JP3947912B2 - Image signal processing device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an image signal processing apparatus suitable for a digital camera or the like, and more particularly to an image signal processing technique for reducing noise in an image signal acquired via an image sensor and achieving high image quality.
[0002]
[Prior art]
In recent years, image quality deterioration due to random noise of an imaging system included in a captured image has become a problem due to S / N degradation accompanying an increase in the number of pixels and sensitivity of an imaging element mounted on a digital camera or the like. As a solution to this problem, conventionally, a method of removing noise by using a low-pass filter or a median filter for an image signal obtained from an imaging system has been proposed (Japanese Patent Laid-Open No. 4-235472). ).
[0003]
[Problems to be solved by the invention]
However, since the low-pass filter and median filter for reducing noise have a large calculation scale, the circuit scale becomes large when hardware is built by LSI or the like, resulting in a cost increase.
[0004]
Further, since the degree of noise generation varies depending on the shooting sensitivity and shooting mode in a digital camera, it is desirable to optimize the characteristics of the noise reduction filter accordingly. However, in order to realize this, it is necessary to prepare a plurality of filters having different characteristics, which causes an increase in cost.
[0005]
The present invention has been made in view of such circumstances, and appropriately performs noise reduction processing according to the degree of noise generation, generates a good image signal, and performs such noise reduction processing on a small scale. An object of the present invention is to provide an image signal processing device that can be realized with a circuit configuration and can minimize cost increase.
[0006]
[Means for Solving the Problems]
In order to achieve the above object, an image signal processing apparatus according to the present invention includes an imaging unit that converts an optical image into an electrical signal, and an image signal that generates an image signal in a digital signal format from the electrical signal output from the imaging unit. A generation unit; a noise reduction unit that reduces a noise component of the generated image signal; a signal transmission unit that re-inputs the image signal output from the noise reduction unit to the noise reduction unit; and the noise reduction unit. A frequency setting means for setting the number of times of noise reduction processing; and a control means for controlling an execution operation of the noise reduction processing by the noise reduction means according to the set number of times, wherein the frequency setting means displays one screen. It is characterized in that it is divided into a plurality of areas and the number of noise reduction processes is changed for each area .
[0007]
According to the present invention, an optical image of a subject is photoelectrically converted by an imaging unit and output as an electrical signal. The output signal of the image pickup means is sent to the image signal generation means, where it is converted into a digital image signal of a predetermined method such as a luminance / color difference signal format. The image signal generated by the image signal generation means is input to the noise reduction means, and noise components are reduced. The image signal whose noise has been reduced by the noise reduction means can be input again to the same noise reduction means via the signal transmission means, and the noise reduction processing can be repeated. The number of times of noise reduction processing can be arbitrarily set by the number of times setting means, and the control means performs control to switch execution / non-execution of noise reduction processing based on the set number of times.
[0008]
As described above, since the number of times of noise reduction processing by one noise reduction unit is made variable and the number of times is controlled, various noise removal characteristics can be realized, and the circuit configuration can be simplified.
[0009]
According to an aspect of the present invention, in addition to the above-described configuration, the information processing apparatus includes a condition information acquisition unit that acquires information related to conditions at the time of shooting, and the number setting unit is based on the condition information acquired by the condition information acquisition unit. It is characterized in that the number of noise reduction processes is determined.
[0010]
The condition information includes various types of information related to the degree of noise generation such as shooting sensitivity, exposure time, and shooting mode, and at least one information content (preferably a plurality of information contents) is acquired. By controlling the number of times of noise reduction processing based on this condition information, it is possible to effectively reduce noise according to the degree of noise generation, and it is possible to generate a good image signal that maintains both image quality maintenance and noise reduction. .
[0011]
In the image signal processing apparatus of the present invention, it is preferable that a filter having a minimum unit configuration that can function as a nonlinear filter such as a low-pass filter or a median filter is used as the noise reduction means. Various filter characteristics can be realized by controlling the number of processings with the minimum configuration, so that the circuit configuration can be further simplified, and the cost of hardware such as LSI can be minimized. .
[0012]
DETAILED DESCRIPTION OF THE INVENTION
Preferred embodiments of an image signal processing apparatus according to the present invention will be described below with reference to the accompanying drawings.
[0013]
FIG. 1 is a block diagram of an image signal processing apparatus according to an embodiment of the present invention. The image signal processing apparatus 10 is applied to, for example, a digital camera. The optical system 14 of the imaging unit 12 includes a photographic lens, a diaphragm, and a shutter mechanism, and light that has passed through the optical system 14 is incident on the imaging device 16. A solid-state imaging device represented by a CCD image sensor or a CMOS image sensor is used for the imaging element 16.
[0014]
A large number of photosensors are arranged in a plane on the light receiving surface of the image pickup device 16, and has a Bayer array G and other predetermined color filter array structures. The optical image of the subject formed on the light receiving surface of the image sensor 16 via the optical system 14 is converted into signal charges of an amount corresponding to the amount of incident light by each photosensor. The signal charge accumulated in each photosensor is sequentially read out as a voltage signal (imaging signal) corresponding to the signal charge based on a pulse supplied from a driver circuit (not shown).
[0015]
A signal output from the imaging element 16 is sent to the imaging processing unit 18. The imaging processing unit 18 includes a sampling hold circuit, a color separation circuit, a gain adjustment circuit, an A / D converter, and the like, and the input imaging signal is subjected to correlated double sampling (CDS) processing and R, G, and the like in the imaging processing unit 18. Each color signal of B is subjected to separation processing, signal level adjustment (pre-white balance processing) of each color signal is performed, converted into a digital signal, and then sent to the image signal processing unit 20.
[0016]
The sensitivity of the imaging system is variable within a range corresponding to ISO 100 to ISO 800, and the user can select a desired imaging sensitivity. The operation unit 22 includes an operation unit for selecting a photographing sensitivity, and the control circuit 24 sets a gain in the imaging processing unit 18 according to the photographing sensitivity designated by the user.
[0017]
The control circuit 24 performs an automatic exposure (AE) calculation based on the imaging signal received via the imaging processing unit 18, and controls the charge accumulation time (exposure time) of the diaphragm and the image sensor 16 according to the calculation result. .
[0018]
In addition to the above-described means for selecting the photographing sensitivity, the operation unit 22 is a means for selecting an operation mode of the camera, a release button for inputting a photographing start instruction, a menu item selection operation (cursor movement operation), and playback. A cross-direction key for inputting an instruction for frame advance / reverse of an image, an execution key for instructing selection (registration) or execution of an operation, a selection item, etc., or a cancellation for canceling an instruction. It includes operation means for the user to input various instructions such as keys. The instruction signal from the operation unit 22 is input to the control circuit 24, and the control circuit 24 controls the operation of the corresponding circuit based on the received instruction signal.
[0019]
The imaging signal input from the imaging processing unit 18 to the image signal processing unit 20 is converted into an image signal according to a predetermined signal format such as a luminance signal (Y signal) and a color difference signal (Cr, Cb signal) in the image signal processing unit 20. Converted. The image signal generated by the image signal processing unit 20 is input to the noise reduction circuit 28 via the selector 26. Note that the present invention is not limited to a mode in which both the luminance signal and the color difference signal are subjected to noise reduction processing, and a part of the image signal generated by the image signal processing unit 20 (for example, only the luminance signal or only the color difference signal) is subjected to noise reduction processing. May be.
[0020]
The selector 26 is means for selectively switching a signal to be sent to the noise reduction circuit 28, and operates according to a command from the control circuit 24.
[0021]
The noise reduction circuit 28 includes, for example, a two-pixel average filter which is a minimum unit of a low-pass filter, or a three-pixel median filter which is a minimum unit of a median filter which is a non-linear filter effective for noise removal (see FIG. 2). Is used.
[0022]
The two-pixel average filter is a filter that outputs an addition average value of input two-pixel data (data for two adjacent pixels) as output data. As shown in FIG. 2, the three-pixel median filter is a filter that rearranges input three-pixel data into three values of a minimum value, a median value, and a maximum value, and outputs the median value as output data. The configuration of the noise reduction circuit 28 is not limited to the above example, and various filters such as a 3 × 3 9-pixel median filter can be applied.
[0023]
As shown in FIG. 1, the signal output from the noise reduction circuit 28 is stored in the memory 30 and is input to the noise reduction circuit 28 via the selector 26 again. The control circuit 24 includes a central processing unit (CPU), controls the number of noise reduction processes by the noise reduction circuit 28 based on a predetermined program, and controls the switching of the selector 26.
[0024]
The number of times of noise reduction processing is variable depending on various conditions (sensitivity, exposure time, shooting mode, etc.) at the time of camera shooting. For example, when the photographing sensitivity is equivalent to ISO 100, the sensitivity is low and the level of noise in the image signal is small. Therefore, the number of times of processing is 0 (no noise reduction processing is performed), and the level of noise increases as the sensitivity increases. The number of processing is controlled according to the degree of noise, such as 3 times for ISO 400, 5 times for ISO 800, and so on.
[0025]
The control circuit 24 functions as a control unit for the number of times of noise reduction processing, and also functions as a control unit for the entire digital camera, and performs autofocus (AF) control, automatic exposure (AE) control, and image signal processing control of the optical system 14. Further, image signal recording control, recording medium read / write control, display control represented by a liquid crystal display (LCD), and the like are performed.
[0026]
Next, the operation of the image signal processing apparatus 10 configured as described above will be described.
FIG. 3 is a flowchart showing a processing procedure of the image signal processing apparatus 10 according to the present embodiment. As shown in the figure, when the process starts, first, the control circuit 24 acquires condition information at the time of photographing (step S110). Here, various condition information such as information on the selected shooting mode, shooting sensitivity, and exposure time is acquired.
[0027]
Next, the number of times of noise reduction processing is set based on the acquired condition information (step S120). The optimum number of processes varies depending on the actual device such as the configuration of the noise reduction circuit to be used and the performance (size and characteristics) of the image sensor. A program for determining the number of processing times is designed in consideration of a specific device configuration.
[0028]
As a common tendency, (1) the number of processing is increased when the photographing sensitivity is increased. (2) Increase the number of processes in the night scene shooting mode. (3) When the brightness is dark (when the exposure time is long), the number of processes is increased. That is, a large number of noise reduction processes is set for a situation where noise is expected to increase.
[0029]
After step S120, noise reduction processing is performed while counting the number of times of noise reduction processing, and it is determined whether or not the number of processing times has been executed (step S130). When the set number of processes has not been executed, the noise reduction process is repeated, and when the set number of processes has been executed, the process ends.
[0030]
Thus, the image signal that has been subjected to the noise reduction processing for the set number of processing times is thereafter output to the display device through necessary signal processing such as contour correction, gamma correction, and color correction, or recorded on a recording medium.
[0031]
4 to 8 are graphs showing the characteristics of the two-pixel average filter. FIG. 4 shows the characteristics of the number of times of processing, and FIGS. 5 to 8 show the characteristics of the number of times of processing of 2 to 5, respectively.
As is apparent from these drawings, various filter characteristics can be realized simply by changing the number of times of processing. The higher the number of times of processing, the higher the effect of reducing high-frequency components and the greater the noise removal effect.
[0032]
In the above embodiment, the number of times of noise reduction processing is set for one screen. However, one screen may be divided into a plurality of areas, and the number of times of noise reduction processing may be changed for each area. For example, when shooting using a strobe, the center of the screen is bright and the surroundings are dark. Therefore, a better image can be obtained by changing the number of times of noise reduction processing according to the area divided by the brightness level.
[0033]
【The invention's effect】
As described above, according to the present invention, the number of times of noise reduction processing by the same noise reduction processing means is variable, and various filter characteristics are realized by controlling the number of times of processing as necessary. Therefore, simplification of the circuit configuration can be achieved, and hardware such as LSI can be realized at low cost. In addition, according to the present invention, the optimum filter characteristics are realized by adjusting the number of times of processing according to the degree of noise generation based on the condition information at the time of shooting, so that noise can be maintained while maintaining good image quality. An image signal from which components are effectively removed can be obtained.
[Brief description of the drawings]
FIG. 1 is a block diagram of an image signal processing apparatus according to an embodiment of the present invention. FIG. 2 is an explanatory diagram showing a processing procedure of a three-pixel median filter. FIG. 4 is a graph showing the characteristics of the two-pixel average filter (the number of times of processing is one). FIG. 5 is a graph showing the characteristics when the two-pixel average filter is used twice. Graph showing the characteristics when used twice. Fig. 7 Graph showing the characteristics when using the two-pixel average filter four times. Fig. 8 Graph showing the characteristics when using the two-pixel average filter five times. Explanation】
DESCRIPTION OF SYMBOLS 10 ... Image signal processing apparatus, 16 ... Imaging device, 18 ... Imaging process part, 20 ... Image signal processing part, 22 ... Operation part, 24 ... Control circuit, 26 ... Selector, 28 ... Noise reduction circuit

Claims (3)

Imaging means for converting an optical image into an electrical signal;
Image signal generating means for generating an image signal in a digital signal format from an electrical signal output from the imaging means;
Noise reduction means for reducing noise components of the generated image signal, signal transmission means for re-inputting the image signal output from the noise reduction means to the noise reduction means,
Number setting means for setting the number of times of noise reduction processing by the noise reducing means;
Control means for controlling an execution operation of noise reduction processing by the noise reduction means according to the set number of times;
Equipped with a,
The number-of-times setting means divides one screen into a plurality of areas, and sets the number of times of noise reduction processing for each area .   The apparatus according to claim 1, further comprising condition information acquisition means for acquiring information related to conditions at the time of shooting, wherein the number setting means determines the number of times of noise reduction processing based on the condition information acquired by the condition information acquisition means. The image signal processing apparatus according to 1.   The image signal processing apparatus according to claim 1, wherein the noise reduction unit is a filter having a minimum unit configuration that can function as a nonlinear filter such as a low-pass filter or a median filter.

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