Patent Publication Number: US-2009220141-A1

Title: Image processing apparatus, image processing method, and image processing program

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image processing apparatus that converts a luminance value of an image, by using a look-up table that indicates a corresponding relationship between a luminance value of an input image and a luminance value of an output image, an image processing method, and an image processing program. 
     2. Description of the Related Art 
     An image correction process that converts a luminance value of an image such as a photographed still image and a moving image has been widely performed in various fields. For example, if a part of an image taken by a digital still camera or a video camera is dark or difficult to see, it is possible to correct brightness and adjust contrast even at home, by scanning the image into a personal computer or the like. 
     The image correction is also carried out with respect to a medical image. For example, a medical image such as an image of subcutaneous veins captured using near infrared rays (see Japanese Patent Application Laid-open No. H8-510393), or an image of internal human body captured using X-rays is used to diagnose disease. However, because the images are of internal human body, it is not necessarily possible to obtain a clear and a user friendly image. Accordingly, the photographed image has been converted into a user friendly image. 
     A known method to correct such an image is a contrast adjusting method. Such methods that adjust the contrast in an image include a “contrast enhancement method”. More specifically, in the contrast enhancement method, the difference between a bright portion and a dark portion in an image is increased, by converting the luminance value and increasing brightness difference (see  FIG. 14 ). In an example of  FIG. 14 , Y indicates a luminance value of an input image, Y′ indicates a converted luminance value, C indicates a parameter of contrast enhancement, and the luminance value Y takes a value of 256 levels of 0 to 255. The contrast of an image is adjusted by appropriately adjusting C. 
     In the contrast enhancement method, a conversion equation shown in  FIG. 14  may be applied to all the pixels in an image. However, in a real process, it takes too long to calculate. Accordingly, a method of creating a conversion table called a look-up table (LUT) is known. 
     The LUT is a conversion table that indicates a corresponding relationship between a luminance value of an input image and a luminance value of an output image (see  FIG. 15 ). A corresponding luminance value is read out from the table with each pixel of the input image, and set to the corresponding pixel of the output image. The image is corrected by converting the luminance value. 
     In the contrast enhancement method (see  FIG. 14 ), all the luminance values in the image are equally enhanced, because the gradient of the whole LUT is changed. Accordingly, a so-called “white-out” condition may occur by exceeding the luminance value of  255 , thereby degrading the quality of image. The output luminance values in an area encircled by a dotted line in  FIG. 16  exceed the maximum luminance value of “255” and, as a result, will be converted into the maximum luminance value of 255. Accordingly, the information related to the luminance values is lost, making the image very difficult to see. Similarly, low luminance values are converted into the luminance value of 0. 
     Therefore, the contrast enhancement method is not usually used alone, but often used with, for example, a γ correction method that adjusts the brightness of image. The γ correction method is also widely used in image processing, and the luminance value Y of an image is converted, based on a conversion equation shown in  FIG. 17 . 
     In an image processing combined with the contrast enhancement method and the γ correction method, the two correction processes are appropriately adjusted. If the contrast adjustment and the γ correction are both carried out at the same time, as shown in  FIGS. 17 and 18 , the differential value of the differential function of the LUT is increased, simply with an increase of the luminance value. This means that the higher the luminance value, the higher the luminance resolution. 
     As an image correction method, a method of detecting information on spatial change (edge) in the luminance value of an image to be corrected, and using the detection result to correct the image is known (see Japanese Patent Application Laid-open No. H7-306938). More specifically, a histogram that indicates a relationship between the luminance value and the edge in the image is created, by searching the edge in the image. 
     The histogram, when a certain luminance value is in focus, shows to what extent an edge or a change in luminance value exists around the luminance value. For example, information such as in a certain image, there are many edges around a pixel with the luminance value of 100, but there is hardly any edge around a pixel with the luminance value of 200 can be obtained from the histogram. 
     By vertically inverting the histogram, a histogram in which the frequency is increased in the luminance value without an edge is generated. By integrating the obtained histogram, an LUT in which the gradient increases with the luminance value without an edge, and the gradient decreases with the luminance value with many edges is generated. 
     In the method that performs image processing by combining the contrast enhancement method and the γ correction method, the two correction processes need to be adjusted appropriately. Accordingly, the adjustment is very difficult. 
     In the technology that performs image processing by using the change in the luminance value, the conversion of the luminance value with respect to the input image is uniquely determined. Accordingly, the contrast cannot be adjusted at will, by a user or according to a purpose. As a result, a flexible image correction cannot be performed. Because the histogram needs to be built by searching the edge in the image, a long processing time is required. 
     SUMMARY 
     It is an object of the present invention to at least partially solve the problems in the conventional technology. 
     According to an aspect of the present invention, an image processing apparatus that converts a luminance value of an image, by using a look-up table that indicates a corresponding relationship between a luminance value of an input image and a luminance value of an output image, includes a setting receiving unit that receives a setting related to a peak height and a peak position of a differential function of the look-up table; a table generating unit that, based on the peak height and the peak position received by the setting receiving unit, defines the differential function of the look-up table, calculates a function by integrating the differential function, and generates the look-up table in a shape indicated by the function; and a luminance value converting unit that, by using the look-up table generated by the table generating unit, converts the luminance value of the image. 
     According to another aspect of the present invention, an image processing method that converts a luminance value of an image, by using a look-up table that indicates a corresponding relationship between a luminance value of an input image and a luminance value of an output image, includes receiving a setting related to a peak height and a peak position of a differential function of the look-up table; defining, based on the received peak height and the received peak position, the differential function of the look-up table; calculating a function by integrating the defined differential function; generating the look-up table in a shape indicated by the function; and converting, by using the generated look-up table, the luminance value of the image. 
     According to still another aspect of the present invention, a computer-readable recording medium stores therein a computer program that implements the above method on a computer. 
     The above and other objects, features, advantages and technical and industrial significance of this invention will be better understood by reading the following detailed description of presently preferred embodiments of the invention, when considered in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a blood vessel imaging device according to a first embodiment of the present invention; 
         FIG. 2  is a schematic of an example of displaying a blood vessel image in which the contrast is enhanced; 
         FIG. 3  is a schematic for explaining an LUT; 
         FIG. 4  is a schematic for explaining a differential function F′(x) of an LUT function F(x); 
         FIG. 5  is a schematic for explaining the LUT function F(x); 
         FIG. 6  is a schematic for explaining a conversion process of a luminance value of an image; 
         FIG. 7  is a flowchart showing an operation of an LUT creation process performed by the blood vessel imaging device according to the first embodiment; 
         FIG. 8  is a flowchart showing an operation of an image display process performed by the blood vessel imaging device according to the first embodiment; 
         FIG. 9  is a block diagram of a medical image display device according to a second embodiment of the present invention; 
         FIG. 10  is a schematic for explaining a differential function F′(x) of an LUT function F(x); 
         FIG. 11  is a schematic for explaining the LUT function F(x); 
         FIG. 12  is a schematic for explaining a contrast enhancement process in a specified area; 
         FIG. 13  is a schematic of a computer that executes an image processing program; 
         FIG. 14  is a schematic for explaining a conventional technology; 
         FIG. 15  is another schematic for explaining the conventional technology; 
         FIG. 16  is still another schematic for explaining the conventional technology; 
         FIG. 17  is still another schematic for explaining the conventional technology; and 
         FIG. 18  is still another schematic for explaining the conventional technology. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Exemplary embodiments of an image processing apparatus, an image processing method, and an image processing program according to the present invention are described below in greater detail with reference to the accompanying drawings. 
     In the following embodiment, a configuration and a processing flow of a blood vessel imaging device according to a first embodiment of the present invention are sequentially described, and advantages of the first embodiment are also described. In the following, the embodiment is applied to a blood vessel imaging device that takes and displays an image of bloods vessel as a medical image. Such a blood vessel imaging device is used for assisting medical practices such as injection, by taking and displaying an image of subcutaneous veins using near infrared rays. 
     With reference to  FIGS. 1 to 6 , a configuration of a blood vessel imaging device  10  will be explained.  FIG. 1  is a block diagram of the blood vessel imaging device  10  according to the first embodiment of the present invention.  FIG. 2  is a schematic of an example of displaying a blood vessel image in which the contrast is enhanced.  FIG. 3  is a schematic for explaining an LUT.  FIG. 4  is a schematic for explaining a differential function F′(x) of an LUT function F(x).  FIG. 5  is a schematic for explaining the LUT function F(x).  FIG. 6  is a schematic for explaining a conversion process of a luminance value of an image. 
     As shown in  FIG. 1 , the blood vessel imaging device  10  includes an adjustment dial  11 , an LCD  12 , a near infrared illuminator  13 , an imaging element  14 , a picked-up image memory  15 , an adjusted image memory  16 , an LUT  17 , a central processing unit  18 , an LUT generating unit  19 , and an image converting unit  20 . The processing performed by each of the units will now be explained. 
     The adjustment dial  11  receives a peak height and a peak position of a differential function F′(x) of an LUT function F(x) that indicates the shape of an LUT, as a set input value. More specifically, the adjustment dial  11  receives an input value for adjusting the intensity of contrast set by a user. With the adjustment dial  11 , it is possible that only the peak height of the differential function F′(x) may be adjusted, and the peak position (in other words, position in the x direction) may be set in advance. In the following example, only the peak height is adjusted. 
     To pick up an image of blood vessels, a subject to be photographed is mainly an arm, and the photographing will presumably take place in a lighting environment where the light of the lighting environment itself is emitted. Accordingly, the lighting is constant as long as there is no strong outside light such as sunlight. Therefore, a luminance value of the subject is more or less determined in advance, and for example, it is possible to set the peak position at the luminance value of about “200”, in advance. The luminance value of “200” corresponds to x=200/255=0.78, if “x” is a normalized luminance value. 
     If an LUT is generated so as the peak of the differential function F′(x) comes to the position of x=0.78, it is possible to improve luminance resolution around the luminance value of “200”, which is the corresponding luminance value. As a result, it is possible to improve the luminance resolution of the skin area and the blood vessel area, thereby improving the visibility of blood vessels. 
     The LCD  12  reads and displays an adjusted image in which the luminance value is converted by the image converting unit  20  and the contrast is enhanced, from the adjusted image memory  16 . For example, the LCD  12 , as shown in  FIG. 2 , displays a blood vessel image in which the contrast is enhanced. As shown in  FIG. 2 , the LCD  12  displays an image, in which the intensity (in other words, peak height of differential function F′(x)) of contrast is appropriately adjusted, based on the input value of the adjustment dial  11 . 
     The near infrared illuminator  13  emits a human body, which is a subject to be photographed, using near infrared rays. The near infrared rays emitted from the near infrared illuminator  13  penetrate into human body, while hemoglobin highly contained in blood absorbs the near infrared rays. Accordingly, in the image of human body to which near infrared rays are emitted, only the vein looks dark. Although the blood vessel looks dark in the image taken using near infrared rays, because the blood vessels are in the human body, if the contrast is not enhanced, the blood vessels are not displayed very clearly. In other words, a difference in luminance values between the portion of surrounding skin and the portion of blood vessels is small. 
     The imaging element  14 , on receiving an image pickup request from a user, picks up a subject emitted by the near infrared illuminator  13  through a lens, and stores in the picked-up image memory  15 . The picked-up image memory  15  stores therein an image picked up by the imaging element  14 . The adjusted image memory  16  stores therein an adjusted image in which the luminance value is converted by the image converting unit  20  and the contrast is enhanced. The picked-up image stored in the picked-up image memory  15  is an image picked up by the imaging element  14  without any change, and a difference in the luminance values between the blood vessel area and the other skin area is not so significant. 
     The LUT  17 , as shown in  FIG. 3 , is a conversion table of a corresponding relationship between the luminance value of an input image and the luminance value of an output image, and generated by the LUT generating unit  19 , which will be described later. The LUT  17  subtracts a corresponding luminance value from the table with each pixel of the input image, and sets to the corresponding pixel of the output image. An image is corrected by converting the luminance value. 
     The central processing unit  18  obtains an input value of the differential function F′(x) from the adjustment dial  11 , and notifies to the LUT generating unit  19 . More specifically, the central processing unit  18 , when the power is turned on, obtains an input value that indicates the peak height of the differential function F′(x) from the adjustment dial  11 . The central processing unit  18 , based on the input value, then sets standard deviations “σ” shown in  FIG. 4 , which will be described later, and notifies the LUT generating unit  19 . 
     A description will now be given by using a specific example. The central processing unit  18 , if a user maximally lowered the contrast by adjusting the adjustment dial  11 , is set to “σ=10.0”, and if a user maximally increased the contrast, is set to “σ=0.2”. A conversion equation may be used, or a conversion table may be created in advance, to determine “σ” based on the input value from the adjustment dial  11 . 
     The LUT generating unit  19 , based on the peak position information and the peak height information, defines the differential function F′(x), and calculates an LUT function F(x) that indicates the shape of the LUT by integrating the differential function F′(x). More specifically, the LUT generating unit  19 , based on the standard deviation “σ” received from the central processing unit  18  and an average value “Xc” of a normal distribution set in advance, defines the differential function F′(x). Then, the LUT generating unit  19  calculates an LUT function F(x) that indicates the shape of the LUT, by integrating the differential function F′(x). The LUT generating unit  19  generates an LUT, based on the LUT function F(x). 
     With reference to  FIG. 4 , the differential function F′(x) of the LUT function F(x) will now be explained. In  FIG. 4 , the differential function F′(x) is expressed using normal distribution. As shown in  FIG. 4 , the differential function F′(x) of the LUT function defined in the LUT generating unit  19  shows the resolution of the corresponding luminance. In other words, as shown in  FIG. 4 , the differential indicates the gradient of the corresponding function. Accordingly, a small difference in the input luminance is enhanced and output, at a portion with a large differential. On the other hand, the change in the input luminance is lowered and output, with the luminance value that has a small differential. 
     In an example of  FIG. 4 , the standard deviation “σ” corresponds to the adjustment parameter of contrast enhancement, when the average value (Xc) of the normal distribution is at the peak position, thereby showing various differential functions with respect to “σ”. “A” is a constant applied to the entire function, and uniquely determined by a normalizing condition, which will be shown below. 
     As shown in  FIG. 5 , the LUT function indicates a relational expression between the input luminance value and the output luminance value, and is shown as F(x). x is a normalized value of a luminance value Y(0-255). In other words, if the luminance value at a certain focus point is Y, it is x=(Y/255). x takes a value in a range between 0.0 to 1.0. By being normalized, it is possible to apply the similar formula, even if the range of the luminance value is outside 0-255. 
     By setting “F( 0 . 0 )= 0 . 0 ” and “F( 1 . 0 )= 1 . 0 ” as a normalizing condition, the LUT function F(x) is uniquely determined as shown in  FIG. 5 , by keeping the output luminance value in the range between “0.0” to “1.0”. However, the calculated value may be kept in a table in advance, instead of integrating. 
     In other words, with respect to the LUT function F(x) that indicates the shape of the LUT, an LUT is generated by defining the differential function F′(x) of F(x) at first, and integrating thereof. Accordingly, it is possible to adjust the luminance resolution around a predetermined luminance value. 
     For example, when the blood vessels are picked up and displayed, the luminance values of the skin and the vessel portion are approximately the same. Accordingly, as shown in  FIG. 6 , by generating the LUT  17  so as a corresponding luminance value (such as luminance value of “200”) comes to the peak position of the differential, a lowered luminance value due to the presence of blood vessels can be selectively enhanced. The left side in  FIG. 6  indicates a state of the section of the luminance value, when an image of human arm is picked up, and the horizontal axis indicates a position and the vertical axis indicates the corresponding luminance value. Around the center of the image is the center portion of the arm, and the luminance lowered due to the blood vessel located there is displayed by increasing the contrast. 
     If the degree of enhancement process is adjusted by the adjustment dial  11 , the peak position of the differential function F′(x) is fixed, and only the peak height changes. This means that the sensitivity of the output luminance value at the luminance value is being adjusted. 
     The image converting unit  20 , by using the LUT  17 , converts the luminance value of a picked-up image. More specifically, the image converting unit  20  reads out the picked-up image stored in the picked-up image memory  15 , converts the luminance value of the picked-up image being read by using the LUT  17 , and stores in the adjusted image memory  16 . 
     With reference to  FIGS. 7 and 8 , a process performed by the blood vessel imaging device  10  according to the first embodiment will be explained.  FIG. 7  is a flowchart showing an operation of an LUT creation process performed by the blood vessel imaging device according to the first embodiment.  FIG. 8  is a flowchart showing an operation of an image display process performed by the blood vessel imaging device according to the first embodiment. 
     As shown in  FIG. 7 , the blood vessel imaging device  10 , on receiving the peak height information of the differential function F′(x) from the adjustment dial  11  (Step S 101 ), defines the differential function F′(x), based on the peak height information and the peak position information of the differential function F′(x) (Step S 102 ). 
     The blood vessel imaging device  10  then calculates an LUT function F(x) that indicates the shape of the LUT, by integrating the defined differential function F′(x) (Step S 103 ). Then, the blood vessel imaging device  10  creates an LUT, based on the LUT function F(x) (Step S 104 ). 
     With reference to  FIG. 8 , an operation of an image display operation performed by the blood vessel imaging device  10  according to the first embodiment will be explained. As shown in  FIG. 8 , the blood vessel imaging device  10 , on receiving an image pickup request from a user (Step S 201 ), picks up an image of a subject emitted by the near infrared illuminator  13  through a lens, and stores in the picked-up image memory  15  (Step S 202 ). 
     The blood vessel imaging device  10  then reads out the picked-up image stored in the picked-up image memory  15 , converts the luminance value of the picked-up image being read using the LUT  17 , and stores in the adjusted image memory  16  (Step S 203 ). The blood vessel imaging device  10  then reads out the adjusted image in which the luminance value is converted and the contrast is enhanced, from the adjusted image memory  16 , and displays the image (Step S 204 ). 
     As described above, the blood vessel imaging device  10  can flexibly control the distribution, by using a method that focuses on the differential function of the LUT function, and sets the differential function into a predetermined shape. In other words, the luminance resolution (the change in output luminance value with respect to the change in input luminance value) is increased in the luminance value that includes the peak of the differential function. On the contrary, the luminance resolution is lowered in a range other than the peak. By defining the peak position (x direction) and the peak height (y direction) using a self-set function, it is possible to perform a flexible image correction process at ease. 
     In the blood vessel imaging device  10 , the peak height is adjusted while maintaining a certain peak position. Accordingly, it is possible to adjust contrast, while maintaining the luminance value with the highest luminance resolution. 
     In the blood vessel imaging device  10 , on determining the LUT function, a normalizing condition is set after the differential function is integrated. Under the condition, it is possible to suppress the number of white-out pixels, while increasing luminance resolution of the luminance value in focus. 
     In the first embodiment, when the image that picked up the blood vessels is processed by using the LUT is explained. However, the present invention is not limited to this, and an image formed by various images (such as medical image of subject emitted by visible light and ultraviolet rays) or by a plurality of planes can be processed using a plurality of LUTs. 
     In the following second embodiment, a configuration of a medical image display device  10   a  according to a second embodiment is explained with reference to  FIG. 9 , as when the embodiment is applied to a medical image display device that displays an image in which a plurality of images (planes) are combined.  FIG. 9  is a block diagram of a medical image display device according to the second embodiment. Descriptions that overlap with the first embodiment will be omitted. 
     As shown in  FIG. 9 , the medical image display device  10   a  is different from the blood vessel imaging device  10  shown in  FIG. 1 , by including an operating terminal  21 , an image recording unit  23 , a plurality of picked-up image memories  24   a  to  24   c , a plurality of adjusted image memories  25   a  to  25   c , a plurality of LUTs  26   a  to  26   c , and a combining unit  30 . In the following, an example of including three LUTs that are suitable for processing each medical image picked up by emitting near infrared rays, visible light, or ultraviolet rays will be explained. 
     The operating terminal  21  receives a request to pick up a medical image of a subject desired by a user, and among the medical images of the subject, receives which of the medical image picked up by emitting near-infrared rays, visible light, or ultraviolet rays is to be combined. 
     The operating terminal  21  inputs the peak height and the peak position of the differential function F′(x) of each image, picked by emitting near-infrared rays, visible light, or ultraviolet rays, as an input value. For example, the operating terminal  21  includes three dials that adjust the respective contrast intensity of the near infrared rays, the visible light, and the ultraviolet rays. Each dial that contains input values of “high” having a high contrast, “low” having a low contrast, and “off” that is not to be combined, is adjusted by a user. 
     The image recording unit  23  stores therein each medical image picked up by emitting near infrared rays, visible light, and ultraviolet rays. The image recording unit  23  stores therein the same subject, respectively, to overlap and combine each of the medical images. 
     The picked-up image memories  24   a  to  24   c , among the medical images stored in the image recording unit  23 , stores therein a medical image of the subject specified by the operating terminal  21  being the medical image to be combined. The picked-up image memory  24   a  stores therein a medical image picked by emitting near infrared rays, the picked-up image memory  24   b  stores therein a medical image picked up by emitting visible light, and the picked-up image memory  24   c  stores therein a medical image picked up by emitting ultraviolet rays. 
     The adjusted image memory  25   a  stores therein the medical image picked up by emitting near infrared rays in which the luminance value is converted by the LUT  26   a  for near infrared rays. The adjusted image memory  25   b  stores therein the medical image picked up by emitting visible light in which the luminance value is converted by an LUT  26   b  for visible light. The adjusted image memory  25   c  stores therein the medical image picked up by emitting ultraviolet rays in which the luminance value is converted by the LUT  26   c  for ultraviolet rays. 
     The LUT  26   a  for near infrared rays stores therein a conversion table that indicates a corresponding relationship between the luminance value of the input image and the luminance value of the output image, to convert the luminance value with respect to the near infrared ray image stored in the adjusted image memory  25   a . The LUT  26   b  for visible light stores therein a conversion table for converting the luminance value with respect to the visible light image stored in the adjusted image memory  25   b . The LUT  26   c  for ultraviolet rays stores therein a conversion table for converting the luminance value with respect to the ultraviolet ray image stored in the adjusted image memory  25   c.    
     The central processing unit  27  obtains each input value (in other words, each piece of peak position information and each piece of peak height information) for near infrared rays, visible light, and ultraviolet rays from the operating terminal  21 , and notifies to an LUT generating unit  28 . 
     The LUT generating unit  28  receives each input value from the central processing unit  27 , generates each of the LUTs for near infrared rays, visible light, and ultraviolet rays based on the input value, and stores them respectively in the LUT  26   a  for near infrared rays, the LUT  26   b  for visible light, and the LUT  26   c  for ultraviolet rays. For an image with which a notification that the image is not to be combined is input from the operating terminal  21 , no input value is notified from the central processing unit  18 , and no LUT is generated. 
     An image converting unit  29  converts the luminance value of each medical image stored in the picked-up image memories  24   a  to  24   c , by using each of the LUTs  26   a  to  26   c . More specifically, the image converting unit  29  reads out the near infrared image stored in the picked-up image memory  24   a , converts the luminance value of the picked-up image being read, by using the LUT  26   a  for near infrared rays, and stores in the adjusted image memory  25   a . The image converting unit  29  also reads out the visible light image stored in the picked-up image memory  24   b , converts the luminance value of the picked-up image being read, by using the LUT  26   b  for visible light, and stores in the adjusted image memory  25   b . The image converting unit  29  also reads out the ultraviolet ray image stored in the picked-up image memory  24   c , converts the luminance value of the picked-up image being read, by using the LUT  26   c  for ultraviolet rays, and stores in the adjusted image memory  25   c.    
     The combining unit  30  reads out each medical image in which the luminance value is converted, from the adjusted image memories  25   a  to  25   c , overlaps and combines the medical images, and displays by transmitting to an LCD  22 . 
     In this manner, a plurality of images is combined by converting the luminance value using the respective LUTs. Accordingly, it is possible to obtain an appropriate image according to a purpose. For example, to pick up images of oxygenated hemoglobin and reduced hemoglobin of which the most absorbing wavelengths are different, the different LUTs are used to correct the image of the oxygenated hemoglobin and the image of the reduced hemoglobin. Subsequently, it is possible to obtain an appropriate image according to a medical purpose, such as an image in which only the distribution of oxygenated hemoglobin is enhanced. 
     While the embodiments of the present invention have been described, it is to be understood that various other modifications may be made to the present invention. The other embodiments included in the present invention will now be described as a third embodiment. 
     In the blood vessel imaging device  10  according to the first embodiment, the peak position of the differential function F′(x) is set in advance. However, the present invention is not limited to this, and the peak position of the differential function F′(x) may be set automatically. For example, an average luminance of a predetermined area around the center of an image is calculated, and the luminance value is set as the peak position. The average luminance is calculated here, to reduce the impact caused by noise. 
     In this manner, the average luminance value of an area that interests a user is calculated in advance (for example, around the center of image), and the calculated luminance value is set as the peak position. Accordingly, it is possible to set the peak position automatically. 
     In the blood vessel imaging device  10  according to the first embodiment, one peak is set. However, the present invention is not limited to this, and a plurality of peaks may be set. For example, a medical X-ray image and the like may be broadly separated into a bright portion such as bones and a dark portion such as the other tissues. Accordingly, it is possible to set the peaks with respect to the portions that have different luminance values within the image. 
     More specifically, the blood vessel imaging device, as shown in  FIG. 10 , defines a differential function F′(x) with two peaks. The blood vessel imaging device, as shown in  FIG. 11 , calculates an LUT function F(x) that indicates the shape of an LUT, by integrating the defined differential function F′(x). Then, the blood vessel imaging device creates an LUT based on the LUT function F(x). 
     In this manner, the peaks are respectively set for the portions that have different luminance values within the image (such as bright portion and dark portion). Accordingly, it is possible to enhance the respective areas. 
     In the blood vessel imaging device  10  according to the first embodiment, the peak position is fixed. However, the present invention is not limited to this, and the peak position may be set at any position specified by a user. 
     For example, the blood vessel imaging device, as shown in  FIG. 12 , displays an X-ray image on a display device. A doctor specifies any position or an area within the image, thereby detecting the luminance value at the specified area. The detected luminance value is set as the peak position of the differential function F′(x). The area that a user desires to focus in the image is specified by using a touch pen, a mouse, and the like (in  FIG. 12 , an area enclosed by a dotted line). 
     In this manner, the luminance of any area specified by a user is set as the peak position. Accordingly, it is possible to display an image in which the contrast of the specified area is enhanced. 
     In the first embodiment, a monochrome image is applied thereto. However, the present invention is not limited to this, and an image formed of a plurality of planes such as a color image in which images of a plurality of wavelengths (such as wavelengths of red (R), green (G), and blue (B)) are overlapped, and a false-color image in which images taken by the wavelength different from that of R, G, and B are overlapped may be applied thereto. 
     As a specific application example, for example, when a plant absorbs light for photosynthesizing, the light with a short wavelength is absorbed more, while light of a near infrared ray area with a long wavelength is not absorbed much but reflected. As a result, on viewing a satellite image taken using near infrared rays, for example, an area with more plants looks bright. By overlapping the image taken using near infrared rays with an image of “red”, which is the most noticeable color to the human eye, it is possible to view the distribution of plant. 
     For a medical purpose, it is also possible to obtain information related to living body, by taking an image using a certain wavelength. For example, in an image taken by the wavelength most absorbed by oxygenated hemoglobin, the less the luminance value, the more oxygenated hemoglobin exists. Therefore, it is possible to visualize the distribution of oxygenated hemoglobin and reduced hemoglobin, for example, by converting the luminance values thereof using the respective LUTs. This is enabled by inverting the converted images (subtract from  255 ), and overlapping and combining the images while allocating red and blue to each of the images. 
     The respective constituents of the illustrated apparatuses are functionally conceptual, and need not necessarily be physically configured as illustrated. In other words, the specific mode of dispersion and integration of each apparatus is not limited to the ones shown in the drawings, and all or a part thereof can be functionally or physically dispersed or integrated in an optional unit, depending on various kinds of load and the status of use. For example, the central processing unit  18  and the LUT generating unit  19  may be integrated in the first embodiment. All or an optional part of the respective processing functions carried out in each apparatus are realized by a central processing unit (CPU) and a computer program analyzed and executed by the CPU, or may be realized as hardware by the wired logic. 
     With each process described in the present embodiments, all or a part of the processes as being described as automatically performed may be manually performed, or all or a part of the processes described as being manually performed may be automatically performed with a known method. The information including the process procedure, the control procedure, specific names, and various kinds of data and parameter shown in the specification or in the drawings can be optionally changed unless otherwise specified. 
     Various kinds of process described in the embodiments can be performed by executing a computer program prepared in advance using a computer. An example of a computer that executes a computer program having the similar function to that of the embodiments will now be explained, with reference to  FIG. 13 .  FIG. 13  is a schematic of a computer that executes an image processing program. 
     As shown in  FIG. 13 , a computer  600  as an image processing apparatus includes a hard disk drive (HDD)  610 , a random access memory (RAM)  620 , a read-only memory (ROM)  630 , and a CPU  640  connected by a bus  650 . 
     The ROM  630  stores therein an image processing program that performs the similar function to that of the embodiments. In other words, as shown in  FIG. 13 , the ROM  630  stores therein an LUT generating program  631  and an image converting program  632  in advance. With the programs  631  and  632 , it is possible to appropriately integrated or dispersed, similar to the respective constituents of the image processing apparatus shown in  FIG. 1 . 
     The CPU  640  reads out the programs  631  and  632  from the ROM  630  and executes. Accordingly, as shown in  FIG. 13 , each of the programs  631  and  632  functions as an LUT generating process  641  and an image converting process  642 . Each of the processes  641  and  642  respectively corresponds to the LUT generating unit  19  and the image converting unit  20 . 
     The RAM  620 , as shown in  FIG. 13 , stores therein an LUT  621  and image data  622 . Based on the stored LUT  621  and the image data  622 , the RAM  620  executes the image converting process shown in  FIG. 1 . 
     In the embodiments, the edge calculation and the like are not required, and the peak position and the peak height of the differential function can be set flexibly. Accordingly, it is possible to advantageously perform a flexible image correction at high speed, with a simple adjustment. 
     Although the invention has been described with respect to specific embodiments for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art that fairly fall within the basic teaching herein set forth.