JPWO2009057478A1 - Image processing apparatus, image processing method, and imaging apparatus - Google Patents

Abstract

The first characteristic conversion unit 110 converts the photoelectric conversion characteristic of the image data output from the image sensor 3 into a predetermined photoelectric conversion characteristic suitable for the dynamic range compression processing by the compression unit 120, and the logarithmic image data (V1). Generate. The exposure correction unit 130 corrects the exposure level of the logarithmic image data (V1). The dynamic range compression unit 140 executes dynamic range compression processing on the logarithmic image data (V1). The second characteristic conversion unit 150 converts the photoelectric conversion characteristic of the logarithmic image data (V3) subjected to the dynamic range compression processing by the dynamic range compression unit 140 to a predetermined photoelectric conversion suitable for processing by the processing unit 160 connected to the subsequent stage. Convert to characteristics.

Description

  The present invention relates to a technique for processing image data output from an image sensor.

  In recent years, imaging devices having various photoelectric conversion characteristics have been adopted in imaging devices such as digital cameras. As the photoelectric conversion characteristics, there are linear characteristics and logarithmic characteristics, but in recent years, a combination of linear characteristics and logarithmic characteristics (for example, Patent Document 1) or a different slope is used in order to achieve a wide dynamic range. A combination of a plurality of types of linear characteristics, a combination of a plurality of types of logarithmic characteristics having different slopes, and the like are known.

  Therefore, Patent Document 2 discloses that predetermined color processing is performed after unifying photoelectric conversion characteristics of image data output from an image sensor having photoelectric conversion characteristics combining logarithmic characteristics and linear characteristics into one of the characteristics. By doing so, it is possible to execute predetermined color processing without causing problems such as color misregistration without separately providing a color processing unit for photoelectric conversion characteristics and a color processing unit for logarithmic characteristics. Is disclosed.

  In addition, Non-Patent Document 1 extracts the illumination component (low frequency component) of the image based on the spatial frequency of the image, compresses the illumination component while maintaining the reflectance component, and compresses the dynamic range of the image. Techniques to do this are disclosed.

  By the way, the dynamic range compression processing as shown in Non-Patent Document 1 is intended for image data having a certain photoelectric conversion characteristic, and the photoelectric conversion characteristic of the image sensor matches the photoelectric conversion characteristic targeted for the dynamic range compression process. If not, accurate dynamic range compression processing cannot be realized. Therefore, in the conventional imaging device, in order to cope with various photoelectric conversion characteristics, dynamic range compression processing corresponding to the type of photoelectric conversion characteristics is executed. As a result, there has been a problem that the circuit scale increases.

  In the imaging apparatus, image data subjected to dynamic range compression processing is subjected to image processing such as white balance correction by a processing unit provided at a later stage. Such image processing is also the same as dynamic range compression processing. If the photoelectric conversion characteristics targeted for image processing and the photoelectric conversion characteristics of the input image data do not match, accurate image processing cannot be realized. Therefore, in the conventional imaging apparatus, image processing corresponding to the type of photoelectric conversion characteristics of input image data is executed, which causes a problem that the circuit scale increases.

On the other hand, although it is possible to perform uniform image processing regardless of the type of photoelectric conversion characteristics of input image data, there arises a problem that accurate image processing cannot be realized.
JP 2006-50544 A JP 2006-270622 A Konica Minolta Technology Report VOL.4 (2007)

  An object of the present invention is to provide an image processing apparatus, an image processing method, and an imaging apparatus capable of realizing accurate image processing without increasing the circuit scale.

  An image processing apparatus according to one aspect of the present invention is an image processing apparatus that processes image data captured by an image sensor, and the photoelectric conversion characteristics of the image data output from the image sensor are set to predetermined first photoelectric conversion characteristics. A first characteristic converter that converts the dynamic range of the image data whose photoelectric conversion characteristics have been converted by the first characteristic converter, and a photoelectric unit of the image data whose dynamic range has been compressed by the compressor. A second characteristic conversion unit that converts the conversion characteristic into a predetermined second photoelectric conversion characteristic; and a processing unit that executes predetermined image processing on the image data whose photoelectric conversion characteristic has been converted by the second characteristic conversion unit. It is characterized by providing.

  An image processing method according to another aspect of the present invention is an image processing method for processing image data picked up by an image pickup device, wherein photoelectric conversion characteristics of image data output from the image pickup device are set to a predetermined value. A first characteristic conversion step for converting the first photoelectric conversion characteristic; a compression step for compressing a dynamic range of the image data whose photoelectric conversion characteristic is converted by the first characteristic conversion step; and a dynamic range is compressed by the compression step. A second characteristic conversion step for converting the photoelectric conversion characteristic of the obtained image data into a predetermined second photoelectric conversion characteristic; and predetermined image processing for the image data whose photoelectric conversion characteristic has been converted by the second characteristic conversion step. And a processing step to be executed.

  An imaging device according to another aspect of the present invention includes the above-described image processing device.

1 shows a block diagram of an imaging apparatus according to an embodiment of the present invention. FIG. FIG. 2 is a block diagram illustrating a detailed configuration of an image processing unit illustrated in FIG. 1. It is the graph which showed the photoelectric conversion characteristic before and behind conversion at the time of converting the photoelectric conversion characteristic of the image data input into a 1st characteristic conversion part into a predetermined logarithmic characteristic. It is the graph which showed the photoelectric conversion characteristic before and behind conversion at the time of converting into a predetermined linear characteristic the photoelectric conversion characteristic of the image data input into a 1st characteristic conversion part. It is the block diagram which showed the detailed structure of the 1st characteristic conversion part. It is the block diagram which showed the detailed structure of the exposure correction part. It is the block diagram which showed the structure of the exposure correction | amendment part when the 1st characteristic conversion part converted image data into a linear characteristic. It is the graph which showed the photoelectric conversion characteristic converted by the 2nd characteristic conversion part. It is the block diagram which showed the structure of the image process part in other one Embodiment of the imaging device of this invention.

  FIG. 1 shows a block diagram of an imaging apparatus according to an embodiment of the present invention. As shown in FIG. 1, the imaging device 1 includes a digital camera, and includes a lens unit 2, an imaging sensor 3, an amplifier 4, an A / D conversion unit 5, an image processing unit 6, an image memory 7, a control unit 8, and a monitor unit. 9 and an operation unit 10.

  The lens unit 2 includes an optical lens system that captures an optical image of a subject and guides it to the image sensor 3. As the optical lens system, for example, a zoom lens, a focus lens, other fixed lens blocks, and the like arranged in series along the optical axis L of the optical image of the subject can be employed. The lens unit 2 includes a diaphragm (not shown) for adjusting the amount of transmitted light, a shutter (not shown), and the like, and the driving of the diaphragm and the shutter is controlled under the control of the control unit 8.

  The imaging sensor 3 (imaging element) photoelectrically converts the optical image formed in the lens unit 2 and has R (red), G (green), and B (blue) color components each having a level corresponding to the amount of light. An image signal is generated and output to the amplifier 4. Here, a CMOS image sensor having photoelectric conversion characteristics as shown in FIGS. 3A to 3D and FIGS. 4A to 4D to be described later is employed as the imaging sensor 3. The photoelectric conversion characteristics are not limited to the types shown in FIGS. 3A to 3D and FIGS. 4A to 4D, and an image sensor having other photoelectric conversion characteristics may be employed. In addition, not only a CMOS image sensor but also an image sensor such as a VMIS image sensor or a CCD image sensor may be employed.

  The amplifier 4 includes, for example, an AGC (auto gain control) circuit and a CDS (correlated double sampling) circuit, and amplifies the image signal output from the imaging sensor 3. The A / D conversion unit 5 converts the R, G, and B color image signals amplified by the amplifier 4 into R, G, and B color digital image data. In the present embodiment, the image signal received by each pixel of the image sensor 3 is converted into image data having, for example, a 12-bit gradation value.

  The image processing unit 6 executes image processing as will be described later. The image memory 7 is composed of, for example, a RAM (Random Access Memory), and stores image data subjected to image processing by the image processing unit 6.

  The control unit 8 includes a ROM (Read Only Memory) that stores various control programs, a RAM that temporarily stores data, a central processing unit (CPU) that reads and executes control programs from the ROM, and the like. It controls the operation of the entire apparatus 1.

  The monitor unit 9 employs, for example, a color liquid crystal display disposed on the back surface of the housing of the imaging apparatus 1, and displays on the monitor image data captured by the imaging sensor 3 or image data stored in the image memory 7. To do.

  The operation unit 10 includes various operation switch groups such as a power switch, a release switch, a mode setting switch for setting various shooting modes, and a menu selection switch. By pressing the release switch, a subject is picked up by the image pickup operation, that is, the image pickup sensor 3, and necessary image processing is performed on the image data obtained by the image pickup and recorded in the image memory 7 or the like. The shooting operation is executed. Instead of storing in the image memory 7 or the like, a series of imaging operations may be completed by outputting as a digital signal from the image processing unit 6 or by performing D / A conversion and outputting as an analog signal such as NTSC.

  FIG. 2 is a block diagram showing a detailed configuration of the image processing unit 6 shown in FIG. The image processing unit 6 includes a first characteristic conversion unit 110, a compression unit 120, a second characteristic conversion unit 150, and a processing unit 160. Since the image processing unit 6 shown in FIG. 2 performs the same image processing on the image data of each color of R, G, and B, the image data of each color of R, G, and B will be described without particular distinction.

  The first characteristic conversion unit 110 compresses the photoelectric conversion characteristics of the image data output from the imaging sensor 3, specifically, the image data output from the A / D conversion unit 5 and input to the first characteristic conversion unit 110. It converts into the predetermined photoelectric conversion characteristic (1st photoelectric conversion characteristic) suitable for the dynamic range compression process by the part 120. FIG.

  Here, the first characteristic conversion unit 110 receives image data having photoelectric conversion characteristics shown in any of FIGS. 3A to 3D and FIGS. 4A to 4D, and receives the input image. The photoelectric conversion characteristic of data may be converted into a predetermined logarithmic characteristic, or may be converted into a predetermined linear characteristic (third linear characteristic). However, the conversion to logarithmic characteristics makes it possible to reduce the width (number of bits) of the image data, and to secure the luminance resolution on the low luminance side when the exposure correction processing described later is executed. Thus, the exposure correction process can be performed with high accuracy.

  As the image sensor 3, the photoelectric conversion characteristic of the output image data has the photoelectric conversion characteristic shown in any of FIGS. 3A to 3D and FIGS. 4A to 4D shown below. Should be adopted. In addition, as the predetermined photoelectric conversion characteristics suitable for the processing of the compression unit 120, the compression unit 120 accurately separates the illumination component and the reflectance component regardless of whether the linear characteristic or the logarithmic characteristic is employed. Thus, it is preferable to employ a photoelectric conversion characteristic that can perform the dynamic range compression process maintaining the reflectance component with high accuracy.

  FIGS. 3A to 3D are graphs showing the photoelectric conversion characteristics before and after conversion when the photoelectric conversion characteristics of the image data input to the first characteristic conversion unit 110 are converted into predetermined logarithmic characteristics. 3A to 3D, the photoelectric conversion characteristics of the input image data have linear characteristics, logarithmic characteristics, characteristics combining linear characteristics and linear characteristics, and characteristics combining linear characteristics and logarithmic characteristics. is doing. In each of the graphs of FIGS. 3A to 3D, the vertical axis indicates the gradation value, the horizontal axis indicates the luminance, the solid line indicates the photoelectric conversion characteristics before conversion, and the dotted line indicates the photoelectric conversion after conversion. The characteristics are shown.

  In FIG. 3A, it can be seen that image data with a linear characteristic having a predetermined inclination is input, and the image data is converted into a logarithmic characteristic indicated by a dotted line. In FIG. 3B, it can be seen that image data having a logarithmic characteristic with a steep slope on the low luminance side is converted into a logarithmic characteristic indicated by a dotted line with a slightly gentle slope on the low luminance side. In FIG. 3C, it can be seen that image data composed of two types of linear characteristics having a large inclination on the low luminance side and a small inclination on the high luminance side are converted into logarithmic characteristics indicated by dotted lines. In FIG. 3D, it can be seen that image data having two types of photoelectric conversion characteristics having linear characteristics on the low luminance side and logarithmic characteristics on the high luminance side is converted into logarithmic characteristics indicated by dotted lines. . Note that the logarithmic characteristics indicated by the dotted lines in FIGS. 3A to 3D are the same.

  4A to 4D show the photoelectric conversion characteristics before and after conversion when the photoelectric conversion characteristics of the image data input to the first characteristic conversion unit 110 are converted into predetermined linear characteristics (third linear characteristics). FIGS. 4A to 4D are graphs in which photoelectric conversion characteristics of input image data are linear characteristics, logarithmic characteristics, characteristics combining linear characteristics and linear characteristics, linear characteristics and logarithmic characteristics, respectively. It has the characteristic which combined.

  In FIG. 4A, it can be seen that image data having a linear characteristic indicated by a solid line having a large inclination is input, and the image data is converted into a linear characteristic having a small inclination indicated by a dotted line. In FIG. 4B, it can be seen that the logarithmic characteristic image data having a steep slope on the low luminance side is converted into a linear characteristic having a gentle slope indicated by a dotted line. In FIG. 4C, image data composed of two types of linear characteristics having a large inclination on the low luminance side and a small inclination on the high luminance side is converted into a linear characteristic with a gentle inclination indicated by a dotted line. I understand. In FIG. 4D, image data having two types of photoelectric conversion characteristics having a linear characteristic on the low luminance side and a logarithmic characteristic on the high luminance side is converted into a linear characteristic with a gentle slope indicated by a dotted line. I understand that. Note that the linear characteristics indicated by the dotted lines in FIGS. 4A to 4D are the same.

  FIG. 5 is a block diagram showing a detailed configuration of the first characteristic converter 110. The first characteristic conversion unit 110 includes an adder 111, a multiplier 112, and an LUT (lookup table) 113. The adder 111 sequentially inputs the image data output from the A / D conversion unit 5 pixel by pixel in a predetermined order (for example, the order in which raster scanning is performed), and adds a correction value to the input image data. , Output to the multiplier 112. The multiplier 112 multiplies the input image data by a correction coefficient and outputs the result to the LUT 113. Here, as the correction value and the correction coefficient, a value for correcting the solid variation of the image sensor 3 is adopted, which is a value measured experimentally in advance, and a value unique to the image sensor 3 is adopted. Yes.

  Then, as preprocessing of the LUT 113, processing that absorbs variations in photoelectric conversion characteristics caused by individual variations in the image sensor 3 of image data to be processed is executed by the adder 111 and the multiplier 112. As a result, image data having a certain photoelectric conversion characteristic can be input to the LUT 113, and high-accuracy photoelectric conversion processing can be realized without having to configure the LUT 113 with different LUTs for each solid in consideration of individual variations. be able to.

  The LUT 113 stores in advance the relationship between photoelectric conversion characteristics before and after conversion shown in any of FIGS. 3A to 3D and FIGS. 4A to 4D, and is output from the multiplier 112. The photoelectric conversion characteristics of the image data are converted into predetermined photoelectric conversion characteristics. Here, the LUT 113 stores in advance the gradation value of the image data after conversion processing at each address associated with each gradation value of the input image data, and sets the gradation value of the input image data as the gradation value of the input image data. A photoelectric conversion characteristic is converted by outputting a corresponding gradation value.

  Returning to FIG. 2, the compression unit 120 includes an exposure correction unit 130 that executes an exposure correction process and a dynamic range compression unit 140 that executes a dynamic range compression process. The exposure correction unit 130 includes a linear characteristic conversion unit 131, a representative value calculation unit 132, and a correction unit 133, and exposure correction processing for setting the exposure level of the image data output from the image sensor 3 to a predetermined target exposure level. Execute.

  The linear characteristic conversion unit 131 sets the image data whose photoelectric conversion characteristic is converted into a predetermined logarithmic characteristic by the first characteristic conversion unit 110 as logarithmic image data (V1), and linearizes the photoelectric conversion characteristic of the logarithmic image data (V1). Convert to a characteristic (second linear characteristic).

  The representative value calculation unit 132 obtains a representative value of the linear image data (V2) output from the linear characteristic conversion unit 131. The correction unit 133 obtains a gain for setting the exposure level of the linear image data (V2) to the target exposure level by comparing the representative value calculated by the representative value calculation unit 132 with the target exposure level. Based on the gain, an offset value for setting the exposure level of the logarithmic image data (V1) to the target exposure level is obtained. The correction unit 133 adds the obtained offset value to the logarithmic image data (V1) output from the first characteristic conversion unit 110, and corrects the logarithmic image data (V1) for exposure.

  FIG. 6 is a block diagram showing a detailed configuration of the exposure correction unit 130. The LUT 201 illustrated in FIG. 6 constitutes the linear characteristic conversion unit 131 illustrated in FIG. Here, the LUT 201 stores in advance the gradation value of the converted image data at each address associated with each gradation value of the input logarithmic image data (V1), and the input logarithmic image data. By outputting a gradation value corresponding to the gradation value of (V1), the photoelectric conversion characteristic is converted from a logarithmic characteristic to a linear characteristic, and linear image data (V2) is output.

The block average calculation unit 202 constitutes the representative value calculation unit 132 shown in FIG. Specifically, the block average calculation unit 202 calculates the average value (Ave) of the gradation values of each pixel constituting the predetermined area of the linear image data (V2) output from the LUT 201. The average value (Ave) is calculated using the following formula (1). Here, as the predetermined region, a partial region of the linear image data may be employed, a plurality of partial regions may be employed, or the entire region of the linear image data may be employed. Good. When a plurality of partial areas are adopted as the predetermined areas, a weighting factor may be adopted for each area so that the exposure level of the area that is important in performing the exposure correction process is increased.
Ave = (ΣV2) / Pix (1)
ΣV2: total tone value Pix of each pixel in the predetermined area Pix: number of pixels in the predetermined area

  The offset calculation unit 203 configures the correction unit 133 illustrated in FIG. 2 and calculates an offset value. Specifically, the offset calculation unit 203 divides the target exposure level (Target) by the average value (Ave), and gain (b) for setting the exposure level of the linear image data (V2) to the target exposure level (Target). = Target / Ave).

Here, V1 and V2 have the following relationship.
V1 = α · Log (Lux) (2)
V2 = a · Lux (3)
α, a: proportionality constant Lux: incident light quantity to the image sensor 3

Substituting equation (3) into equation (2) yields equation (4).
V1 = α · Log (V2 / a)
= Α · (Log (V2) −Log (a)) (4)

The gain (b) calculated from the linear image data (V2) is a gain for the linear image data (V2). Logarithmic image data (V1) offset value when logarithmic image data (V3) is calculated by converting linear image data (b · V2) obtained by multiplying linear image data (V2) by gain (b) to logarithmic characteristics (Β) is obtained by the following equation.
β = α · Log (b · V2 / a) −V1 (5)

When Expression (4) is substituted into V1 of Expression (5), Expression (6) is obtained.
β = α · Log (b) (6)

  Therefore, the offset calculation unit 203 calculates the offset value (β) of each pixel constituting the logarithmic image data (V1) using Expression (6).

  The addition unit 204 configures the correction unit 133 illustrated in FIG. 2 and adds a corresponding offset value (β) to each pixel constituting the logarithmic image data (V1), that is, calculates V1 + β. Logarithmic image data (V3) in which the exposure level of the logarithmic image data (V1) is corrected is calculated.

  Here, the linear characteristic conversion unit 131 shown in FIG. 2 converts the photoelectric conversion characteristic from the logarithmic characteristic to the linear characteristic as in the second characteristic conversion unit 150. In the exposure correction process, the block average calculation unit The average value of the linear image data is adopted by 202, and each pixel value of the linear image data (V2) is rounded, so that the highly accurate conversion processing is not required as with the second characteristic conversion unit 150. Therefore, the circuit scale of the LUT 201 can be reduced.

  Note that when the first characteristic conversion unit 110 converts the photoelectric conversion characteristic of the input image data from the logarithmic characteristic to the linear characteristic (third linear characteristic) as shown in FIGS. The correction unit 130 may adopt the configuration shown in FIG. 7 instead of the configuration shown in FIG. FIG. 7 is a block diagram illustrating a configuration of the exposure correction unit 130 when the first characteristic conversion unit 110 converts image data into linear characteristics.

  As shown in FIG. 7, the exposure correction unit 130 includes a representative value calculation unit 132 and a correction unit 133 without the linear characteristic conversion unit 131 shown in FIG. 6. The representative value calculation unit 132 includes a block average calculation unit 202, and obtains a representative value of the image data (linear image data (V1 ′)) output from the first characteristic conversion unit 110. Note that the processing of the representative value calculation unit 132 is the same as the processing of the representative value calculation unit 132 shown in FIG.

  The correction unit 133 includes a gain calculation unit 301 and a multiplier 302. The gain calculation unit 301 compares the representative value calculated by the representative value calculation unit 132 with the target exposure level, thereby obtaining a gain (b) for setting the exposure level of the linear image data (V1 ′) to the target exposure level. Ask. The gain (b) calculation method is the same as that of the offset calculation unit 203 shown in FIG. The multiplier 302 multiplies the linear image data (V1 ′) by the gain (b) calculated by the gain calculation unit 301 to calculate linear image data (V3 ′) whose exposure level is corrected.

  Returning to FIG. 2, the dynamic range compression unit 140 performs dynamic range compression processing on the logarithmic image data (V3) output from the correction unit 133. Here, the dynamic range compression unit 140 can employ the dynamic range compression processing described in Konica Minolta Technology Report VOL.4 (2007).

  Specifically, downsampling processing using a two-dimensional low-pass filter having a relatively small size such as 3 × 3, 5 × 5, or the like is repeated a plurality of times for one piece of logarithmic image data (V3). After the original image has been converted to multi-resolution, the edge-preserving low-pass filter processing is executed to perform upsampling multiple times while replacing the edge portion of the lower resolution image with the upper resolution image on the upper layer. Thus, the illumination component is extracted. The extracted illumination component is compressed according to a predetermined compression characteristic, and the compressed illumination component is multiplied by a reflectance component obtained by dividing the original image by the illumination component before compression. Thereby, the image data subjected to the dynamic range compression process is generated.

  According to this dynamic range compression processing, it is possible to obtain a large blurred image with a relatively small size low-pass filter by multi-resolution, and to extract illumination components with high accuracy while reducing the amount of calculation. Can do. In addition, since edge-preserving low-pass filter processing is performed, it is possible to suppress a problem called a halo effect that occurs near the edge.

  The conventional dynamic range compression unit performs the dynamic range compression processing described above for each photoelectric conversion characteristic in order to execute a dynamic range compression process suitable for the type of photoelectric conversion characteristic of the input image data. A different process was being executed. Therefore, there is a problem that the circuit scale increases. In the imaging apparatus 1 according to the present embodiment, the image data input to the dynamic range compression unit 140 is converted into a predetermined photoelectric conversion characteristic by the first characteristic conversion unit 110, so that the dynamic range compression is performed. The unit 140 only needs to perform dynamic range compression processing corresponding to one type of photoelectric conversion characteristic, and as a result, the circuit scale can be reduced.

  The second characteristic conversion unit 150 converts the photoelectric conversion characteristic of the logarithmic image data (V3) subjected to the dynamic range compression processing by the dynamic range compression unit 140 to a predetermined photoelectric conversion suitable for processing by the processing unit 160 connected to the subsequent stage. Conversion to characteristics (second photoelectric conversion characteristics). Here, as the photoelectric conversion characteristics suitable for the processing of the processing unit 160, linear characteristics (first linear characteristics and fourth linear characteristics) having higher luminance resolution on the higher luminance side than the logarithmic characteristics before conversion may be adopted. preferable.

  8A and 8B are graphs showing the photoelectric conversion characteristics converted by the second characteristic conversion unit 150, and FIG. 8A shows the case where logarithmic image data (V3) is input. FIG. 8B shows a case where linear image data (V3 ′) is input. In the graphs shown in FIGS. 8A and 8B, the vertical axis indicates the gradation value and the horizontal axis indicates the luminance.

  As shown in FIG. 8A, DR1 indicates the dynamic range of the logarithmic image data (V3) before the dynamic range compression processing is performed by the dynamic range compression unit 140, and DR2 is the dynamic range compression processing. The logarithmic image data (V3) after that, that is, the dynamic range of the logarithmic image data (V3) input to the second characteristic converter 150 is shown.

  As shown in FIG. 8A, the logarithmic image data (V3) input to the second characteristic conversion unit 150 has a gentle slope on the high luminance side and a luminance on the high luminance side as shown by a solid line. It can be seen that the resolution is reduced. Such a decrease in luminance resolution causes a decrease in the accuracy of image processing such as white balance correction in the processing unit 160 at the subsequent stage. Therefore, the second characteristic converter 150 converts the photoelectric conversion characteristic of the logarithmic image data (V3) into a linear characteristic indicated by a dotted line as shown in FIG. 8A to generate linear image data (V4). Specifically, linear image data (V4) having a linear characteristic connecting the origin O1 shown in FIG. 8A and the intersection P1 between the maximum luminance and the saturation level in the dynamic range DR2 is generated. A predetermined value is used as the saturation level.

  As a result, since the inclination of the photoelectric conversion characteristic on the high luminance side is larger than the inclination of the photoelectric conversion characteristic before conversion, the luminance resolution on the high luminance side is increased, and the processing unit 160 performs image processing such as white balance correction. Can be performed with high accuracy.

  When the linear image data (V3 ′) is input to the second characteristic conversion unit 150, the second characteristic conversion unit 150 generally performs more than the linear characteristic indicated by the solid line as illustrated in FIG. 8B. Image data (V4) having a linear characteristic indicated by a dotted line having a large inclination, specifically, a straight line connecting the origin O1 and the intersection P1 is generated. As a result, the luminance resolution of the photoelectric conversion characteristics is increased as a whole, and the processing unit 160 can perform image processing such as white balance correction with high accuracy.

  Returning to FIG. 2, the processing unit 160 performs predetermined image processing different from the dynamic range compression processing on the image data (linear image data (V4)) whose photoelectric conversion characteristics have been converted by the second characteristic conversion unit 150. To do. Here, the predetermined image processing includes at least one of white balance correction, color correction, γ correction, and noise cancellation correction.

The white balance correction corrects the photoelectric conversion characteristics of the image data of R, G, and B colors so that the reference white color is reproduced. The color correction is a process for correcting the hue (color balance: saturation) of the image data for each of the R, G, and B colors, and specifically, conversion that converts the level ratio of the image data for each of the R, G, and B colors. Coefficients are prepared, and the level ratio of R, G, and B image data is converted using a preferable conversion coefficient according to the shooting scene to correct the hue of the image data. For example, using a total of nine conversion coefficients a1 to c3, the image data is converted using the following color correction conversion formula.
R '= a1, R + a2, G + a3, B
G '= b1, R + b2, G + b3, B
B '= c1, R + c2, G + c3, B

  The γ correction corrects the inclination of the photoelectric conversion characteristics of input image data so that the output image is reproduced more naturally. Noise cancellation correction removes noise contained in input image data.

  The conventional processing unit 160 executes different processes for each photoelectric conversion characteristic in order to perform image processing suitable for the type of photoelectric conversion characteristic of the input image data when performing the various image processes described above. It was. Therefore, there is a problem that the circuit scale increases. In the imaging apparatus 1, the image data input to the processing unit 160 is converted into a predetermined photoelectric conversion characteristic by the second characteristic conversion unit 150. Therefore, the processing unit 160 has one type of photoelectric conversion. As a result of performing image processing according to the characteristics, the circuit scale can be reduced.

  Further, since the second characteristic conversion unit 150 converts the logarithmic image data (V3) into linear characteristics as shown in FIGS. 8A and 8B and inputs them to the processing unit 160, the processing unit 160 Therefore, it is possible to realize more accurate image processing. Note that the various image processes of the processing unit 160 described above are examples, and other image processes may be included.

  Hereinafter, the operation of the image processing unit 6 will be described with reference to FIG. First, the photoelectric conversion characteristic of the image data input by the first characteristic conversion unit 110 is converted into a predetermined logarithmic characteristic and is output as logarithmic image data (V1). Next, the logarithmic image data (V1) is converted into image data having a predetermined linear characteristic by the linear characteristic conversion unit 131 and output as linear image data (V2). Next, for the linear image data (V2), the representative value calculation unit 132 calculates the average value (Ave) of the gradation values of the pixels constituting the predetermined area as the representative value. Next, the correction unit 133 divides the target exposure level (Target) by the representative value (Ave), calculates the gain (b), substitutes the gain (b) into the equation (6), and sets the offset value (β). Calculated. Next, the correction unit 133 adds the offset value (β) to the logarithmic image data (V1) output from the first characteristic conversion unit 110 for each pixel, and corrects the exposure level of the logarithmic image data (V1). Logarithmic image data (V3) is output. Next, the logarithmic image data (V3) is subjected to compression of the dynamic range by the dynamic range compression unit 140, and then the photoelectric conversion characteristics are converted into linear characteristics by the second characteristic conversion unit 150, and the linear image data (V4) and And output to the processing unit 160. The linear image data (V4) is subjected to the above-described image processing by the processing unit 160, and the processing ends.

  As described above, according to the imaging device 1, the image data output from the imaging sensor 3 is converted into a predetermined photoelectric conversion characteristic suitable for compression processing by the compression unit 120 after the photoelectric conversion characteristic is converted into the compression unit. Since the dynamic range compression process is executed by 120, the photoelectric conversion characteristics suitable for the dynamic range compression process are obtained. As a result, the compression unit 120 does not have to deal with photoelectric conversion characteristics other than the photoelectric conversion characteristics, and can realize dynamic range compression processing with high accuracy without increasing the circuit scale.

  In addition, since the image data subjected to the dynamic range compression processing is converted into a predetermined linear characteristic suitable for processing by the processing unit 160 provided at the subsequent stage, the photoelectric conversion characteristic other than the photoelectric conversion characteristic is converted into the photoelectric conversion characteristic. It is not necessary to correspond. As a result, accurate image processing can be realized without increasing the circuit scale of the processing unit 160.

  The present invention may adopt the following aspects. FIG. 9 is a block diagram illustrating a configuration of the image processing unit 6 of the imaging device 1 according to another embodiment of the present invention. The image processing unit 6 illustrated in FIG. 9 is different from the image processing unit 6 illustrated in FIG. 2 in that a linear characteristic conversion unit 131 is connected between the output side of the correction unit 133 and the input side of the representative value calculation unit 132. It is characterized by being.

  That is, in the image processing unit 6 shown in FIG. 9, the linear characteristic conversion unit 131 converts the photoelectric conversion characteristic to a predetermined linear characteristic (second linear characteristic) with respect to the logarithmic image data (V3) whose exposure level is corrected by the correction unit 133. Linear image data (V2) is generated, the representative value calculation unit 132 calculates a representative value from the linear image data (V2), and the correction unit 133 corrects the exposure level from the representative value. A gain (b) is obtained, and an offset value (β) is obtained from the gain (b) and stored in a RAM (not shown) or the like. When the next log image data (V1) is output from the first characteristic converter 110, an offset value (β) is added to each pixel of the log image data (V1), and the log image data ( The exposure level of V1) is corrected.

  That is, in the image processing unit 6 shown in FIG. 2, the image data from which the offset value (β) is calculated and the image data to be corrected are the same, and the exposure level is corrected in a feed forward manner. The image processing unit 6 shown in FIG. 9 calculates an offset value (β) using the previous image data, and adds the offset value (β) to the next image data in a feedback manner such that the offset level (β) is added. Has been corrected. By doing so, the correction unit 133 can quickly correct the exposure level when the image data is output from the first characteristic conversion unit 110, so that the speed of the exposure correction process can be increased.

  The technical features of the above-described embodiment can be summarized as follows.

  (1) The image processing apparatus is an image processing apparatus that processes image data picked up by an image pickup device, and converts photoelectric conversion characteristics of image data output from the image pickup element into predetermined first photoelectric conversion characteristics. A first characteristic conversion unit that compresses the dynamic range of the image data whose photoelectric conversion characteristics have been converted by the first characteristic conversion unit, and a photoelectric conversion characteristic of the image data whose dynamic range has been compressed by the compression unit A second characteristic conversion unit that converts the image into a predetermined second photoelectric conversion characteristic, and a processing unit that executes predetermined image processing on the image data whose photoelectric conversion characteristic has been converted by the second characteristic conversion unit. It is characterized by.

  According to this configuration, the image data output from the image sensor is converted into the predetermined first photoelectric conversion characteristic suitable for the compression processing by the compression unit in the photoelectric conversion characteristic. It is not necessary to deal with photoelectric conversion characteristics other than the above, and accurate dynamic range compression processing can be realized without increasing the circuit scale.

  In addition, since the image data subjected to the dynamic range compression process is converted into a predetermined photoelectric conversion characteristic suitable for processing by a processing unit provided at a later stage, the photoelectric conversion characteristic other than the photoelectric conversion characteristic is converted into the photoelectric conversion characteristic. It is not necessary to deal with this problem, and accurate image processing can be realized in the subsequent processing unit without increasing the circuit scale.

  (2) In the above configuration, the compression unit includes a dynamic range compression unit that compresses a dynamic range of image data captured by the image sensor, and an exposure level of the image data output from the image sensor. It is preferable to include an exposure correction unit for setting the level.

  According to this configuration, the compression unit compresses the dynamic range of the image data captured by the image sensor, and the exposure level of the image data output from the image sensor is set to a predetermined target exposure level. become.

  (3) In the above configuration, it is preferable that the first photoelectric conversion characteristic is a predetermined logarithmic characteristic, and the second photoelectric conversion characteristic is a predetermined first linear characteristic.

  According to this configuration, since the photoelectric conversion characteristic of the image data output from the image sensor is converted into a predetermined logarithmic characteristic and input to the compression unit, the image data is compared with the case where the linear characteristic image data is subjected to exposure correction processing. It is possible to reduce the data width (number of bits) and to ensure the luminance resolution on the low luminance side. As a result, the exposure correction process in the compression unit can be accurately performed using a small-scale circuit.

  Further, since image data having a predetermined first linear characteristic is input to the processing unit, it has a preferable linear characteristic (for example, a linear characteristic having a relatively high luminance resolution) for performing image processing such as white balance correction. Image processing such as white balance correction can be performed on the image data, and accurate image processing can be realized.

  (4) In the above configuration, the exposure correction unit converts a photoelectric conversion characteristic of logarithmic image data whose photoelectric conversion characteristic is converted into the logarithmic characteristic by the first characteristic conversion unit into a predetermined second linear characteristic. A conversion unit; a representative value calculation unit for obtaining a representative value of the linear image data obtained by the linear characteristic conversion unit; and comparing the representative value with the target exposure level, thereby adjusting the exposure level of the linear image data. A gain for obtaining the target exposure level is obtained, an offset value for obtaining the exposure level of the logarithmic image data is set to the target exposure level based on the obtained gain, and the offset value is added to the logarithmic image data. It is preferable to provide a correction unit.

  According to this configuration, the photoelectric conversion characteristic of the logarithmic image data is once converted into the second linear characteristic by the linear conversion characteristic unit to obtain linear image data. Further, the representative value calculation unit obtains the representative value of the linear image data. Further, the correction unit compares the representative value with the target exposure level to obtain a gain for setting the exposure level of the linear image data to the target exposure level, and outputs the gain from the first characteristic conversion unit based on the obtained gain. An offset value for setting the exposure level of the logarithmic image data to the target exposure level is obtained. Further, the correction unit adds the offset value to the logarithmic image data to perform exposure correction processing. Therefore, exposure correction processing for logarithmic image data can be performed with high accuracy.

  (5) In the above configuration, it is preferable that the conversion accuracy of the second linear characteristic is lower than the conversion accuracy of the first linear characteristic.

  According to this configuration, in the exposure correction processing, the representative value of the linear image data is adopted, and each pixel value of the linear image data is rounded. Therefore, the conversion accuracy to the second linear characteristic of the linear characteristic conversion unit is improved. Thus, it is possible to lower the conversion accuracy of the second characteristic conversion unit to the first linear characteristic. Therefore, the circuit scale of the linear characteristic conversion unit can be reduced.

  (6) In the above configuration, it is preferable that the first photoelectric conversion characteristic is a predetermined third linear characteristic, and the second photoelectric conversion characteristic is a predetermined fourth linear characteristic.

  According to this configuration, the photoelectric conversion characteristics of the image data are converted into the predetermined third linear characteristics suitable for the dynamic range compression process, and are input to the compression unit. Therefore, even if the circuit scale of the compression unit is reduced, highly accurate dynamic range compression processing can be realized. Further, the photoelectric conversion characteristics of the image data compressed by the compression section are converted into a predetermined fourth linear characteristic suitable for processing by the processing section and input to the processing section. Therefore, even if the circuit scale of the compression unit is reduced, highly accurate dynamic range compression processing can be realized.

  (7) The exposure correction unit includes a representative value calculation unit that obtains a representative value of linear image data in which photoelectric conversion characteristics are converted to the third linear characteristic by the first characteristic conversion unit, and the representative value is used as the target exposure. In order to obtain the gain for setting the exposure level of the linear image data to the target exposure level by comparing with the level, and to set the exposure level of the linear image data to the target exposure level based on the obtained gain It is preferable to provide a correction unit that obtains the offset value and adds the offset value to the linear image data.

  According to this configuration, the representative value calculation unit obtains the representative value of the linear image data output from the first characteristic conversion unit. The correction unit compares the representative value with the target exposure level to obtain a gain for setting the exposure level of the linear image data to the target exposure level. Based on the obtained gain, the exposure level of the linear image data is determined. An offset value for obtaining the target exposure level is obtained. Further, the correction unit adds the offset value to the linear image data output from the first characteristic conversion unit to perform exposure correction processing. Therefore, it is possible to accurately perform the exposure correction process on the linear image data.

  (8) The exposure correction unit adds an offset value for setting the exposure level of the logarithmic image data to the target exposure level in the logarithmic image data in which the photoelectric conversion characteristic is converted into the logarithmic characteristic by the first characteristic conversion unit. A correction unit that adds and outputs to the dynamic range compression unit, a linear characteristic conversion unit that converts the photoelectric conversion characteristic of the logarithmic image data to which the offset value is added by the correction unit into a predetermined second linear characteristic, A representative value calculation unit that obtains a representative value of the linear image data obtained by the linear characteristic conversion unit, and the correction unit compares the representative value with the target exposure level, thereby exposing the logarithmic image data. A gain for setting the level to the target exposure level is obtained, the offset value is obtained based on the obtained gain, and the logarithmic image data of this time is obtained from the first characteristic conversion unit. Is input, it is preferable to adding the offset value obtained for the previous logarithmic image data wherein the current logarithmic image data.

  According to this configuration, when the logarithmic image data of this time is input from the first characteristic conversion unit, the offset value obtained with respect to the logarithmic image data of the previous time is added to the logarithmic image data of the current time, and exposure correction processing is performed. It has been broken. Therefore, exposure correction processing can be performed at high speed.

  (9) The compression unit extracts an illumination component from the image data captured by the image sensor, compresses the extracted illumination component according to a predetermined compression characteristic, and converts the compressed illumination component into the illumination component from the image data. It is preferable to compress the dynamic range by multiplying the reflectance component obtained by dividing the dynamic range.

  According to this configuration, the photoelectric conversion characteristic is converted into the photoelectric conversion characteristic suitable for the processing of the compression unit by the first characteristic conversion unit. Therefore, the process for compressing the dynamic range can be performed with high accuracy without increasing the circuit scale.

Claims (11)

An image processing apparatus that processes image data captured by an image sensor,
A first characteristic converter that converts photoelectric conversion characteristics of image data output from the image sensor into predetermined first photoelectric conversion characteristics;
A compression unit that compresses the dynamic range of the image data whose photoelectric conversion characteristics are converted by the first characteristic conversion unit;
A second characteristic conversion unit that converts photoelectric conversion characteristics of image data whose dynamic range is compressed by the compression unit into predetermined second photoelectric conversion characteristics;
An image processing apparatus comprising: a processing unit that performs predetermined image processing on the image data whose photoelectric conversion characteristics have been converted by the second characteristic conversion unit. The compression unit is
A dynamic range compression unit that compresses a dynamic range of image data captured by the image sensor;
The image processing apparatus according to claim 1, further comprising: an exposure correction unit that sets an exposure level of the image data output from the image sensor to a predetermined target exposure level. The first photoelectric conversion characteristic is a predetermined logarithmic characteristic,
The image processing apparatus according to claim 2, wherein the second photoelectric conversion characteristic is a predetermined first linear characteristic. The exposure correction unit
A linear characteristic conversion unit that converts a photoelectric conversion characteristic of logarithmic image data obtained by converting the photoelectric conversion characteristic into the logarithmic characteristic by the first characteristic conversion unit into a predetermined second linear characteristic;
A representative value calculator for obtaining a representative value of the linear image data obtained by the linear characteristic converter;
By comparing the representative value with the target exposure level, a gain for setting the exposure level of the linear image data to the target exposure level is obtained, and based on the obtained gain, the exposure level of the logarithmic image data is determined. The image processing apparatus according to claim 3, further comprising: a correction unit that obtains an offset value for obtaining the target exposure level and adds the offset value to the logarithmic image data.   The image processing apparatus according to claim 4, wherein the conversion accuracy of the second linear characteristic is lower than the conversion accuracy of the first linear characteristic. The first photoelectric conversion characteristic is a predetermined third linear characteristic,
The image processing apparatus according to claim 2, wherein the second photoelectric conversion characteristic is a predetermined fourth linear characteristic. The exposure correction unit
A representative value calculation unit for obtaining a representative value of linear image data in which photoelectric conversion characteristics are converted into the third linear characteristic by the first characteristic conversion unit;
By comparing the representative value with the target exposure level, a gain for setting the exposure level of the linear image data to the target exposure level is obtained, and the exposure level of the linear image data is determined based on the obtained gain. The image processing apparatus according to claim 6, further comprising: a correction unit that obtains an offset value for obtaining the target exposure level and adds the offset value to the linear image data. The exposure correction unit
The dynamic range compression unit by adding an offset value for setting the exposure level of the logarithmic image data to the target exposure level to the logarithmic image data obtained by converting the photoelectric conversion characteristic into the logarithmic characteristic by the first characteristic conversion unit. A correction unit to output to
A linear characteristic conversion unit that converts a photoelectric conversion characteristic of logarithmic image data to which the offset value is added by the correction unit into a predetermined second linear characteristic;
A representative value calculation unit for obtaining a representative value of the linear image data obtained by the linear characteristic conversion unit,
The correction unit obtains a gain for setting the exposure level of the logarithmic image data to the target exposure level by comparing the representative value with the target exposure level, and based on the obtained gain, the offset value When the current log image data is input from the first characteristic conversion unit, the offset value obtained for the previous log image data is added to the current log image data. Item 6. The image processing apparatus according to Item 3.   The compression unit extracts an illumination component from the image data captured by the image sensor, compresses the extracted illumination component according to a predetermined compression characteristic, and removes the compressed illumination component from the image data. The image processing apparatus according to claim 1, wherein the dynamic range is compressed by multiplying the obtained reflectance component. An image processing method for processing image data captured by an image sensor,
A first characteristic conversion step of converting a photoelectric conversion characteristic of image data output from the image sensor into a predetermined first photoelectric conversion characteristic;
A compression step of compressing the dynamic range of the image data whose photoelectric conversion characteristics have been converted by the first characteristic conversion step;
A second characteristic conversion step of converting the photoelectric conversion characteristic of the image data whose dynamic range is compressed by the compression step into a predetermined second photoelectric conversion characteristic;
And a processing step of executing predetermined image processing on the image data whose photoelectric conversion characteristics have been converted by the second characteristic conversion step. An image sensor;
An imaging apparatus comprising: the image processing apparatus according to claim 1.

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