JP2009017167A - Image processor, image processing method and imaging device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To preferably detect and remove a flicker from an image signal when a high-speed imaging mode is turned on under a photographing environment which generates a fixed flicker stripe in a normal photographing. <P>SOLUTION: A weighted average is obtained with respect to a waveform advancing by a half-cycle of a previous sampling period in a region corresponding to a flicker phase 0 to π, and the weighted average is obtained with respect to the waveform delaying by the half-cycle of the previous sampling period in a region corresponding to the flicker phase π to 2π. Further, a finite difference is obtained with respect to the waveform prior by the half-cycle of the previous sampling period in the region corresponding to the flicker phase 0 to π, and the finite difference is obtained with respect to the waveform behind by a half-cycle of the previous sampling period in the region corresponding to the flicker phase π to 2π. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an image processing apparatus and image processing for detecting and correcting fluorescent lamp flicker generated on a captured image when a subject is photographed by a video camera under illumination of a blinking light source such as a fluorescent lamp that is turned on by a commercial AC power source. The present invention relates to a method and an imaging apparatus, and in particular, an image processing apparatus for detecting and correcting horizontal stripe-like fluorescent lamp flicker generated when an image is taken with an XY address scanning solid-state imaging apparatus such as a CMOS image sensor under fluorescent lamp illumination. The present invention relates to an image processing method and an imaging apparatus.

  More specifically, the present invention relates to an image processing apparatus and image for detecting and correcting flicker fringes that do not fit into one screen within one screen, which occurs at the time of high-speed shooting in which a subject is imaged at a frame rate higher than that of a standard television signal. The present invention relates to a processing method and an imaging apparatus, and in particular, an image processing apparatus and image processing for detecting and correcting flicker from an image signal when entering a high-speed shooting mode in a shooting environment where fixed flicker fringes occur during normal shooting. The present invention relates to a method and an imaging apparatus.

  Recently, in place of a silver salt camera that shoots using a film or a photosensitive plate, an image is captured by a solid-state imaging device in which a light receiving portion of a pixel array that performs photoelectric conversion and accumulation is configured by a photodiode, and is subjected to digital code processing and stored. Digital cameras are widespread. According to the digital camera, there is an advantage that a digitally encoded image can be stored in a memory, image processing and image management by a computer can be performed, and there is no problem of film life.

  Examples of the solid-state imaging device include a CCD (Charge Coupled Device) and a CMOS (Complementary Metal Oxide Semiconductor: complementary metal oxide semiconductor). Among them, the CMOS image sensor is configured to sequentially read and transfer signals by a matrix of MOS transistor switches and to perform sequential scanning by a vertical and horizontal shift register. However, the manufacturing cost is lower than that of a CCD. There is an advantage that it is power consumption.

  By the way, when a subject is photographed by a video camera under the illumination of a blinking light source such as a fluorescent lamp that is turned on by a commercial AC power supply, the difference between the luminance change frequency (light intensity change) frequency of the light source and the vertical synchronization frequency of the camera. It is known that a temporal change in light and darkness that occurs on a captured image, that is, a fluorescent lamp flicker occurs. In particular, when an XY address scanning type imaging device such as the above-described CMOS type image sensor is used, the flicker on the captured image has a periodic luminance level in the vertical direction because the exposure timing for each horizontal line is different. Alternatively, it is observed as a striped pattern due to a change in hue. As a method for removing such flicker components from the captured image signal, mainly a “shutter correction method” for correcting based on the relationship between the shutter speed and the flicker level, and detecting a flicker waveform. A “gain correction method” is known in which the inverse waveform is applied to an image signal as a correction gain.

  On the other hand, with the demand for higher functionality for digital video cameras and the like, recently, development of a camera having a function of imaging a subject at a screen rate faster than a standard television signal has been progressing. In the following, a flicker reduction technique in a camera having such a high-speed imaging function will be considered.

  FIG. 15 shows a computer simulation result regarding the relationship between the flicker level and the shutter speed when a camera having an XY address scanning type imaging device is used for photographing under fluorescent lamp illumination. In the graph shown in the drawing, the horizontal axis represents the shutter speed and the vertical axis represents the flicker level. The graph corresponds to the shutter speed when shooting under non-inverter fluorescent lamp illumination in an area where the commercial AC power supply frequency is 50 Hz. It shows the flicker level change. From the figure, there is a relationship between the shutter speed and the flicker level. In particular, flicker can be completely suppressed when the shutter speed is N / 100 (where N is a positive integer). When the power frequency of the fluorescent lamp is set to f [Hz], the flickerless shutter speed S_fkless can be generally expressed by the following equation (1).

  In the shutter correction method, based on such properties, when it is detected that flicker is generated by some method, the shutter speed is set to S_f k les s in the above equation (1). The occurrence of flicker can be avoided. However, this method has a problem that the degree of freedom of AE (Auto Exposure) control is reduced because the shutter speed is limited.

  For example, it is easy to imagine that the shutter speed that can be set for the camera is limited to at least higher than 1/240 seconds during high-speed shooting where the frame rate is four times that of NTSC (240 fps). As the frame rate further increases, the shutter speed that can be set for the camera can only be selected at a higher shutter speed than the frame rate. In other words, for a camera that performs high-speed shooting, there is no option of avoiding flicker by the shutter correction method.

  On the other hand, in the gain correction method, since the flicker waveform is detected and the inverse waveform is applied to the image signal as a correction gain, it is released from the shutter speed limitation in the shutter correction method. It is very difficult to detect.

  As an example of the gain correction method, the step of integrating the input image signal over a time of one horizontal period or more, the step of normalizing the integration value or the difference value of the integration value in the adjacent field or frame, A step of extracting a spectrum of an integral value or a difference value, a step of estimating a flicker component from the extracted spectrum, and a step of calculating the estimated flicker component and the input image signal so as to cancel the estimated flicker component A method for reducing fluorescent flicker components included in an image signal photographed by an XY address scanning type imaging device under fluorescent lamp illumination has been proposed (see, for example, Patent Document 1). In short, the flicker fringe detection system in this method samples one period of flicker fringes while processing the input signal into an appropriate shape, and uses flicker fringes having a DFT (Discrete Fourier Transform) period as a fundamental wave from this sampling data. It comprises the steps of calculating the frequency spectrum and estimating the flicker waveform using only the lower order terms.

  According to the above-described flicker reduction method, a signal that can easily estimate the flicker component with high accuracy by removing signal components other than the flicker component as an integrated value or difference value after normalization is obtained. The flicker component can be estimated with high accuracy by extracting a spectrum up to an appropriate order of the integral value or the difference value, and the flicker component can be calculated from the input image signal by calculating the estimated flicker component and the input image signal. Can be reliably and sufficiently reduced. However, since the flicker detection system presupposes sampling for one cycle of flicker from one screen, it is not an effective flicker correction algorithm when one cycle of flicker does not fit within one screen during high-speed shooting. .

  When the number of lines in the vertical synchronization period is M, the number of lines corresponding to one cycle of flicker stripes is T_fk, and the frame rate is FPS, the relationship represented by the following equation (2) is generally established.

  When shooting at high speed such that the frame rate satisfies the condition shown in the following equation (3), the relationship between the flicker fringe period and the number of lines is expressed by the following equation (4) from the above equation (2).

  The above equation (4) means that one flicker cycle can no longer fit in one screen.

  FIG. 16 shows the relationship between the vertical synchronization frequency and the flicker waveform during normal imaging and during high-speed imaging, respectively.

  FIG. 16A shows the relationship between the vertical synchronization signal VD of 60 fps, which is the frame rate of the NTSC system (National Television Standards Committee), and the flicker waveform (the number of lines corresponding to one cycle of flicker stripes in the same figure). T_fk is determined by the above equation (2)). In this case, while the vertical synchronization period is 1/60 second, one cycle of flicker fringes is 1/100 second, and one cycle of flicker fringes is contained in one screen. Therefore, the flicker reduction method described above is used. The waveform can be detected and flicker correction can be performed effectively. In the figure, a range surrounded by a broken line is a detection frame unit composed of one cycle of flicker fringes, and flicker estimation processing is performed by distinguishing flicker and background in this unit.

  On the other hand, FIG. 16B shows the relationship between the vertical synchronization signal and the flicker waveform when imaging is performed at 240 fps, which is a standard four times the frame rate, as an example of high-speed imaging (in FIG. 16B, flicker fringes in the figure). The number of lines T_fk corresponding to one period is determined by the above equation (2)). In this case, one cycle of flicker stripes is 1/100 second as in FIG. 16A, but the vertical synchronization period is 1/240 seconds. Therefore, as shown in the above equation (3), flicker appears within one screen. Since one fringe period cannot be accommodated, the flicker reduction method described above cannot detect the flicker waveform, and flicker correction cannot be performed effectively. In the same figure, the range enclosed by a broken line is a detection frame unit consisting of one cycle of flicker fringe. However, flicker fringe estimation is performed by distinguishing flicker from the background in this unit using multiple screens. Perform processing.

  In addition to the above flicker reduction methods, many flicker correction algorithms use the fact that the flicker phenomenon has field repeatability (for example, the flicker in the 50 Hz power supply area of NTSC becomes the same flicker waveform after 3 fields). For example, it is necessary to extract only the background component from the average value of the flicker waveforms of three screens. However, since the number of repeated fields varies depending on the frame rate FPS during high-speed imaging, this kind of The flicker correction algorithm cannot be applied during high-speed imaging.

  As another example of the gain correction method, image information for a plurality of fields corresponding to a time in which flicker fringes are included for one period or more is obtained under a condition that flicker fringes are no longer included for one period on one screen. An image processing apparatus has been proposed that applies a flicker detection and correction algorithm using flicker repeatability after collecting and pre-processing the information so that flicker fringe information is always included in one cycle or more (for example, (See Patent Document 2). According to this image processing apparatus, a continuous image signal including a flicker component for one cycle is reliably an object to be analyzed for flicker regardless of the screen rate of the image signal, and flicker detection accuracy is improved.

  Here, the case where high-speed imaging is performed under different imaging conditions will be further considered. For example, when shooting under a fluorescent lamp in a region where the power supply frequency is 60 Hz, the shutter speed that can be set in the camera is basically limited to a higher speed than 1/240 seconds (same as above). That is, even under this shooting condition, there is no option of avoiding flicker by shutter correction. Accordingly, the image information for a plurality of fields corresponding to the time in which the flicker fringe is included for one period or more is collected, and the flicker correction is performed after pre-processing so as to always include the flicker fringe information for one period or more. Let's consider the application of the flicker detection and correction algorithm.

  FIG. 17 shows the relationship between the vertical synchronization frequency and the flicker waveform when normal imaging and high-speed imaging are performed under a fluorescent lamp in a region where the power supply frequency is 60 Hz.

  FIG. 17A shows the relationship between the vertical synchronization signal VD of 60 fps, which is the frame rate of the NTSC system, and the flicker waveform (in FIG. 17A, the number of lines T_fk corresponding to one cycle of flicker fringes is expressed by the equation (2) )). In this case, while the vertical synchronization period is 1/60 seconds, one cycle of flicker stripes is 1/120 seconds, and one cycle of flicker stripes fits in one screen. However, since the time corresponding to one cycle of the frame rate and flicker is exactly an integer multiple, when viewed in the detection frame unit, the flicker fringes of each detection frame unit are in exactly the same position with respect to the vertical synchronization period. In the same figure where a fixed flicker fringe is generated, the range surrounded by a broken line is a detection frame unit consisting of one cycle of the flicker fringe, but the flicker phase is exactly the same for each detection frame unit as shown in the figure, Cannot distinguish between flicker and background. However, flicker can be avoided by the shutter correction method in the normal shooting mode.

  On the other hand, FIG. 17B shows the relationship between the vertical synchronization signal and the flicker waveform when imaging is performed at 240 fps, which is a frame rate four times the standard as an example of high-speed imaging. The vertical synchronization period is 120 seconds compared to 1/120 seconds (in the figure, the number of lines T_fk corresponding to one flicker fringe period is determined by the above equation (2)). In the figure, the range surrounded by the broken line is a detection frame unit consisting of one cycle of flicker fringes, but it is handled as one cycle of flicker fringes using a plurality of screens. Actually, under this shooting condition, one period of flicker fringe is just included in one screen and flicker information for one period is included, so it seems that there is no problem at first glance (although it is a fixed flicker condition, This is not a fixed flicker when considering each screen). Certainly, each screen contains information for one cycle. However, since the time corresponding to one cycle of the frame rate and flicker is an integer multiple, as can be seen from FIG. 17B, the flicker fringes of each detection frame unit are vertical when viewed in the detection frame unit. Fixed flicker fringes that are exactly the same position with respect to the synchronization period occur. In such a case, the flicker fringe information is lost together with the background when the difference between the fields is taken when performing the normalization process. That is, under the above conditions where the vertical synchronization and the flicker stripes are synchronized, even if the flicker has repeatability, it cannot be distinguished from the background. Therefore, the repeatability of the flicker phenomenon is used to distinguish between the background and the flicker stripes. It is not possible to apply the flicker detection algorithm described above.

  Of course, even during normal imaging, fixed flicker fringes are generated as described above under imaging conditions in which the time corresponding to one cycle of the frame rate and flicker is an integer multiple (see FIG. 17A). . However, since there is an avoidance method using the shutter correction method (described above) during normal imaging, flicker can be eliminated if it is known that the shooting conditions cause fixed flicker by any method.

  Alternatively, a method of providing a shutter speed constraint that does not cause the fixed flicker problem in advance has been proposed (see, for example, Patent Document 3). However, flicker cannot be avoided by the shutter correction method in the high-speed imaging mode. is there. Also, as can be seen from FIG. 17B, when viewed on a screen basis, the image is no longer a fixed flicker fringe, but is very ugly as an image in which the flicker fringe flows in the vertical direction.

JP 2004-222228 A JP 2006-345368 A Japanese Patent No. 3826604

  An object of the present invention is to suitably detect horizontal flicker-like fluorescent lamp flicker that occurs when photographing with an XY address scanning solid-state imaging device such as a CMOS image sensor under fluorescent lamp illumination by a commercial AC power source, and from the image signal. An object of the present invention is to provide an excellent image processing apparatus, image processing method, and imaging apparatus capable of removing flicker.

  A further object of the present invention is to suitably detect flicker fringes that do not fit in one period in one screen, which occurs when shooting a subject at a higher frame rate than a standard television signal, and to detect flicker from the image signal. An object of the present invention is to provide an excellent image processing apparatus, image processing method, and imaging apparatus that can be removed.

  A further object of the present invention is to provide an excellent image processing apparatus capable of suitably detecting and removing flicker from an image signal when entering a high-speed shooting mode in a shooting environment where fixed flicker stripes are generated during normal shooting. And an image processing method and an imaging apparatus.

The present invention has been made in view of the above problems, and is an image processing apparatus that processes an image signal including a flicker component,
An integration processing unit that integrates an image signal for each line over a sampling period having a length of one flicker or longer;
An integration value holding unit that holds an integration value by the integration processing unit in a past sampling period;
An average value calculation unit that averages integral values in two or more consecutive sampling periods;
A difference calculation unit that takes a difference between integral values in two or more consecutive sampling periods;
A normalization calculation unit that normalizes the difference value calculated by the difference calculation unit using the average value obtained by the average value calculation unit;
Frequency analysis means for frequency analysis of the difference value of the normalized integral value;
Flicker detection means for detecting a flicker component included in the image signal based on the result of frequency analysis by the frequency analysis means;
A calculation unit for performing a calculation for removing the flicker component detected by the flicker detection unit from the image signal;
With
Under a shooting environment in which the time corresponding to one period of the frame rate and the flicker waveform is exactly an integer multiple, or the flicker waveform has the same phase each time with respect to the vertical synchronization period,
The average value calculation unit shifts the sampling phase of one of the integral values in two consecutive sampling periods by a half period of the flicker period, and performs averaging.
The difference calculation unit obtains a difference by shifting one sampling phase of the integral value in two consecutive sampling periods by a half period of the flicker period.
An image processing apparatus characterized by this.

  In recent years, digital cameras that capture images using a solid-state image sensor have become widespread. However, when a subject is photographed under fluorescent lamp lighting that is lit by a commercial AC power supply, the frequency of the luminance change (light quantity change) of the light source and the camera It is known that a flicker phenomenon in which temporal brightness changes on a captured image occurs due to a difference from the vertical synchronization frequency. In particular, when an XY address scanning type imaging device such as a CMOS type image sensor is used, the exposure timing for each horizontal line is different, and therefore flicker on the captured image has a periodic luminance level or hue in the vertical direction. Observed as a striped pattern due to fluctuations.

  A flicker component can be removed by a gain correction method in which a flicker waveform is detected from an image signal and the inverse waveform is applied to the image signal as a correction gain. For example, the image signal is integrated over a period of one horizontal period or more, the integral value or the difference value of the integral value in an adjacent field or frame is normalized, and the spectrum extracted from the normalized integral value or difference value after the normalization is obtained. There is a known method for estimating the flicker component, and the frequency spectrum of the flicker whose fundamental wave is the DFT period is calculated from the sampling data for one period of the flicker fringe, and the flicker waveform is increased using only the low-order terms. The accuracy can be estimated (see Patent Document 1).

  Also, during high-speed shooting, one cycle of flicker does not fit within one screen, but image information for a plurality of fields corresponding to a time in which flicker fringes are included for one cycle or more is collected, and information on flicker fringes is always included for one cycle or more. After pre-processing in such a form, as described above, as a normalized integral value or difference value, a signal component other than the flicker component is removed to obtain a signal that can estimate the flicker component with high accuracy, The flicker component can be estimated with high accuracy by extracting a spectrum up to an appropriate order of the normalized integral value or difference value after the normalization (see Patent Document 2).

  However, for example, when performing high-speed shooting with a frame rate of 240 fps under a fluorescent lamp in a region where the power supply frequency is 60 Hz, the time corresponding to one cycle of the frame rate and flicker is exactly an integer multiple. A “fixed flicker fringe” is generated in which the flicker fringes of each detection frame unit are exactly at the same position with respect to the vertical synchronization period. In such a case, when the difference between the fields is taken when performing normalization processing, the flicker fringe information is lost together with the background, and even if the flicker has repeatability, it cannot be distinguished from the background. That is, the above-described flicker detection algorithm that distinguishes the background from the flicker fringes using the repeatability of the flicker phenomenon cannot be applied.

  On the other hand, it is known that there is a relationship between the shutter speed and the flicker level, and using the characteristic that flicker can be completely suppressed when the shutter speed is N / 100 (where N is a positive integer), A shutter correction method that avoids flicker by using a shutter speed S_fkless at which flicker is reduced is also known. However, since the shutter speed is limited and the degree of freedom of AE control is reduced, there is no option to avoid flicker by the shutter correction method for a camera that performs high-speed shooting. In other words, fixed flicker fringes resulting from the fact that the time corresponding to one cycle of the frame rate and flicker is an integral multiple is an inevitable problem during high-speed shooting.

  On the other hand, the image processing apparatus according to the present invention basically applies the above-described flicker detection algorithm for distinguishing between the background and flicker fringes using the repeatability of the flicker phenomenon. An integration processing unit that integrates an image signal for each line over a sampling period having the above length, an average value calculation unit that averages integration values in two or more consecutive sampling periods, and the two or more consecutive samplings A difference calculation unit that takes a difference between integral values in a period, a normalization calculation unit that normalizes a difference value calculated by the difference calculation unit using an average value obtained by the average value calculation unit, and the normalization Frequency analysis means for frequency analysis of the difference value of the integrated integral value, and flicker included in the image signal based on the result of frequency analysis by the frequency analysis means And flicker detection means for detecting a minute, and a calculation unit for performing an operation for removing a flicker component detected by the flicker detection unit from the image signal.

  Here, in a shooting environment where the frame rate and the time corresponding to one cycle of the flicker waveform are exactly an integer multiple, and the flicker waveform has the same phase every time in the vertical synchronization period and a fixed flicker occurs. The average value calculation unit obtains a weighted average of signals that are the same in both the flicker and the background by shifting and averaging one sampling phase of the integral value in two consecutive sampling periods by a half cycle of the flicker period. To avoid that. In addition, the difference calculation unit obtains a difference between signals that are the same in both the flicker and the background by shifting the sampling phase of one of the integral values in two consecutive sampling periods by a half cycle of the flicker period. It is designed to avoid it.

  In this way, by performing averaging and difference processing by shifting one sampling phase of the integral value in two consecutive sampling periods by a half cycle of the flicker cycle, the time corresponding to one cycle of the frame rate and flicker is exactly the same. Since the relationship is an integral multiple, only the background component and the flicker component can be extracted from the input image signal even under shooting conditions in which fixed flicker occurs.

  Of course, in a shooting environment where the flicker waveform does not have the same phase with respect to the vertical synchronization period and no fixed flicker occurs, averaging processing is performed or the difference is taken with the combination of sampling phases to be added changed. There is no need to perform difference processing in a state where the combination of sampling phases is changed. In such a case, the average value calculation unit adds and averages the integration values in two or more consecutive sampling periods in the order of sampling phases, and the difference calculation unit calculates the difference between the integration values in two consecutive sampling periods. Should be taken in the order of the sampling phase.

  In short, the image processing apparatus according to the present invention performs averaging by shifting one sampling phase of the integrated value in two sampling periods in which the average value calculation unit is continuous by a half cycle of a flicker cycle, and the difference calculation unit. The first normalized integral value calculation mode for taking a difference by shifting one sampling phase of the integral values in two consecutive sampling periods by a half cycle of the flicker period, and two or more samplings in which the average value calculation unit is continuous The integration value in the period is added and averaged in the order of the sampling phase, and the difference calculation unit includes a second normalized integration value calculation mode in which the difference between the integration values in two consecutive sampling periods is taken in the order of the sampling phase. When the subject is located in the 50 Hz region or the 60 Hz region, that is, when one cycle of the flicker waveform based on the frequency of the commercial AC power supply and the frame rate have an integer multiple relationship, It is sufficient to set to the integrated integral value calculation mode, and to set to the second normalized integrated value calculation mode in other cases.

  In addition, mode switching between the first normalized integral value calculation mode and the second normalized integral value calculation mode according to the commercial AC power supply currently used can be automatically performed. For example, first, the second normalized integral value calculation mode is set, and detection of a flicker component included in the image signal is attempted. At this time, when the frequency spectrum corresponding to the flicker component included in the image signal is constantly generated by the frequency analysis means, it is estimated that the area is 50 Hz, so the second normalized integral value calculation mode is set. Leave it set. On the other hand, when the frequency spectrum corresponding to the flicker component included in the image signal is not constantly generated by the frequency analysis means, it is estimated that the region is 60 Hz. To switch to.

  Subsequently, in the second normalized integral value calculation mode, a stationary frequency spectrum does not occur, and since it is estimated that the region is 60 Hz, when switching to the first normalized integral value calculation mode, an image signal is displayed. If a frequency spectrum corresponding to the included flicker component is steadily generated by the frequency analysis means, it is determined that the region is 60 Hz, so that the first normalized integral value calculation mode remains set. To do. On the other hand, when the frequency spectrum corresponding to the flicker component included in the image signal is not constantly generated by the frequency analysis means, it is not a problem of the cycle of the flicker component based on the power supply frequency, and the flicker is originally uttered. It is estimated that there is no state. In such a case, the flicker detection value is not output in any calculation mode, and as a result, no unnecessary correction is applied to the input image signal.

  The gist of the present invention is not limited to the case where high-speed shooting is performed such as 240 fps in a 60 Hz region. The present invention can be applied in the same manner even under conditions where vertical synchronization and flicker fringe are synchronized during normal shooting, and the sampling phase of one of the integral values in two consecutive sampling periods is shifted by a half period of the flicker period. Thus, by calculating the averaging and the difference, it is possible to realize high-precision flicker detection and correction processing by the gain correction method.

  According to the present invention, horizontal stripe-like fluorescent lamp flicker generated when an image is taken with an XY address scanning solid-state imaging device such as a CMOS image sensor under fluorescent lamp illumination by a commercial AC power source is suitably detected from the image signal. It is possible to provide an excellent image processing apparatus, image processing method, and imaging apparatus that can remove flicker.

  In addition, according to the present invention, flicker fringes that do not fit in one period within one screen and that occur when shooting a subject at a frame rate higher than that of a standard television signal are preferably detected to detect flicker from the image signal. It is possible to provide an excellent image processing apparatus, image processing method, and image pickup apparatus that can eliminate the above.

  In addition, according to the present invention, excellent image processing that can suitably detect and remove flicker from an image signal when entering a high-speed shooting mode in a shooting environment where fixed flicker fringes occur during normal shooting. An apparatus, an image processing method, and an imaging apparatus can be provided.

  Other objects, features, and advantages of the present invention will become apparent from more detailed description based on embodiments of the present invention described later and the accompanying drawings.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 1 schematically shows the configuration of an imaging apparatus suitable for applying the present invention. The illustrated imaging apparatus includes an optical block 11, a driver 11a, a CMOS image sensor 12, a timing generator (TG) 12a, an analog front end (AFE) circuit 13, a camera processing circuit 14, A system controller 15, an input unit 16, a graphic I / F (interface) 17, and a display 17a are provided. Hereinafter, each part will be described.

  The optical block 11 includes a lens for condensing light from a subject on the CMOS sensor 12, a drive mechanism for moving the lens to perform focusing and zooming, a shutter mechanism, an iris mechanism, and the like. . The driver 11 a drives each mechanism in the optical block 11 based on a control signal from the system controller 15.

  The CMOS sensor 12 includes a plurality of photodiodes (photogates), transfer gates (shutter transistors), switching transistors (addressed transistors), amplification transistors, reset transistors (reset gates), etc. on a CMOS substrate. These pixels are formed in a two-dimensional arrangement, and a vertical scanning circuit, a horizontal scanning circuit, an image signal output circuit, and the like are also formed on the same CMOS substrate. The CMOS sensor 12 is driven based on the timing signal output from the timing generator 12a, and converts incident light from the subject into an electrical signal. The timing generator 12 a outputs a timing signal under the control of the system controller 15.

  For example, when outputting a pixel signal for one line, the CMOS sensor 12 adds the signals of adjacent pixels of the same color on the image sensor and outputs the signals simultaneously, thereby synchronizing the frequency when reading out the pixel signals. Without increasing the screen, the screen switching rate is increased. This also reduces the image size (resolution) without changing the angle of view. In the present embodiment, the CMOS sensor 12 has a high-speed imaging mode for high-speed imaging at a frame rate higher than 60 fps (for example, 240 fps), in addition to a normal imaging mode at a normal speed of 60 fps, which is the NTSC specification. And

  The AFE circuit 13 samples and holds the image signal output from the CMOS sensor 12 so as to maintain a good S / N (Signal / Noise) ratio by CDS (Correlated Double Sampling) processing. Further, the gain is controlled by AGC (Auto Gain Control) processing, A / D conversion is performed, and a digital image signal is output. The AFE circuit 13 is configured as, for example, one IC (Integrated Circuit), or a circuit for performing CDS processing may be formed on the same CMOS substrate as the CMOS sensor 12.

  The camera processing circuit 14 is configured as one IC, for example, and performs AF (Auto Focus), AE (Auto Exposure), AWB (Auto White Balance) on the image signal from the AFE circuit 13. Various camera signal processing such as (adjustment) or a part of the processing is executed. In the present embodiment, the camera processing circuit 14 includes a flicker reducing unit 20 that reduces the flicker signal component generated on the screen during imaging under a fluorescent lamp from the image signal.

  The system controller 15 is constituted by a microprocessor including, for example, a CPU (Central Processing Unit), a ROM (Read Only Memory), a RAM (Random Access Memory), and the like, and by executing a program stored in the ROM or the like, Each part of this imaging apparatus is controlled in an integrated manner.

  The input unit 16 includes various operation keys such as a shutter release button, a lever, a dial, and the like, and outputs a control signal corresponding to an input operation by the user to the system controller 15.

  The graphic I / F 17 generates an image signal to be displayed on the display 17a from the image signal supplied from the camera processing circuit 14 via the system controller 15, and supplies this signal to the display 17a. Display. The display 17a is composed of, for example, an LCD (Liquid Crystal Display), and displays a camera-through image being captured, a reproduced image based on data recorded on a recording medium (not shown), and the like. Further, on the display 17a, in response to a request from the system controller 15, a menu screen, various setting screens, various warning information, and the like are combined and displayed and output as an OSD (On Screen Display).

  In the illustrated image pickup apparatus, signals received and photoelectrically converted by the CMOS sensor 12 are sequentially supplied to the AFE circuit 13, subjected to CDS processing and AGC processing, and then converted into digital signals. The camera processing circuit 14 performs image quality correction processing on the digital image signal supplied from the AFE circuit 13, and finally converts the digital image signal into a luminance signal (Y) and a color difference signal (RY, BY) and outputs the result.

  The image data output from the camera processing circuit 14 is supplied to the graphic I / F 17 via the system controller 15 and converted into a display image signal, whereby a camera-through image is displayed on the display 17a. When the system controller 15 is instructed to record an image by a user input operation or the like from the input unit 16, the image data from the camera processing circuit 14 is supplied to an encoder (not shown) and is subjected to predetermined compression. The encoding process is performed and the data is recorded on a recording medium (not shown). At the time of recording a still image, image data for one frame is supplied from the camera processing circuit 14 to the encoder, and at the time of recording a moving image, an image subjected to camera signal processing such as AF, AE, AWB, etc. Data is continuously supplied to the encoder.

  FIG. 2 shows the internal configuration of the camera processing circuit 14. The illustrated camera processing circuit 14 includes a reference signal generation unit 30 that generates a reference signal for the entire circuit, and a plurality of processes for various camera signal processing that operate upon receiving a reference signal from the reference signal generation unit 30. Blocks 31, 32, 33... Are provided. In the present embodiment, a flicker reduction unit 20 is provided as one of the processing blocks.

  The reference signal generator 30 synchronizes with a reference signal supplied from a source oscillator (not shown) to the camera processing circuit 14, and operates the reference signals SIG_REF_1, SIG_REF_2, SIG_REF_3 for operating the processing blocks 31, 32, 33,. Generate and output…. Each of the reference signals SIG_REF_1, SIG_REF_2, SIG_REF_3,... Is output in consideration of a delay that occurs between the processing blocks 31, 32, 33,. Further, the processing blocks 31, 32, 33,... Generate a reference signal that controls the operation timing in detail based on the supplied reference signals SIG_REF_1, SIG_REF_2, SIG_REF_3, etc. (none of them are shown). Are provided.

  Similarly, the flicker reducing unit 20 that is one of the processing blocks also generates an internal reference signal generating unit that generates a reference signal that controls operation timing inside the flicker reducing unit 20 based on the reference signal SIG_REF_FK from the reference signal generating unit 30. 21 and a detection / reduction processing unit 22 that operates using the generated reference signal.

  The reference signal generation unit 30 uses, as a reference signal SIG_REF_FK for the flicker reduction unit 20, a vertical synchronization signal VD, a horizontal synchronization signal HD, two types of enable signals VEN1 and VEN2 (to be described later) indicating the effective period of the image signal in the vertical direction, and horizontal An enable signal HEN indicating the effective period of the image signal with respect to the direction is output. The internal reference signal generation unit 21 generates various reference signals and count values for the detection / reduction processing unit 22 based on these signals.

  The internal reference signal generation unit 21 includes a counter 21a. The counter 21a outputs a count value VCOUNT indicating the number of lines in an effective period in one vertical period. When the counter 21a receives the setting of the imaging mode from the system controller 15, the counter 21a selects either the enable signal VEN1 or VEN2 according to the frame rate corresponding to the setting. Then, the internal reference signal generation unit 21 outputs the count number of the horizontal synchronizing signal HD during the period when the selected enable signal is at the high level as VCOUNT, and resets the count value VCOUNT when the enable signal changes to the low level. To do.

  Each unit in the detection / reduction processing unit 22 is controlled based on the count value VCOUNT for the operation timing of the image signal in the vertical direction. The internal reference signal generation unit 21 compares, for example, the count value VCOUNT with a predetermined value, thereby enabling an enable signal that becomes high level only during a certain period, a pulse signal that becomes high level at a predetermined interval during a certain period, and the like. Can be freely generated, and the operation timing of the detection / reduction processing unit 22 can be controlled.

  In the present embodiment, the internal reference signal generation unit 21 calculates a count number (T_fk described later) corresponding to one cycle of flicker fringes according to the imaging mode set by the system controller 15, and enables the enable signal VEN1. Alternatively, the enable signal DETECT_EN is generated and detected until the VEN2 is at the high level and the count value VCOUNT reaches the count number (T_fk: the number of lines corresponding to one cycle of flicker stripes). Supply to the reduction processing unit 22. The enable signal DETECT_EN indicates a sampling period of the image signal in the detection / reduction processing unit 22, and a detection block (described later) in the detection / reduction processing unit 22 operates based on this sampling period.

  The internal reference signal generation unit 21 does not set the count number (T_fk) that determines the period during which the enable signal DETECT_EN is at the high level according to the imaging mode notified from the system controller 1, but the system controller 15 May directly set the count number (T_fk).

  The detection / reduction processing unit 22 samples the input image signal in the sampling period asserted by the enable signal DETECT_EN, estimates the flicker waveform from the sampling data, adjusts the gain of the image signal, and extracts the flicker component. A process to reduce or eliminate is executed. A series of processing relating to detection and reduction of flicker components is executed based on various reference signals from the internal reference signal generation unit 21 such as the enable signal DETECT_EN and the count value VCOUNT.

  FIG. 3 shows an internal configuration of the detection / reduction processing unit 20. Hereinafter, the configuration of each unit in the detection / reduction processing unit 20 and the sampling operation of the image signal will be described with reference to the drawings.

  The illustrated detection / reduction processing unit 22 detects an image signal, normalizes the detection value and outputs the normalized detection value, and a DFT processing unit 120 performs DFT processing on the normalized detection value. , A flicker generation unit 130 that estimates a flicker component from the result of spectrum analysis by DFT, a buffer 140 that temporarily stores an estimated value of the flicker component, and a calculation unit 150 that removes the estimated flicker component from the image signal. ing.

  The normalized integral value calculation unit 110 includes an integration processing unit 111, an integration value holding unit 112, an average value calculation unit 113, a difference calculation unit 114, and a normalization processing unit 115.

  The integration processing unit 111 integrates the input image signal In ′ (x, y) including the flicker component for each line over a sampling period (described above) in which the enable signal DETECT_EN is at a high level. The integral value holding unit 112 temporarily holds the integral values Fn_1 (y) and Fn_2 (y) calculated in two sampling periods.

  The average value calculation unit 113 is a functional block that averages the integration values calculated during the most recent sampling period. The output from the integration processing unit 111 is output from the past integration processing unit 111 corresponding to the flicker fringe repetition period. Are added to each other (given from the integrated value holding unit 112), and the average value is output.

  The difference calculation unit 114 is a functional block that calculates the difference between the integration values calculated in the two most recent sampling periods. From the output from the integration processing unit 111, the output (integration) one field before from the integration processing unit 111 is calculated. The background component is removed by subtracting (provided from the value holding unit 112), and only the flicker component is extracted.

  The normalization processing unit 115 normalizes the difference value calculated by the difference calculation unit 114 using the average value obtained by the average value calculation unit 113.

  The DFT processing unit 120 performs DFT on the normalized difference value and performs frequency analysis to estimate the flicker component amplitude γm and the initial phase φmn. The flicker generation unit 130 calculates a correction coefficient indicating the ratio of flicker components included in the image signal from the estimated value obtained by frequency analysis. The calculation unit 150 performs a calculation for removing the flicker component from the input image signal In ′ (x, y) based on the calculated correction coefficient, and the image signal In (x) in which the flicker component is reduced or removed. , Y).

  FIG. 4 shows an internal configuration of the average value calculation unit 113. In order to extract only the background component from the input image signal, the average value calculation unit 113 outputs to the output from the integration processing unit 111 the output from the past integration processing unit 111 corresponding to the flicker fringe repetition cycle (integrated value). And the average value is output. In the present embodiment, the integrated value Fn (y) of the input image signal of each line y in the current sampling period and the integrated values Fn_1 (y) and Fn_2 (y) of the same line y in the two most recent sampling periods. In parallel with the averaging process A corresponding to the 3V average, an averaging process B corresponding to the 2V average of the integrated value for each line in the current sampling period and the immediately preceding sampling period is provided, and a selection unit (SEL) However, the output of either of the averaging processes A or B can be selected as the output Ave [Fn (y)] of the average value calculation unit 113 by an instruction from the outside.

  In the averaging process A, the three waveforms Fn (y), Fn_1 (y), and Fn_2 (y) are simply added and averaged in the order of the sampling phase (see FIG. 5A). That is, the averaging process A is expressed by the following formula (5).

  On the other hand, in the averaging process B, instead of simply averaging the two waveforms Fn (y) and Fn_1 (y) in the order of the sampling phase, the sampling phase is shifted and averaged by shifting the half of the flicker period. That is, in the averaging process B, the calculation formula differs according to the sampling phase of the line y. In the region of 0 ≦ y <T_fk / 2 corresponding to the flicker phase 0 to π, the previous sampling is performed by the following formula (6). Although averaging with the previous waveform Fn_1 (y + T_fk / 2) is performed for a half cycle of the period (see FIG. 5B), in the region of T_fk / 2 ≦ y <T_fk corresponding to flicker phases π to 2π By (7), averaging is performed with the waveform Fn_1 (y-T_fk / 2) after a half cycle of the previous sampling period (see FIG. 5C). The purpose of the averaging process B is to avoid taking a weighted average of signals that are the same for both flicker and background. In other words, this is equivalent to taking an average of signals having the same background and having the flicker signal in reverse phase (described later).

  The averaging process is performed in such a state that the combination of sampling phases to be added is changed. The phase shift side may be essentially the integrated value Fn (y) in the current sampling period, but in the following, as shown in the above equations (6) and (7), A description will be given assuming that the phase of the integration value Fn_1 (y) in the sampling period is shifted.

  FIG. 6 shows an internal configuration of the difference calculation unit 114. In order to extract only the flicker component, the difference calculation unit 114 subtracts the output from the previous integration processing unit 111 (given from the integration value holding unit 112) from the output from the integration processing unit 111, thereby extracting the background component. And only the flicker component is extracted. In the present embodiment, in parallel with the difference processing A in which the integral value Fn_1 (y) of the same line y in the immediately preceding sampling period is subtracted from the integral value Fn (y) of the input image signal of each line y in the sampling period. Then, a difference process B for performing a difference for each line in the current sampling period and the immediately preceding sampling period is provided, and the selection unit (SEL) outputs either of the difference processes A or B according to an instruction from the outside. Can be selected as the output [Fn (y) -Fn_1 (y)] of the difference calculation unit 114.

  In the difference process A, only the difference between the two waveforms Fn (y) and Fn_1 (y) is taken in the order of the sampling phase (see FIG. 5A). That is, the difference process A is expressed by the following equation (8).

  On the other hand, in the difference processing B, instead of simply taking the difference between the two waveforms Fn (y) and Fn_1 (y) in the order of the sampling phase, the difference is obtained by shifting the sampling phase by a half cycle of the flicker cycle. That is, in the difference process B, the calculation formula differs depending on the sampling phase of the line y, and in the region of 0 ≦ y <T_fk / 2 corresponding to the flicker phase 0 to π, the previous sampling period is obtained by the following formula (9). (See FIG. 5B), but in the region of T_fk / 2 ≦ y <T_fk corresponding to the flicker phase π to 2π, the following equation (10 ) To obtain a difference from the waveform Fn_1 (y-T_fk / 2) after a half cycle of the previous sampling period (see FIG. 5C). The difference process B is intended to avoid taking a difference between signals that are the same for both flicker and background. In other words, this is equivalent to taking the difference between the signals having the same background and the flicker signal having the opposite phase (described later).

  Difference processing is performed in such a state that the combination of sampling phases for obtaining the difference is changed. The phase shift side may be essentially the integrated value Fn (y) in the current sampling period, but in the following, as shown in the above equations (9) and (10), A description will be given assuming that the phase of the integration value Fn_1 (y) in the sampling period is shifted.

  Note that at least a part of the processing by each functional block shown in FIG. 3 can also be realized in the form of software processing in the system controller 15. Further, in the imaging apparatus shown in FIG. 1, processing by each functional block shown in FIG. 3 is executed for each luminance signal and color difference signal constituting the image signal. Alternatively, at least the luminance signal may be processed by each functional block shown in FIG. 3, and may be executed for the color difference signal and each color signal as necessary. Further, with respect to the luminance signal, processing by each functional block shown in FIG. 3 may be executed at the stage of the color signal before being combined with the luminance signal. Further, in the process at the stage of the color signal, the process by each functional block shown in FIG. 3 may be executed at any stage of the color signal by the primary color and the color signal by the complementary color. When these color signals are processed by the functional blocks shown in FIG. 3, the processing is executed for each color signal.

  Next, an operation for detecting flicker during high-speed shooting by the detection / reduction processing unit 22 shown in FIG. 3 will be described. Below, each example at the time of performing 240 fps imaging under the illumination of the fluorescent lamp by the commercial alternating current power supply of each frequency 50Hz and 60Hz is given.

  First, a sampling operation in the case of imaging at 240 fps as an example of a high-speed imaging mode in a power supply frequency 50 Hz region will be described with reference to FIGS.

  FIG. 7 shows a first timing chart for explaining the sampling operation in the high-speed imaging mode (240 fps). In the illustrated example, as the frame rate increases, the length of the effective data area of the image signal in the vertical direction indicated by the enable signal VEN1 also decreases. Here, let us consider a case where the counter 21a of the internal reference signal generation unit 21 selects VEN1 as the enable signal and counts the horizontal synchronization signal HD within a period in which the enable signal VEN1 is at a high level. The count value VCOUNT is repeatedly counted up to the number M of lines for each period of the effective data area, as in the normal imaging mode.

  However, as shown in FIG. 16B, when the relationship of FPS> 2 × f is established (however, the power supply frequency of the fluorescent lamp is set to f and the frame rate is set to FPS), from one cycle of flicker The period of the effective data area of one field becomes shorter. For this reason, the count value VCOUNT is reset before being counted up to T_fk corresponding to one flicker cycle, so that it is impossible to generate sampling timing for one flicker cycle. That is, the detection system block of the detection / reduction processing unit 22 cannot process the image signal every flicker period.

  FIG. 8 shows a second timing chart for explaining the sampling operation in the high-speed imaging mode (240 fps). In the illustrated example, in addition to the enable signal VEN1 indicating the effective data area corresponding to the frame rate, the enable signal VEN2 indicating the effective data area in the normal imaging mode of 60 fps is sent from the reference signal generation unit 30 to the flicker reduction unit 20. Supply. When the counter 21a of the internal reference signal generation unit 21 is set to the high-speed imaging mode in which FPS> 2 × f is established, VEN2 is selected as the input enable signal, and the enable signal VEN2 is set to the high level. During this period, the horizontal synchronizing signal HD is counted and the count value VCOUNT is output.

  Therefore, the detection / reduction processing unit 22 performs the sampling operation over a sampling period having a length equal to or longer than one period of flicker in which the enable signal VEN2 becomes high level. Therefore, as described with reference to FIG. In addition, since the period of the effective data area of one field is shorter, the problem that the image signal cannot be processed every one flicker period can be solved.

  In the example shown in FIG. 8, the enable signal VEN2 is generated by counting the synchronization timing, for example, based on the enable signal VEN1. However, the enable signal VEN2 may be a signal that is always generated based on the synchronization signal in the normal imaging mode and that accurately indicates the effective data area in the normal imaging mode. In other words, the enable signal VEN2 that defines the effective data area in the high-speed shooting mode may be a signal that becomes a high level only for a period of one flicker period or more from the start of the effective data area in a certain field. That's fine. In addition, the counter 21a of the internal reference signal generation unit 21 may be configured to generate the count value VCOUNT based on the always enable signal VEN2 instead of selecting the enable signal.

  By using such an enable signal VEN2, the upper limit value of the count value VCOUNT becomes T_fk or more corresponding to one flicker cycle. The enable signal DETECT_EN becomes high level from when the count value VCOUNT starts to be counted up until the count value reaches T_fk. Therefore, the detection / reduction processing unit 22 uses such an enable signal DETECT_EN to always capture an image signal including a flicker component for one cycle or more in a functional block of each detection system, and perform a flicker component detection process. Can be performed.

  However, during the period in which the enable signal DETECT_EN is at the high level, there is a vertical blanking period in which the image data is invalid, and thus the sampling value of the image signal is uncertain during this period. For this reason, in the present embodiment, in the final stage of the sampling (integration) processing block, the image signal in the invalid period is interpolated from the preceding and succeeding signals so that the flicker components for one cycle are smoothly connected. Details of this interpolation processing will be described later.

  Next, a sampling operation in the case of imaging at 240 fps as an example of the high-speed imaging mode in the power supply frequency 60 Hz region will be described with reference to FIGS. 9 and 10.

  FIG. 9 shows a first timing chart for explaining the sampling operation in the high-speed imaging mode (240 fps). Similar to the timing chart shown in FIG. 7, as the frame rate increases, the length of the effective data area of the image signal in the vertical direction indicated by the enable signal VEN1 also decreases. Here, let us consider a case where the counter 21a of the internal reference signal generation unit 21 selects VEN1 as the enable signal and counts the horizontal synchronization signal HD within a period in which the enable signal VEN1 is at a high level. The count value VCOUNT is repeatedly counted up to the number M of lines for each period of the effective data area, as in the normal imaging mode.

  However, as shown in FIG. 16B, since the relationship of FPS> 2 × f is established, the period of the effective data area of one field becomes shorter than one flicker period. For this reason, the count value VCOUNT is reset before being counted up to T_fk corresponding to one flicker cycle, so that it is impossible to generate sampling timing for one flicker cycle. That is, the detection system block of the detection / reduction processing unit 22 cannot process the image signal for each flicker cycle (same as above).

  FIG. 10 shows a second timing chart for explaining the sampling operation in the high-speed imaging mode (240 fps). In the illustrated example, in addition to the enable signal VEN1 indicating the effective data area corresponding to the frame rate, the enable signal VEN2 indicating the effective data area in the normal imaging mode of 60 fps is sent from the reference signal generation unit 30 to the flicker reduction unit 20. Supply. When the counter 21a of the internal reference signal generation unit 21 is set to the high-speed imaging mode in which FPS> 2 × f is established, VEN2 is selected as the input enable signal, and the enable signal VEN2 is set to the high level. During this period, the horizontal synchronizing signal HD is counted and the count value VCOUNT is output.

  Accordingly, the detection / reduction processing unit 22 performs the sampling operation over a sampling period having a length equal to or longer than one period of flicker in which the enable signal VEN2 becomes high level. Therefore, as described in FIG. In addition, since the period of the effective data area of one field is shorter, the problem that the image signal cannot be processed every one flicker period can be solved.

  In the example shown in FIG. 10, the enable signal VEN2 is generated by counting the synchronization timing, for example, based on the enable signal VEN1. However, the enable signal VEN2 may be a signal that is always generated based on the synchronization signal in the normal imaging mode and that accurately indicates the effective data area in the normal imaging mode. In other words, the enable signal VEN2 that defines the effective data area in the high-speed shooting mode may be a signal that becomes a high level only for a period of one flicker period or more from the start of the effective data area in a certain field. That's fine. The counter 21a of the internal reference signal generation unit 21 may be configured to generate the count value VCOUNT based on the enable signal VEN2 at all times, instead of selecting the enable signal (same as above).

  By using such an enable signal VEN2, the upper limit value of the count value VCOUNT becomes T_fk or more corresponding to one flicker cycle. The enable signal DETECT_EN becomes high level from when the count value VCOUNT starts to be counted up until the count value reaches T_fk. Therefore, the detection / reduction processing unit 22 uses such an enable signal DETECT_EN to always capture an image signal including a flicker component for one cycle or more in a functional block of each detection system, and perform a flicker component detection process. (Same as above).

  However, since there is a vertical blanking period during which the image data is invalid during the period when the enable signal DETECT_EN is at the high level, the image signal in the invalid period is before and after that in the final stage of the sampling (integration) processing block. Is interpolated from the signal (same as above).

  By the way, comparing the sampling operation under the high-speed shooting mode in the power supply frequency 50 Hz region shown in FIG. 8 with the sampling operation under the high-speed shooting mode in the power supply frequency 60 Hz region shown in FIG. In this case, the waveform sampled during the DETECT_EN period is a “fixed flicker fringe” that has the same phase every time in the vertical synchronization period. This is because the time corresponding to one cycle of the frame rate and flicker is exactly an integer multiple. In other words, although flicker has repeatability, flicker occurs at the same phase every time when considered in terms of detection units, so when the difference between fields is taken during normalization processing, there is no flicker fringe information along with the background. (See above).

  Specifically, when the averaging process A is performed according to the above equation (5), the three waveforms Fn (y), Fn_1 (y), and Fn_2 (y) that are the same signal in the flicker and the background are averaged. The target background component cannot be extracted. Further, when the difference processing A is performed according to the above equation (8), the difference between the two waveforms Fn (y) and Fn_1 (y) having the same signal in both the flicker and the background is obtained. (Removal of background component) cannot be performed. In other words, the algorithm that distinguishes the background from the flicker fringe using the repeatability of the flicker phenomenon is distinguished from the background even if there is repeatability under the above conditions where the vertical synchronization and the flicker fringe are synchronized. The desired detection operation cannot be performed.

  Therefore, in this embodiment, the time corresponding to one cycle of the frame rate and flicker is exactly an integer multiple, and the average value calculation unit 113 performs averaging under shooting conditions in which vertical synchronization and flicker fringe are synchronized. While performing the process B, the difference calculation unit 114 performs the difference process B.

  As already described with reference to FIG. 4, the averaging process B avoids taking a weighted average of signals that are the same in both flicker and background. The calculation formula differs depending on the sampling phase of the line y, and in the region of 0 ≦ y <T_fk / 2 corresponding to the flicker phase 0 to π, the following waveform (11) indicates the waveform ahead by the half cycle of the previous sampling period. Fn_1 (y + T_fk / 2) is averaged (see FIG. 5B), but in the region of T_fk / 2 ≦ y <T_fk corresponding to the flicker phase π to 2π, the following equation (12) Averaging with the waveform Fn_1 (y-T_fk / 2) after a half cycle of the sampling period is performed (see FIG. 5C).

  Further, as already described with reference to FIG. 6, in the difference processing B, the difference is obtained by shifting the phase by a half cycle of the flicker cycle, thereby taking the difference between the signals that are the same in both the flicker and the background. is doing. The calculation formula differs depending on the sampling phase of the line y, and in the region of 0 ≦ y <T_fk / 2 corresponding to the flicker phase 0 to π, the following waveform (13) indicates the waveform ahead by the half cycle of the previous sampling period. The difference from Fn_1 (y + T_fk / 2) is obtained (see FIG. 5B), but in the region of T_fk / 2 ≦ y <T_fk corresponding to the flicker phase π to 2π, the previous sampling is performed by the following equation (14). The difference from the waveform Fn_1 (y-T_fk / 2) after a half period is taken (see FIG. 5C).

  FIG. 11 shows a state of a detection waveform obtained by performing the averaging process B and the difference process B in the high-speed imaging mode (240 fps) in the power supply frequency 60 Hz region. However, the sampling operation of the flicker waveform is the same as the timing chart shown in FIG. For convenience of explanation, the vertical blanking period is omitted in FIG.

  In the averaging process B, in the region of 0 ≦ y <T_fk / 2 corresponding to the flicker phase 0 to π, the waveform Fn_1 (y + T_fk / 2) which is the half cycle of the previous sampling period is obtained by the following equation (15). In the region of T_fk / 2 ≦ y <T_fk corresponding to the flicker phase π to 2π, the waveform Fn_1 (y−T_fk / 2) after the half cycle of the previous sampling period is obtained by the following equation (16). ) And the weighted average.

  Therefore, in the averaging process B, the portion corresponding to 0 to π of the flicker phase becomes “(C) + (B)” in FIG. 11, and the portion corresponding to π to 2π of the flicker phase is shown in FIG. “(D) + (A)”.

  Further, in the difference processing B, in the region of 0 ≦ y <T_fk / 2 corresponding to the flicker phase 0 to π, the waveform Fn_1 (y + T_fk / 2) ahead by the half cycle of the previous sampling period by the following equation (17). In the region of T_fk / 2 ≦ y <T_fk corresponding to the flicker phase π to 2π, the waveform Fn_1 (y−T_fk / 2) after the half cycle of the previous sampling period is obtained by the following equation (18). ) And the difference.

  Therefore, in the difference processing B, the portion corresponding to 0 to π of the flicker phase is “(C) − (B)” in FIG. 11, and the portion corresponding to π to 2π of the flicker phase is “ (D)-(A) ".

  As can be seen from FIG. 11, the combinations of (A) and (D) and (B) and (C) have the same background and the reverse flicker phase. Therefore, when averaging and difference calculation are performed with a combination of (A) and (D), and (B) and (C), the time corresponding to one cycle of the frame rate and flicker is exactly an integral multiple. Therefore, only the background component and the flicker component can be extracted from the input image signal even under shooting conditions in which fixed flicker occurs.

  FIG. 11 shows an example in which the background is flat for convenience of explanation. Since the background is actually reflected, the flat background component and the beautiful flicker waveform as shown in FIG. 11 are not obtained, but high-precision flicker detection and flicker correction are realized by the subsequent normalization processing and DFT processing. Please understand that you can. In addition, as can be seen from FIG. 11, the flicker component obtained by the difference processing B has an amplitude that is doubled. Therefore, when calculating the correction coefficient, the correction coefficient is calculated in consideration of that amount. Caution must be taken.

  As described above, according to the present embodiment, since the time corresponding to one cycle of the frame rate and flicker is exactly an integer multiple, averaging and difference calculations are appropriately performed even under shooting conditions in which fixed flicker occurs. Can be implemented. For details of normalization processing, DFT processing, flicker generation processing, and flicker correction processing thereafter, refer to paragraphs 0092 to 0111 of JP-A-2004-222228 (Patent Document 1) already assigned to the present applicant, for example. Please refer.

  In addition, when the high-speed imaging mode is entered in a state where either the 50 Hz region or the 60 Hz region is known in advance, which of the averaging processing A and B or the difference processing A and B should be used is as described above. It is. Even in a state where the power supply frequency is unknown, for example, after entering high-speed imaging, appropriate correction can be automatically applied by following the procedure below.

(1) First, flicker detection is performed with settings of averaging process A and difference process A. If the place where the subject is photographed is a 50 Hz region, the flicker component can be detected with this setting.
(2) At this time, when the DFT spectrum corresponding to the flicker component does not constantly occur, it is estimated that the flicker is in the 60 Hz region or the flicker is not generated. Therefore, the setting is switched to the setting of the averaging process B and the difference process B, and the apparatus is set in a state where the flicker component can be detected in the 60 Hz region.
(3) At this time, when the DFT spectrum corresponding to the flicker component does not constantly occur, it is determined that the flicker is not generated. Therefore, the flicker detection value is not output, and as a result, unnecessary correction is not applied to the input image signal.

  In the case of the configuration of the camera processing circuit 14 shown in FIG. 2, as can be seen from the timing chart shown in FIG. 10, DETECT_EN that defines the sampling period using the enable signal VEN <b> 2 indicating the effective data area in the normal imaging mode of 60 fps. Is generated. In such a case, the generation interval of DETECT_EN is limited to 1/60 second interval which is the generation interval of VEN2, but in the case of flicker occurring in the 60 Hz region, it is not necessary to set DETECT_EN to 1/60 second interval. This is because, as can be seen from FIG. 10 or FIG. 11, the same flicker phase is obtained at 1/120 second intervals, and even if sampling processing is not performed at 1/60 second intervals, it is 1/120 seconds. Equivalent processing may be performed at intervals.

  FIG. 12 shows a modification of the camera processing circuit 14. In the illustrated camera processing circuit 14, in addition to VEN1 and VEN2, the reference signal generation unit 30 supplies VEN3 (a signal that becomes high level for a time corresponding to exactly half of VEN2) to the counter 21a. The counter 21a of the internal reference signal generation unit 21 selects VEN3 instead of VEN2 as an input enable signal when the high-speed imaging mode in which FPS> 2 × f is established, and the enable signal VEN3 is high. The horizontal synchronization signal HD is counted during the level period and the count value VCOUNT is output.

  FIG. 13 shows a sampling operation in the high-speed imaging mode (240 fps) in the region where the power supply frequency is 60 Hz when the camera processing circuit 14 shown in FIG. 12 is used. In the example shown in the figure, VEN3 that becomes high level for a time corresponding to exactly half of VEN2 is supplied from the reference signal generation unit 30 to the flicker reduction unit 20. When the counter 21a of the internal reference signal generation unit 21 is set to the high-speed imaging mode in which FPS> 2 × f is established, VEN3 is selected as the input enable signal, and the enable signal VEN3 is set to the high level. During this period, the horizontal synchronizing signal HD is counted and the count value VCOUNT is output. In this case, the detection / reduction processing unit 22 can perform the sampling operation over a sampling period having a length equal to or longer than one cycle of flicker in which the enable signal VEN3 becomes high level.

  In the timing chart shown in FIG. 10, the averaging / difference calculation is performed between flicker waveforms separated by one period. On the other hand, in the timing chart shown in FIG. 13, since the calculation is performed for averaging / difference between the latest flicker waveforms, the detection accuracy is increased (in the direction of being stronger against the background motion).

  Further, in both of the embodiments shown in FIGS. 10 and 13, shooting is performed at 240 fps in a 60 Hz region, but the gist of the present invention is not limited to high-speed shooting.

  For example, even in normal shooting such as shooting at 60 fps in a 60 Hz region, when the flicker waveform is viewed in units of detection frames, fixed flicker fringes in which the flicker patterns in each detection frame unit are exactly the same position with respect to synchronization are generated. For this reason, if the normalization process is performed by simply taking the difference between the integration values for each sampling period in the order of the sampling phase, the flicker fringe information is lost together with the background when the difference between the fields is obtained. In other words, when the vertical synchronization and the flicker fringe are synchronized with the algorithm that distinguishes the background from the flicker fringe using the repeatability, even if the flicker waveform is repeatable, it cannot be distinguished from the background.

  On the other hand, in the present invention, the sampling phase of one of the integral values in two consecutive sampling periods is similarly shifted by a half period of the flicker period even under the condition that the vertical synchronization and the flicker fringe are synchronized during normal shooting. Thus, by calculating the averaging and the difference, it is possible to realize high-precision flicker detection and correction processing by the gain correction method.

  FIG. 14 shows an example of the sampling operation in the normal imaging mode (60 fps) in the region where the power frequency is 60 Hz. However, although the sampling operation shown in the figure can be applied to a uniform subject, it is difficult to apply to a natural image (the direct cause is that the calculation is performed with different background parts). Is).

  In the illustrated example, in addition to the enable signal VEN1 indicating the effective data area corresponding to the frame rate, the enable signal VEN2 indicating the effective data area in the normal imaging mode of 60 fps is sent from the reference signal generation unit 30 to the flicker reduction unit 20. Supply. The counter 21a of the internal reference signal generation unit 21 selects VEN1 as an input enable signal when the normal photographing mode in which FPS> 2 × f is not established, and the enable signal VEN1 becomes a high level. The horizontal synchronization signal HD is counted during the period, and the count value VCOUNT is output. Then, the detection / reduction processing unit 22 can perform a sampling operation for one cycle of flicker over a sampling period in which DETECT_EN is at a high level.

  Here, it can be seen from the flicker waveforms Fn (y) and Fn_1 (y) in FIG. 14 that the relationship between the flicker waveforms Fn (y) and Fn_1 (y) shown in FIG. 10 is exactly the same. In other words, for the problem that the background and flicker cannot be separated in addition averaging and difference processing under fixed flicker generation conditions, a method of calculating addition averaging and difference by shifting the sampling phase by half a flicker cycle is applied. By doing so, it is possible to separate the image signal into the background and the flicker. Therefore, high-precision flicker detection / correction can be used by a method of estimating the flicker component based on the spectrum extracted from the normalized integral value or difference value (see Patent Document 1).

  In the embodiment shown in FIG. 14 as well, as in the embodiment shown in FIG. 10, when the high-speed imaging mode is entered in a state where either 50 Hz region or 60 Hz region is known in advance, averaging is performed. Which of the processes A and B or the difference processes A and B should be used is as described above. Even in a state where the power supply frequency is unknown, for example, after entering high-speed imaging, appropriate correction can be automatically applied by following the procedure below.

(1) First, flicker detection is performed with settings of averaging process A and difference process A. If the place where the subject is photographed is a 50 Hz region, the flicker component can be detected with this setting.
(2) At this time, when the DFT spectrum corresponding to the flicker component does not constantly occur, it is estimated that the flicker is in the 60 Hz region or the flicker is not generated. Therefore, the setting is switched to the setting of the averaging process B and the difference process B, and the apparatus is set in a state where the flicker component can be detected in the 60 Hz region.
(3) At this time, when the DFT spectrum corresponding to the flicker component does not constantly occur, it is determined that the flicker is not generated. Therefore, the flicker detection value is not output, and as a result, unnecessary correction is not applied to the input image signal.

  The present invention has been described in detail above with reference to specific embodiments. However, it is obvious that those skilled in the art can make modifications and substitutions of the embodiment without departing from the gist of the present invention.

  In the present specification, the embodiment applied to the digital camera has been mainly described, but the gist of the present invention is not limited to this. For example, the present invention can be similarly applied to various information devices having a mobile phone, a PDA (Personal Digital Assistant), or other camera functions.

  In short, the present invention has been disclosed in the form of exemplification, and the description of the present specification should not be interpreted in a limited manner. In order to determine the gist of the present invention, the claims should be taken into consideration.

FIG. 1 is a diagram schematically showing a configuration of an imaging apparatus suitable for applying the present invention. FIG. 2 is a diagram showing an internal configuration of the camera processing circuit 14. FIG. 3 is a diagram showing an internal configuration of the detection / reduction processing unit 20. FIG. 4 is a diagram showing an internal configuration of the average value calculation unit 113. FIG. 5A is a diagram for explaining the mechanism of the averaging process and the difference process. FIG. 5B is a diagram for explaining the mechanism of the averaging process and the difference process. FIG. 5C is a diagram for explaining the mechanism of the averaging process and the difference process. FIG. 6 is a diagram illustrating an internal configuration of the difference calculation unit 114. FIG. 7 is a diagram showing a first timing chart for explaining the sampling operation in the high-speed imaging mode (240 fps) in the region where the power frequency is 50 Hz. FIG. 8 is a diagram showing a second timing chart for explaining the sampling operation in the high-speed imaging mode (240 fps) in the region where the power supply frequency is 50 Hz. FIG. 9 is a diagram showing a first timing chart for explaining the sampling operation in the high-speed imaging mode (240 fps) in the power supply frequency 60 Hz region. FIG. 10 is a diagram showing a second timing chart for explaining the sampling operation in the high-speed imaging mode (240 fps) in the power supply frequency 60 Hz region. FIG. 11 is a diagram illustrating a state of a detection waveform obtained by performing the averaging process B and the difference process B in the high-speed imaging mode (240 fps) in the power supply frequency 60 Hz region. FIG. 12 is a diagram showing a modification of the camera processing circuit 14. FIG. 13 is a diagram showing a timing chart for explaining the sampling operation in the high-speed imaging mode (240 fps) in the region where the power supply frequency is 60 Hz when the camera processing circuit 14 shown in FIG. 12 is used. FIG. 14 is a diagram for explaining the sampling operation in the normal imaging mode (60 fps) in the region where the power frequency is 60 Hz. FIG. 15 is a diagram showing a computer simulation result regarding the relationship between the flicker level and the shutter speed when a camera having an XY address scanning type image pickup device is used for photographing under fluorescent lamp illumination. FIG. 16A is a diagram showing the relationship between the vertical synchronization signal VD of 60 fps, which is the NTSC frame rate, and the flicker waveform. FIG. 16B is a diagram illustrating a relationship between a vertical synchronization signal and a flicker waveform when imaging is performed at 240 fps, which is a frame rate four times higher than a standard, as an example of high-speed imaging. FIG. 17A is a diagram showing the relationship between the vertical synchronization signal VD of 60 fps, which is the NTSC frame rate, and the flicker waveform. FIG. 17B is a diagram illustrating a relationship between a vertical synchronization signal and a flicker waveform when imaging is performed at 240 fps, which is a frame rate four times the standard as an example of high-speed imaging.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 ... Optical block 11a ... Driver 12 ... CMOS type image sensor 12a ... Timing generator 13 ... Analog front end (AFE) circuit 14 ... Camera processing circuit 15 ... System controller 16 ... Input part 17 ... Graphic I / F (interface)
17a ... Display 20 ... Flicker reduction unit 21 ... Internal reference signal generation unit 21a ... Counter 22 ... Detection / reduction processing unit 110 ... Normalized integration value calculation unit 111 ... Integration processing unit 112 ... Integration value holding unit 113 ... Average value calculation unit 114: Difference calculation unit 115 ... Normalization processing unit 120 ... DFT processing unit 130 ... Flicker generation unit 140 ... Buffer 150 ... Calculation unit

Claims (13)

An image processing apparatus for processing an image signal including a flicker component,
An integration processing unit that integrates an image signal for each line over a sampling period having a length of one flicker or longer;
An integration value holding unit that holds an integration value by the integration processing unit in a past sampling period;
An average value calculation unit that averages integral values in two or more consecutive sampling periods;
A difference calculation unit that takes a difference between integral values in two or more consecutive sampling periods;
A normalization calculation unit that normalizes the difference value calculated by the difference calculation unit using the average value obtained by the average value calculation unit;
Frequency analysis means for frequency analysis of the difference value of the normalized integral value;
Flicker detection means for detecting a flicker component included in the image signal based on the result of frequency analysis by the frequency analysis means;
A calculation unit for performing a calculation for removing the flicker component detected by the flicker detection unit from the image signal;
With
In a shooting environment where the frame rate of the image signal and the time corresponding to one cycle of the flicker waveform are exactly an integer multiple, or the flicker waveform is in the same phase each time with respect to the vertical synchronization period,
The average value calculation unit shifts the sampling phase of one of the integral values in two consecutive sampling periods by a half period of the flicker period, and performs averaging.
The difference calculation unit obtains a difference by shifting one sampling phase of the integral value in two consecutive sampling periods by a half period of the flicker period.
An image processing apparatus. Processing an image signal obtained by photographing with an XY address scanning image pickup device under illumination of a blinking light source such as a fluorescent lamp that is turned on by a commercial AC power source;
The image processing apparatus according to claim 1. In the shooting environment where the flicker waveform is not in the same phase with respect to the vertical synchronization period,
The average value calculation unit adds and averages integration values in two or more consecutive sampling periods in the order of sampling phases,
The difference calculation unit takes a difference between integral values in two consecutive sampling periods in the order of sampling phases.
The image processing apparatus according to claim 1. The sampling phase of one of the integral values in the two sampling periods in which the average value calculation unit is continuous is shifted and averaged by a half cycle of the flicker cycle, and the integration value in the two sampling periods in which the difference calculation unit is continuous is calculated. A first normalized integral value calculation mode in which one sampling phase is shifted by a half cycle of the flicker period and a difference is obtained, and an integral value in two or more sampling periods in which the average value calculation unit is continuous is added and averaged in the order of sampling phases And a mode switching means for switching a second normalized integral value calculation mode in which the difference calculation unit takes the difference between the integral values in two consecutive sampling periods in the order of the sampling phase.
The image processing apparatus according to claim 3. Processing an image signal obtained by photographing with an XY address scanning image sensor under illumination of a blinking light source such as a fluorescent lamp that is turned on by a commercial AC power source,
The mode switching means sets the first normalized integral value calculation mode when one cycle of the flicker waveform based on the frequency of the commercial AC power supply and the frame rate have an integer multiple relationship, and otherwise Set to the second normalized integral value calculation mode,
The image processing apparatus according to claim 4. The mode switching unit first sets the second normalized integral value calculation mode, tries to detect a flicker component included in the image signal, and a frequency spectrum corresponding to the flicker component included in the image signal is the frequency analysis unit. However, a frequency spectrum corresponding to a flicker component included in the image signal was not generated constantly by the frequency analysis means. Sometimes switch to the first normalized integral value calculation mode,
The image processing apparatus according to claim 4. An image processing method for processing an image signal including a flicker component,
An integration step for integrating the image signal line by line over a sampling period having a length of one flicker period or more;
An average value calculating step for averaging the integral values in two or more consecutive sampling periods;
A difference calculating step for calculating a difference between integral values in the two or more consecutive sampling periods;
A normalization calculation step for normalizing the difference value calculated in the difference calculation step using the average value obtained in the average value calculation step;
A frequency analysis step for frequency analysis of the difference value of the normalized integral value;
A flicker detection step for detecting a flicker component included in the image signal based on a result of frequency analysis by the frequency analysis step;
A calculation step for performing a calculation for removing the flicker component detected by the flicker detection step from the image signal;
With
In a shooting environment where the frame rate of the image signal and the time corresponding to one cycle of the flicker waveform are exactly an integer multiple, or the flicker waveform is in the same phase each time with respect to the vertical synchronization period,
In the average value calculating step, one of the sampling phases of the integral values in two consecutive sampling periods is shifted and averaged by shifting by a half period of the flicker period,
In the difference calculation step, a difference is obtained by shifting one sampling phase of the integral value in two consecutive sampling periods by a half period of the flicker period.
An image processing method. Processing an image signal obtained by photographing with an XY address scanning image pickup device under illumination of a blinking light source such as a fluorescent lamp that is turned on by a commercial AC power source;
The image processing method according to claim 7. In the shooting environment where the flicker waveform is not in the same phase with respect to the vertical synchronization period,
In the average value calculating step, the integration values in two or more consecutive sampling periods are averaged in the order of sampling phases,
In the difference calculation step, a difference between integral values in two consecutive sampling periods is taken in the order of sampling phases.
The image processing method according to claim 7. In the mean value calculating step, one sampling phase of the integrated values in two consecutive sampling periods is shifted by half a flicker period and added and averaged, and in the difference calculating step, the integrated values in two consecutive sampling periods are calculated. A first normalized integral value calculation mode in which one sampling phase is shifted by a half cycle of the flicker period and a difference is obtained, and an integral value in two or more consecutive sampling periods in the average value calculating step is added and averaged in the order of the sampling phase. And a mode switching step of switching a second normalized integral value calculation mode in which the difference between integral values in two consecutive sampling periods in the difference calculation step is taken in the order of sampling phases.
The image processing method according to claim 9. Processing an image signal obtained by photographing with an XY address scanning image sensor under illumination of a blinking light source such as a fluorescent lamp that is turned on by a commercial AC power source,
In the mode switching step, the first normalized integral value calculation mode is set when one cycle of the flicker waveform based on the frequency of the commercial AC power supply and the frame rate have an integer multiple relationship, and otherwise Is set to the second normalized integral value calculation mode,
The image processing method according to claim 10. In the mode switching step, first, the second normalized integral value calculation mode is set, detection of a flicker component included in the image signal is attempted, and a frequency spectrum corresponding to the flicker component included in the image signal is detected in the frequency analysis step. However, a frequency spectrum corresponding to a flicker component included in the image signal was not generated constantly by the frequency analysis step. Sometimes switch to the first normalized integral value calculation mode,
The image processing method according to claim 10. An XY address scanning type imaging device;
Integration processing for integrating an image signal for each line over a sampling period having a length of one cycle or more of a flicker waveform included in a harmful image signal, obtained by photographing an object using the image sensor And
An integration value holding unit that holds an integration value by the integration processing unit in a past sampling period;
An average value calculation unit that averages integral values in two or more consecutive sampling periods;
A difference calculation unit for calculating a difference between integral values in two or more consecutive sampling periods;
A normalization calculation unit that normalizes the difference value calculated by the difference calculation unit using the average value obtained by the average value calculation unit;
Frequency analysis means for frequency analysis of the difference value of the normalized integral value;
Flicker detection means for detecting a flicker component included in the image signal based on the result of frequency analysis by the frequency analysis means;
A calculation unit for performing a calculation for removing the flicker component detected by the flicker detection unit from the image signal;
With
In a shooting environment in which the frame rate of the image signal and the time corresponding to one cycle of the flicker waveform are exactly an integral multiple, or the flicker waveform is in the same phase each time with respect to the vertical synchronization period,
The average value calculation unit shifts the sampling phase of one of the integral values in two consecutive sampling periods by a half period of the flicker period, and performs averaging.
The difference calculation unit obtains a difference by shifting one sampling phase of the integral value in two consecutive sampling periods by a half period of the flicker period.
An imaging apparatus characterized by that.

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