JPH10191374A - Digital still camera - Google Patents

Abstract

PROBLEM TO BE SOLVED: To compress image data efficiently by detecting the luminance of an image or a color characteristic and compressing the luminance signal and the chrominance signal depending on the detected luminance of the image or the color characteristic so as to prevent required information from being lost by the compression. SOLUTION: Each element of a quantization matrix is changed for each frame based on color component information obtained from the output of a white balance detection circuit 16, luminance information obtained from the output of a luminance detection circuit 17, and high frequency luminance information obtained from a high frequency luminance detection circuit 18. Thus, the compression rate of a luminance signal and the compression rate of a chrominance signal are changed depending on the luminance or the color tone of the image. For example, even in the case of a black/white image without color contrast or a color image, when there is a large change in the hue and the saturation, the content of each element of the quantization matrix with respect to the chrominance signal is increased to compress the chrominance signal at a high compression rate and the content of each element of the quantization matrix with respect to the luminance signal is decreased to compress the luminance signal at a low compression rate.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a digital still camera, and more particularly to compression of an image signal.

[0002]

2. Description of the Related Art In a digital still camera, a video signal output from a solid-state image pickup device is digitally processed into component signals of a luminance signal and a color signal, and these signals are recorded on a recording medium. At the time of recording, signal compression is performed to effectively use the capacity of the recording medium. Signal compression is performed in the same processing procedure using the same parameters when capturing any image.

In general, to improve the usability, the number of compressed images to be recorded on a recording medium is generally determined in advance. For this reason, the recording capacity allocated to one frame of image is constant regardless of the type of image, and any image data is compressed data close to the fixed amount within a range not exceeding the fixed amount. Such a method of compressing data is called constant rate control. In the conventional compressed data constant rate control, a component signal is input to a compression processing circuit, a part or all of an input image is compressed by a predetermined algorithm, a data amount after compression is measured, and a result is obtained. The compression was repeated by changing parameters used for compression based on the compression.

[0004]

However, the component signals are different for each image to be captured, and if the component signals are compressed by the same processing procedure using the same parameters regardless of the characteristics of the image to be captured, the individual Optimum compression cannot be performed for the image. For example, when an image without color information is processed as color information, compressed data is assigned to a color without information, even though originally the largest possible compressed data should be assigned to luminance information. become. That is,
The data amount of the luminance signal is reduced by the data amount of the color signal, and a part of the luminance information is lost, thereby deteriorating the quality of the image.

[0005] Even in the constant rate control of compressed data, since a predetermined algorithm is not suitable for each input image, all the images cannot be satisfactorily compressed by the algorithm. In addition, it takes time to change parameters in a certain manner and repeat compression until obtaining compressed data of a certain amount or less, and it takes a certain amount of time while minimizing information lost due to compression within a limited time. It is difficult to obtain the following compressed data.

An object of the present invention is to provide a digital still camera that efficiently compresses image data while preventing necessary information from being lost due to compression.

[0007]

In order to achieve the above object, according to the present invention, there is provided a solid-state imaging device for converting received light into an analog image signal and outputting the same, and an analog image output from the solid-state imaging device. A conversion circuit for converting a signal into a digital image signal and outputting the image signal; an image processing unit for generating and outputting a luminance signal and a color signal from the image signal output from the conversion circuit; and a luminance signal output from the image processing unit A digital still camera having compression means for compressing and outputting the color signal and recording means for recording the luminance signal and the color signal output from the compression means, wherein the image processing means converts the luminance signal and the color signal from the image signal. In the process of generation and output, the luminance or color characteristics of the image are detected, and the compression unit compresses the luminance signal and the color signal according to the luminance or color characteristics of the image detected by the image processing unit. And things.

[0008] The image processing means generates a luminance signal and a color signal from the digitized image signal and outputs the signal, while detecting characteristics relating to luminance or color of the image. The compression unit compresses the luminance signal and the chrominance signal output from the image processing unit, but compresses the luminance signal and the chrominance signal differently for each image. Since the compression of the luminance signal and the color signal of each image is performed according to the luminance or color characteristics detected by the image processing means for the image, compression suitable for the characteristics of the image can be performed.

In the above digital still camera, the image processing means detects the white balance of the image signal output from the conversion circuit and adjusts the white balance.
A luminance signal and a color signal are generated and output from the image signal after white balance adjustment, and the compression unit detects a color component from the white balance detected by the image processing unit,
It is preferable to compress the luminance signal and the color signal according to the detected color component.

[0010] By detecting and adjusting the white balance, it is possible to record an image having a good color tone regardless of the color temperature of external light. On the other hand, the color components of the image can be detected from the detected white balance, and the luminance signal and the chrominance signal are compressed accordingly, whereby the compression according to the color characteristics of the image can be performed. For example, in an image having a large change in hue or saturation, the color signal is not compressed so much as to leave sufficient color information. Conversely, in an image having little change in hue or saturation, the color signal is sufficiently compressed because little color information is lost by compression.

The image processing means detects a luminance difference between image regions from the image signal output from the conversion circuit, and the compression means converts a luminance signal and a chrominance signal in accordance with the luminance difference detected by the image processing means. You may make it compress.

For an image having a large luminance difference, that is, a high contrast, the luminance signal is not compressed so much and luminance information is sufficiently left. On the other hand, images with low contrast
Since the necessary luminance information is not lost even if it is compressed, the luminance signal is sufficiently compressed. The luminance difference detected by the image processing means is
Adjustment of the amount of light received by the solid-state imaging device, that is, exposure control is also possible.

The image processing means detects the high frequency range of the generated luminance signal to correct the luminance signal, and outputs the corrected luminance signal in order to enhance the outline of the image. The luminance signal and the chrominance signal may be compressed according to the high frequency range detected by the image processing means.

The image processing means outputs a corrected luminance signal which emphasizes the outline of the image, and this is compressed together with the color signal. At this time, the compression unit can compress the luminance signal and the color signal without impairing the effect of the outline enhancement according to the high frequency band detected for the outline enhancement.

In the present invention, the compression means changes the rate at which the luminance signal is compressed and the rate at which the chrominance signal is compressed in accordance with the luminance or color characteristics of the image detected by the image processing means. It is assumed that the ratio between the amount of the signal and the amount of the color signal after compression is changed.

By changing the relative amount ratio of the luminance signal and the color signal after compression, even when the total amount of the compressed signal is determined, the luminance information and the color information are more important for image reproduction. Can be left more. For example, in the case of an image having a large contrast and a monotonous color tone, the color signal is further compressed, whereby the amount of the luminance signal after compression is increased and sufficient luminance information is left. Conversely, in the case of an image having a small contrast and a large change in color tone, the luminance signal is further compressed, thereby increasing the amount of the color signal after compression and leaving sufficient color information.

The compression means changes the compression ratio of the luminance signal and the chrominance signal until the sum of the amount of the luminance signal after the compression and the amount of the color signal after the compression becomes a predetermined amount or less. Compression may be repeated.

[0018] Regardless of the difference in image characteristics, the signal amount after compression of all the images becomes equal to or less than a certain amount, and the recording means can record a certain number of images.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the digital still camera according to the present invention will be described below with reference to the drawings. FIG. 1 shows a schematic configuration relating to generation and processing of an image signal of the digital still camera according to the present embodiment. This camera is
A photographing lens 1, a mechanical diaphragm 2, a CCD 3 which is a solid-state image sensor, an analog signal processing circuit (AGC / CDS) 4 for performing automatic gain control and sample hold, an AD converter 5, a digital signal processing circuit 6, an image compression processing circuit 7, It comprises a recording medium 8, a timing generator for supplying various control signals such as field shift pulses to the CCD 3, a driver 9, and a microcomputer 10 for controlling the entire camera.

The photographing lens 1 uses light from a photographing object as a CCD.
An image is formed on the light receiving surface 3 and the aperture 2 is moved from the photographing lens 1 to the CC.
The light flux reaching D3 is regulated to adjust the amount of light received by CCD3. The CCD 3 converts the received light into charges for each pixel and stores the charges, and outputs the stored charges as an analog image signal.

FIG. 3 shows a pixel arrangement of the CCD 3. Of the three color pixels of red (R), green (G), and blue (B), G-
Horizontal lines composed of R pairs and horizontal lines composed of GB pairs are alternately arranged vertically. The CCD 3 outputs pixel signals of one horizontal line during one horizontal scanning period, sequentially changes the output line for each horizontal scanning period, and outputs pixel signals of all horizontal lines during one vertical scanning period. An image of one frame is composed of pixel signals output in one vertical scanning period. FIG. 4 shows pixel signals output during two consecutive horizontal scanning periods.

The output signal of the CCD 3 is sampled by the analog signal processing circuit 4, amplified, and output to the AD converter 5. The AD converter 5 converts an analog signal input from the analog signal processing circuit 4 into a digital signal and outputs the digital signal to the digital signal processing circuit 6.

FIG. 2 shows the configuration of the digital signal processing circuit 3. The digital signal processing circuit 3 outputs a white balance (W
B) Adjustment circuit 11, delay / interpolation circuit 12 for performing delay and interpolation for each of R, G, and B planes, gamma correction circuit 1
A matrix circuit 14 for generating a luminance signal (Y) and a color signal (C) from three color signals of R, G, and B; an enhancer circuit 15 for emphasizing the contour of the luminance signal; The balance detection circuit 16 includes a high-frequency luminance detection circuit 18 that counts luminance high-frequency information output from the enhancer circuit 15.

The output signal of the AD converter 5 is input to a white balance adjustment circuit 11. The white balance adjustment circuit 11 individually adds a gain to each of the R, G, and B signals in order to correct a difference in light receiving sensitivity due to the color temperature between the R, G, and B pixels.

In order to set the gain to be added to each signal, for example, a white screen is photographed in advance, and the R, G, B signals are converted into I / Q axes by the white balance detection circuit 16 and are set for each predetermined area. And stored in the microcomputer 10. The microcomputer 10 compares the output of the white balance detection circuit 16 with the stored information at the time of actual photographing, calculates a gain to be added to each of the R, G, and B signals, and Control.

In calculating the gain, the microcomputer 10 obtains information on the color components of the image such as hue and saturation from the output of the white balance detection circuit 16.
This information is used for compression of an image signal described later.

The output of the white balance adjustment circuit 11 is input to the delay / interpolation circuit 12. Delay / interpolation circuit 12
Uses a plurality of horizontal line delay circuits to interpolate discontinuous R, G, and B signals with upper, lower, left, and right pixel signals, respectively, to generate R, G, and B planes.

The luminance detecting circuit 17 includes the delay / interpolating circuit 12
A luminance signal is generated from the output R, G, and B signals, integrated for each predetermined area, and provided to the microcomputer 10. The microcomputer 10 obtains the amount of light received by the CCD 3 from the given luminance signal, and controls the aperture 2 so that the CCD 3 receives an appropriate amount of light.

Here, the luminance signal supplied from the luminance detecting circuit 17 to the microcomputer 10 indicates the luminance for each region of the image, and the luminance difference between the regions can be determined from the luminance signal. This information is used for compression of an image signal described later.

R, G, and R generated by the delay / interpolation circuit 12
The signal of the B plane is gamma-corrected by the gamma correction circuit 13 and then input to the matrix circuit 14. The matrix circuit 14 generates and outputs a luminance signal and a chrominance signal from the three color signals of R, G, and B. The luminance signal is input to the enhancer circuit 15. The enhancer circuit 15 detects a high-frequency component of the image from the luminance signal for each pixel, corrects the luminance signal, and enhances the outline of the image.

The high-frequency components detected by the enhancer circuit 15 are integrated for each predetermined area by the high-frequency luminance detection circuit 18 and provided to the microcomputer 10 as high-frequency component information. This high frequency component information is also used for compressing the image signal.

The color signal generated by the matrix circuit 14 and the luminance signal corrected by the enhancer circuit 15 are output from the digital signal processing circuit 6 as signals of FIG.
Is input to the image compression processing circuit 7. The image compression processing circuit 7 performs a discrete cosine transform on each of 8 × 8 pixels for each of the luminance signal and the chrominance signal using, for example, a JPEG (Joint Photographic Expert Group) baseline algorithm, and converts the conversion result to 8 × The quantization result is quantized by a quantization matrix of 8 and entropy coding is performed on the quantization result. As a result, the luminance signal and the chrominance signal are each compressed. For the entropy coding, for example, the Huffman coding method is used. The luminance signal and the chrominance signal compressed by the encoding are recorded on the recording medium 8.

The quantization is a process of dividing each element of the matrix subjected to the discrete cosine transform by the corresponding element of the quantization matrix and discarding the fractional part. If each element of the quantization matrix is large, the data after compression becomes small (high compression ratio), but on the other hand, the information lost due to truncation increases. Conversely, if each element of the quantization matrix is small, the amount of lost information decreases, but the compression ratio decreases. Even if the same compression method is used, the compression ratio changes depending on the quantization matrix.

FIG. 5 shows an example of processing up to quantization performed by the image compression processing circuit 7. In this figure, S1 is an 8 × 8 matrix which is a part of an input luminance signal, S2 is a matrix obtained by performing a discrete cosine transform of the matrix S1, and M
Is a quantization matrix, and S3 is a matrix obtained by quantizing the matrix S2 by the quantization matrix M. The upper left part of the matrix S2 after the discrete transformation represents the low frequency component of the luminance signal, and the lower right part represents the high frequency component. In this example, since there is not so much difference between the intensity of the luminance signal, that is, the elements of the matrix S1, as shown in the matrix S3, the high frequency components are lost by the quantization.

By performing inverse quantization in which each element of the quantized matrix S3 is multiplied by each element of the quantization matrix M, a matrix approximate to the matrix S2 can be reproduced. By performing the conversion, a matrix approximate to the matrix S1 of the original luminance signal can be reproduced. Therefore, the recording medium 8
After decoding the compressed signal recorded in, the inverse quantization and inverse transform are performed, the luminance signal and the chrominance signal are reproduced, and an image close to the image before the compression processing is obtained.

In the digital still camera of the present embodiment, the quantization matrix is obtained by using color component information obtained from the output of the white balance detection circuit 16, luminance information obtained from the output of the luminance detection circuit 17, and a high-frequency luminance detection circuit. The change is made for each frame based on the high frequency component information obtained from the output of No. 18. Thereby, the compression ratio of the luminance signal and the compression ratio of the color signal are changed according to the luminance and the color tone of the image.

For example, when there is no significant change in hue and saturation even in a black-and-white image having no color shading or a color image, the elements of the quantization matrix for the color signal are increased to compress the color signal at a high compression rate. Compression is performed, and the luminance signal is compressed at a low compression ratio by reducing the elements of the quantization matrix for the luminance signal. As a result, a portion having no information in the color signal can be discarded, and luminance information necessary for reproducing an image can be sufficiently stored.

For an image having a small luminance difference, the compression ratio of the luminance signal is increased by increasing the elements of the quantization matrix for the luminance signal, and a portion of the luminance signal having no information is discarded. Conversely, in an image having a large luminance difference, the elements of the quantization matrix for the luminance signal are reduced to lower the compression rate of the luminance signal. As a result, the luminance information is sufficiently held, and the brightness of the image is reproduced well.

When there are many high-frequency components in the luminance signal, the elements of the low-frequency portion (upper left portion of the matrix M in FIG. 5) of the quantization matrix for the luminance signal are increased and the high-frequency components (the same matrix M (Lower right of the element). As a result, it is possible to faithfully record an image in which the outline is emphasized or an image in which the contrast is finely changed.

If the values of the elements of one of the quantization matrix for the luminance signal and the quantization matrix for the chrominance signal are changed, the ratio of the compressed data amount of the luminance signal to the chrominance signal changes. In particular, if the elements of these two quantization matrices are changed in the opposite direction so that one becomes larger and the other becomes smaller, the ratio of the data amount after compression changes significantly. If the signal is compressed in this way, it is easy to keep the total amount of compressed data substantially constant while preserving more of either the luminance information or the color information.

In the digital still camera of the present embodiment,
It is possible to perform a constant rate control of compressed data in which the total amount of data after compression is equal to or less than a predetermined amount for all frames, and a variable rate control of compressed data in which such an upper limit is not provided for the amount of data after compression. Even when performing variable rate control,
Judging from the luminance and color characteristics of the image, a signal having no information can be discarded, so that the compression ratio increases. Therefore, the recording medium 8 can be used effectively.

Even in the constant rate control, it is possible to judge the importance of information from the luminance characteristics and color characteristics of the image and discard unnecessary or less important information, so that the total amount of data after compression can be quickly reduced. To a predetermined amount or less. That is, convergence is faster than in the conventional constant rate control using the same quantization matrix for all frames.

Although the quantization matrix is necessary for inverse quantization, in the present digital still camera, the quantization matrix is not constant but differs for each frame, so that the quantization matrix used for the luminance signal and the chrominance signal are different. The quantization matrix used for this is recorded on the recording medium 8 together with the compressed luminance signal and color signal. The recording medium 8 will be used more than before by the amount of data of these matrices, but the amount is as small as 1 kilobyte at most per frame.
Considering that it is necessary to compress a × 8 matrix by 10,000 or more per frame, there is no problem.

[0044]

According to the digital still camera of the first aspect, since the luminance signal and the color signal can be compressed in accordance with the luminance or color characteristics of the image, the information about the luminance and the information about the color are lost by the compression. Is suppressed. Therefore, a high quality image can be reproduced from the compressed signal. Further, since the compression ratio can be increased by discarding unnecessary information according to the characteristics of the image, it is possible to record images of many frames.

According to the digital still camera of the second aspect, the color of the object to be photographed can be correctly recorded irrespective of the color temperature of the external light, and the signal can be compressed according to the color characteristics of the image. For example, for an image with little change in hue or saturation, even if the color signal is compressed at a high compression ratio, information about the color is not lost, so the signal amount after compression should be reduced while maintaining high image quality. Can be. Further, since the amount of the color signal after compression is reduced, the amount of the luminance signal after compression can be increased, and more information about the luminance can be retained after the compression, thereby improving the image quality. . on the other hand,
For images with large changes in hue and saturation, reducing the compression ratio of the color signal allows more information about the color to be retained even after compression, and reproduces high-quality images from the compressed signal. Is possible.

In the digital still camera according to the third aspect, signal compression according to the contrast of an image can be performed. For example, it is possible to sufficiently retain luminance information by reducing the compression ratio of a luminance signal for an image having a large contrast, and it is possible to record a high-quality image that faithfully represents light and dark. Conversely, for an image with a small contrast, even if the luminance signal is compressed at a high compression ratio, information regarding the luminance is not lost, so that the signal amount after compression can be reduced. In addition, the compression rate of the color signal can be reduced and a large amount of color information can be held, so that the reproduced image has high quality that faithfully represents the color.

According to the digital still camera of the fourth aspect, it is possible to generate a signal in which the outline is enhanced, and the effect of the outline enhancement is not lost even by the compression. Therefore, an image reproduced from the signal after compression is a sharp image with a clear outline.

According to the digital still camera of the fifth aspect, even when the total amount of the compressed signal is determined,
Of the luminance information and the color information, more important ones for representing image characteristics can be left. For example, for an image having a large contrast, the compression ratio of the luminance signal is relatively lowered to sufficiently retain the luminance information, and for an image with a large change in color tone, the compression ratio of the color signal is relatively reduced to obtain the color information. Can be recorded without losing the features of the image.

According to the digital still camera of claim 6, 1
Compressed data constant rate control for setting an upper limit on the amount of signal per frame after compression is performed. At this time, it is possible to compress the signal in accordance with the luminance and color characteristics of the image, thereby compressing the signal at a high compression rate without losing information. Can be reduced to a predetermined amount or less. In addition, each of the recorded frames faithfully reproduces the contrast and color, and there is no difference in image quality between the frames.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a schematic configuration relating to generation and processing of an image signal of a digital still camera.

FIG. 2 is a block diagram illustrating a configuration of a digital signal processing circuit.

FIG. 3 is a diagram showing a pixel arrangement of a CCD.

FIG. 4 is a diagram showing pixel signals output from a CCD during two horizontal scanning periods.

FIG. 5 is a diagram illustrating an example of processing up to quantization performed by an image compression processing circuit.

[Explanation of symbols]

 Reference Signs List 1 shooting lens 2 mechanical aperture 3 CCD 4 analog signal processing circuit 5 AD converter 6 digital signal processing circuit 7 image compression processing circuit 8 recording medium 9 timing generator and driver 10 microcomputer 11 white balance adjustment circuit 12 delay / interpolation circuit 13 gamma correction Circuit 14 Matrix circuit 15 Enhancer circuit 16 White balance detection circuit 17 Luminance detection circuit 18 High frequency luminance detection circuit

Claims (6)

[Claims] 1. A solid-state imaging device that converts received light into an analog image signal and outputs the same, and a conversion circuit that converts an analog image signal output from the solid-state imaging device into a digital image signal and outputs the digital image signal. Image processing means for generating and outputting a luminance signal and a color signal from the image signal output from the conversion circuit, and compression means for compressing and outputting the luminance signal and the color signal output from the image processing means, In a digital still camera having recording means for recording a luminance signal and a color signal output from the compression means, the image processing means generates a luminance signal and a color signal from the image signal and outputs the luminance signal and the color signal. Alternatively, the compression unit detects color characteristics, and the compression unit compresses a luminance signal and a color signal according to the luminance or color characteristics of the image detected by the image processing unit. Mera. 2. The image processing means detects a white balance of an image signal output from the conversion circuit, adjusts the white balance, and generates a luminance signal and a color signal from the image signal after the white balance adjustment. The compression unit detects a color component from the white balance detected by the image processing unit, and compresses a luminance signal and a color signal according to the detected color component. A digital still camera according to item 1. 3. The image processing means detects a luminance difference between image regions from an image signal output from the conversion circuit, and the compression means responds to a luminance difference detected by the image processing means. The digital still camera according to claim 1, wherein the luminance signal and the chrominance signal are compressed. 4. The image processing means detects a high frequency band of the generated luminance signal to correct the luminance signal, and outputs the corrected luminance signal in order to enhance the outline of the image. 2. The digital still camera according to claim 1, wherein the compression unit compresses a luminance signal and a chrominance signal in accordance with a high frequency band detected by the image processing unit. 5. The compressed luminance signal by changing a rate of compressing a luminance signal and a rate of compressing a color signal in accordance with luminance or color characteristics of an image detected by the image processing means. 2. The digital still camera according to claim 1, wherein a ratio between the amount of the color signal and the amount of the color signal after compression is changed. 6. The compression means changes the luminance signal and chrominance signal compression ratio until the sum of the amount of the luminance signal after compression and the amount of the color signal after compression becomes a predetermined amount or less. The digital still camera according to claim 1, wherein compression of the color signal is repeated.