B are converted into C, M, and C by the processing of C = W−R M = W−G Y = W−B.
Convert to Y DC data. W in the above equation is the maximum value that can be taken by the R, G, B data. In the processing of the above equation, when the R, G, B data is binary digital data, all bits are inverted and the 1's complement is calculated. It corresponds to the process of taking. DC color correction unit 1
After performing the above conversion processing, the reference numeral 02 performs color correction processing according to the characteristics of the color printer 104 that outputs the color correction DC component data 1 for each CMY color as in the embodiment of FIG.
Create 07. The color correction DC component data 107 is then input to the decoding unit 103.

In the case of the embodiment shown in FIG. 1, the AC component data 108 of RGB is directly input to the decoding unit 103.
In this embodiment, the AC component conversion unit 201 is entered once. Here, the conversion formula of Cac = -Rac Mac = -Gac Yac = -Bac is performed on the AC components Rac, Gac, and Bac of each RGB color, and the RGB AC component data is converted into the CMY AC component data Cac, Mac, Converted AC converted to Yac
The component data 202 is output. Converted AC component data 20
2 is input to the decoding unit 103, and the following is the same processing as that of the embodiment of FIG. 1 to generate the decoded color data 109 for the printer 104.

In the above-described embodiment, the color correction calculation is performed using the equation 3
Although the × 3 matrix calculation process is used, any color conversion process such as the color correction process shown in Expression 4 may be used.

Further, in the above embodiment, one pixel is 8 × 8 pixels.
An example of image data that has been orthogonally transformed is described as a block, but the pixel size of one block is 4 × 4, 16 ×
Any number such as 16 may be used. Further, it may be a number other than a power of 2 such as 10 × 10.

In the above embodiment, the output device is a color printer, but it may be another output device such as a CRT display.

As described above, in the present invention, instead of performing the color correction processing on the original image data obtained by decoding the DCT data, the color correction is performed only on the DC component before the decoding processing. In the case of DCT data with 8 × 8 pixels as one block, the number of DC component data is 1/64 of the total frequency data, and the number of data that needs color correction calculation is greatly reduced. As described above, the DC component is a value corresponding to the average value of the data in the block. For this reason, the color correction accuracy in an image block with many high-frequency components is slightly reduced, but in the image portion where such high-frequency components are originally large and the color changes rapidly within the block, the color may be slightly displaced. Does not matter so much. In addition, when a natural image is handled as image data, the amount of AC component is generally not so large, so that the problem caused by color correction only for the DC component does not occur much, and compared to when no color correction is performed, Greatly improved color reproducibility.

[0034]

As described above, since the image conversion apparatus of the present invention performs the color correction calculation only for the DC component in the frequency data, it can be used when the DCT block size is 8 × 8. The amount of color correction processing, which is 1/64, has been drastically reduced, and it has become possible to perform color correction tailored to the output device at a sufficiently high speed without using special hardware.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention.

FIG. 2 is a block diagram showing another embodiment of the present invention.

FIG. 3 is a diagram showing DCT conversion.

[Explanation of symbols]

 101 data input unit 102 DC color correction unit 103 decoding unit

Claims (3)

[Claims] 1. An image output device using, as input image data, frequency data obtained by converting gradation color image data composed of color components of at least three primary colors into spatial frequency components for each color component. A DC color correction unit that performs color correction processing on the DC component and outputs corrected DC component data; AC component data in the frequency data and the color correction DC.
An image conversion device comprising a decoding unit that performs a decoding process based on component data. 2. The image conversion apparatus according to claim 1, wherein the frequency data is data obtained by orthogonally converting gradation image data. 3. The image conversion device according to claim 1, wherein the frequency data is data obtained by performing discrete cosine conversion on gradation image data.