Patent Publication Number: US-10765297-B2

Title: Image processing apparatus, image processing method, and computer readable recording medium

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of PCT International Application No. PCT/JP2015/086425 filed on Dec. 25, 2015, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     The present disclosure relates to an image processing apparatus, an image processing method, and a computer readable recording medium. 
     In the related art, a technique for recognizing a specific region, such as an abnormal region, using a recognition criterion in an intraluminal image that is obtained by imaging inside a lumen (inside a gastrointestinal tract) of a living body by using a medical observation apparatus, such as an endoscope, has been known. The recognition criterion used in this technique is usually generated based on a wide range of variations of images of a normal mucosal region or an abnormal region that are extracted as learning samples from intraluminal images. 
     As a technique related to image recognition, for example, U.S. Pat. No. 8,903,167 discloses a technique for generating a new image by performing processing of changing a position, a size, and an orientation of any region of interest in an image acquired as a learning sample, and a technique for generating a recognition criterion by calculating a feature amount from the new image and an original image. 
     SUMMARY 
     An image processing apparatus according to one aspect of the present disclosure includes: a surface shape estimation unit configured to estimate a surface shape of a target that appears in an intraluminal image of a living body; an imaging viewpoint changing unit configured to change an imaging viewpoint with respect to the surface shape from an imaging viewpoint used for estimation; and an image generation unit configured to generate a virtual image of the target for a case of imaging the target from the changed imaging viewpoint. 
     The above and other features, advantages and technical and industrial significance of this disclosure will be better understood by reading the following detailed description of presently preferred embodiments of the disclosure, when considered in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram illustrating a configuration of an image processing apparatus according to a first embodiment; 
         FIG. 2  is a flowchart illustrating an outline of processing performed by the image processing apparatus according to the first embodiment; 
         FIG. 3  is a diagram illustrating an example of an intraluminal image; 
         FIG. 4  is a flowchart illustrating an outline of surface shape estimation processing in  FIG. 2 ; 
         FIG. 5  is a schematic view for explaining a surface coordinates estimation method performed by an imaging distance estimation unit; 
         FIG. 6  is a flowchart illustrating an outline of imaging viewpoint change processing in  FIG. 2 ; 
         FIG. 7  is a schematic view for explaining a method of changing a relative angle by a relative angle changing unit; 
         FIG. 8  is a flowchart illustrating an outline of image generation processing in  FIG. 2 ; 
         FIG. 9  is a diagram illustrating an example of a virtual image; 
         FIG. 10  is a diagram illustrating another example of a virtual image; 
         FIG. 11  is a block diagram illustrating a configuration of a surface shape estimation unit according to a first modification of the first embodiment; 
         FIG. 12  is a flowchart illustrating an outline of surface shape estimation processing performed by the surface shape estimation unit according to the first modification of the first embodiment; 
         FIG. 13  is a block diagram illustrating a configuration of an image generation unit according to a second modification of the first embodiment; 
         FIG. 14  is a flowchart illustrating an outline of image generation processing performed by the image generation unit according to the second modification of the first embodiment; 
         FIG. 15  is a block diagram illustrating a configuration of an imaging viewpoint changing unit according to a second embodiment; and 
         FIG. 16  is a block diagram illustrating a configuration of an arithmetic unit according to a third embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of an image processing apparatus, an image processing method, and a program according to the present disclosure will be described below with reference to the drawings. The present disclosure is not limited by the embodiments below. Further, the same components are denoted by the same reference signs throughout the drawings. 
     First Embodiment 
     Configuration of Image Processing Apparatus 
       FIG. 1  is a block diagram illustrating a configuration of an image processing apparatus according to a first embodiment. An image processing apparatus  1  according to the first embodiment is, as one example, an apparatus that performs image processing of generating a new image (virtual image) with a different imaging viewpoint from an original intraluminal image, based on a surface shape of a target that appears in the intraluminal image that is acquired by imaging a lumen of a living body by an endoscope (an endoscopy scope, such as a flexible endoscope or a rigid endoscope) or a capsule endoscope (hereinafter, these are collectively and simply referred to as an “endoscope”). The intraluminal image is usually a color image with pixel levels (pixel values) corresponding to wavelength components of red (R), green (G), and blue (B) at each of pixel positions. 
     The image processing apparatus  1  illustrated in  FIG. 1  includes an image acquisition unit  2  that acquires, from an endoscope or from outside, image data corresponding to an intraluminal image captured by the endoscope, an input unit  3  that receives an input signal that is input through external operation, a display unit  4  that displays the intraluminal image and various kinds of information, a recording unit  5  that records the image data acquired by the image acquisition unit  2  and various programs, a control unit  6  that controls the whole operation of the image processing apparatus  1 , and an arithmetic unit  7  that performs predetermined image processing on the image data. 
     The image acquisition unit  2  is appropriately configured in accordance with a mode of a system including the endoscope. For example, when a portable recording medium is used for transmission and reception of image data to and from the endoscope, the image acquisition unit  2  is configured as a reader device to which the recording medium is detachably attachable and which may read the recorded image data. Further, when a server for recording the image data captured by the endoscope is used, the image acquisition unit  2  is configured with a communication device or the like capable of performing bi-directional communication with the server, and acquires the image data by performing data communication with the server. Furthermore, the image acquisition unit  2  may be configured with an interface device or the like to which the image data is input from the endoscope via a cable. 
     The input unit  3  is realized by, for example, an input device, such as a keyboard, a mouse, a touch panel, or various switches, and outputs the input signal that has been received in accordance with external operation to the control unit  6 . 
     The display unit  4  is realized by a display device, such as a liquid crystal display panel or an organic electro luminescence (EL) display panel, and displays various screens including the intraluminal image under the control of the control unit  6 . 
     The recording unit  5  is realized by any kind of integrated circuit (IC) memory, such as a flash memory, a read only memory (ROM), and a random access memory (RAM), and an internal hard disk, a hard disk connected via a data communication terminal, or the like. The recording unit  5  records a program for operating the image processing apparatus  1  and causing the image processing apparatus  1  to implement various functions, data used during execution of the program, and the like, in addition to the image data acquired by the image acquisition unit  2 . For example, the recording unit  5  records an image processing program  51  for generating a new virtual image (learning sample) with a different imaging viewpoint from the intraluminal image, various kinds of information used during execution of the program, and the like. 
     The control unit  6  is realized by a central processing unit (CPU) or the like, and configured to read various programs recorded in the recording unit  5  and comprehensively control the whole operation of the image processing apparatus  1  by transferring an instruction, data, and the like to each of the units of the image processing apparatus  1  in accordance with the image data input from the image acquisition unit  2 , the input signal input from the input unit  3 , or the like. 
     The arithmetic unit  7  is realized by a CPU or the like, and configured to read the image processing program  51  recorded in the recording unit  5  and perform image processing of generating a virtual image with a different imaging viewpoint with respect to a target that appears in the intraluminal image. 
     Detailed Configuration of Calculation Unit 
     Next, a detailed configuration of the arithmetic unit  7  will be described. 
     The arithmetic unit  7  includes a surface shape estimation unit  10 , an imaging viewpoint changing unit  11 , and an image generation unit  12 . 
     The surface shape estimation unit  10  estimates a surface shape of a target that appears in an intraluminal image of a living body. The surface shape estimation unit  10  includes an imaging distance estimation unit  20 . 
     The imaging distance estimation unit  20  estimates an imaging distance to the target that appears at each of pixels of the intraluminal image. The imaging distance estimation unit  20  includes a low absorption wavelength component selection unit  201 . 
     The low absorption wavelength component selection unit  201  selects a low absorption wavelength component, for which a degree of absorption and dispersion inside a living body is low, in the intraluminal image. 
     The imaging viewpoint changing unit  11  changes an imaging viewpoint with respect to the surface shape estimated by the surface shape estimation unit  10 . The imaging viewpoint changing unit  11  includes a relative angle changing unit  30  and a relative position changing unit  31 . 
     The relative angle changing unit  30  changes a relative angle between the surface shape estimated by the surface shape estimation unit  10  and the imaging viewpoint. The relative angle changing unit  30  includes a rotation unit  300  that rotates any of roll, yaw, and pitch with respect to the imaging direction of the endoscope. 
     The relative position changing unit  31  changes relative positions of the surface shape estimated by the surface shape estimation unit  10  and the imaging viewpoint. The relative position changing unit  31  includes a horizontal/vertical position changing unit  311  that changes horizontal positions and/or vertical positions of the surface shape and the imaging viewpoint, and an imaging distance changing unit  312  that changes an imaging distance from the surface shape to the imaging viewpoint. 
     The image generation unit  12  generates a virtual image of the target for a case of imaging the target from the changed imaging viewpoint. The image generation unit  12  includes a pixel value estimation unit  40  that estimates a pixel value of each of pixels of the virtual image based on pixel values of pixels of an intraluminal image that is projected on the virtual image due to a change in the imaging viewpoint, and a lost pixel value interpolation unit  41  that, when a pixel value of any of the pixels of the virtual image is lost, interpolates the pixel value of the subject pixel from pixels around the pixel whose pixel value is lost. 
     The pixel value estimation unit  40  includes a search unit  401  that searches for pixels of the intraluminal image that are projected around each of the pixels of the virtual image due to a change in the imaging viewpoint, a pixel value interpolation unit  402  that interpolates the pixel value of each of the pixels of the virtual image based on pixel values of the pixels of the intraluminal image obtained by search by the search unit  401 , a shielded region elimination unit  403  that eliminates a pixel corresponding to a shielded region in the virtual image among the pixels of the intraluminal image projected on the virtual image, and a distance-corresponding-pixel-value correction unit  404  that corrects the pixel value of each of the pixels of the virtual image based on the imaging distance from the surface shape to the imaging viewpoint. 
     Processing Performed by Image Processing Apparatus 
     An image processing method performed by the image processing apparatus  1  configured as above will be described below.  FIG. 2  is a flowchart illustrating an outline of processing performed by the image processing apparatus  1 . 
     As illustrated in  FIG. 2 , first, the image processing apparatus  1  acquires an intraluminal image corresponding to image data, which is captured by the endoscope or the like, from outside via the image acquisition unit  2 , and records the acquired intraluminal image in the recording unit  5  (Step S 101 ).  FIG. 3  illustrates an example of an intraluminal image W 1  captured by the endoscope or the like from outside via the image acquisition unit  2 . 
     Subsequently, the surface shape estimation unit  10  acquires the image data of the intraluminal image recorded in the recording unit  5 , and performs surface shape estimation processing of estimating a surface shape of a target that appears in the acquired intraluminal image (Step S 102 ). 
       FIG. 4  is a flowchart illustrating an outline of the surface shape estimation processing at Step S 102  in  FIG. 2 . 
     As illustrated in  FIG. 4 , the low absorption wavelength component selection unit  201  selects a low absorption/low dispersion wavelength component inside a living body (Step S 201 ). Specifically, an R component, for which the degree of absorption and dispersion inside a living body is the lowest, is selected. This is to obtain information on a pixel value that is correlated with an imaging distance to the best mucosal surface for which a reduction in the pixel value due to a blood vessel or the like that appears in the mucosal surface is prevented, to thereby improve the accuracy of imaging distance estimation to be performed in the subsequent stage. 
     Subsequently, the imaging distance estimation unit  20  estimates an imaging distance a to a target that appears at each of the pixels of the intraluminal image (Step S 202 ). Specifically, the imaging distance is estimated in accordance with Equation (1) below using an assumed uniform diffuser based on the pixel value of the low absorption wavelength component. 
     
       
         
           
             
               
                 
                   
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                         θ 
                       
                       
                         L 
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     Here, α (x i , y i ) represents an imaging distance to a target that appears at the coordinates (x i , y i ), I represents radiation intensity of a light source (measured in advance), K represents a diffuse reflection coefficient of a mucosal surface (an average value is measured in advance), θ represents an angle between a normal vector of the mucosal surface and a vector from the mucosal surface to the light source (which is a value determined based on a positional relationship between the light source at a distal end of the endoscope and the mucosal surface; in this example, an average value is set in advance), and L (x i , y i ) represents a pixel value of the low absorption wavelength component (R component) of the pixel at the coordinates (x i , y i ). 
     Thereafter, the surface shape estimation unit  10  estimates the surface coordinates of the target that appears at each of the pixels of the intraluminal image (Step S 203 ). The coordinates (x i , y i ) in the image W 1  (the origin is set at the center of the image) and surface coordinates (X i , Y i , Z i ) of the target that appears at this coordinates have a relationship as illustrated in  FIG. 5 . Here, XYZ is a surface coordinate system with the XY axis parallel to the xy axis of the image W 1  and the Z axis that passes through the center of the image. Further, f is a value that is determined based on a pixel pitch of a sensor, characteristics of an imaging system, or the like. α is the imaging distance estimated by the imaging distance estimation unit  20 . 
     Equation (2) and Equation (3) below are obtained from the relationship illustrated in  FIG. 5 . 
                     α   r     =         X   i       x   i       =         Y   i       y   i       =       Z   i     f                 (   2   )               r   =         x   i   2     +     y   i   2     +     f   2                 (   3   )               
Accordingly, Equation (4) below is obtained from Equation (2) and Equation (3).
 
                     (           X   i               Y   i               Z   i           )     =       α         x   i   2     +     y   i   2     +     f   2           ⁢     (           x   i               y   i             f         )               (   4   )               
In this manner, the surface shape estimation unit  10  performs conversion to the surface coordinate system based on the imaging distance to the target that appears at each of the pixels of the intraluminal image W 1  and based on the coordinates of each of the pixels, and estimates the surface coordinates (surface shape) of the target (subject) that appears at each of the pixels. After Step S 203 , the image processing apparatus  1  returns to the main routine in  FIG. 2 .
 
     Referring back to  FIG. 2 , processes from Step S 103  will be described. 
     At Step S 103 , the imaging viewpoint changing unit  11  performs imaging viewpoint change processing for changing an imaging viewpoint with respect to the surface shape, e.g., for changing the imaging viewpoint from a first imaging viewpoint to a second imaging viewpoint. 
       FIG. 6  is a flowchart illustrating an outline of the imaging viewpoint change processing at Step S 103  in  FIG. 2 . 
     As illustrated in  FIG. 6 , the relative angle changing unit  30  changes a relative angle between the surface shape and the imaging viewpoint (any of a roll angle, a yaw angle, and a pitch angle with respect to the imaging direction of the endoscope) (Step S 301 ). Specifically, as illustrated in  FIG. 7 , a relative angle between a surface shape Q 1  and an imaging viewpoint P 1  is changed. More specifically, the rotation unit  300  performs conversion from the surface coordinate system XYZ to a barycentric coordinate system X0Y0Z0 with the origin at the center of mass of the surface shape, in accordance with Equation (5) below. 
                     (           X   ⁢           ⁢     0   i                 Y   ⁢           ⁢     0   i                 Z   ⁢           ⁢     0   i             )     =       (           X   i               Y   i               Z   i           )     -     (           X   c               Y   c               Z   c           )               (   5   )               
Here, (X c , Y c , Z c ) represents barycentric coordinates of the surface shape.
 
Further, the rotation unit  300  changes a relative angle between the surface shape and the imaging viewpoint by X-axis rotation (pitch), Y-axis rotation (yaw), and Z-axis rotation (roll), in accordance with Equation (6) below. In  FIG. 7 , T 1  indicates a surface shape that is obtained when the surface shape Q 1  is changed as described above.
 
                     (           X   ⁢           ⁢     0   i   ′                 Y   ⁢           ⁢     0   i   ′                 Z   ⁢           ⁢     0   i   ′             )     =       (           cos   ⁢           ⁢     θ   z               -   sin     ⁢           ⁢     θ   z           0             sin   ⁢           ⁢     θ   z             cos   ⁢           ⁢     θ   z           0           0       0       1         )     ⁢     (           cos   ⁢           ⁢     θ   y           0         sin   ⁢           ⁢     θ   y               0       1       0               -   sin     ⁢           ⁢     θ   y           0         cos   ⁢           ⁢     θ   y             )     ⁢     (         1       0       0           0         cos   ⁢           ⁢     θ   x               -   sin     ⁢           ⁢     θ   x               0         sin   ⁢           ⁢     θ   x             cos   ⁢           ⁢     θ   x             )     ⁢     (           X   ⁢           ⁢     0   i                 Y   ⁢           ⁢     0   i                 Z   ⁢           ⁢     0   i             )               (   6   )               
Here, θx represents a rotation angle with respect to the X-axis, θy represents a rotation angle with respect to the Y-axis, and θz represents a rotation angle with respect to the Z-axis.
 
     Subsequently, in the relative position changing unit  31 , the horizontal/vertical position changing unit  311  changes relative positions (horizontal/vertical positions) of the surface shape and the imaging viewpoint (Step S 302 ), and the imaging distance changing unit  312  changes an imaging distance from the surface shape to the imaging viewpoint (Step S 303 ). Specifically, the horizontal/vertical position changing unit  311  changes the horizontal/vertical positions of the surface shape and the imaging viewpoint in accordance with Equation (7) below, and the imaging distance changing unit  312  changes the imaging distance from the surface shape to the imaging viewpoint in accordance with Equation (8). Then, conversion from the barycentric coordinate system X0Y0Z0 to the surface coordinate system XYZ is performed in accordance with Equation (9) below. In  FIG. 7 , T 2  indicates a surface shape that is obtained when the surface shape T 1  is changed as described above. 
                     (           X   ⁢           ⁢     0   i   ″                 Y   ⁢           ⁢     0   i   ″             )     =       (           X   ⁢           ⁢     0   i   ′                 Y   ⁢           ⁢     0   i   ′             )     +     (           X   s               Y   s           )               (   7   )                 Z   ⁢           ⁢     0   i   ″       =       Z   ⁢           ⁢     0   i   ′       +     Z   s               (   8   )                 (             X   ⁢             i   ″                 Y   ⁢             i   ″                 Z   ⁢             i   ″           )     =       (           X   ⁢           ⁢     0   i   ″                 Y   ⁢           ⁢     0   i   ″                 Z   ⁢           ⁢     0   i   ″             )     +     (           X   c               Y   c               Z   c           )               (   9   )               
Here, X s  represents an amount of change in the position in the X direction (horizontal direction), Y s  represents an amount of change in the position in the Y direction (vertical direction), and Z s  represents an amount of change in the position in the Z direction (amount of change in the imaging distance). After Step S 303 , the image processing apparatus  1  returns to the main routine in  FIG. 2 .
 
     Referring back to  FIG. 2 , processes from Step S 104  will be described. 
     At Step S 104 , the image generation unit  12  performs image generation processing of generating a virtual image of a target for a case of imaging the target from the changed imaging viewpoint. 
       FIG. 8  is a flowchart illustrating an outline of the image generation processing at Step S 104  in  FIG. 2 . 
     As illustrated in  FIG. 8 , first, the pixel value estimation unit  40  calculates the coordinates of each of the pixels of the intraluminal image (original image) that is projected on an imaging plane after the change in the imaging viewpoint (Step S 401 ). Specifically, coordinates (x i ″, y i ″) of each of the pixels of the original image that is projected on the imaging plane (virtual image) after the change in the imaging viewpoint is calculated in accordance with Equation (10) below. 
     
       
         
           
             
               
                 
                   
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     Subsequently, the search unit  401  searches for pixels of the original image that are projected around each of the pixels of the virtual image (Step S 402 ). An integer coordinate in the xy coordinate system corresponds to the position of each of the pixels of the virtual image. 
     Thereafter, the shielded region elimination unit  403  detects and eliminates data (a pixel) corresponding to occlusion (shielded portion) in the virtual image from among the pixels of the original image projected on the virtual image (Step S 403 ). Specifically, the shielded portion is determined and eliminated based on the imaging distances to the pixels of the original image projected around each of the pixels of the virtual image. 
     Subsequently, the pixel value interpolation unit  402  calculates a value of each of the pixels of the virtual image by performing interpolation based on the pixels obtained by search by the search unit  401  (Step S 404 ). 
     Thereafter, the distance-corresponding-pixel-value correction unit  404  corrects the pixel value of each of the pixels of the virtual image based on the imaging distance from the surface shape to the imaging viewpoint (Step S 405 ). Specifically, the pixel value is corrected so as to be increased with a decrease in the imaging distance obtained after the change in the imaging viewpoint, and so as to be decreased with an increase in the imaging distance obtained after the change in the imaging viewpoint. Accordingly, it is possible to generate an image with the changed imaging viewpoint, such as a virtual image W 2  illustrated in  FIG. 9  and a virtual image W 3  illustrated in  FIG. 10 . 
     Subsequently, if the pixel value of any of the pixels of the virtual image is lost (Yes at Step S 406 ), the lost pixel value interpolation unit  41  interpolates the pixel value of the subject pixel from pixels around the pixel whose pixel value is lost (Step S 407 ). After Step S 406 , the image processing apparatus  1  returns to the main routine in  FIG. 2 , and terminates the processing. In contrast, if the pixel value of any of the pixels of the virtual image is not lost (No at Step S 406 ), the image processing apparatus  1  returns to the main routine in  FIG. 2 , and terminates the processing. 
     According to the first embodiment, when a virtual image is generated for a case of imaging a target that appears in the intraluminal image from an imaging viewpoint that is different from an actual imaging viewpoint, and even if image distortion due to the characteristics of the imaging system of the endoscope occurs, even if a pixel value varies due to a change in the imaging distance, or even if occlusion (shielded portion) occurs due to a surface shape of the target, it is possible to generate a virtual image (learning sample) that appropriately reflects the above-described states inside a lumen. 
     First Modification 
     Next, a first modification of the first embodiment will be described. In the first modification of the first embodiment, a surface shape estimation unit has a different configuration and performs different processing. In the following, a configuration of the surface shape estimation unit according to the first modification of the first embodiment will be first described, and thereafter, processing performed by the surface shape estimation unit according to the first modification of the first embodiment will be described. The same components as those of the image processing apparatus  1  of the first embodiment described above will be denoted by the same reference signs, and explanation thereof will be omitted. 
       FIG. 11  is a block diagram illustrating the configuration of the surface shape estimation unit according to the first modification of the first embodiment. A surface shape estimation unit  10   a  illustrated in  FIG. 11  includes an imaging distance estimation unit  20   a  instead of the imaging distance estimation unit  20  of the first embodiment described above. 
     The imaging distance estimation unit  20   a  further includes a low spatial frequency component calculation unit  202  that calculates a low spatial frequency component for which a spatial frequency is low, in addition to the low absorption wavelength component selection unit  201 . 
     Next, surface shape estimation processing performed by the surface shape estimation unit  10   a  will be described.  FIG. 12  is a flowchart illustrating an outline of the surface shape estimation processing performed by the surface shape estimation unit  10   a . In the first modification of the first embodiment, processing other than the surface shape estimation processing performed by the surface shape estimation unit  10   a  is the same as the processing performed by the image processing apparatus  1  of the first embodiment described above (see  FIG. 2 ), and therefore, explanation thereof will be omitted. Further, in  FIG. 12 , Step S 501 , Step S 503 , and Step S 504  respectively correspond to Step S 201 , Step S 202 , and Step S 203  in  FIG. 4  described above, and therefore, explanation thereof will be omitted. 
     At Step S 502 , the low spatial frequency component calculation unit  202  calculates a low spatial frequency component for which a spatial frequency is low. 
     Specifically, a low spatial frequency component for which a spatial frequency is low is calculated using well-known smoothing processing or the like, and a noise component is eliminated. After Step S 502 , the image processing apparatus  1  proceeds to Step S 503 . 
     According to the first modification of the first embodiment as described above, it is possible to estimate a surface shape with reduced noise components, so that it is possible to generate a learning sample that appropriately reflects a state inside a lumen based on the estimated surface shape. 
     Second Modification 
     Next, a second modification of the first embodiment will be described. In the second modification of the first embodiment, an image generation unit has a different configuration and performs different processing. In the following, a configuration of the image generation unit according to the second modification of the first embodiment will be first described, and thereafter, processing performed by the image generation unit according to the second modification of the first embodiment will be described. The same components as those of the image processing apparatus  1  of the first embodiment described above will be denoted by the same reference signs, and explanation thereof will be omitted. 
       FIG. 13  is a block diagram illustrating a configuration of the image generation unit according to the second modification of the first embodiment. An image generation unit  12   a  illustrated in  FIG. 13  includes a pixel value estimation unit  40   a  instead of the pixel value estimation unit  40  of the first embodiment described above. 
     The pixel value estimation unit  40   a  includes a weight adding unit  406  that obtains a pixel value of each of the pixels of the virtual image by adding a weight, which corresponds to a distance between each of the pixels of the intraluminal image that is projected on the virtual image due to a change in the imaging viewpoint and each of the pixels of the virtual image, to the pixel value of each of the pixels of the intraluminal image, instead of the search unit  401  and the pixel value interpolation unit  402  of the first embodiment described above. Further, the weight adding unit  406  includes a weight adjustment unit  406   a  that adjusts the weight corresponding to the distance of each of the pixels of the virtual image, based on density information on the pixels of the intraluminal image projected on the virtual image. 
     Next, image generation processing performed by the image generation unit  12   a  will be described.  FIG. 14  is a flowchart illustrating an outline of the image generation processing performed by the image generation unit  12   a . In the second modification of the first embodiment, processing other than the image generation processing performed by the image generation unit  12   a  is the same as the processing performed by the image processing apparatus  1  of the first embodiment described above (see  FIG. 2 ), and therefore, explanation thereof will be omitted. Further, in  FIG. 14 , Step S 601 , Step S 603 , and Step S 605  respectively correspond to Step S 401 , Step S 403 , and Step S 405  in  FIG. 8  described above. 
     At Step S 602 , the weight adjustment unit  406   a  adjusts the weight used by the weight adding unit  406  based on the density information on the pixels of the original image around each of the pixels of the virtual image. The weight is set in accordance with a distance between each of the pixels of the intraluminal image (original image) projected on the virtual image and each of the pixels of the virtual image. More specifically, the weight is set so as to be increased with a decrease in the distance and so as to be decreased with an increase in the distance by use of a Gaussian function or the like. The weight adjustment unit  406   a  adjusts the width of the Gaussian function, i.e., the degree of change in the weight according to the distance. More specifically, the weight is adjusted such that the width of the Gaussian function is increased when the density of pixels of the original image around a target pixel of the virtual image for which the pixel value is to be obtained through weighted addition is low, and the width of the Gaussian function is decreased when the density is high. After Step S 602 , the image processing apparatus  1  proceeds to Step S 603 . 
     At Step S 604 , the weight adding unit  406  calculates the value of each of the pixels of the virtual image by performing addition to each of the pixels of the original image based on the distance between each of the pixels of the original image projected on the virtual image and each of the pixels of the virtual image and based on the weight adjusted by the weight adjustment unit  406   a . After Step S 604 , the image processing apparatus  1  proceeds to Step S 605 . 
     According to the second modification of the first embodiment as described above, it is possible to generate a learning sample that appropriately reflects a state inside a lumen. 
     Second Embodiment 
     Next, a second embodiment will be described. An image processing apparatus according to the second embodiment is different from the image processing apparatus  1  according to the first embodiment described above in that the imaging viewpoint changing unit  11  has a different configuration. In the following, a configuration of an imaging viewpoint changing unit according to the second embodiment will be described. The same components as those of the image processing apparatus  1  of the first embodiment described above will be denoted by the same reference signs, and explanation thereof will be omitted. 
       FIG. 15  is a block diagram illustrating a configuration of the imaging viewpoint changing unit according to the second embodiment. An imaging viewpoint changing unit  11   a  illustrated in  FIG. 15  further includes a change amount control unit  32 , in addition to the components of the imaging viewpoint changing unit  11  of the first embodiment described above. 
     The change amount control unit  32  controls an amount of change in the imaging viewpoint such that data loss does not occur in the virtual image generated by the image generation unit  12  described above. Specifically, the same processing as the processing performed by the pixel value estimation unit  40  described above is performed to control an amount of change in the relative angle to be changed by the relative angle changing unit  30  and an amount of change to be performed by the relative position changing unit  31  such that the shielded portion (data lost portion) in the original image does not appear in the virtual image. 
     According to the second embodiment as described above, the change amount control unit  32  controls an amount of change in the imaging viewpoint such that data loss does not occur in the virtual image generated by the image generation unit  12 . Therefore, it is possible to generate a learning sample that appropriately reflects a state inside a lumen. 
     Third Embodiment 
     Next, a third embodiment will be described. An image processing apparatus according to the third embodiment is different from the image processing apparatus  1  according to the first embodiment in that the arithmetic unit  7  has a different configuration. In the following, a configuration of an arithmetic unit according to the third embodiment will be described. The same components as those of the image processing apparatus  1  of the first embodiment described above will be denoted by the same reference signs, and explanation thereof will be omitted. 
       FIG. 16  is a block diagram illustrating a configuration of the arithmetic unit according to the third embodiment. An arithmetic unit  7   c  illustrated in  FIG. 16  further includes an image generation frequency control unit  13 , a learning unit  14 , and a recognition unit  15 , in addition to the components of the arithmetic unit  7  of the first embodiment described above. 
     The image generation frequency control unit  13  controls generation of a plurality of virtual images so as to increase the frequency of generation of a virtual image from an imaging viewpoint that tends to be used in actual imaging of an intraluminal image. Here, the imaging viewpoint that tends to be used in actual imaging of the intraluminal image is an imaging viewpoint at which a region from a mucosal surface that is located on the front side of a tract at a short imaging distance to a mucosal surface that is located in a deep part of the tract at a long imaging distance appears and at which a luminal wall appears in a lower part of the intraluminal image, for example. Specifically, the image generation frequency control unit  13  controls the imaging viewpoint changing unit  11  based on image information generated by the image generation unit  12 , and increases the frequency of generation of a virtual image from the imaging viewpoint that tends to be used in actual imaging of the intraluminal image. 
     The learning unit  14  learns a parameter used for recognition of the intraluminal image based on the virtual image. 
     The recognition unit  15  recognizes an intraluminal image based on the parameter learned by the learning unit  14 . 
     According to the third embodiment as described above, it is possible to generate a large number of learning samples from the imaging viewpoint that tends to be used in actual imaging of the intraluminal image. Therefore, it is possible to appropriately learn a parameter (for example, a recognition criterion for a color, a contour (edge), a pixel value surface shape (pixel value gradient), texture, or the like) of the recognition unit  15  using the learning sample that appropriately reflects the state. As a result, it is possible to improve the accuracy of the recognition unit  15  using the appropriately learned parameter. 
     OTHER EMBODIMENTS 
     The present disclosure may be realized by causing a computer system, such as a personal computer or a workstation, to execute an image processing program recorded in a recording apparatus. Further, the computer system may be used by being connected to other computer systems or other devices, such as servers, via a local area network (LAN), a wide area network (WAN), or a public line, such as the Internet. In this case, the image processing apparatus according to the first to third embodiments and the modifications may be configured to acquire image data of an intraluminal image via the above-described networks, output an image processing result to various output devices, such as a viewer or a printer, connected via the above-described networks, or store the image processing result in a storage device connected via the above-described networks, e.g., a recording medium that may be read by a reading device connected to the above-described networks, or the like. 
     The present disclosure is not limited to the first to third embodiments and the modifications. Variations may be made by appropriately combining a plurality of constituent elements disclosed in the embodiments and the modifications described above. For example, some constituent elements may be deleted from all of the constituent elements described in the embodiments and the modifications described above, or the constituent elements described in the embodiments and the modifications may be appropriately combined. 
     According to the present disclosure, it is possible to generate a learning sample that appropriately reflects a state inside a lumen. 
     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the disclosure in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.