JP2006128872A - Exposure control device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exposure control device capable of accurately adjusting an exposure amount independently of brightness of a field. <P>SOLUTION: Y data generated by an image sensor are subjected to bit shift by a bit shift circuit 28a. A switch 28b turns on/off the bit shift circuit 28a in accordance with brightness of the field. A CPU evaluates a luminance evaluation value on the basis of Y data subjected to clip processing. The evaluated luminance evaluation value is multiplied by 2 in a multiplier 28g. A switch 28i turns on/off the multiplier 28g in accordance with brightness of the field. An exposure of the image sensor is adjusted by the CPU on the basis of a value resulting from multiplication in the multiplier 28g. Thus the exposure can be accurately adjusted independently of brightness of the field. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an exposure control apparatus, and more particularly to an exposure control apparatus that is applied to a video camera and adjusts exposure.

An example of a conventional device of this type is disclosed in Patent Document 1. According to this prior art, when the area of the high luminance region is large, the peak luminance in the screen is made constant, and when the area of the high luminance region is small, the average luminance of the entire screen is made constant. As a result, appropriate exposure can be obtained even when the subject moves in a backlight environment or an excessively forward light environment in which a high-luminance portion exists in the screen.
JP-A-9-46581 [H04N 5/238, G03B 7/28]

  However, in the prior art, when evaluating the brightness of a captured image, the brightness is evaluated by a common procedure regardless of the object scene. For this reason, depending on the brightness of the object scene, the luminance cannot be accurately evaluated, and the exposure amount may not be adjusted accurately.

  Therefore, a main object of the present invention is to provide an exposure control apparatus capable of accurately adjusting an exposure amount regardless of the brightness of the object scene.

  An electronic camera according to a first aspect of the invention includes an imaging means for capturing a scene, generation means for generating luminance data indicating the brightness of the scene captured by the imaging means, and luminance data generated by the generation means. Attenuating means that attenuates according to a gain that decreases as the brightness of the field increases, evaluation means that evaluates the brightness of the object field based on luminance data attenuated by the attenuating means, and an evaluation value obtained by the evaluating means A calculation unit that multiplies a coefficient that increases as the brightness of the object field increases, and an adjustment unit that adjusts the exposure amount of the imaging unit based on the multiplication value obtained by the calculation unit.

  The generation unit generates luminance data indicating the brightness of the scene captured by the imaging unit. The generated luminance data is attenuated by the attenuation means according to a gain that decreases as the brightness of the object scene increases. The evaluation means evaluates the brightness of the scene based on the attenuated luminance data, and the calculation means multiplies the evaluation value obtained by the evaluation means by a coefficient that increases as the brightness of the scene increases. To do. The exposure amount of the image pickup unit is adjusted by the adjustment unit based on the multiplication value obtained by the calculation unit.

As described above, the luminance data is attenuated according to the gain that decreases as the brightness of the object field increases, and the evaluation value is obtained based on the attenuated luminance data. This makes it possible to accurately evaluate the brightness even when the brightness of the object scene fluctuates greatly. The obtained evaluation value is multiplied by a coefficient that increases as the brightness of the object field increases. As a result, the deviation of the evaluation value due to the gain is eliminated, and the exposure amount can be adjusted by a common procedure.
An electronic camera according to a second aspect of the invention is dependent on the first aspect, wherein the attenuation means turns on / off the bit shift means for halving the data value by applying a bit shift to the luminance data, and turns on / off the bit shift means according to the brightness. First control means for turning off is included. Thereby, a different attenuation operation is realized depending on the brightness.

  An electronic camera according to a third aspect of the invention is dependent on the second aspect, wherein the calculating means is a multiplying means for multiplying the evaluation value by "2", and a second control means for turning on / off the multiplying means in accordance with the brightness. including. As a result, different multiplication operations are obtained depending on the brightness.

  An electronic camera according to a fourth aspect of the invention is dependent on the first to third aspects, wherein a plurality of blocks are assigned to the object scene, and the evaluation means integrates the luminance data for each block assigned to the object scene. Including means. Thereby, the brightness of the object scene can be accurately evaluated.

  An electronic camera according to a fifth aspect of the present invention is dependent on the fourth aspect, wherein the gain is determined based on a comparison unit that compares each of a plurality of integrated values obtained by the integration unit with a threshold value, and a comparison result of the comparison unit. And a gain determining means.

An electronic camera according to a sixth aspect of the invention is dependent on the fifth aspect, and further includes coefficient determination means for determining a coefficient based on a comparison result of the comparison means.
The exposure control apparatus described.

  According to the present invention, the luminance data is attenuated according to the gain that decreases as the brightness of the object field increases, and the evaluation value is obtained based on the attenuated luminance data. This makes it possible to accurately evaluate the brightness even when the brightness of the object scene fluctuates greatly. The obtained evaluation value is multiplied by a coefficient that increases as the brightness of the object field increases. As a result, the deviation of the evaluation value due to the gain is eliminated, and the exposure amount can be adjusted by a common procedure. Therefore, the exposure amount can be accurately adjusted regardless of the brightness of the object scene.

  The above object, other objects, features and advantages of the present invention will become more apparent from the following detailed description of embodiments with reference to the drawings.

  Referring to FIG. 1, the digital camera 10 of this embodiment has an autofocus function, and an optical image of a subject forms a CCD (Charge Coupled Device) through an opening 14 a of a focus lens 12 and a mechanical shutter 14. The image sensor 16 is irradiated. On the imaging surface, a charge corresponding to the optical image of the object scene image, that is, a raw image signal is generated by photoelectric conversion.

  When the shooting mode is selected by the mode switch 34, through image processing, that is, processing for displaying a real-time moving image (through image) of the object scene image on the liquid crystal monitor 42 is executed. First, the CPU 30 instructs the TG 24 to repeat pre-exposure and thinning-out reading. The TG 24 repeatedly executes pre-exposure of the image sensor 16 and thinning out of the raw image signal generated thereby. Pre-exposure and thinning readout are executed in response to a vertical synchronization signal generated every 1/30 seconds. As a result, a low-resolution raw image signal corresponding to the optical image of the object scene is output from the image sensor 16 at a frame rate of 30 fps.

  The output raw image signal of each frame is subjected to a series of processes of noise removal, level adjustment and A / D conversion by the CDS / AGC / AD circuit 18. From the CDS / AGC / AD circuit 18, raw image data which is a digital signal is obtained. The raw image data is subjected to signal processing such as white balance adjustment, color separation, and YUV conversion by the signal processing circuit 26, and thereby converted into image data in the YUV format. The signal processing circuit 26 provides a predetermined amount of image data to the memory control circuit 36 through the bus 52, and issues a write request for the predetermined amount of image data to the memory control circuit 36. A predetermined amount of image data is written into the SDRAM 38 by the memory control circuit 36. Thus, the image data is stored in the SDRAM 38 by a predetermined amount.

  The video encoder 40 repeatedly issues a read request to the memory control circuit 36 in order to read the image data stored in the SDRAM 38 at a rate of 1 frame per 1/30 second. The memory control circuit 36 reads a predetermined amount of image data from the SDRAM 38. The read image data is given to the video encoder 40 through the bus 52. The image data stored in the SDRAM 38 is thus given to the video encoder 40 by a predetermined amount.

  Note that the transfer speed of the image data on the bus 52 is much faster than the processing speed of each of the signal processing circuit 26 and the video encoder 40. For this reason, the image data does not collide on the bus 52, and the data transfer process does not fail.

  The video encoder 40 converts the image data supplied from the memory control circuit 36 into a composite video signal conforming to the NTSC format, and supplies the converted composite video signal to the liquid crystal monitor 42. As a result, a through image of the scene is displayed on the monitor screen.

  The Y data (= 8 bits) forming the image data generated by the signal processing circuit 26 is also given to the luminance evaluation circuit 28. The luminance evaluation circuit 28 integrates the given Y data in response to the vertical synchronization signal, and calculates the luminance evaluation value of each frame.

  The CPU 30 executes through image AE processing based on the luminance evaluation value evaluated by the luminance evaluation circuit 28, and calculates the optimum exposure period Tpre. The calculated optimal period Tpre is set to TG24. The driver 22 executes pre-exposure according to the set optimum exposure period Tpre. As a result, the brightness of the through image displayed on the liquid crystal monitor 42 is adjusted.

  When the shutter button 32 is half-pressed, the CPU 30 captures the brightness evaluation value in response to the vertical synchronization signal, and executes the recorded image AE process based on the captured brightness evaluation value. As a result, the optimum exposure period Tmain can be obtained in a short time.

  When the shutter button 32 is fully pressed, the CPU 30 executes photographing / recording processing. First, the CPU 30 instructs the TG 24 to execute main exposure and all pixel readout, and instructs the driver 20 to drive the mechanical shutter 14 when the optimum exposure period Tmain has elapsed from the start of the main exposure.

  The TG 24 performs the main exposure on the image sensor 16, and the mechanical shutter 14 blocks light from entering the image sensor 16 at a desired timing. The TG 24 reads all the charges generated by the main exposure after the mechanical shutter 14 is driven.

  Next, the CPU 30 instructs the JPEG codec 44 to perform JPEG compression. The JPEG codec 44 reads high-resolution image data stored in the SDRAM 38 through the memory control circuit 36, performs JPEG compression on the read image data, and writes the compressed image data into the SDRAM 38 through the memory control circuit 36.

  When the compressed image data is secured in the SDRAM 38, the CPU 30 reads the compressed image data through the memory control circuit 36. The read image file including the compressed image data is recorded on the recording medium 48 through the I / F circuit 46.

  The luminance evaluation circuit 28 is configured as shown in FIG. When the power source of the digital camera 10 is turned on, the switch 28b is connected to the terminal T1. The Y data output from the signal processing circuit 26 is given to the bit shift circuit 28a. The switch 28b is connected to the terminal T2 when an SW1 switching command (SW1 = T2) is given from the CPU 30. The Y data output from the signal processing circuit 26 is given to the clip circuit 28c. Further, when an SW1 switching command (SW1 = T1) is given from the CPU 30, the switch 28b is again connected to the terminal T1.

  The bit shift circuit 28a shifts the applied Y data downward by 1 bit and assigns 0 to the most significant bit. As a result, the numerical value indicated by the Y data is halved. The Y data output from the bit shift circuit 28a is applied to the clip circuit 28c. The clip circuit 28c invalidates Y data indicating a threshold value (LH) or more.

  That is, when the switch 28b is connected to the terminal T2, Y data to which a gain “1” is given is output from the switch 28b. On the other hand, when the switch 28b is connected to the terminal T1, Y data to which the gain “0.5” is given is output from the switch 28b. The switch 28b is connected to the terminal T2 when the brightness of the object scene is small, and is connected to the terminal T1 when the brightness of the object scene is large. As brightness increases, it decreases.

  FIG. 3 shows the relationship between the Y data input from the signal processing circuit 26 and the Y data output from the clip circuit 28c. When SW1 = T1, Y data in the range of RY1 is distributed in the range indicated by RC after the output of the clip circuit 28c. A solid line L1 indicates an effective value of the Y data output from the clip circuit 28c when SW1 = T1. When SW = T2, the Y data in the range of RY2 is distributed in the range indicated by RC after the output of the clip circuit 28c. A solid line L2 indicates an effective value of the Y data output from the clip circuit 28c when SW2 = T2.

  Thereby, when the switch 28b is connected to the terminal T2, Y data in the range of RY2, that is, image data with low luminance, can receive an accurate luminance evaluation. On the other hand, when the switch 28b is connected to the terminal T1, the high-brightness image data is converted into image data of appropriate brightness that is not saturated by the bit shift circuit 28a.

  As shown in FIG. 4A, the object scene is divided into 16 parts in each of the horizontal direction and the vertical direction, and a total of 256 divided areas are distributed on the object field. The SRAM 28e is divided into 256 blocks as shown in FIG. 4B by the controller 28f. Each of the plurality of divided blocks corresponds to the plurality of divided areas.

  Returning to FIG. 2, the controller 28f specifies the pixel position of the Y data output from the clip circuit 28c, and reads the luminance evaluation value of the block corresponding to the specified pixel position from the SRAM 28e. The read luminance evaluation value is added by the adder 28d with the Y data output from the clip circuit 28c. The luminance evaluation value updated by the addition process is written into the SRAM 28e by the controller 28f. The writing destination block matches the block from which the luminance evaluation value before update is read. The controller 28f receives a vertical synchronization signal from the TG 24. The luminance evaluation value written in the SRAM 28e is reset after a predetermined delay time by the controller 28f every time a vertical synchronization signal is generated.

  The switch 28i receives a SW2 switching command from the CPU 30. The S2 switching command received by the switch 28i is synchronized with the SW1 switching command given to the switch 28b. When SW1 = T1, the switch 28i is connected to the terminal T3, and when SW1 = T2, the switch 28i is connected to the terminal T4.

  When the switch 28i is connected to the terminal T3, the controller 28f gives the luminance evaluation value given from the SRAM 28e to the multiplier 28g. The register 28h receives “2” from the CPU 30 and supplies the value to the multiplier 28g. The multiplier 28g outputs a value obtained by multiplying the luminance evaluation value by “2” to the CPU 30. On the other hand, when the switch 28i is connected to the terminal T4, the controller 28f gives the luminance evaluation value given from the SRAM 28e to the CPU 30.

  That is, when the switch 28i is connected to the terminal T4, the luminance evaluation value is output from the switch 28i as it is. On the other hand, when the switch 28i is connected to the terminal T3, the luminance evaluation value multiplied by the coefficient “2” is output from the switch 28i. Since the switch 28i is connected to the terminal T4 when the brightness of the object scene is low, and is connected to the terminal T3 when the brightness of the object scene is high, the coefficient given to the luminance evaluation value is It increases as the brightness increases.

  CPU30 performs the imaging | photography task shown in FIGS. The CPU 30 is a multitask CPU equipped with a multitask OS such as μITRON, and executes a shooting task in parallel with a plurality of tasks (not shown). A control program corresponding to such a task is stored in the flash memory 50.

  Referring to FIG. 5, in step S1, initialization processing is performed, and switch 28b is connected to terminal T1, and switch 28i is connected to terminal T3. In step S3, through image processing is performed, and a through image of the object scene is displayed on the liquid crystal monitor. In step S5, it is determined whether or not the shutter button 32 has been half-pressed. If NO is determined in step S5, the process proceeds to step S7, a luminance evaluation process described later is executed, and the process proceeds to step S9. In step S9, through image AE processing is performed, and the process returns to step S5. As a result of the series of operations from step S5 to step S7, the optimum exposure period Tpre for pre-exposure is updated. As a result, the brightness of the through image is appropriately adjusted. If YES is determined in the step S5, the process proceeds to a step S11 so as to execute a recording image AE process. As a result, the optimum exposure period Tmain is accurately obtained.

  In step S13, it is determined whether or not the shutter button 32 has been fully pressed. In step S15, it is determined whether or not the shutter button has been released. When the operation of the shutter button 32 is released, the process returns to step S5, and when the shutter button 32 is fully pressed, the photographing / recording process is performed in step S17. By the photographing / recording process, the main exposure according to the optimum exposure period Tmain is performed based on the average photometry, and the image data of the object scene photographed thereby is recorded on the recording medium 48 in a compressed state. When step S17 is completed, the process returns to step S3.

  The luminance evaluation process in step S7 follows a subroutine shown in FIG. In step S21, initialization processing is executed. Specifically, LH is set to 128, N is set to 0, cnt is set to 0, Block is set to 256, and Peak is set to 205. LH is a threshold value of Y data when the switch 28a is connected to the terminal T2, cnt indicates the number of blocks whose luminance evaluation value is equal to or smaller than the threshold value, and N is output from the luminance evaluation circuit 28. This is the identification number of the block to which the luminance evaluation value belongs. Block is the number of blocks formed in the SRAM 28e, and Peak is a threshold used for brightness determination.

  In step S23, it is determined whether a vertical synchronizing signal has been generated. If YES, it is determined in step S25 whether N is smaller than Block. When the determination result is YES, the process proceeds to step S27. In step S27, it is determined whether or not the luminance evaluation value (IyN) of the Nth block is equal to or less than a threshold value (LH). When the determination result is NO, the process proceeds to step S31. On the other hand, if the determination result is YES, the process proceeds to step S29, cnt is incremented, and the process proceeds to step S31. In step S31, N is incremented, and the process returns to step S25 again.

  If the determination result is NO in step S25, the luminance evaluation value comparison process is completed, and the process proceeds to step S33. In step S33, it is determined whether cnt is larger than Peak. If the determination result is YES, it is determined that the captured object scene image is dark, the switch 28b is connected to the terminal T2, and the switch 28i is connected to the terminal T4. On the other hand, if the determination result is NO, it is determined that the captured object scene image is not dark, the switch 28b is connected to the terminal T1, and the switch 28i is connected to the terminal T3. When the setting of the switch 28b and the switch 28i is completed, the process returns to the upper hierarchy routine.

  As can be seen from the above description, the signal processing circuit 26 generates Y data indicating the brightness of the object scene captured by the image sensor 16. The generated Y data is bit-shifted by the bit shift circuit 28a. The switch 28b turns on / off the bit shift circuit 28a according to the brightness of the object scene. The CPU 30 evaluates the luminance evaluation value based on the Y data subjected to the bit shift. The luminance evaluation value obtained by the CPU 30 is multiplied by “2” by the multiplier 28g. The switch 28i turns on / off the multiplier 28g according to the brightness of the object scene. The exposure amount of the image sensor 16 is adjusted by the CPU 30 based on the multiplication value obtained by the multiplier 28g.

  Thus, the Y data is shifted in accordance with the shift bit circuit 28a that shifts the bit when the object scene is bright, and the luminance evaluation value is obtained based on the shifted Y data. This makes it possible to accurately evaluate the brightness even when the brightness of the object scene fluctuates greatly. The obtained luminance evaluation value is multiplied by “2” by the multiplier 28g when the object scene is bright. As a result, the deviation of the luminance evaluation value caused by the shift bit circuit 28a is eliminated, and the exposure amount can be adjusted by a common procedure.

  In this embodiment, the above-described threshold values LH and Peak have been described, but values other than those described may be set as appropriate.

  In this embodiment, exposure is set by average photometry, but center-weighted photometry or spot photometry may be adopted instead of average photometry.

  Further, in this embodiment, the object scene image is photographed by the CCD imager, but a CMOS imager may be employed instead of the CCD imager.

  In this embodiment, the case where the present invention is applied with the Y data set to the 8-bit rate has been described. However, the present invention can also be applied to other rates such as the 16-bit rate.

  In this embodiment, the case where the present invention is applied to the digital camera 10 has been described. However, the present invention can also be applied to other cameras such as a video camera.

It is a block diagram which shows the structure of one Example of this invention. It is a block diagram which shows a part of operation | movement of FIG. 1 Example. It is an illustration figure which shows an example of operation | movement of the luminance evaluation circuit applied to the FIG. 1 Example. (A) is an illustrative view showing a mapping state of a plurality of divided areas assigned to the object scene, and (B) is an illustrative view showing one example of an SRAM divided area. It is a flowchart which shows a part of operation | movement of CPU applied to the FIG. 1 Example. It is a flowchart which shows a part of other operation | movement of CPU applied to the FIG. 1 Example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Digital camera 16 ... Image sensor 26 ... Signal processing circuit 28 ... Luminance evaluation circuit 30 ... CPU
36 ... Memory control circuit 38 ... SDRAM
40 ... Video encoder 44 ... JPEG codec

Claims (6)

Imaging means for capturing the object scene,
Generating means for generating luminance data indicating the brightness of the object scene captured by the imaging means;
Attenuating means for attenuating the luminance data generated by the generating means according to a gain that decreases as the brightness of the object scene increases;
Evaluation means for evaluating the brightness of the object scene based on the luminance data attenuated by the attenuation means;
An arithmetic means for multiplying the evaluation value obtained by the evaluation means by a coefficient that increases as the brightness of the object field increases, and an exposure amount of the imaging means based on the multiplication value obtained by the arithmetic means. An electronic camera comprising adjusting means for adjusting.   2. The attenuation means includes bit shift means for bit-shifting the luminance data to halve the data value, and first control means for turning on / off the bit shift means in accordance with the brightness. Electronic camera.   The electronic camera according to claim 2, wherein the calculation means includes a multiplication means for multiplying the evaluation value by “2” and a second control means for turning on / off the multiplication means in accordance with the brightness. Multiple blocks are assigned to the first scene,
The electronic camera according to claim 1, wherein the evaluation unit includes an integration unit that integrates the luminance data for each block assigned to the object scene. 5. The electronic camera according to claim 4, further comprising: a comparison unit that compares each of a plurality of integration values obtained by the integration unit with a threshold; and a gain determination unit that determines the gain based on a comparison result of the comparison unit. . Coefficient determination means for determining the coefficient based on the comparison result of the comparison means is further provided.
The electronic camera according to claim 5.

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