JP2011199787A - Image processing apparatus and image processing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To output a more natural image by considering noise depending on luminance and suppressing a discontinuous change of noise volume in specific luminance.SOLUTION: An image processing apparatus includes: a reference gradation conversion characteristic-setting unit 50 for setting a reference gradation conversion characteristic which is a reference in gradation conversion processing; a contrast amplification factor calculator 51 for calculating a contrast amplification factor for a luminance value based on the reference gradation conversion characteristic; a composition number determination unit 52 for determining the number of composite images corresponding to the luminance value based on the contrast amplification factor; and a composition ratio determination unit 53 for determining a composition ratio so that the composition ratio is continuously changed in a predetermined luminance value range including the luminance value in which the number of composite images is changed.

Description

  The present invention relates to an image processing apparatus and an image processing method, and more particularly to an image processing apparatus and an image processing method for performing gradation conversion processing on an image signal.

  In general, it is known that an imaging device such as a CCD sensor or a CMOS sensor applied to a digital camera has a narrower dynamic range than an actual shooting scene or a dynamic range in human visual characteristics as an inspector. For this reason, various techniques have been proposed to interpolate this difference. In addition, like surveillance cameras, a technology for tone mapping that converts a wide dynamic range image into lower dynamic range data has been developed to detect and recognize the subject regardless of the viewer's visual perception. Has been. Further, as a secondary problem, if an image with a wide dynamic range is transferred as it is, a larger number of arithmetic units and data buses are required, and a technique for saving these is also disclosed.

  As a specific example of the above-described technique, Patent Document 1 (Japanese Patent Laid-Open No. 8-125924) discloses a case where the signal intensity after A / D conversion exceeds a predetermined number of bits by linking the number of times of multiple exposure and the dynamic range. In addition, there is disclosed a technique for changing the number of bits for A / D conversion and the exposure time for shooting for each pixel for shooting a subject with a wide dynamic range. Patent Document 2 (Japanese Patent Laid-Open No. 2003-259200) discloses a technique for expanding a dynamic range by capturing and combining a plurality of images with different exposures. Furthermore, Patent Document 3 (Japanese translations of PCT publication No. 2004-530368) discloses a technique for performing a local histogram averaging process and expanding an apparent dynamic range as compared with gradation correction by non-local gamma conversion. ing. Furthermore, in Patent Document 4 (Japanese Patent No. 4111980), as non-local tone conversion, an input image is corrected for each pixel to generate a corrected image signal, and the pixel and A technique for calculating the gain by calculating the average luminance using the information of the surrounding pixels is disclosed.

By the way, in an image processing apparatus that generates such an image with an expanded dynamic range, noise and gradation skip are suppressed, noise amount variation according to luminance is smoothed, and the number of digits in the middle of calculation It is also a problem to suppress this.
When the number of digits of usable data between the computing units is sufficient, for example, in a data bus and computing unit capable of transmitting or computing 20-bit image data, 16 pieces of 16-bit data use any area. Even if it does not overflow. In the image data, the high-luminance area has a relatively small amount of noise, and therefore does not require an average of more than a predetermined amount of data. On the other hand, in addition to a relatively large amount of noise in the low luminance region, the gradation amplification is usually increased by gradation conversion. In order to prevent an unnatural image such as gradation skipping due to a large amplification factor of gradation conversion from being obtained, the requirement for accuracy with respect to luminance increases in the lower luminance region, and more Data by superimposing images and data with a larger exposure time are required.
However, when trying to perform high-speed processing with low power consumption, it is not a good idea to expand the data bus unnecessarily, and it is desirable to obtain sufficient gradation reproducibility by effectively using the existing data bus. .
In addition to the above-mentioned viewpoint of data depth, in the alignment and addition processing of a plurality of sheets, considering the influence of sharpness deterioration due to the mismatch of the alignment processing of a plurality of sheets, the amount of noise is relatively small. For the luminance region, it is preferable not to perform alignment processing more than necessary.

  Therefore, Patent Document 5 (Japanese Patent Application Laid-Open No. 2008-60855) adaptively specifies the number of images used at the time of gradation conversion for a plurality of input images in accordance with the gradation conversion characteristics of a predetermined image. A technique for selecting is disclosed. Specifically, when processing based on the gradation conversion characteristics of the output gradation (luminance) Q (I) is performed on the input gradation (luminance) I as shown in FIG. From the tone conversion characteristic Q (I), a contrast amplification factor R (I) indicating how much the level of the input gradation has been extended is obtained as shown in Equation 1 below (see FIG. 19B). .

  For the contrast amplification factor R (I), the number of images F (I) used at the time of gradation conversion is defined as the following equation 2 (see FIG. 19C).

ここで、Ｎ１、Ｎ２は定数である。

Here, N1 and N2 are constants.

  The correction coefficient G (I) is given by the following equation 3 for the number of images F (I) used at the time of gradation conversion (see FIG. 19D).

  At this time, the gradation conversion characteristic Q "(I) that fluctuates smoothly as a whole is obtained as shown in Equation 4 below.

JP-A-8-125924 JP 2003-259200 A JP-T-2004-530368 Japanese Patent No. 4111980 JP 2008-60855 A

  However, in the invention disclosed in Patent Document 5 described above, since the number of combined images corresponds to the amplification factor, gradation conversion characteristics that change smoothly as a whole can be obtained by the correction coefficient G (I). However, this has a problem that noise is not taken into consideration. That is, in consideration of noise, the noise variance σ (N) of the added image signal with respect to the noise variance σ of one image when the number of added images is N is expressed by the following formula 5.

  Therefore, in a region having a specific luminance in the image, noise dispersion, that is, a step in noise amount becomes conspicuous before and after N is switched. Specifically, for example, in the luminance A (near I = 70) in which the number of added images shown in FIG. 19D changes, two images are added at a composite ratio of 0.5. In an area having a luminance lower than the luminance, the three images are added at a composition ratio of 0.33. For this reason, in a region where the luminance continuously changes like gradation in the image, a large step occurs in the noise variance from σ / √2 to σ / √3 before and after the change number region. There was a problem of becoming a natural image.

  The present invention has been made to solve the above-described problem. In consideration of luminance-dependent noise, a more natural image is output by suppressing a discontinuous change in the amount of noise at a specific luminance. It is an object of the present invention to provide an image processing apparatus and an image processing method that can be used.

In order to solve the above problems, the present invention employs the following means.
The present invention provides a reference for setting a reference gradation conversion characteristic that is a reference in the gradation conversion process in an image processing apparatus that performs gradation conversion processing and generates a single composite image by combining a plurality of images. A gradation conversion characteristic setting means; a contrast amplification factor calculation means for calculating a contrast amplification factor for a luminance value based on the reference gradation conversion characteristic; and an image synthesis according to the luminance value based on the contrast amplification factor. A composite number determining means for determining the number of sheets; a composite ratio determining means for determining the composite ratio so that the composite ratio continuously changes in a predetermined luminance value range including a luminance value at which the composite number changes; An image processing apparatus is provided.

  The present invention also provides a reference gradation conversion characteristic that is a reference in the gradation conversion processing in an image processing method for performing gradation conversion processing and generating a single composite image by combining a plurality of images. A reference gradation conversion characteristic setting step, a contrast amplification factor calculation step for calculating a contrast amplification factor for the luminance value based on the reference gradation conversion characteristic, and an image corresponding to the luminance value based on the contrast amplification factor A composite number determining step for determining the composite number, and a composite ratio determining step for determining the composite ratio so that the composite ratio continuously changes in a predetermined luminance value range including a luminance value at which the composite number changes. An image processing method is provided.

  According to the present invention, in consideration of noise depending on luminance, it is possible to suppress a noise amount from changing discontinuously at a specific luminance, and to output a more natural image.

1 is a block diagram illustrating a schematic configuration of an imaging apparatus to which an image processing apparatus according to a first embodiment of the present invention is applied. 1 is a block diagram illustrating a schematic configuration of a signal processing circuit according to a first embodiment of the present invention. It is a reference figure which shows an example of a gradation conversion characteristic curve. FIG. 5 is a reference diagram illustrating a concept of gradation conversion correction using a plurality of images. It is a block diagram which shows schematic structure of the gradation conversion characteristic determination part of this invention. 4 is a flowchart illustrating a process of calculating a gradation conversion characteristic in the first embodiment of the present invention. It is explanatory drawing which shows the calculation result according to the flowchart of FIG. It is a figure which shows the gradation conversion curve of each image. It is explanatory drawing which shows the detail of FIG. 1 is a block diagram illustrating a schematic configuration of a gradation correction synthesis circuit according to a first embodiment of the present invention. It is a block diagram which shows schematic structure of the gradation correction | amendment synthetic | combination circuit in the modification of the 1st Embodiment of this invention. It is a reference figure which shows the relationship of the noise amount with respect to a signal level. It is a block diagram which shows schematic structure of the imaging device to which the image processing apparatus in the 2nd Embodiment of this invention is applied. 6 is a flowchart illustrating a process of calculating gradation conversion characteristics in the second embodiment of the present invention. It is explanatory drawing which shows the calculation result according to the flowchart of FIG. It is a figure which shows the gradation conversion curve of each image. It is a block diagram which shows schematic structure of the gradation conversion characteristic determination part in the 3rd Embodiment of this invention. It is explanatory drawing which shows the gradation conversion characteristic curve calculated in the 3rd Embodiment of this invention. It is explanatory drawing in the case of calculating the conventional gradation conversion characteristic.

  Embodiments of an image processing apparatus according to the present invention will be described below with reference to the drawings.

[First Embodiment]
FIG. 1 is a block diagram showing a schematic configuration of a so-called digital camera, that is, an imaging apparatus as an example to which the image processing apparatus according to the first embodiment of the present invention is applied.

  As shown in FIG. 1, the imaging apparatus 101 realizes an operation on the display element 2001, a release switch 3001 for imaging a subject, a menu switch 3002 that can display a menu screen of the imaging apparatus 101 on the display element 2001, and the menu screen. And an imaging mode selection dial 3004 capable of setting the contents in accordance with the determined position.

  In addition, the imaging apparatus 101 is arranged at a system controller 100 that controls each part of the imaging apparatus 101, a lens 1001 that collects and forms an image of a subject, and an imaging position of the lens 1001, and captures the subject image. From the image pickup circuit 1003 and the image pickup circuit 1003 configured to output an image pickup signal corresponding to an image of a subject formed on the solid-state image pickup element 1002 and the solid-state image pickup element 1002 and have a luminance correction function, and configured by an analog front end processor or the like An A / D conversion circuit 1004 that converts an analog imaging signal that is output into digital image data and outputs it, a blur detection circuit 4000 that detects blur in image data, and corrects the relative positions of a plurality of image data. A shake correction mechanism 4002 is provided.

  The blur detection circuit 4000 calculates the shift amount of the solid-state imaging device 1002 and mechanically shifts the solid-state imaging device 1002 or, in the case of an XY address system such as a MOS imager, the read start position. Image data blur is detected by shifting or the like. The shake correction mechanism 4002 corrects the relative positions of a plurality of pieces of image data by performing shake correction processing on the image data based on the detection result of the shake detection circuit 4000. As a means for detecting blurring, a configuration in which image data itself is not used, such as a gyro, or a unit that detects by calculation from image data, such as a motion vector between frames, may be used.

  Further, the imaging apparatus 101 accumulates image data output from the signal processing circuit 1005 that performs predetermined signal processing including gradation conversion processing on the image data output from the A / D conversion circuit 1004, and the signal processing circuit. Memory 1006, 1008, display element 2001 for displaying the image data stored in the memory 1006, image compression processing is performed on the image data stored in the memory 1006, and the image data after the image compression processing is portable A compression circuit 1007 for outputting to a recording medium 2002 having

  Hereinafter, the signal processing circuit 1005 that functions as the image processing apparatus of the present invention in the imaging apparatus 101 configured as described above will be described. FIG. 2 is a block diagram illustrating a schematic configuration of the signal processing circuit 1005. As shown in FIG. 2, the signal processing circuit 1005 includes a demosaicing circuit 1051 that performs demosaicing processing on the image data recorded in the memory 1008, gradation conversion processing on image data that has been subjected to demosaicing processing, and The gradation correction characteristics at the time of gradation conversion processing in the gradation correction composition circuit 1052 that performs image composition processing, the color signal processing circuit 1053 that performs predetermined color signal processing on image data, and the gradation correction composition circuit 1052 A gradation conversion characteristic determining unit 1054 is provided.

  FIG. 3 is a reference diagram showing an example of tone conversion characteristics, that is, a tone conversion curve, and FIG. 4 schematically shows tone conversion processing and image synthesis processing performed in the tone correction synthesis circuit 1052. FIG. In the gradation correction composition circuit 1052, gradation conversion processing and image composition processing are performed on the image data. At this time, contrast amplification occurs. This is because the gradation of the image data before processing is, for example, a change of 1 LSB with 8 bits (1 gradation when all gradations are 256 gradations), 2 gradations with an output gradation, etc. It shows that the difference is amplified. Here, the amplification of contrast is obtained by differentiating the gradation conversion curve by gradation. Therefore, in FIG. 3, the gradation conversion characteristic amplifies the contrast of the region having a relatively low luminance value. In order to suppress the gradation skip due to the amplification of the trust, It is necessary to perform gradation conversion processing using more images.

  Specifically, as shown in FIG. 4, for example, the gradation correction synthesis circuit 1052 applies this luminance value to a pixel present at a position A having a relatively low luminance value among the positions on the image. It is necessary to perform gradation correction processing while using a relatively large number of images so that the amplification factor is relatively high. Further, as shown in FIG. 4, the gradation correction / combination circuit 1052 amplifies the luminance value for, for example, a pixel existing at a position B having a relatively high luminance value among the positions on the image. The gradation correction processing can be performed while using a relatively small number of images so that the rate is relatively low.

  For this reason, the gradation conversion characteristic determination unit 1054 uses a relatively large number of images so that the amplification factor of the luminance value is relatively high at a position where the luminance is relatively low on the image. In the gradation conversion characteristic memory 1008, a relatively small number of images are used so that the amplification factor of the luminance value is relatively low at a position where the luminance value is relatively high. A unique gradation conversion characteristic is set for each of a plurality of stored images.

  Specifically, as shown in FIG. 5, the gradation conversion characteristic determination unit 1054 includes a reference gradation conversion characteristic setting unit 50 that sets a reference gradation conversion characteristic that serves as a reference in the gradation conversion processing, and a reference gradation conversion. A contrast amplification factor calculating unit 51 that calculates a contrast amplification factor for a luminance value based on the characteristics, a composite number determining unit 52 that determines a composite number of images according to the luminance value based on the contrast amplification factor, and a composite number change A plurality of images based on a reference ratio conversion characteristic and a combination ratio, a combination ratio determination unit 53 that determines a combination ratio so that the combination ratio continuously changes within a predetermined luminance value range including the luminance value to be A gradation conversion characteristic setting unit 54 is provided for setting gradation conversion characteristics for each image.

  Then, the gradation conversion characteristic determination unit 1054 sets unique gradation conversion characteristics for each of a plurality of images stored in the gradation conversion characteristic memory 1008 according to the flowchart shown in FIG. That is, in step S1, the reference gradation conversion characteristic setting unit 50 performs gradation conversion processing in the image processing apparatus, and the reference gradation conversion characteristics used as a reference when generating a composite image are shown in FIG. ) Is set. FIG. 7A shows the input luminance value I on the x-axis and the output luminance value Q (I) on the y-axis. The setting of the gradation conversion curve can be calculated each time with respect to the luminance gradation of the obtained image, but can also be stored in advance, for example, using a lookup table.

  Subsequently, in step S2, the contrast amplification factor calculation unit 51 calculates the contrast amplification factor for the luminance value from the gradation conversion curve set in step S1, as shown in FIG. 7B, and in step S3. Then, by quantizing the contrast amplification factor, the quantized contrast amplification factor for the luminance value as shown in FIG. 7C is calculated. As shown in FIG. 4, the method for estimating the number of necessary images when generating a composite image calculates how many times one gradation of input luminance is increased from contrast amplification. The required number of images can be estimated from the quantized contrast amplification factor shown in FIG. That is, for example, the number of sheets can be estimated by rounding up and discretizing the following Expression 6.

ここで、Ｎは１以上の整数である。

Here, N is an integer of 1 or more.

  The graph of FIG. 7C shows the number of sheets necessary for performing gradation conversion and generating one composite image from the contrast amplification factor R (I). That is, in the region of R ′ (I) = 1, the gradation conversion uses only one image, and in the region of R ′ (I) = 2, the gradation conversion uses two images. Yes.

  Next, in step S4, the composite number determination unit 52 calculates the composite number. FIG. 7 (d) shows the addition ratio of a plurality of sheets calculated from FIG. 7 (c). That is, if the result of quantization in FIG. 7C is “1”, one image is used, and if it is “2”, two images are used, and after gradation conversion and addition, 1/2 is used. That means That is, in FIG. 7D, for example, the composite number of images near I = 0.26 is changed from one to two. For the same brightness, when one image is used and when two images are used, the shot noise (noise component depending on the brightness) of the image differs by √2 times, so that I = 0. Although the brightness is almost the same in the vicinity of 26, only the noise amplitude is different by √2 times, and an unnatural image is obtained. Therefore, in step S5, in order to continuously change the composition ratio after gradation conversion by the composition ratio determination unit 53, smoothing processing is performed using the luminance value of FIG. The composition ratio W (I, k) of each image to be calculated is calculated (see FIG. 7E). As the smoothing, there can be considered a method of limiting the numerical value of the luminance value by a look-up table or the like, and a method of mathematically specifying the luminance value as a function of the luminance value.

  FIG. 9 (a) shows details of FIG. 7 (d), and FIG. 9 (b) shows details of FIG. 7 (e). In the range where the luminance value is high, one image is used. In FIG. 9, one image is used in the range indicated as the intermediate region I which is a range including the luminance value in which the number of combined images changes from one to two. The composition ratio of the eye image and the second image is continuously changed. In the range where the number of images to be combined is two, the first and second images have the same composition ratio. In the intermediate region II, which is a range including a luminance value in which the number of combined images changes from 2 to 3, the composition ratio of the third image is continuously increased gradually. Hereinafter, the composite ratio of a plurality of sheets is changed depending on the luminance, and a range (intermediate region) in which the composite ratio continuously changes is provided.

  In step S6, the gradation conversion characteristic Q (I, k) for each image according to Equation 7 based on the reference gradation conversion curve of FIG. 7A and the composition ratio W (I, k) obtained in step S5. Are calculated respectively.

  FIG. 8 shows the tone conversion characteristics of each image.

  By the way, as shown in FIG. 10, the gradation correction synthesis circuit 1052 includes the same number of images as the number of images obtained from the gradation conversion processing unit 57 that performs gradation conversion processing on each image. A gradation composition processing unit 58 that generates a composite image from an image that has been subjected to gradation conversion processing by the conversion processing unit 57 is provided. Prior to the gradation conversion processing corresponding to each image, luminance correction processing corresponding to exposure data (shutter speed, aperture, sensitivity) at the time of capturing each image is performed. Therefore, based on the gradation conversion characteristic Q (I, k) for each image, the gradation conversion processing unit 57 performs gradation conversion processing on each of the plurality of images, and the plurality of images after gradation conversion processing are converted into a scale. By adding by the tone synthesis processing unit 58, one synthesized image is generated. The generated image is temporarily stored in the memory 1006 and displayed on the display element 2001 or compressed by the compression circuit 1007 and stored in the storage medium 2002.

Thus, according to the present embodiment, tone conversion curves corresponding to a plurality of images are calculated, and tone conversion processing is performed on each image based on the calculated curves. This gradation conversion curve for each image is set so that gradation conversion output is performed while the contribution of each image changes continuously according to the input gradation, so it is used at a specific input gradation. The number of images to be switched is switched, the contribution of each image does not change discontinuously, the noise amount can be suppressed from changing discontinuously at a specific gradation, and a more natural image can be obtained.
In addition, by performing tone conversion on each of the plurality of images, the data amount of each image is reduced, and the bus line can be saved.

[Modification of First Embodiment]
In the first embodiment of the present invention described above, the gradation correction / combination circuit 1052 includes a plurality of gradation conversion processing units 57, and adds the short guns to the images that have undergone the gradation conversion processing in each gradation conversion processing unit 57. As a result, one composite image was obtained. This modification is different in that the tone correction / synthesis circuit 1052 includes only one tone conversion processing unit 59 as shown in FIG. Hereinafter, the description of the present modification example that is common to the first embodiment will be omitted, and different points will be described.

  As shown in FIG. 11, the tone correction synthesis circuit 1052 includes one tone conversion processing unit 59, a plurality of synthesis ratio multiplication units 60 corresponding to a plurality of images, and a tone synthesis processing unit 58. Yes. Prior to the gradation conversion processing corresponding to each image, luminance correction processing corresponding to exposure data (shutter speed, aperture, sensitivity) at the time of capturing each image is performed. The gradation conversion processing unit 59 performs gradation conversion processing on all the images stored in the memory 1008 based on the reference gradation conversion characteristics, and outputs each image to the respective composition ratio multiplication unit 60. In each composition ratio multiplication unit 60, the gradation conversion characteristic Q (I, k) in the first embodiment described above is multiplied by multiplying the image after the gradation conversion processing by the composition ratio W (I, k). Based on this, it is possible to obtain the same effect as that obtained by performing gradation conversion processing on each of a plurality of images.

  Then, gradation conversion processing is performed, and a plurality of images multiplied by the composition ratio are added by the gradation composition processing unit 58, thereby generating one composite image. The generated image is temporarily stored in the memory 1006 and displayed on the display element 2001 or compressed by the compression circuit 1007 and stored in the storage medium 2002.

[Second Embodiment]
In the present embodiment, in addition to the configuration of the above-described embodiment, a noise amount depending on the luminance value is predicted, and the number of synthesized images and the synthesized ratio are determined in consideration of this noise amount.
That is, when the noise amount N is Gaussian noise, the noise variance is expressed by the following equation 8 modeled by a quadratic function.

Here, α, β, and γ are constant terms. From this _equation 8, it can be seen that the noise N increases in a quadratic curve with respect to the luminance signal L ₀ of the imager output.
However, the amount of noise varies not only with the signal level but also with the temperature and gain of the element. FIG. 12 shows, as an example, a plot of noise amounts with respect to three types of ISO sensitivities 100, 200, and 400 related to gain at a certain temperature t. The individual curves in FIG. 11 are in the form shown in Eq. 8, but the coefficients differ depending on the ISO sensitivity related to the gain. Therefore, when the temperature is t and the gain is g, and the model is formulated in consideration of these, the following formula 9 is obtained.

Here, α _gt , β _gt , and γ _gt are constant terms determined by gain and temperature. This noise model can also be applied independently for RGB.

Knee processing or γ conversion in a processing system in which a digital signal processing input signal is a 12-bit output signal and an 8-bit processing system is a conversion that compresses the gradation for a high-brightness area in 8 bits. In general, the signal processing is often used. Therefore, in consideration of the luminance value-noise dispersion value characteristic immediately after A / D conversion and the Knee process as in equations (15) and (16), the luminance gradation and noise dispersion after 8-bit conversion. When the values are matched, it becomes a unimodal curve.
FIG. 13 is a diagram illustrating a configuration in the present embodiment, which includes a noise amount estimation unit 4003, and the noise amount estimation unit gives a noise estimation amount to a gradation conversion characteristic determination unit included in the signal processing unit 1005.

  FIG. 14 is a flowchart showing a calculation process for setting gradation conversion characteristics in the present embodiment, and FIG. 15 is a graph showing a calculation process for setting gradation conversion characteristics in the present embodiment.

  In step S11 and step S12, as in the first embodiment described above, a reference gradation conversion curve as a reference gradation conversion characteristic is set, and then the contrast amplification factor is calculated (see FIG. 15A). . In step S13, as shown in FIG. 15 (b) with the data given from the noise amount estimation unit 4003, the noise amount based on the above equation 9 is set. In step S14, a noise model considering contrast amplification is calculated from FIGS. 15 (a) and 15 (b). FIG. 15C shows a noise model considering the contrast amplification factor. In step S15, as in the first embodiment, in order to calculate the number of combined images, the noise model considering the contrast amplification factor is quantized (see FIG. 15D), and in the next step S16, In addition to calculating the number of images to be combined and the composition ratio, smoothing is performed so as to obtain a smooth composition ratio change by applying a smoothing process to FIG. In step S17, a composite ratio curve (W (I, k)) for each image is calculated. FIG. 15E shows the synthesis ratio W (I, k) corresponding to the noise model. In step S18, a gradation conversion characteristic Q (I, k) for each image shown in FIG.

  In this way, the number of composite images and the composite ratio are calculated in consideration of the contrast amplification factor and the amplitude of noise depending on the brightness. By synthesizing the number of images, it is possible to suppress the amplification of noise after the gradation change.

[Third Embodiment]
In the above-described embodiment, when a reference gradation conversion characteristic (first reference gradation conversion characteristic) different from the reference gradation conversion characteristic is given, a plurality of images determined based on the reference gradation conversion characteristic and the composition ratio Based on the gradation conversion characteristic Q (I) and the first reference gradation conversion characteristic for each of the plurality of images, a first value corresponding to the first reference gradation conversion characteristic is provided for each of the plurality of images. The gradation conversion characteristic Q ₂ (I) can be calculated. More specifically, in the above-described embodiment, the gradation conversion characteristic Q (I) for each image has already been obtained, and the inverse variable Q ⁻¹ (I) of the gradation conversion characteristic Q (I) is defined. In addition, when the mapping exchange rule holds, another gradation conversion characteristic Q ₂ (I) can be calculated from the gradation conversion characteristic Q (I) (see FIG. 17). That is, when the above conditions are satisfied, the luminance I can be defined by the following formula 11.

Therefore, the gradation conversion characteristic Q ₂ (I, k) is expressed by the following equation (12).

When the exchange rule is applied, Equation 13 is obtained.

When the gradation conversion characteristics Q (I) and Q ₂ (I) assume a plurality of gradation conversions, the gradation conversion characteristics Q (I, k) and Q ₂ (I, k) are numbers. 14 can be defined,

Ｑ_２（Ｉ，ｋ）は、数１５及び数１６となる。

Q ₂ (I, k) is expressed by Equation 15 and Equation 16.

Therefore, each image corresponding to the reference gradation conversion characteristic Q (I) is calculated without calculating the contrast amplification factor and the number of images necessary for obtaining the composition ratio W (I, k) as in the above-described embodiment. gradation conversion characteristic Q (I, k) can get a gradation conversion characteristic Q _{2 (I,} k) of each image output different gradation conversion characteristics Q _{2 (I)} used. Note that a lookup table may be used for the calculation of gradation conversion. FIG. 17 is a block diagram illustrating a schematic configuration of a gradation conversion characteristic determination unit 1054 as an arithmetic unit for calculating gradation conversion characteristics. Reference numeral 701 denotes a reference gradation conversion curve. Similar to the embodiment described above, the gradation conversion curve of each image is calculated with respect to the reference gradation conversion curve. The inverse function calculation unit 702 calculates the characteristics of the inverse function with respect to the reference gradation conversion curve. The gradation conversion calculation means 704 gives the inverse function characteristic calculated by the inverse function calculation means 702 to the argument of the target gradation conversion characteristic, and Q ₂ (Q ⁻¹ (I )) Is calculated. The substitution calculation means 706 gives Q ₂ (Q ⁻¹ (I)) as an argument of the gradation conversion characteristic of each image corresponding to the reference gradation conversion characteristic acquired by the gradation conversion characteristic setting means 705 of each image, Equation 15 is calculated.

  As a reference example, FIG. 18 shows a reference gradation conversion curve A (FIG. 18A), a target gradation conversion characteristic B (FIG. 18B), and each image corresponding to the reference gradation conversion characteristic. The gradation conversion characteristics (FIG. 18C) and the gradation conversion characteristics (FIG. 18D) of each image corresponding to the gradation conversion characteristics B acquired by the above method are shown. Further, instead of the above-described equation 16, the equation 17 can be used.

  The composition ratio W (I, k) can be calculated by Equation 17 and applied to the configuration shown in FIG. 11, that is, gradation conversion processing for all of a plurality of images based on the reference gradation conversion characteristics. The image after gradation conversion processing may be multiplied by the composition ratio W (I, k).

  By doing this, it is not necessary to calculate the contrast amplification factor and to calculate the number of images to be combined and the combination ratio from the contrast amplification factor, and the calculation can be speeded up.

  In the above-described embodiment, processing by hardware is assumed as the image processing apparatus, but it is not necessary to be limited to such a configuration. For example, a configuration in which processing is performed separately by software is also possible. In this case, a general-purpose or dedicated computer equipped with a CPU (Central Processing Unit), ROM (Read Only Memory), RAM (Random Access Memory), etc. can realize all or part of the above processing by the CPU. Similar to the above-described image processing apparatus by reading a program recorded on a computer-readable recording medium in which the program is recorded, expanding the program in a ROM, a RAM, etc., and executing information processing / calculation processing Realize the process.

Here, the computer-readable recording medium means a magnetic disk, a magneto-optical disk, a CD-ROM, a DVD-ROM, a semiconductor memory, or the like. Alternatively, the computer program may be distributed to the computer via a communication line, and the computer that has received the distribution may execute the program.
As mentioned above, although embodiment of this invention was explained in full detail with reference to drawings, the concrete structure is not restricted to this embodiment, The design change etc. of the range which does not deviate from the summary of this invention are included.

50 Reference gradation conversion characteristic setting unit (reference gradation conversion characteristic setting means)
51 Contrast amplification factor calculation unit (contrast amplification factor calculation means)
52 composite number determination unit (composite number determination means)
53 Composite ratio determining unit (Composite ratio determining means)
54 gradation conversion characteristic setting sections 57 and 59 gradation conversion processing section 58 gradation composition processing section 1054 gradation conversion characteristic determining section

Claims (8)

In an image processing apparatus that performs gradation conversion processing and generates a single composite image by combining a plurality of images,
A reference gradation conversion characteristic setting means for setting a reference gradation conversion characteristic serving as a reference in the gradation conversion process;
A contrast amplification factor calculating means for calculating a contrast amplification factor for a luminance value based on the reference gradation conversion characteristic;
A composite number determination means for determining the composite number of images according to the luminance value based on the contrast amplification factor;
A composition ratio determining means for determining the composition ratio so that the composition ratio continuously changes in a predetermined luminance value range including a luminance value in which the composition number changes;
An image processing apparatus comprising: A noise amount prediction means for predicting a noise amount depending on the luminance value;
The image processing apparatus according to claim 1, wherein the composite number determination unit determines a composite number according to a luminance value based on the noise amount and the contrast amplification factor.   The image processing apparatus further comprises gradation conversion characteristic setting means for setting gradation conversion characteristics for each of the plurality of images based on the reference gradation conversion characteristics and the composition ratio. The image processing apparatus according to claim 1 or 2. Gradation conversion processing means for performing gradation conversion processing on the plurality of images;
A composite image generating means for generating a composite image by combining the plurality of images;
The gradation conversion processing unit performs gradation conversion processing on the plurality of images based on reference gradation conversion characteristics, and the composite image generation unit includes the plurality of images subjected to gradation conversion processing. The image processing apparatus according to claim 1, wherein a combined image is generated by combining the number of images according to the number of images to be combined and the combination ratio. Gradation conversion processing means for performing gradation conversion processing on the plurality of images;
A composite image generating means for generating a composite image by combining the plurality of images;
The gradation conversion processing unit performs gradation conversion processing on the plurality of images based on the gradation conversion characteristics, and the composite image generation unit includes the plurality of sheets on which the gradation conversion processing has been performed. The image processing apparatus according to claim 3, wherein a combined image is generated by adding the images.   The image processing apparatus according to claim 4, further comprising a luminance correction unit that corrects a luminance difference between the plurality of images when the synthetic image is generated by the synthetic image generation unit.   Based on the first reference gradation conversion characteristic different from the reference gradation conversion characteristic, the reference gradation conversion characteristic, and the composition ratio, the first reference gradation conversion characteristic for each of the plurality of images. 7. The image processing apparatus according to claim 1, further comprising a calculation unit configured to calculate a first gradation conversion characteristic corresponding to the first gradation conversion characteristic. In an image processing method for performing gradation conversion processing and generating a single composite image by combining a plurality of images,
A reference gradation conversion characteristic setting step for setting a reference gradation conversion characteristic serving as a reference in the gradation conversion process;
A contrast amplification factor calculating step for calculating a contrast amplification factor for a luminance value based on the reference gradation conversion characteristic;
A composite number determination step for determining a composite number of images according to a luminance value based on the contrast amplification factor;
A composition ratio determining step for determining the composition ratio so that the composition ratio continuously changes in a predetermined luminance value range including a luminance value in which the composition number changes;
An image processing method comprising:

Images