JP2000175061A - Color transformation processing method and device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an excellent device to transform an input color in a prescribed color space into an output color in other color space by separating an input color image into a luminance component and a chrominance component, using the chrominance component to obtain a color transformation function where the luminance component is an input and an output color space is an output, and applying the luminance component to the color transformation function. SOLUTION: A color transformation processor consists of an input buffer 2, an output buffer 3, a color transformation function decision means 1 and a color transformation means 22. The color transformation processor receives a color image where a chrominance component is down-sampled and stores an input color space value to the input buffer 2 tentatively. The color transformation function decision means 1 uses the chrominance component other than a luminance component to obtain a color transformation function, where the luminance component is an input and an output color space is an output. The color transformation means 22 applies the color transformation function obtained by the color transformation function decision means 1 to the luminance component of a sampling factor to transform the color.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a color image processing method and apparatus for converting an input color in a certain color space into an output color in another color space, and more particularly, to a method for expanding a compressed image. The present invention relates to a color conversion processing method and apparatus for converting into a full-color image.

More specifically, the present invention relates to a color conversion for separating a color space of a color image into a brightness component and a chrominance component, and down-sampling the chrominance component to expand a compressed image to convert it into a full-color image. The present invention relates to a processing method and apparatus.

[0003]

2. Description of the Related Art With the recent development of information processing and information communication technology, in addition to character-based computer data, various types of data such as images and voices are also stored in computer data.
It began to be handled on the system.

However, since a full-color image obtained by digitizing a photograph or the like has a large data amount, a large storage capacity is required when, for example, storing image data in a storage device. Alternatively, LAN (Local Area N)
In the case where image data is transmitted between independent computer systems via a network such as a network, the network load is high, the bandwidth is consumed, and the transfer time is increased. To reduce the amount of data, recently, JPEG
(Joint Photographic Expe
An irreversible compressed image format such as “rts Group” is generally used.

Regarding JPEG, for example, James
D. Murray and William va
"Encyclopedia of G" by nRyper
graphics File Formats ", O '
Reilly & Associates, 199
4, p. 159-171.

Here, the compression / expansion processing in the JPEG format will be described with reference to FIG.

The original image data, that is, the uncompressed color image is first subjected to color conversion in the color conversion section 53. Color conversion refers to a process of converting an input color in a specific color space (for example, RGB) into an output color (for example, YCrCb) in another color space.

Next, down-sampling is performed in the down-sampling section 54. In the JPEG format, the color space of a color image is separated into a brightness component and a chrominance component other than brightness in order to increase the data compression rate. Is generally down-sampled to a quarter or a half.
In the case of the YCrCb format color space, the brightness component corresponds to Y, and the chrominance components other than brightness correspond to Cr and Cb. In FIG. 1, a portion 50 surrounded by a broken line constitutes a color conversion processing device.

[0009] Next, the down-sampled image data is processed by using a DCT (Discrete Cos
neTransform (discrete cosine transform), quantization, and encoding.
Here, the DCT is an orthogonal transform coding system that converts information about an image into information about a frequency component by applying a DCT function to a pixel block, and is executed by the DCT unit 55. In addition, quantization refers to each DCT
This is a process in which coefficients are sequentially divided by corresponding quantization thresholds and quantized, and are performed in the quantization unit 56 in order to obtain high encoding efficiency. In addition, encoding means that each of the quantized DCs is encoded using a predetermined code table.
This is a process of assigning a code to a series of T coefficients, and is performed by the encoding unit 57. One example of encoding is Huffman encoding.

The JPEG compressed color image output from the encoding unit 57 is stored in a large-capacity storage device such as a hard disk device, or transferred to another device via a transmission medium such as a LAN.

On the other hand, when an image compressed in the JPEG format is decompressed and converted into a full-color image, it is realized by a procedure substantially opposite to that in the above-described encoding. That is, the decoding unit 58 decodes the coded compressed image to return to the quantized DCT coefficients,
9 and inverse IDCT (inverse discrete cosine transform) in the IDCT unit 60 to obtain the original n
Restore to image data of × n pixel blocks.

Next, upsampling is performed. This is because the chrominance component is downsampled in the image compressed in the JPEG format (described above). The chrominance component is returned to the same number of pixels as the brightness component by up-sampling four or two times in the up-sampling unit 61.

Conventionally, a simple method of copying pixels has been generally used for upsampling.

After up-sampling, the color space conversion is performed by using a conversion function as shown in the following equation (1), thereby converting the YCrCb color space value into the RGB color space value (however, each color of YCrCb and RGB is converted). Spatial values are both 8
Bit unsigned integer).

[0015]

(Equation 1)

However, in the conventional method, downsampling is reduced to one fourth (sampling factor SF = 4).
In this case, one set of chrominance components is copied three times, for example, six times in total in order to perform upsampling and color space conversion for four pixels. Was.

Further, in order to perform the operation described in the above equation (1), a large number of multiplications have to be performed, and the calculation load is high. If the amount of calculation is large, the calculation time inevitably becomes long. Further, the color space conversion processing is performed by an LSI (L
In the case of implementation using “Arrange Scale Integration”, since the multiplier requires a larger number of gates, the circuit scale is enlarged and the manufacturing cost is increased.

[0018]

SUMMARY OF THE INVENTION An object of the present invention is to provide an excellent color conversion process for converting an input color in one specific color space (for example, RGB) into an output color (for example, YCrCb) in another color space. It is to provide a method and an apparatus.

It is a further object of the present invention to provide an excellent color conversion processing method and apparatus for decompressing a compressed image and converting it to a full color image.

A further object of the present invention is to separate the color space of a color image into a brightness component and a chrominance component, downsample the chrominance component, expand the compressed image, and convert it to a full-color image. To provide a color conversion processing method and apparatus.

It is a further object of the present invention to provide a color conversion processing method and apparatus which can convert an image compressed in the JPEG format into a full-color image at high speed and at low cost after decompression.

[0022]

SUMMARY OF THE INVENTION The present invention has been made in consideration of the above problems, and a first aspect of the present invention is to convert an input color in a specific color space into an output color in another color space. Wherein the input color image is separated into a brightness component L and a chrominance component C, and the chrominance component C is downsampled, and (a) a brightness component of the input color space value A color conversion function determining step or means for obtaining a color conversion function using L as an input and an output color space value as an output using a chrominance component C other than the brightness component L in the input color space value; A color conversion step or means for performing color conversion by applying the brightness components L of the factor SF to the color conversion function obtained in the color conversion function determination step.

The color conversion processing method or apparatus according to the first aspect of the present invention uses, for example, JPEG (JPEG) as an input color image.
point Photographic Experts
(Group) format.

According to a second aspect of the present invention, there is provided a color conversion method or apparatus for converting an input color in a specific color space into an output color in another color space, wherein the input image has a first component. And (a) a color conversion function that takes the first component of the input color space value as an input and outputs the output color space value as an input color A color conversion function determining step or means determined by using the second component of the spatial value; and (b) applying the first components of the SF sampling factors to the color conversion function determined by the color conversion function determining step. And a color conversion step or means for color conversion.

[0025]

The present invention utilizes the property that the above equation (1) can be converted into the following equation (2).

[0026]

(Equation 2)

If the above equation (2) is more generally considered to be a conversion from an input color space separated into a brightness component and a chrominance component to an output color space, it can be expressed as the following equation (3). You.

[0028]

(Equation 3)

According to the present invention, in a color image in which a chrominance component C such as JPEG is down-sampled, a color conversion function having a brightness component L as an input and an output color space value as an output is converted into a set of chrominance components at the time of decompression. A step (or means) for determining a color conversion function obtained from C is provided. Then, in the color conversion step (or means), the brightness components L of the sampling factors SF are
Color space conversion is achieved by applying this color conversion function.

According to the present invention, since upsampling is not required at the time of decompression, high-speed and low-cost color image processing can be realized.

Still other objects, features and advantages of the present invention are:
It will become apparent from the following more detailed description based on the embodiments of the present invention and the accompanying drawings.

[0032]

Embodiments of the present invention will be described below in detail with reference to the drawings.

Embodiment 1 First, in the YCrCb color space, the chrominance components Cr and Cb are down-sampled by a factor of two each in both the horizontal and vertical directions with respect to the brightness component Y (in other words, a quarter). For example, a case will be described in which the sampling factor SF is 4) a compressed image in the JPEG format is expanded into an RGB full-color image.

In this specification, the inverse discrete cosine transform (I
nverse DiscreteCosine Tra
nsform: IDCT) and restored 8 × 8
A pixel block is defined as a minimum coding unit (Minimum C)
oded Unit (hereinafter, referred to as “MCU”).

As shown in FIG. 2, each of the RGB components of the color image before compression is composed of a 16 × 16 pixel block, that is, four MCUs. The YCrCb color space immediately after the color space conversion is performed on the 16 × 16 pixel block of the RGB original image includes a brightness component Y composed of four MCUs and a chrominance component composed of four MCUs. You. Further, by performing downsampling, the chrominance components Cr and Cb are downsized to one MCU each.

FIG. 3 schematically shows a configuration of a color conversion processing apparatus 100 used in the embodiment of the present invention. The color conversion processing device 100 includes an input buffer 2 and an output buffer 3
And a color conversion function determining unit 1 and a color conversion unit 22. Hereinafter, each unit will be described.

The color conversion processing device 100 has as input a color image in which the chrominance component C has been downsampled, and the input color space values are temporarily stored in the input buffer 2.

The color conversion function determining means 1 uses a chrominance component C other than the brightness component L to obtain a color conversion function that inputs the brightness component L and outputs the output color space value. If the output color space is in the RGB format, the obtained color conversion function is described as in the above equation (2). Further, the color conversion function can be represented by a general expression as shown in the above expression (3).

The color conversion means 22 is a color conversion function determining means 1 for the brightness components L of the sampling factor SF.
The color conversion of the input image is performed by applying the color conversion function obtained by (1).

The output color space value after the color conversion is temporarily stored in the output buffer 3 and then output outside the device 100.

FIG. 4 is a flowchart showing a color conversion processing procedure executed by the color conversion processing apparatus 100. Hereinafter, each step will be described.

First, initialization is performed in step S1. The initialization processing includes addressing of the input color space values Y, Cr, and Cb stored in the input buffer 2.

Next, in step S2, it is checked whether or not all of the input color space values have undergone color conversion. If the color conversion has been completed, the process proceeds to Yes, and the processing procedure illustrated in FIG. 4 ends. On the other hand, if not completed, the process goes to the branch No and proceeds to the next step S3.

In step S3, the color conversion function determining means 1
Extracts the respective values of the chrominance components Cr and Cb from the input color space value from the input buffer 2, and advances each address by one.

In step S4, a color conversion function is determined. In this embodiment, since the color conversion function is based on the above equation (2), the constants ConstR and ConstR are calculated by multiplying the chrominance components Cr and Cb by the conversion coefficients.
It is necessary to find G and ConstB. For example, Cr =
50, Cb = 212, ConstR = −10
9.356, ConstG = 26.796, Const
B = 148.848. According to the color conversion function as shown in the above equation (2), the RGB output color space can be obtained by using only the brightness component Y of the input color space values as input.

Next, in step S5, step S4
Is set in the color conversion means 22.

Next, in step S6, the loop counter i is initialized to zero.

Next, in step S7, the loop counter i and the sampling factor SF (in this example, SF =
4) is compared. If the loop counter i is smaller, the process proceeds to the next step S8; otherwise, the process proceeds to the step S8.
2 and the same processing as described above is repeated.

In step S8, the color conversion means 22 takes out only one value of the brightness component Y from the input buffer 2.

Next, in step S9, using the value of the extracted brightness component Y and applying it to the above equation (2),
Find output color space values.

In step S10, the RGB output color space value is set in the output buffer 3, and the address indicating the storage position of the brightness component Y and the loop counter i are each set to 1
Then, the process returns to step S7 to repeat the same processing as described above.

Next, the improvement effect of the present embodiment over the conventional example will be described.

FIG. 5 is a table showing the main operations required to obtain the RGB output color space values for four pixels and the number of operations in the conventional example and the present embodiment.

In the conventional example, the chrominance components Cr and C
b had to be copied three times each, for a total of six times. On the other hand, in the present embodiment, the number of times of copying is zero. Further, in the conventional example, 16 multiplications are required, whereas in the present embodiment, the number can be greatly reduced to 4 times.

<< Embodiment 2 >> In the first embodiment, YCrCb
Has been specifically described in the case where a color space having a color component as an input and a color space having a color component of RGB as an output. On the other hand, in the second embodiment, an arbitrary color space (CIEL * a * b) in which the brightness component L and the chrominance component C are separated.
*, Etc.) as input and an arbitrary color space as output.

In short, this is a case where an arbitrary color conversion function f _C shown in the above equation (3) is used.

Generally, the color conversion function f _C tends to change gradually with respect to the brightness component L of the input color space value. Utilizing this property, the color conversion means 2 is obtained by using the polynomial approximation shown in the "color conversion apparatus" described in Japanese Patent Application No. 10-289153 already assigned to the present applicant.
2 can be used.

[Supplement] The present invention has been described in detail with reference to the specific embodiments. However, it is obvious that those skilled in the art can modify or substitute the embodiment without departing from the spirit of the present invention. That is, the present invention has been disclosed by way of example, and should not be construed as limiting. In order to determine the gist of the present invention, the claims described at the beginning should be considered.

[0059]

As described above in detail, according to the present invention,
An excellent color conversion processing method and apparatus for converting an input color in a specific color space (for example, RGB) into an output color (for example, YCrCb) in another color space can be provided.

Further, according to the present invention, it is possible to provide an excellent color conversion processing method and apparatus for decompressing a compressed image and converting it to a full-color image.

Further, according to the present invention, there is provided a method for separating a color space of a color image into a brightness component and a chrominance component, down-sampling the chrominance component, expanding the compressed image, and converting the compressed image into a full-color image. Color conversion processing method and apparatus can be provided.

Further, according to the present invention, it is possible to provide a high-speed and low-cost color conversion processing method and apparatus for expanding an image compressed in the JPEG format and converting the image into a full-color image.

In the present invention, a color conversion function that receives a brightness component L as an input and an output color space value as an output is obtained from a chrominance component C other than the brightness component L. Then, color conversion is performed by applying the brightness components L of the sampling factors SF to the color conversion function. Therefore, according to the color conversion processing method and apparatus of the present invention, upsampling is not required at the time of decompression, so that color conversion processing can be performed at high speed, and low-cost color conversion processing with a small amount of calculation is realized. can do.

[Brief description of the drawings]

FIG. 1 is a diagram showing a procedure of JPEG compression.

FIG. 2 is a diagram showing sampling of a chrominance component.

FIG. 3 is a color conversion processing apparatus 10 provided for implementing the present invention.
FIG. 2 is a block diagram showing a schematic configuration of a zero.

FIG. 4 is a flowchart illustrating a processing procedure in the color conversion processing apparatus 100 according to the present embodiment.

FIG. 5 is a diagram showing the number of operations in the first embodiment.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Color conversion function determination means, 2 ... Input buffer, 3 ... Output buffer, 22 ... Color conversion means, 100 ... Color conversion processing device.

Claims (6)

[Claims] 1. A color conversion processing method for converting an input color in a specific color space into an output color in another color space, wherein an input color image is separated into a brightness component L and a chrominance component C and a chrominance component C is down-sampled, and (a) a color conversion function that receives a brightness component L of an input color space value as an input and outputs an output color space value as a chrominance component of the input color space value other than the brightness component L A color conversion function determining step obtained by using C; and (b) a color conversion step of performing color conversion by applying the brightness components L of the sampling factors SF to the color conversion function obtained by the color conversion function determining step. And a color conversion processing method. 2. An input color image is a JPEG (Joint) image.
Photographic Experts Grou
The color conversion processing method according to claim 1, wherein the image is an image compressed in p) format. 3. A color conversion processing method for converting an input color in a specific color space into an output color in another color space, wherein the input color image is separated into a first component and a second component, and 2 is down-sampled, and (a) a color conversion function using the first component of the input color space value as an input and the output color space value as an output using the second component of the input color space value Determining a color conversion function to be determined, and (b) performing a color conversion by applying the first components of the sampling factor SF to the color conversion function determined by the color conversion function determination step. A color conversion processing method. 4. A color conversion processing device for converting an input color in a specific color space into an output color in another color space, wherein the input color image is separated into a brightness component L and a chrominance component C and a chrominance component. C is down-sampled, and (a) a color conversion function that receives a brightness component L of an input color space value as an input and outputs an output color space value as a chrominance component of the input color space value other than the brightness component L C: a color conversion function deciding means obtained by using C; and (b) a color conversion means for performing color conversion by applying the brightness components L of the sampling factors SF to the color conversion function obtained by the color conversion function deciding means. And a color conversion processing device. 5. An input color image is JPEG (Joint).
Photographic Experts Grou
The color conversion processing device according to claim 4, wherein the image is an image compressed in p) format. 6. A color conversion processing apparatus for converting an input color in a specific color space into an output color in another color space, wherein the input color image is separated into a first component and a second component, and 2 is down-sampled, and (a) a color conversion function using the first component of the input color space value as input and outputting the output color space value as output using the second component of the input color space value Color conversion function determining means to be obtained; and (b) color conversion means for performing color conversion by applying the SF first components to the color conversion function obtained in the color conversion function determining step. A color conversion processing device characterized by the above-mentioned.