JP2008292541A - Automatic focus adjustment device and automatic focus adjustment method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an automatic focus adjustment device and an automatic focus adjustment method by which focusing is easily determined by using digital image data passing through a noise lowering part. <P>SOLUTION: The automatic focus adjustment device 1 is equipped with the noise lowering part 17 to output a digital image signal after lowering the noise of the digital image signal, and a means (image processing part 20 and color separation part 18 or the like) to determine focusing in accordance with output of the image signal from the noise lowering part. The noise lowering part removes the noise of the digital image data received from an imaging device 15 through a CDS/ADC or the like 16. The focusing is determined based on the image output the noise of which is lowered. Since image processing is performed on the basis of the image signal the noise of which is lowered, the degree of freedom of the image processing part 20 is increased. Generally speaking, the lowering amount of the noise becomes large when a subject is dark. By setting a threshold corresponding thereto and increasing chances to determine focusing, an effect that focusing is facilitated is obtained. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an automatic focus adjustment apparatus and an automatic focus adjustment method used in an imaging apparatus such as a digital camera, and more particularly to an automatic focus adjustment apparatus and an automatic focus adjustment method including a noise reduction circuit.

  Recently, demand for digital cameras is increasing, such as digital cameras being used for mobile terminals such as mobile phones. In general, an automatic focus adjustment device used for a portable terminal such as a cellular phone detects contrast and performs autofocus control (hereinafter abbreviated as AF (Automatic Focus) control). Conventional problems of this AF control include improvement of focusing characteristics in an environment where the surrounding environment is dark or low in contrast, and provision of good image quality with less noise. In addition, these digital cameras have basic requirements such as low cost, small size, and ease of use.

  In addition, the automatic focus adjustment device used for digital cameras of mobile terminals such as mobile phones such as mobile phones, where there are many opportunities for design changes and functional requirements are changing, is not limited to its own configuration. From the wide viewpoint of linked camera system control, there is a demand for resource saving and generalization of memories and semiconductor devices as the entire video engine. Hereinafter, before describing the problem to be solved by the present invention, a brief description will be given of the related arts.

  Non-Patent Document 1 introduces a configuration example of a mobile phone camera video engine. An example is shown in FIGS. FIG. 1 shows an example in which an imaging device such as a lens and CCD and a camera video engine such as a camera DSP are incorporated as a camera module.

  FIG. 2 discloses an example in which the camera video engine is divided into functional blocks. As a functional block, it is introduced that an imaging control unit, a signal processing unit, and a CPU are provided to perform imaging control. The imaging control unit is also called a preprocessor, and performs measurement processing for AE (Automatic Exposure) and AWB (Automatic White Balance) and detection processing for AF (Automatic Focus) in RGB (red, green, blue) signal format. ing. The signal processing unit is also referred to as a post processor, and uses three types of information: YUV signal format (luminance signal (Y), difference between luminance information and blue component (U), and difference between luminance signal and red component (V)). The image forming system, the image correcting system, and the output control system are processed. Further, the RGB signal is converted into a YUV signal at a connection point between the imaging control unit and the signal processing unit.

  In the signal processing unit, the image correction system performs various processes such as an edge correction process and a noise suppression process. Among the image correction processing, noise suppression processing is important in connection with the handling of digital data. Note that the noise suppression processing is processing for removing and reducing noise, and is also referred to as noise removal processing and noise reduction processing. Hereinafter, this specification is referred to as noise reduction processing.

  In summary, functional parts that perform AF control and the like are integrated in the imaging control unit, and parts that perform noise reduction processing and the like are performed as part of digital signal processing after the processing such as AF control is completed. It is a conventional method. A dedicated LSI chip that performs such imaging processing, signal processing, and the like has also been developed as a camera engine as described above.

  In AF control, for example, the amount of high-frequency components contained in the output from the CCD or the like (imaging signal or the like) is integrated for, for example, one field period, and the value is set as an AF evaluation value, and the focus lens is adjusted so that it becomes the largest. A method of moving in the optical axis direction is often performed.

  However, in contrast detection methods generally used in AF control, the amount of high-frequency components such as imaging signals from a CCD or the like is used as an AF evaluation value, so that a dark subject or a change in contrast is small. The focus position could not be detected for the subject.

  In order to solve these problems, the technique disclosed in Patent Document 1 will be briefly described with reference to FIGS. FIG. 3 shows a typical configuration, which discloses that a digital gain adjustment unit is provided for each of RGB and that image data is then input to the YUV conversion unit. Further, as shown in FIG. 4, the brightness of the image signal acquired by imaging in advance is detected (SA4), and if it is below the reference, the R gain of the image signal to be evaluated for AF is calculated. A technique for increasing the overall luminance component of an image signal to be subjected to AF evaluation by setting the gain higher than G, B gain (SA7) is disclosed.

  Further, it is disclosed that, with the above-described configuration, a focused point can be detected and AF control can be performed even for a dark subject.

  Japanese Patent Application Laid-Open No. 2004-228561 discloses that a focus is set to a focus position with high probability even for a subject with little change, that is, a subject with low contrast, that is, the focus characteristic is improved. This disclosure will be briefly described with reference to FIG. It is disclosed that the signal processing circuit receives the image data output from the CDS / AGC / AD circuit, performs processing such as white balance adjustment, color separation, and YUV conversion to generate image data in the YUV format.

  Further, the AF evaluation circuit extracts the high frequency component from the Y data (luminance data) forming the output image data and takes in the AF evaluation value. At this time, the noise component included in the AF evaluation value is removed. Is disclosed. Here, the CDS circuit is a circuit for removing noise caused by a known analog signal amplifier.

Further, it is disclosed that, when removing noise components, a luminance evaluation value, an AGC gain from a luminance evaluation circuit, and temperature information from a temperature sensor are taken in, and a corrected AF evaluation value is calculated. It is disclosed that the focus lens is set at a position where the corrected AF evaluation value is maximized, that is, this position is the in-focus position. And it is disclosed that this improves the focusing characteristics.
State-of-the-art technology for digital cameras, in particular, pages 171 to 176, Technical Information Association, first edition published on October 29, 2004 Japanese Patent Laid-Open No. 2002-287013 JP 2006-79069 A

  As described in the above non-patent documents, a noise reduction processing circuit for digitally converted raw images is generally provided in a signal processing circuit. However, recent customer demands for image quality are widespread, and noise reduction processing is also affected by these image quality processing, so noise reduction processing is also required when image quality processing corresponding to strict image quality requirements is required. The situation was complicated.

  In order to avoid this, it is effective to perform the noise reduction process before the signal processing circuit. In many cases, imaging control and signal processing are integrated and used as an image processing unit in a camera engine or the like. Considering these factors, placing noise reduction processing in the previous stage of the image processing unit, that is, in the previous stage of the imaging control unit, increases the degree of freedom of the image processing unit and increases the economy. It is effective.

  However, in front of the imaging control unit, in other words, before AF control. As a result, AF control is performed based on the data after the noise suppression processing, and the conventional AF control method cannot be used as it is. That is, a new problem arises that AF control considering noise reduction processing is necessary.

  Therefore, the present invention provides an automatic focus adjustment device that obtains high-precision AF control in consideration of noise reduction processing, that is, high-precision focusing using digital image data that has passed through the noise reduction processing circuit, It is an object to provide a focusing method.

  In order to solve the above problems, an automatic focus adjustment apparatus of the present invention includes a noise reduction unit that reduces and outputs noise of a digital image signal, and a unit that performs focus determination according to an image signal output from the noise reduction unit. It is characterized by providing. The present invention can increase the opportunities for focus determination by performing focus determination based on the image output from the noise reduction unit. For example, in general, when the subject is dark, the amount of noise reduction increases and the image output decreases. Therefore, the focus determination opportunity can be increased by reducing the focus determination threshold correspondingly. .

  An automatic focus adjustment method according to the present invention is an automatic focus adjustment method using an automatic focus adjustment device including a noise reduction unit of a digital image signal subjected to noise reduction processing and digital conversion processing on an analog image signal, In-focus determination is performed according to an image signal output from the noise reduction unit. In the present invention, the focus determination is performed based on the image output from the noise reduction unit. For example, when the subject is generally dark, the amount of noise reduction increases, and the image output is reduced. By reducing the threshold value, the opportunity for in-focus determination is increased.

  As described above, according to the present invention, it is possible to obtain an automatic focus adjustment device that enables high-precision focusing using digital image data that has passed through a noise reduction unit (noise reduction processing circuit unit).

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 6 is an example of an embodiment of the automatic focus adjustment apparatus 1 of the present invention. The automatic focus adjustment apparatus 1 of the present invention includes a focus lens 10, a motor 11 for a focus lens, a motor driver 12, an image sensor 15, an image sensor sensor driver 13, a CPU (Central Processing Unit) 14, an image. A circuit unit 16 and a noise reduction unit (NR) 17 for performing known CDS (Correlated Double Sampling), AGC (Automatic Gain Control), and ADC (Analog / Digital Conversion) processing for removing noise from analog raw image data from the sensor. A color separation unit 18, an AF evaluation value calculation unit 19, and an image processing unit 20.

  In the present specification, the CDS, AGC (Automatic Gain Control), and ADC (analog / digital conversion) circuit unit 16 will also be referred to as a raw image data processing unit 16 for the sake of simplicity.

  In the configuration of the present invention, characteristic parts are a noise reduction unit (NR) 17, a color separation unit 18, and an AF evaluation value calculation unit 19 in FIG. The noise reduction unit (NR) 17 has the same function as the noise reduction processing unit referred to in the background art. Since the description of the noise reduction processing unit is redundant, it is referred to as a noise reduction unit (hereinafter, the description of (NR) is also deleted for the sake of simplicity). Therefore, these parts and the characteristic parts of the operation will be described in detail later. The image processing unit 20 according to the embodiment of the present invention performs image processing corresponding to the signal processing unit in addition to image processing of the imaging control unit referred to in the background art, that is, imaging processing (not illustrated) such as AE and AF. , White balance, correction and other various image processing. As these external outputs, image data is output to a compression unit, a memory unit, a display unit, etc. (not shown). Further, the CPU 14 integrates and adjusts the operation of each unit including these and performs various controls.

  Next, the noise reduction unit 17, the color separation unit 18, and the AF evaluation value calculation unit 19, which are characteristic parts of the present invention, will be described with reference to FIG. The noise reduction unit 17 performs noise suppression processing. Raw image data such as R, G, and B from the image sensor 15 such as a CCD or CMOS is passed through a raw image data processing unit 16 including CDS, AGC, or ADC. Receiving digitally converted R, G and B data, respectively. Although the noise reduction part 17 can also be comprised with one semiconductor chip, this is not essential. That is, the noise reduction unit 17 can be configured by one semiconductor chip together with the color separation unit 18 and the AF evaluation value calculation unit 19. In addition, the color separation unit 18 and the AF evaluation value calculation unit 19 may be configured by one semiconductor chip together with the image processing unit 20. In the case of configuring from the beginning, the whole (the noise reduction unit 17, the color separation unit 18, the AF evaluation value calculation unit 19, and the image processing unit 20) including the CPU 14 can be configured by one semiconductor chip.

  The basic operation such as noise suppression of the noise reduction unit 17 is not different from the conventional one, but the respective data are output to the image processing unit 20 and the color separation unit 18 as shown in FIG. It is different. These data are used for in-focus determination after receiving the processing described later.

  The color separation unit 18 outputs only the output result from the noise reduction unit 17, that is, one of RGB output data from the sensor raw image processing unit 16 in which noise is suppressed, in this embodiment, only G is an AF evaluation value. Send to the calculator. Specifically, the G output data is sent to the AF evaluation value calculation unit 23 using the color separation unit 18, and the AF evaluation value is calculated using the G output data.

  Next, the operation of the present invention will be described in detail with reference to the flowchart of FIG. 7 in addition to FIG. FIG. 7 is a flowchart in the case of performing focus determination in the present invention. Specifically, in addition to the noise reduction unit 17, the color separation unit 18, and the AF evaluation value calculation unit 19, the operation is performed including the image processing unit 20, the CPU 14, and the like.

  When performing AF evaluation, the CPU 14 issues an AF STRAT (S1: step 1) instruction. The image processing unit 20 receives this instruction and performs AE lock (S2: Step 2). This is to prevent the AF processing time from being significantly extended if it is left to automatic processing. Specifically, a threshold is provided for the shutter speed (TV value), and exposure is adjusted by gain.

  That is, when the adjustment range in AGC in 16 of FIG. 6 is set within a certain range and the amount of light is insufficient such as the subject is dark, it is not amplified to a certain value as in the conventional case, and noise reduction is performed. The output from the unit may be low. As a result, the AF processing time is extended within a predetermined range.

  Next, AF processing (S3: Step 3), lens movement (S4: Step 4), and AF evaluation value acquisition (S5: Step 5) are performed. As shown in part in FIG. 8, the AF evaluation value for each lens stop position is acquired while performing a scan step drive within the AF search range, specifically, the center of the subject as in the known technique. . However, in the present embodiment, as described later, the AF evaluation value acquisition or the like is performed using only the G data.

  Then, the acquired AF evaluation value is compared before and after to perform peak position detection (S6: Step 6). Thereby, the AF evaluation value acquisition is completed (S7: Step 7).

  That is, in this embodiment, only the G component data is selected from the R, B, and G component data described above, and the AF evaluation is performed using only the G component data. Specifically, as shown in FIG. 6, the RBG component data output from the noise reduction unit 17 is received by the color separation unit 18, and only the G component is sent to the AF evaluation value calculation unit 19. Is going. As shown in FIG. 10, the reason for performing AF evaluation using only the G component data is that the G component is located in the center of RGB and has a long wavelength, and therefore is acquired in a form including the luminance wavelengths of the R component and the B component. Because AF determines in-focus / out-of-focus by contrast, it is more accurate to determine from the luminance difference due to the G component than to determine the focus from the R pixel component. This is because it can be acquired.

  Next, NR intensity is acquired (S8: Step 8). Here, the NR (Noise Reduction) strength is the noise reduction value (suppression value) in the noise reduction unit 17, in other words, the strength of the excluded digital noise signal. Specifically, the acquisition of the NR intensity is performed according to the following procedure. First, the RGB output is sent from the noise reduction unit 17 to the image processing unit 20, and the image processing unit 20 determines the luminance in the shooting environment. Based on the brightness determination result, the CPU 14 sets the NR intensity. It should be noted that the relationship between the luminance and the NR intensity is preferably determined in advance at the design stage, but it goes without saying that the program setting of the CPU 14 may be changed if necessary.

  When the relationship between the brightness of the subject and the NR intensity is described, for example, when the subject is dark, noise is conspicuous. Therefore, it is necessary to strengthen the function of the noise reduction unit 17. Also, when the subject is bright, noise is not noticeable. Accordingly, the function of the noise reduction unit 17 may be weakened. That is, when the subject is dark, the noise reduction unit 17 is strengthened so that the NR intensity value is increased. In general, when the subject is bright, the NR intensity value is small.

  Next, the focus determination threshold value is changed according to the NR intensity (S9: Step 9). This is performed inside the CPU 14. This will be described in more detail with reference to FIG. 9. The focus determination threshold value is based on the absence of NR intensity (FIG. 9 (a)), and when the NR intensity is weak, FIG. 9 (b) shows. . Further, when the NR is strong, FIG. Thus, the level of focus determination is lowered by the strength of the NR intensity.

  Next, the CPU 14 performs in-focus determination. Here, the in-focus determination is a value obtained by adding the NR intensity to the AF evaluation value at the peak position, in other words, as shown in the schematic diagrams of FIGS. 9A, 9B, and 9C. A value obtained by subtracting a value corresponding to the NR intensity from the focus determination threshold value when NR is zero is used as a new focus determination threshold value, and focus determination is performed using this threshold value.

  When the AF evaluation value obtained by the AF evaluation value calculation unit 19 is equal to or greater than the focus determination threshold, the CPU 14 determines that the focus is achieved. When the AF evaluation value is less than the focus determination threshold, the CPU 14 determines that the focus is not in focus (S10: Step 10).

  If determined to be in focus, the CPU 14 corrects the focus position (S11: step 11). That is, the CPU 14 issues a predetermined focus position correction command to the motor driver 12 and the like. Based on this command, the motor driver 12 moves the motor 11 to move the lens position to a predetermined location (S12: step 12).

  If it is determined that the image is out of focus, for example, the lens is moved to the out of focus position, and an operation corresponding to the out of focus specific to each camera system is performed (S13: Step 13). The focus determination operation ends when the lens is moved to either in-focus or out-of-focus (S14: Step 14).

  The focus determination is completed as described above. The completion of the focus determination is also transmitted from the CPU 14 to the AF evaluation value calculation unit 19 as a command to stop the subsequent calculation processing of the AF evaluation value calculation unit. A schematic diagram of the focus determination is shown in FIG. This is a schematic diagram of focusing as in the prior art, but focusing determination is performed by comparing the peak position of the AF evaluation value with the focusing determination threshold as an AF evaluation value used for focusing determination as shown in FIG. Is going. That is, in the present invention, the focus determination threshold value is set in consideration of the NR intensity as described above. In other words, the focus determination is performed according to the image output from the noise reduction unit. There is a feature in the point.

  As described above, the present invention has the special effect that the focus determination can be performed smoothly even when the peak of the AF evaluation value is low by selecting an appropriate focus determination threshold according to the strength of the NR. Thus, there is a special effect that the focus determination can be performed easily and quickly even when the subject is dark, the contrast of the subject is low, or when there is a lot of noise for other reasons.

  Needless to say, the present invention is not limited to the above embodiment. For example, although the embodiment has shown that the AF evaluation is performed using the data of the G component, it is needless to say that the evaluation may be performed using only the data of other components according to the necessity of the image characteristics. In addition, changes and the like can be freely made within the scope of the idea of the present invention.

Explanatory drawing of the camera module of the conventional mobile phone with a camera. Explanatory drawing explaining the process of the image data in FIG. 1 from a software viewpoint. The block diagram of the conventional automatic focusing apparatus. It is a flowchart of the conventional focusing determination by the apparatus of FIG. It is a block diagram of the automatic focusing apparatus of another conventional electronic camera. It is a figure of one Embodiment of the automatic focus adjustment apparatus of this invention. It is a follow chart of the focus determination of this invention. It is a schematic diagram of focusing determination. It is a schematic diagram explaining the effect of the focus determination of this invention. It is explanatory drawing which uses G pixel component for the 1st Embodiment of this invention.

Explanation of symbols

1 is a diagram showing a main part of a digital camera using an automatic focusing device of the present invention.
10 focus lens 11 motor 12 motor driver 13 sensor driver 14 CPU
15 Image sensor 16 Sensor raw image processing unit 17 Noise reduction unit (NR)
18 color separation unit 19 AF evaluation value calculation unit 20 image processing unit

Claims (6)

A noise reduction unit that reduces and outputs digital image signal noise;
Means for performing in-focus determination according to an image signal output from the noise reduction unit;
An automatic focus adjustment device comprising:
2. The automatic focus adjustment apparatus according to claim 1, wherein the digital image signal is obtained by subjecting an analog image signal to noise reduction processing and digital conversion processing.
The means is
An image processing unit that receives an image signal from the noise reduction unit and detects luminance used for in-focus determination;
A color separation unit that receives an image signal from the noise reduction unit, separates it into R, G, and B signal components and outputs an AF evaluation value using at least one of these signal components to an AF evaluation value calculation unit When,
The automatic focus adjustment apparatus according to claim 1 or 2, characterized by comprising:
The means sets a threshold value for focusing determination based on the luminance detected by the image signal processing unit, and performs a focusing determination by comparing with the focusing evaluation value (CPU). The automatic focusing apparatus according to claim 3 or 4, further comprising:
6. The automatic focus adjustment apparatus according to claim 4, wherein an AF evaluation value is obtained using the G signal component.
An automatic focus adjustment method using an automatic focus adjustment device including a noise reduction unit of a digital image signal subjected to noise reduction processing and digital conversion processing on an analog image signal,
An automatic focus adjustment method, comprising: performing in-focus determination according to an image signal output from the noise reduction unit.

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