JP4441219B2 - Digital camera - Google Patents

Description

  The present invention relates to a digital camera, and more particularly to a digital camera that performs shading correction of a captured image.

  Since light passing through the camera lens is spherically attenuated (shaded) around the optical axis of the lens, the image taken by the camera is a lens beam in an image range 100 as shown in FIG. In general, the area near the center of the axis is the brightest, and it becomes darker as it goes to the periphery. This differs depending on the lens area and the effect of light shielding by mirror copper. However, in principle, the amount of light around the optical axis of the lens is inevitably reduced, and there is a limit to simply devising the lens.

  In the case of a digital camera, shading can be corrected by multiplying the pixel value of each pixel of the captured image by a correction coefficient corresponding to the pixel position. The calculation of the coefficient requires second-order to fourth-order calculations such as spherical calculation. For this reason, enormous calculations are required for multi-pixel images exceeding megapixels.

For this reason, conventionally, a lookup table (LUT) that stores the correspondence between pixel positions and correction coefficients is prepared in advance, and shading correction is performed using this LUT (for example, Patent Documents 1 to 3). reference).
JP 2000-165624 A JP 2000-125156 A JP 2002-325166 A

However, since the technique described in Patent Document 1 uses a two-dimensional LUT that stores correction coefficients in a one-to-one correspondence with pixel positions, the LUT capacity may increase. In particular, in the image of the number of pixels exceeding megapixels, the capacity of the LUT becomes enormous, there is a problem that.

  Further, the technique described in Patent Document 2 has a configuration in which a light detection unit that outputs a signal indicating the degree of shading is provided in the periphery of the light receiving surface of the image sensor, which complicates the apparatus and increases the cost. There was a problem.

  Furthermore, the technique described in Patent Document 3 performs shading correction of a line image read by a line sensor, and is difficult to apply to a two-dimensional image taken by a digital camera.

  The present invention has been made in view of the above-described facts, and provides a digital camera capable of performing two-dimensional shading correction with a simple configuration by reducing the capacity of an LUT necessary for calculation of shading correction. With the goal.

In order to achieve the above object, an invention according to claim 1 is directed to a photographing means for photographing a subject, a photographing optical system including a lens for imaging light from the subject on the photographing means, and a photographed image of the subject. The correspondence relationship between the first pixel position in the first direction and the first correction coefficient determined according to the curvature of the lens in the first direction is stored, and the input first first A first lookup table that outputs a first correction coefficient corresponding to a pixel position, a second pixel position in a second direction of the captured image, and a curvature of the lens in the second direction. A second look-up table that stores a correspondence relationship with the determined second correction coefficient and outputs a second correction coefficient corresponding to the input second pixel position; and Correction coefficient and the second A total correction coefficient calculating means for multiplying the correction coefficient and outputting a total correction coefficient; and an adjustment correction coefficient for multiplying the total correction coefficient by an adjustment gain determined according to the zoom magnification of the lens and outputting an adjustment correction coefficient And a correction pixel value calculation unit that outputs a correction pixel value by multiplying the pixel value of the captured image by the adjustment correction coefficient, and the curvature of the lens in the first direction, The curvature of the lens in the second direction is the same, the length of the captured image in the first direction and the length in the second direction are different, and the first look-up table and the second One of the lookup tables corresponding to the direction of the shorter length is also used as the other lookup table corresponding to the direction of the longer length. Characterized in that it.

The light from the subject is imaged on the photographing means by the photographing optical system including the lens, and the subject is photographed by the photographing means, but the amount of light of the photographed image decreases as it goes around the optical axis of the photographing optical system. It is necessary to correct shading.

For this reason, the present invention includes a first lookup table and a second lookup table. The first lookup table shows a correspondence relationship between the first pixel position in the first direction of the captured image of the subject and the first correction coefficient determined according to the curvature of the lens in the first direction. I remember it. Then, when the first pixel position is input, the first lookup table outputs a first correction coefficient corresponding to the first pixel position.

Similarly, the second lookup table is determined according to the second pixel position in the second direction of the captured image, for example, the direction orthogonal to the first direction, and the curvature of the lens in the second direction . The correspondence relationship with the second correction coefficient is stored. Then, when the second pixel position is input, the second lookup table outputs a second correction coefficient corresponding to the second pixel position. That is, the first lookup table and the second lookup table are each a one-dimensional lookup table.

The total correction coefficient calculation means multiplies the first correction coefficient output from the first lookup table and the second correction coefficient output from the second lookup table, and outputs a total correction coefficient.
The adjustment correction coefficient calculation means outputs the adjustment correction coefficient by multiplying the total correction coefficient by an adjustment gain determined according to the zoom magnification of the lens .
Specifically, the shading characteristics are determined in accordance with, for example, the lens characteristics of a lens used in the photographing optical system, the zoom magnification at the time of photographing, the photographing characteristics of the photographing means, etc. when using a zoom lens.

The corrected pixel value calculation unit multiplies the pixel value of the captured image by the adjustment correction coefficient and outputs a corrected pixel value.

Thus, instead of using a two-dimensional lookup table that stores correction coefficients corresponding to pixel positions in a one-to-one relationship, a one-dimensional lookup table that stores correction coefficients corresponding to pixel positions in each direction is used. Since the total correction coefficient is obtained by multiplying the correction coefficients in the respective directions output from each lookup table using two, the capacity of the lookup table is reduced as compared with the case of using a two-dimensional LUT. In addition, the total correction coefficient can be obtained with a simple configuration. In particular, in a digital camera having a high pixel count of, for example, 10 million pixels or more, the capacity of the lookup table can be greatly reduced, and the effect is remarkable.
Also, the adjustment correction coefficient is obtained by multiplying the total correction coefficient by the adjustment gain corresponding to the zoom magnification of the lens , and this is multiplied by the pixel value of the photographed image, so that the correction pixel value is obtained. Therefore, shading correction can be performed more accurately.

Further, the curvature of the lens in the first direction and the curvature of the lens in the second direction are the same, and the length in the first direction and the length in the second direction of the photographed image And one of the first look-up table and the second look-up table corresponding to the direction of the shorter length corresponds to the direction of the longer length. Also used as the other lookup table. As a result, the apparatus can be simplified and can be configured at low cost .

Furthermore, as described in claim 2 , pixel positions input to at least one of the first look-up table and the second look-up table according to the amount of deviation of the optical axis of the photographing optical system. It may be configured to further include offset setting means for offsetting.

  In this way, by offsetting the pixel position according to the amount of deviation of the optical axis of the photographing optical system, even when the optical axis is displaced due to an assembly error of the apparatus, the shading can be corrected accurately.

  As described above, according to the present invention, there is an excellent effect that the capacity of the LUT necessary for calculation of shading correction can be reduced and shading correction can be performed with a simple configuration.

  Hereinafter, an example of an embodiment of the present invention will be described in detail with reference to the drawings. In the present embodiment, a case where the present invention is applied to a digital still camera will be described.

  As shown in FIG. 1, a digital still camera (hereinafter referred to as a “digital camera”) 10 according to the present embodiment includes an optical unit 12 including a lens for forming a subject image, and an optical unit 12. A CCD (Charge Coupled Device) 13 disposed behind the optical axis, an A / D converter 14 that converts an analog image signal output from the CCD 13 into a digital signal, and a lens drive that drives the lens of the optical unit 12 And a drive unit 15 including a CCD drive unit for driving the CCD 13, an LCD 16 for displaying images and various information obtained by photographing with the digital camera 10, and an operation unit 18 operated by a photographer. And an SDR as a main storage device for temporarily storing digital image data obtained mainly by photographing by the CCD 13 And M (Synchronous Dynamic Random Access Memory) 20, various programs, and external ROM22 etc. parameters are stored in advance, is configured to include a main control unit 24, the responsible for the digital camera 10 as a whole operation. The SDRAM 20 functions as a main memory.

  Note that the operation unit 18 is a shooting that is operated to select any one of a shutter button, a still image shooting mode, a moving image shooting mode, and a playback mode that are operated when instructing execution of still image or moving image shooting. A mode selector as a mode selection means, various parameters are set, a cursor button operated to specify an image to be reproduced when the reproduction mode is selected, and an operation to turn on / off the power of the digital camera 10 Power switch.

  The main control unit 24 also includes an image signal processing unit 30 that performs various processes on the digital signal input from the A / D converter 14, and the digital camera 10 in accordance with the operation status of the operation unit 18 by the photographer. A CPU 32 for controlling the overall operation of the image forming apparatus, an input / output port 34 for performing input / output control with respect to the drive unit 15 and the operation unit 18, and the like, a JPEG circuit 36 for compressing / decompressing and encoding image data according to JPEG format, A memory controller 38 for controlling reading / writing of data to / from the SDRAM 20 and reading of data to the ROM 22, a relatively small internal RAM 40, a DMA controller (DMAC) 42 for performing DMA (Direct Memory Access) transfer, and a digital camera 10 and a personal computer connected via a predetermined cable such as a USB cable. An external communication control unit 44 for transmitting / receiving various data to / from an external device such as a printer, an LCD signal processing unit 46 for displaying an image on the LCD 16, and a media control unit for controlling reading / writing of various data with respect to a recording medium 48 are connected to each other by a bus 50.

  In the present embodiment, the main control unit 24 is configured as a one-chip LSI (Large Scale Integrated circuit), whereby the digital camera 10 can be reduced in size, increased in reliability, and reduced in cost. ing.

  The optical unit 12 includes a zoom lens group and a focus lens (not shown), and includes a lens moving mechanism that moves them in the optical axis direction, and is configured as a zoom lens that can change (magnify) the focal length. .

  In the optical unit 12, the zoom lens group is moved in the optical axis direction (focal length variable lens) so as to obtain a desired zoom magnification under the control of the drive unit 15, and incident light indicating a subject image that has passed through the lens is incident on the CCD 13. The focus lens is moved in the optical axis direction so as to form an image on the light receiving surface (autofocus (AF) mechanism). As a result, the CCD 13 captures the subject based on the incident light indicating the subject image that has passed through the lens of the optical unit 12 and outputs an analog image signal indicating the subject image.

  In the present embodiment, as the focus control, a so-called TTL (Through The Lens) method is adopted in which the lens position is adjusted so that the contrast of an image obtained by photographing is maximized. In-focus control is automatically performed so that a subject existing at a predetermined position (AF frame) is in focus. Specifically, when the still image shooting mode for shooting a still image is selected by the operation of the mode switch on the operation unit 18 by the photographer, the shutter button is pressed halfway to automatically In-focus control is performed. On the other hand, when a moving image shooting mode for shooting a moving image is selected, the focusing control is continuously performed after the shutter button is fully pressed and shooting is started.

  The output end of the CCD 13 is connected to the A / D converter 14. That is, the A / D converter 14 converts an analog image signal indicating a subject image photographed and output by the CCD 13 into digital image data (hereinafter referred to as “CCD data”). The output end of the A / D converter 14 is connected to the image signal processing unit 30, and the CCD data is temporarily stored in the SDRAM 20 by DMA transfer via the memory controller 38.

  The image signal processing unit 30 reads out the CCD data once stored in the SDRAM 20 by DMA transfer, and performs resizing processing as necessary. The image signal processing unit 30 includes a shading correction unit 60 that performs shading correction, which will be described later, and a white balance adjustment unit that performs white balance adjustment by applying a digital gain corresponding to a light source type, for example. Various processing units such as a gamma processing unit that performs correction, a sharpness processing unit that performs sharpness processing, and a YC conversion processing unit are provided.

  The image data subjected to the image processing is subjected to YC conversion processing in a YC conversion processing unit. Thereby, luminance data Y and chroma data Cr, Cb (hereinafter referred to as “YC data”) are generated from the R, G, B digital image data, and the generated YC data is transferred to the SDRAM 20 via the memory controller 38. Once stored.

  The YC data stored in the SDRAM 20 is read out by DMA transfer by the image signal processing unit 30 via the memory controller 38 and input to the JPEG circuit 36. The JPEG circuit 36 compresses and encodes the input YC data according to the JPEG method, which is an international standard for color still image encoding. The compressed data compressed by the JPEG circuit 36 is temporarily stored in the SDRAM 20 by DMA transfer via the media control unit 48 and recorded on a recording medium (not shown) loaded in the digital camera 10.

  The recording media can be various forms such as smart media, PC card, micro drive, multimedia card (MMC), magnetic disk, optical disk, magneto-optical disk, memory stick, etc., depending on the media used. The signal processing means and interface are applied.

  The YC data stored in the SDRAM 20 is read by the image signal processing unit 30 via the memory controller 38 and input to the LCD signal processing unit 46.

  The LCD signal processing unit 46 includes a video encoder (not shown), and converts the input YC data into an NTSC (National TV Standards Committee) signal. The LCD signal processing unit 46 is connected to the LCD 16 and supplies the converted NTSC signal to the LCD 16 to display an image on the LCD 16.

  Further, the YC data based on the image signal output from the CCD 13 is periodically rewritten, and a video signal generated from the YC data is supplied to the LCD 16 so that the subject image captured by the CCD 13 is converted into a moving image in substantially real time. Or, it is displayed on the LCD 16 as a continuous image. That is, the LCD 16 can be used as an electronic viewfinder, and the photographer can check the shooting angle of view not only with a normal optical viewfinder (not shown) but also with a display image (so-called through image) of the LCD 16. . Then, in response to a predetermined recording instruction (shooting instruction) operation such as a shutter switch pressing operation, the capturing of digital image data for recording is started.

  Further, at the time of image reproduction, compressed data to be reproduced stored in the recording medium is read by the media control unit 48, decompressed by the JPEG circuit 36, and transferred to the LCD signal processing unit 46. Thereby, an image based on the compressed data acquired by photographing and stored in the recording medium can be reproduced and displayed on the LCD 16.

  The digital camera 10 can record a still image on the recording medium by compressing the still image by the JPEG method in the still image shooting mode, and can record a moving image on the recording medium in the moving image shooting mode.

  A moving image is recorded in a relatively small size such as a QVGA size (320 × 240 pixels) or a VGA size (640 × 480 pixels), and a still image is recorded in an SVGA size (in addition to the QVGA size or VGA size). 800 × 600 pixels), XGA size (1024 × 768 pixels), SXGA size (1280 × 1024 pixels) and the like can be recorded.

  In the present embodiment, in the moving image shooting mode, a moving image is compression-encoded by a so-called motion JPEG method. The motion JPEG method is a method for realizing moving image display by compressing and recording a photographed image by a JPEG method every predetermined time and continuously reproducing the plurality of still images. This motion JPEG method is easier to digitize video than encoding methods with advanced compression algorithms such as MPEG (Moving Picture coding Experts Group), etc., and it is easier to process each image frame for editing work etc. There are features.

  That is, the processing of each frame image in the moving image shooting mode is the same as in the case of shooting in the still image shooting mode, and moving image data is generated based on the image data of a plurality of still images corresponding to the shooting time. Recorded on the recording medium.

  FIG. 2 is a block diagram illustrating a schematic configuration of the shading correction unit 60. The shading correction unit 60 includes an X counter 62, an X-LUT 64, a Y counter 66, a Y-LUT 68, and multipliers 70, 72, and 74.

  The X counter 62 receives an X count and an X synchronization signal. For example, as shown in FIG. 3, the X count value is a coordinate (pixel position) in the X direction when the position of the upper left corner of the captured image area 80 is the coordinate (0, 0) of the origin of the captured image area 80. . As an example, the aspect ratio of the captured image region 80 is 4: 3.

  Image data is sequentially input to the shading correction unit 60 as shown in FIG. That is, the pixel data of the origin are input in the X direction (first direction) and the Y direction (second direction) in order. The X count value is sequentially input in synchronization with the input of the image data. In other words, the pixel position in the X direction is sequentially input to the X counter 62 in order from the X coordinate of the origin position.

  The X counter 62 uses the input of the X synchronization signal as a trigger, converts the X count value into the address of the X-LUT 64 in which the correction coefficient corresponding to the X count value is stored, and outputs the correction coefficient at that address. Instructs the X-LUT 64. As a result, the X correction coefficient (first correction coefficient) corresponding to the X count value is output from the X-LUT 64 to the multiplier 70.

  The X-LUT 64 stores a one-dimensional X correction coefficient in the X direction in advance. As shown in FIG. 3, for example, the correction coefficient increases toward the both ends with the center position Xc in the X direction of the captured image region 80 being 1 (times), and becomes 2 (times) at both ends. That is, as shown in FIG. 7, the center of the image range 100 is brightest, and the amount of light decreases as it moves away from the center position Xc. The coefficient is increased.

  On the other hand, the Y counter 66 receives the Y count value and the Y synchronization signal. The Y count value is a coordinate in the Y direction orthogonal to the X direction shown in FIG.

  The Y count value is sequentially input in synchronization with image data input. That is, pixel positions in the Y direction are sequentially input to the Y counter 66 in order from the Y coordinate of the origin position.

  Using the input of the Y synchronization signal as a trigger, the Y counter 66 converts the Y count value into the address of the Y-LUT 68 in which the correction coefficient corresponding to the Y count value is stored, and outputs the correction coefficient at that address. The Y-LUT 68 is instructed. As a result, the Y correction coefficient (second correction coefficient) corresponding to the Y count value is output from the Y-LUT 68 to the multiplier 70.

  The Y-LUT 68 stores a one-dimensional Y correction coefficient in the Y direction in advance. As shown in FIG. 3, the Y correction coefficient increases toward the both ends, for example, assuming that the center position Yc in the Y direction of the captured image region 80 is 1 (times), and becomes 1.5 (times) at both ends. That is, the correction coefficient is increased in order to increase the degree of compensation of the light amount as the distance from the center position Yc increases.

  In FIG. 3, the density is increased toward the center of the image, and the density is decreased from the center of the image to the periphery. However, the amount of light correction is smaller as the center of the image is smaller, and the density is lower. The image shows that it is necessary to increase the amount of correction of the light amount toward the periphery.

  Thus, the correction amount changes in an annular shape from the center position (Xc, Yc) of the image. That is, as shown in FIG. 3, the correction coefficient in each direction is determined according to the curves 82X and 82Y having a predetermined curvature. The curvatures of the curves 82X and 82Y are determined according to the lens characteristics of the optical unit 12.

  Multiplier 70 multiplies the X-direction correction coefficient output from X-LUT 64 and the Y-direction correction coefficient output from Y-LUT 68, and outputs the total correction coefficient to multiplier 72. For example, as shown in FIG. 3, when the correction coefficient in the X direction of the coordinates (Xa, Ya) is 1.3 and the correction coefficient in the Y direction is 1.2, the total correction coefficient is 1.56.

  The multiplier 72 multiplies the total correction coefficient by the input adjustment gain, and outputs the adjustment correction coefficient to the multiplier 74. The adjustment gain is determined according to the shading characteristics of the digital camera. Specifically, for example, the lens characteristics of the lens used in the optical unit 12 or the zoom magnification at the time of shooting when a zoom lens is used. It is determined according to the etc. In this case, the adjustment gain according to the lens characteristics and the adjustment gain according to the zoom magnification are stored in advance in the storage means such as the ROM 22, and the adjustment gain is determined by the zoom magnification at that time, for example, according to an instruction from the CPU 32, and multiplied. Output to the device 72.

  The multiplier 74 multiplies the input image data (pixel data) by the adjustment correction coefficient, and outputs the result to a subsequent circuit.

  In this way, instead of using a two-dimensional LUT that stores a correction coefficient corresponding to the coordinate position in a one-to-one relationship, it corresponds to a one-dimensional X-LUT 64 and a Y coordinate that store a correction coefficient corresponding to the X coordinate. Since the total correction coefficient is obtained by multiplying the correction coefficient in each direction output from each LUT using the one-dimensional LUT 68 storing the correction coefficient, the capacity of the LUT is increased as compared with the case of using the two-dimensional LUT. It is possible to reduce the total correction coefficient with a simple configuration. In particular, in a digital camera having a high pixel count of, for example, 10 million pixels or more, such as for studio photography, the LUT capacity can be greatly reduced.

  When the curvatures of the curves 82X and 82Y are the same, the correction coefficients are the same when the distances from the center positions Xc and Yc in each direction are the same. In such a case, the L-LUT 68 having a shorter length, that is, the Y-LUT 68 that is the Y-direction LUT may be used as the X-LUT 64 as shown in FIG. Thereby, the apparatus can be simplified and configured at low cost.

  Further, the position of the optical axis does not always coincide with the center position (Xc, Yc) of the captured image region 80. For example, the optical axis in the X direction extends in the X direction of the captured image region 80 as shown in FIG. There is a case where the center position Xc is shifted to Xc ′.

  Therefore, the amount of deviation of the optical axis in each direction is determined in advance as an X offset value and a Y offset value, for example, stored in the storage means in the ROM 22 or the like, and as shown in FIG. The offset value may be added and input to the X counter 62, and the Y count value and the Y offset value may be added by the adder 86 and input to the Y counter 66. In this case, for example, as shown in FIG. 5, when the optical axis in the X direction is shifted from Xc to Xc ′, the correction coefficient differs at both ends of the captured image region 80.

  In this way, since the amount of deviation of the optical axis is used as an offset value and the count value input to each counter is offset, shading correction can be accurately performed even when the position of the optical axis is displaced. In this case, it is necessary to prepare an LUT corresponding to a range that is slightly larger than the captured image area 80 in order to correspond to the amount of deviation of the optical axis.

  In the present embodiment, the case where the present invention is applied to a digital camera has been described. However, it is needless to say that the present invention can also be applied to a mobile terminal device having a photographing unit such as a digital video camera or a camera-equipped mobile phone. Absent.

It is a block diagram of a digital camera. It is a schematic block diagram of a shading correction | amendment part. It is an image figure for demonstrating a correction coefficient. It is a schematic block diagram of the shading correction | amendment part which concerns on a modification. It is an image figure for demonstrating a correction coefficient. It is a schematic block diagram of the shading correction | amendment part which concerns on a modification. It is an image figure for demonstrating the fall of a light quantity.

Explanation of symbols

10 Digital Camera 12 Optical Unit (Shooting Optical System)
13 CCD (photographing means)
14 A / D converter 15 Drive unit 16 LCD
18 Operation unit 20 SDRAM
22 ROM
30 Image signal processor 34 Input / output port 36 JPEG circuit 38 Memory controller 40 Built-in RAM
42 DMAC
44 External communication control unit 46 LCD signal processing unit 48 Media control unit 60 Shading correction unit 62 X counter 64 X-LUT (first lookup table)
66 Y counter 68 Y-LUT (second lookup table)
70 multiplier (total correction coefficient calculation means)
72 Multiplier (Adjustment correction coefficient calculation means)
74 Multiplier (correction pixel value calculation means)
80 Captured image range 84, 86 Adder (offset setting means)

Claims (2)

Photographing means for photographing the subject;
A photographing optical system including a lens that forms an image of light from a subject on the photographing unit;
The correspondence between the first pixel position in the first direction of the photographed image of the subject and the first correction coefficient determined according to the curvature of the lens in the first direction is stored and input. A first look-up table for outputting a first correction coefficient corresponding to the first pixel position,
The correspondence between the second pixel position in the second direction of the photographed image and the second correction coefficient determined according to the curvature of the lens in the second direction is stored and inputted. A second look-up table for outputting a second correction coefficient corresponding to the second pixel position;
Total correction coefficient computing means for multiplying the first correction coefficient and the second correction coefficient to output a total correction coefficient;
An adjustment correction coefficient computing means for multiplying the total correction coefficient by an adjustment gain determined according to the zoom magnification of the lens and outputting an adjustment correction coefficient;
Correction pixel value calculation means for multiplying the pixel value of the photographed image by the adjustment correction coefficient and outputting a correction pixel value;
Equipped with a,
The curvature of the lens in the first direction and the curvature of the lens in the second direction are the same, and the length of the captured image in the first direction and the length of the second direction are different. , One of the first lookup table and the second lookup table corresponding to the shorter direction of the length is replaced with the other lookup table corresponding to the direction of the longer length. A digital camera that doubles as a lookup table . The apparatus further comprises offset setting means for offsetting a pixel position input to at least one of the first lookup table and the second lookup table in accordance with a deviation amount of the optical axis of the photographing optical system. claim 1 Symbol placement of the digital camera, characterized.