Patent Publication Number: US-11047793-B2

Title: Signal processing apparatus, signal processing method, and progress

Description:
TECHNICAL FIELD 
     The present disclosure relates to a signal processing apparatus, a signal processing method, and a program. Particularly, the present disclosure relates to a signal processing apparatus, a signal processing method, and a program in which growth conditions of vegetation can be easily confirmed. 
     BACKGROUND ART 
     In the past, a normalized difference vegetation index (NDVI) has been used an index indicative of a distribution condition or degree of activity of plants. 
     In PTL 1, for example, an information processing system that calculates a growth index of crops has been disclosed on the basis of an NDVI image obtained from an RGB image and near-infrared ray image in which crops are captured. 
     CITATION LIST 
     Patent Literature 
     [PTL 1] 
     JP 2014-183788A 
     SUMMARY 
     Technical Problem 
     However, for example, in a case where an environmental condition is changed when performing an inspection of vegetation, an NDVI fluctuates. Therefore, in some cases, it is difficult to confirm growth conditions of vegetation in a conventional NDVI image. 
     The present disclosure has been made in a consideration of such a circumstance and is aimed at enabling the growth conditions of vegetation to be easily confirmed. 
     Solution to Problem 
     A signal processing apparatus according to one aspect of the present disclosure includes a relative value calculation section configured to calculate a relative value to an average of an index from the index indicative of a state of an inspection object, which is calculated on the basis of a sensing signal; and a display processing section configured to perform processing to allow an image indicative of the state of the inspection object on the basis of the relative value to be displayed. 
     A signal processing method or a program according to another aspect of the present disclosure includes the steps of: calculating a relative value to an average of a vegetation index from the vegetation index indicative of a state of an inspection object, which is calculated on the basis of a sensing signal; and performing processing to allow an image indicative of the state of the inspection object on the basis of the relative value to be displayed. 
     According to another aspect of the present disclosure, there is calculated a relative value to an average of a vegetation index from a state indicative of growth conditions of an inspection object, which is calculated on the basis of a sensing signal; and there is performed processing to allow an image indicative of the state of the inspection object on the basis of the relative value to be displayed. 
     Advantageous Effect of Invention 
     According to an aspect of the present disclosure, growth conditions of vegetation can be easily confirmed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram illustrating a configuration example of an embodiment of a vegetation inspection apparatus to which the present technology is applied. 
         FIG. 2  is a diagram illustrating an arrangement example of an IR (near-infrared) pixel. 
         FIG. 3  is a block diagram illustrating a configuration example of a signal processing block. 
         FIG. 4  is a flowchart describing signal processing performed in the signal processing block. 
         FIG. 5  is a diagram illustrating an example of a conventional NDVI image and that of a relative NDVI image. 
         FIG. 6  is a diagram illustrating an example in which the relative NDVI image is generated in each area of interest. 
         FIG. 7  is a block diagram illustrating a configuration example of a vegetation inspection system. 
         FIG. 8  is a block diagram illustrating a configuration example of the embodiment of a computer to which the present technology is applied. 
     
    
    
     DESCRIPTION OF EMBODIMENT 
     Hereinafter, a specific embodiment to which the present technology is applied will be described in detail with reference to the drawings. 
     &lt;Embodiment of Vegetation Inspection Apparatus&gt; 
       FIG. 1  is a block diagram illustrating a configuration example of an embodiment of a vegetation inspection apparatus to which the present technology is applied. 
     As illustrated in  FIG. 1 , a vegetation inspection apparatus  11  performs an inspection of grass as an inspection object in a state in which environmental light such as sunlight is radiated. Then, the vegetation inspection apparatus  11  displays an image (e.g., a relative NDVI image described below) indicative of growth conditions such as a grass state or a degree of activity on a display section  12 . 
     For example, the vegetation inspection apparatus  11  includes an optical system  21 , an aperture diaphragm  22 , a spectral sensor  23 , a signal processing block  24 , and a control block  25 . Further, the spectral sensor  23  has a spectroscope  31  and a sensing device  32 . 
     The optical system  21  has one or plural pieces of lenses. For example, the optical system  21  collects light incident to the vegetation inspection apparatus  11 , such as reflected light in which the environmental light is reflected on the grass. Further, the optical system  21  provides an image of a photographic subject on a detector plane of the sensing device  32  of the spectral sensor  23 . 
     The aperture diaphragm  22  controls amount of light collected into the spectral sensor  23  via the optical system  21 . Thereby, the aperture diaphragm  22  adjusts an exposure of an image acquired by the spectral sensor  23 . 
     The spectral sensor  23  detects components in plural different wavelength regions of the reflected light in which the environmental light is reflected on the grass. Specifically, the spectral sensor  23  separates the reflected light into light in plural wavelength regions by using the spectroscope  31 . Further, the spectral sensor  23  provides a detection signal in which brightness of light (spectrally split component) of respective wavelength regions is detected in each pixel of the sensing device  32  for the signal processing block  24 . 
     The spectroscope  31  has a configuration in which a plurality of optical filters that transmit light of predetermined wavelength regions are arranged in each pixel of the sensing device  32 . Then, spectroscope  31  separates light radiated on the detector plane of the sensing device  32  by using respective optical filters. Note that the above optical filter is referred to as a color filter that separates visible light into each color. 
     In  FIG. 1 , an example of the optical filter that is arranged as the spectroscope  31  is illustrated. For example, eight pixels such that the number of vertical pixels×the number of horizontal pixels is 2×4 is defined as a measurement unit. Further, eight kinds of optical filters that transmit light in respective different wavelength regions are arranged in a manner corresponding to each pixel that configures the measurement unit. Specifically, in the order corresponding to a short wavelength in a manner corresponding to eight pixels of the measurement unit, an optical filter B 1  that transmits first blue light, an optical filter B 2  that transmits second blue light, an optical filter G 1  that transmits first green light, an optical filter G 2  that transmits second green light, an optical filter R 1  that transmits first red light, an optical filter R 2  that transmits second red light, an optical filter IR 1  that transmits first infrared light, and an optical filter IR 2  that transmits second infrared light are arranged. 
     While using as one measurement unit the above optical filter of eight pixels, the spectroscope  31  has a configuration in which the optical filter for n measurement units (n is a natural number equal to or greater than 1) is continuously arranged in the entire plane of the detector plane of the sensing device  32 . Note that the measurement unit of the optical filter is not limited to a configuration in which eight pixels are defined as one measurement unit. In the measurement unit of the optical filter, other modes such as a configuration in which four pixels (R, G, B, IR) are defined as one measurement unit can be adopted. 
     For example, in a configuration that is a so-called Bayer arrangement and in which an R pixel, a G pixel, and a B pixel are arranged, the optical filter of an arrangement example in which a portion of pixels are replaced with IR pixels may be used. Specifically, in the G pixel in which the number of pixels is plentiful in the Bayer arrangement, as illustrated in  FIG. 2A , the optical filter in which the G pixels arranged in a row of the R pixels are replaced with the IR pixels can be used. Further, the optical filter in which the B pixel in which visibility is low is replaced with the IR pixel can be used. In this case, as illustrated in  FIG. 2B , a configuration in which all the B pixels are not replaced with the IR pixels but a portion of B pixels are replaced with the IR pixels can be used. 
     The sensing device  32  is, for example, an image pickup device having a configuration in which a plurality of pixels are arranged in a matrix pattern in the detector plane. Further, the sensing device  32  detects brightness of the spectrally split component spectrally split by each optical filter of the spectroscope  31  in each pixel. Further, the sensing device  32  outputs the detection signal (sensing signal) in accordance with the brightness of each spectrally split component. 
     Note that, in addition to an area sensor that gets an object by a plane, as the sensing device  32 , a line sensor that gets the object by a line can be used. Further, even in the case where the R pixel and the IR pixel are arranged only one by one in the sensing device  32 , a mechanism for moving a sensor or a measurement object is provided to thereby scan the object. 
     The signal processing block  24  performs signal processing on the detection signal (that is, a sensing image sensed by the sensing device  32 ) output from the spectral sensor  23 . Thereby, the signal processing block  24  generates an image indicative of a result in which the growth conditions of the grass are inspected and displays the image on the display section  12 . Note that a detailed configuration of the signal processing block  24  will be described below with reference to  FIG. 3 . 
     The control block  25  performs control for each block that configures the vegetation inspection apparatus  11  such as the sensing device  32  and the signal processing block  24  so that the growth conditions of the grass can be preferably inspected in the vegetation inspection apparatus  11 . 
     The vegetation inspection apparatus  11  configured as described above can inspect the growth conditions of the grass by using a normalized difference vegetation index NDVI that is numerically indicative of the growth conditions. Further, the vegetation inspection apparatus  11  can acquire an NDVI image constructed by the normalized difference vegetation index NDVI as a result in which the growth conditions of the grass are inspected. The normalized difference vegetation index NDVI is obtained by calculating the following formula (1) by using a pixel value R of a pixel in which red light is detected in the sensing device  32  and a pixel value IR of a pixel in which near-infrared light is detected. 
     
       
         
           
             
               
                 
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                     ⁢ 
                     1 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                       
                   
                   ⁢ 
                   
                     NDVI 
                     = 
                     
                       
                         IR 
                         - 
                         R 
                       
                       
                         IR 
                         + 
                         R 
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     Here, the normalized difference vegetation index NDVI is used as an index of growth of stems and leaves. Note that a reflectance (pixel value IR) of the near-infrared light and a reflectance (pixel value R) of the red light are calculated by obtaining as an incident light intensity a red light intensity and near-infrared ray intensity in an area such as sky and by obtaining as a reflected light intensity the red light intensity and near-infrared ray intensity in an object area in an RGB image and a near-infrared ray image in an area that is not the object area. Further, in the reflectance of the near-infrared light and that of the red light, the incident light intensity may be measured by using a diffusion plate having a known reflectance as a reference. Further, a reflection coefficient may be calculated on the basis of a ratio between the incident light intensity and reflection luminance of an object and then the reflectance of the near-infrared light and that of the red light may be obtained by converting the reflection coefficient into a reflectance. Further, the vegetation inspection apparatus  11  calculates the NDVI image by using an average, dispersion, high-order dispersion, or the like of NDVI only in the object area. Through the process, the NDVI image is calculated from only information obtained from pixels in the object area. Thereby, the NDVI image can be calculated with higher accuracy. 
     Incidentally, the vegetation inspection apparatus  11  performs an inspection of the grass in an outdoor environment. Thereby, for example, the detection signal detected by the spectral sensor  23  may fluctuate, for example, in accordance with a change in environmental conditions such as a position of sun, weather (fine/cloudy), or a direction of grass grain. In the result, the NDVI image is generated by using the detection signal output from the spectral sensor  23  directly. In such a case, it is assumed that, in some cases, it is difficult to determine the growth conditions of the grass by an influence in which the fluctuation of the detection signal is given to the NDVI image. 
     To solve the above problem, in the vegetation inspection apparatus  11 , the signal processing is performed on the detection signal by the signal processing block  24  so that the influence given to the NDVI image is suppressed by the fluctuation in the detection signal due to the change in the environmental conditions. The process permits the growth conditions of vegetation to be easily confirmed. For example, it is possible to easily make a distinction between a location in which the grass is preferably grown or a location in which the grass is not preferably grown. 
       FIG. 3  is a block diagram illustrating a configuration example of the signal processing block  24 . 
     As illustrated in  FIG. 3 , the signal processing block  24  includes an NDVI average calculation section  41 , a correlation coefficient calculation section  42 , an NDVI relative value calculation section  43 , and a display processing section  44 . 
     By using the detection signal acquired by actually sensing the grass by the sensing device  32  illustrated in  FIG. 1 , the NDVI average calculation section  41  calculates an NDVI average Na obtained by averaging the normalized difference vegetation index NDVI of the entire grass and provides the NDVI average Na for the correlation coefficient calculation section  42 . 
     For example, the NDVI average calculation section  41  calculates an average Ra of all the pixel values R of the red light detected in a grass area in the pixel value R of the red light included in the detection signal output from the spectral sensor  23 . Similarly, the NDVI average calculation section  41  calculates an average IRa of all the pixel values IR of the near-infrared light detected in the grass area in the pixel value IR of the near-infrared light included in the detection signal output from the spectral sensor  23 . Further, the NDVI average calculation section  41  can obtain the NDVI average Na to the entire grass from the average Ra of the red light and the average IRa of the near-infrared light on the basis of the following formula (2). 
     
       
         
           
             
               
                 
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                     Math 
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                     ⁢ 
                     2 
                   
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                   ⁢ 
                   
                     Na 
                     = 
                     
                       
                         IRa 
                         - 
                         Ra 
                       
                       
                         IRa 
                         + 
                         Ra 
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     The correlation coefficient calculation section  42  calculates a correlation coefficient α by which the NDVI average Na obtained by the NDVI average calculation section  41  is matched with a predetermined NDVI specified value Nd and provides the correlation coefficient α for the NDVI relative value calculation section  43 . Here, it is assumed that in the vegetation inspection apparatus  11 , an average of the normalized difference vegetation indexes NDVI to the entire grass is the NDVI specified value Nd determined to be previously a specified value. Further, as represented in the following formula (3), the NDVI specified value Nd is represented by using the average Ra of the red light, the average IRa of the near-infrared light, and the correlation coefficient α. 
     
       
         
           
             
               
                 
                   [ 
                   
                     Math 
                     . 
                     
                         
                     
                     ⁢ 
                     3 
                   
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                       
                   
                   ⁢ 
                   
                     Nd 
                     = 
                     
                       
                         IRa 
                         - 
                         
                           α 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           Ra 
                         
                       
                       
                         IRa 
                         + 
                         
                           α 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           Ra 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     Further, from formula (3), the correlation coefficient α that allows the average of the normalized difference vegetation indexes NDVI that is originally the NDVI average Na to be matched with the NDVI specified value Nd is represented by the following formula (4). 
     
       
         
           
             
               
                 
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                     ⁢ 
                     4 
                   
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                   ⁢ 
                   
                     α 
                     = 
                     
                       
                         IRa 
                         ⁡ 
                         
                           ( 
                           
                             1 
                             - 
                             Nd 
                           
                           ) 
                         
                       
                       
                         Ra 
                         ⁡ 
                         
                           ( 
                           
                             1 
                             + 
                             Nd 
                           
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     In each measurement unit of the detection signal acquired by actually sensing the grass by the sensing device  32  illustrated in  FIG. 1  (for example, on the basis of the sensing signal for each pixel and not the entire NDVI image), the NDVI relative value calculation section  43  applies the correlation coefficient α calculated by the correlation coefficient calculation section  42  and calculates an NDVI relative value Nr relative to the NDVI average Na. Note that the NDVI average Na is used in order to describe the correlation coefficient α and is not required for an operation to obtain the NDVI relative value Nr. Specifically, as represented in the following formula (5), the NDVI relative value calculation section  43  can obtain the NDVI relative value Nr (x, y) on the basis of the pixel value R (x, y) of the red light and pixel value IR (x, y) of the infrared light for each measurement unit (x, y) of the sensing device  32 . 
     
       
         
           
             
               
                 
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                   ⁢ 
                   
                     
                       Nr 
                       ⁡ 
                       
                         ( 
                         
                           x 
                           , 
                           y 
                         
                         ) 
                       
                     
                     = 
                     
                       
                         
                           IR 
                           ⁡ 
                           
                             ( 
                             
                               x 
                               , 
                               y 
                             
                             ) 
                           
                         
                         - 
                         
                           α 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             R 
                             ⁡ 
                             
                               ( 
                               
                                 x 
                                 , 
                                 y 
                               
                               ) 
                             
                           
                         
                       
                       
                         
                           IR 
                           ⁡ 
                           
                             ( 
                             
                               x 
                               , 
                               y 
                             
                             ) 
                           
                         
                         + 
                         
                           α 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             R 
                             ⁡ 
                             
                               ( 
                               
                                 x 
                                 , 
                                 y 
                               
                               ) 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
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                   5 
                   ) 
                 
               
             
           
         
       
     
     The display processing section  44  generates the NDVI image based on the NDVI relative value Nr calculated by the NDVI relative value calculation section  43  and performs display processing in which the NDVI image is displayed on the display section  12 . For example, the display processing section  44  maps a color set in accordance with the NDVI relative value Nr (x, y) in each measurement unit (x, y) like a heat map to thereby generate the NDVI image. Note that, as described above, the NDVI relative value Nr (x, y) is relative to the NDVI average Na. Hereinafter, the NDVI image generated from the NDVI relative value Nr (x, y) is appropriately referred to as the relative NDVI image. 
     As described above, the vegetation inspection apparatus  11  is configured and the relative NDVI image indicative of a result in which the growth conditions of the grass are inspected is displayed on the display section  12 . 
     Further, in the vegetation inspection apparatus  11 , for example, the NDVI specified value Nd can be set (for example, set to 0.5 to 0.7 when the inspection object is the grass) in a portion in which a change in the normalized difference vegetation index NDVI is large. Thereby, the NDVI relative value Nr is calculated by using the NDVI specified value Nd as a reference. Therefore, the relative NDVI image in which it is easy to confirm a portion in which a change in the normalized difference vegetation index NDVI is large, that is, a portion in which a change in the growth conditions of the grass is large can be generated. The above relative NDVI image is generated, and thereby it is possible for the vegetation inspection apparatus  11  to confirm the growth condition of the grass more easily than a conventional NDVI image in which the normalized difference vegetation index NDVI is directly used. 
     Further, the vegetation inspection apparatus  11  can realize quantization and visualization by using the NDVI relative value Nr for the growth conditions of the grass in a level that is incapable of being visually confirmed in the conventional NDVI image. That is, the vegetation inspection apparatus  11  matches the NDVI average Na with the NDVI specified value Nd (to be always constant), and thereby subjects the detection signal output from the sensing device  32  to a signal normalization. Thereby, the vegetation inspection apparatus  11  can suppress an influence that is given to the detection signal due to a change in the environmental conditions and generate the relative NDVI image in which it is easy to confirm the growth conditions of the grass. 
     &lt;Regarding Signal Processing&gt; 
     Next, the signal processing that is performed in the signal processing block  24  will be described with reference to a flowchart illustrated in  FIG. 4 . 
     In step S 11 , for example, the detection signal for a piece of relative NDVI image is provided from the sensing device  32  to the signal processing block  24 . Then, the NDVI average calculation section  41  acquires the detection signal. In step S 12 , the NDVI average calculation section  41  obtains the average Ra of the red light and the average IRa of the near-infrared light from the detection signal provided from the sensing device  32 . Further, the NDVI average calculation section  41  calculates the NDVI average Na on the basis of formula (2) described above. 
     In step S 13 , the correlation coefficient calculation section  42  calculates the correlation coefficient α that matches the NDVI average Na obtained by the NDVI average calculation section  41  in step S 11  with the NDVI specified value Nd on the basis of formula (4) described above. 
     In step S 14 , the NDVI relative value calculation section  43  applies the correlation coefficient α obtained by the correlation coefficient calculation section  42  in step S 12  and calculates the NDVI relative value Nr on the basis of formula (5) described above in each measurement unit of the detection signal provided from the sensing device  32 . 
     In step S 15 , the display processing section  44  generates the relative NDVI image on the basis of the NDVI relative value Nr obtained in each measurement unit by the NDVI relative value calculation section  43  in step S 13  and performs display processing in which the relative NDVI image is displayed on the display section  12 . 
     After the process of step S 15 , the signal processing in the signal processing block  24  is completed. 
     As described above, the signal processing block  24  can suppress an influence that is given to the detection signal due to a change in the environmental conditions and generate the relative NDVI image in which the growth conditions of the grass are confirmed more easily than the conventional NDVI image. 
     In  FIG. 5 , for example, an example of the conventional NDVI image and an example of the relative NDVI image are illustrated. 
     On the upper side of  FIG. 5 , the conventional NDVI image is illustrated, and on the lower side of  FIG. 5 , the relative NDVI image is illustrated. Both the conventional NDVI image and the relative NDVI image illustrated in  FIG. 5  are generated by using the same detection signal. 
     For example, the vegetation inspection apparatus  11  may display only the relative NDVI image on the display section  12 . Alternatively, the vegetation inspection apparatus  11  may simultaneously display the conventional NDVI image and the relative NDVI image on the display section  12  while disposed to thereby compare them. Further, the vegetation inspection apparatus  11  may switch a display between the conventional NDVI image and the relative NDVI image through the user operation. Further, the conventional NDVI image and the relative NDVI image are not limited to a simultaneous display of them or a switching display of them. For example, the vegetation inspection apparatus  11  can display a live image photographed by a normal image pickup apparatus, other images, or the like simultaneously or while switching. 
     The vegetation inspection apparatus  11  calculates the normalized difference vegetation index NDVI from an image obtained through sensing. Thereby, the vegetation inspection apparatus  11  can display a distribution or degree of activity of the grass as the heat map as illustrated in the upper side of  FIG. 5 . As described above, the vegetation inspection apparatus  11  further obtains the NDVI relative value Nr so as to be relative to the NDVI average Na. Thereby, as illustrated in the lower side of  FIG. 5 , the vegetation inspection apparatus  11  can generate the relative NDVI image that emphatically indicates a portion in which the growth conditions of the grass are satisfactory and a portion in which the growth conditions of the grass are unsatisfactory. For example, the vegetation inspection apparatus  11  appropriately sets the NDVI specified value Nd to thereby generate the relative NDVI image such that the portion in which the growth conditions of the grass are unsatisfactory is noticeable. 
     Here, with regard to a color display of the heat map, as a highlight of the satisfactory portion and unsatisfactory portion of the growth conditions of the grass, for example, the satisfactory portion of the growth conditions of the grass can be displayed so as to have a blue color. The unsatisfactory portion of the growth conditions of the grass can be displayed so as to have a red color. Further, in accordance with a predetermined standard, the color display may be performed only on the satisfactory portion or unsatisfactory portion of the growth conditions of the grass. Note that the highlight is not limited to the color display, and further shading, luminance, or saturation of an image may be changed to thereby present the highlight. A degree of the display modes can be switched through setting or operations of the user. Particularly, the user changes the NDVI specified value Nd or multiplies the NDVI specified value Nd by the correlation coefficient α to thereby change how to highlight the unsatisfactory portion of the growth conditions of the grass. 
     Incidentally, in  FIG. 5 , the relative NDVI image is generated over the entire grass. Further, the vegetation inspection apparatus  11  may generate the relative NDVI image in each specified area of interest while paying attention to the specified area. 
     As illustrated in  FIG. 6A , for example, the vegetation inspection apparatus  11  can specify a sunny area (area without hatching) in which sunlight is radiated on the grass and a shadow area (area with hatching) in which sunlight is not radiated on the grass. Further, the vegetation inspection apparatus  11  can individually generate the respective relative NDVI images of the sunny area and the shadow area. 
     Specifically, from the entire grass sensed by the spectral sensor  23 , the vegetation inspection apparatus  11  can set the area of interest on the basis of the brightness and allow the signal processing block  24  to perform the signal processing in each area of interest. In this case, the NDVI average calculation section  41  calculates the NDVI average Na concerning the area of interest of the grass. The correlation coefficient calculation section  42  calculates the correlation coefficient α concerning the area of interest of the grass. Further, the NDVI relative value calculation section  43  can calculate the NDVI relative value Nr relative to the NDVI average Na in each area of interest of the grass. 
     Through the process, the vegetation inspection apparatus  11  can inspect the growth conditions of the grass more particularly in each area of interest, for example, in each of the sunny area and the shadow area. Note that specification of the area of interest is not limited to the sunny area and the shadow area. An arbitrary area can be specified, for example, a half of a tournament-quality court is specified, and the like. 
     Further, as illustrated in  FIG. 6B , the vegetation inspection apparatus  11  can distinguish the grass area (area without hatching) and an area other than the grass (area with hatching) in the entire image. Further, the vegetation inspection apparatus  11  can specify only the grass area as the area of interest and generate the relative NDVI image of the grass area. At this time, the specification of the grass area can be performed on the basis of a normal image recognition or on the basis of the normalized difference vegetation index NDVI. As described above, the vegetation inspection apparatus  11  can automatically determine and specify the area of interest. In addition, the vegetation inspection apparatus  11  can specify an arbitrary area in the entire image as the area of interest, for example, in accordance with setting of the user. 
     Note that, in the present embodiment, descriptions will be made with reference to the normalized difference vegetation index NDVI. Further, in the vegetation inspection apparatus  11 , the vegetation index (for example, an RVI (Ratio Vegetation Index), a GNDVI (Green NDVI), and the like) other than the normalized difference vegetation index NDVI may be used. Further, the vegetation inspection apparatus  11  can cover a forest, agricultural crops, and the like in addition to the grass as described above as an inspection object to be inspected on the growth conditions. 
     As other vegetation indexes, for example, the ratio vegetation index (RVI) calculated by performing an operation of the following formula (6), a difference vegetation index (DVI) calculated by performing an operation of the following formula (7), or the like can be used. 
     
       
         
           
             
               
                 
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                     6 
                   
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                   ⁢ 
                   
                     RVI 
                     = 
                     
                       IR 
                       R 
                     
                   
                 
               
               
                 
                   ( 
                   6 
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                   [ 
                   
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                     ⁢ 
                     7 
                   
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                     DVI 
                     = 
                     
                       IR 
                       - 
                       R 
                     
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
     Note, however, that, in formulas (6) and (7), IR represents a reflectance (pixel value of a pixel that detects the near-infrared light) of a near-infrared area and R represents a reflectance (pixel value of a pixel that detects the red light) of red in a visible area. Note that, here, only the vegetation index in which IR and R are used as a parameter is exemplified. Further, it is as a matter of course possible to measure other vegetation indexes by using, as a parameter, reflectances, etc. of other light other than red in the visible area. In addition, a spectral ratio is not limited to a combination of R and IR. 
     Further, the present technology can be applied to a vegetation inspection system connected through a network, for example, in addition to an apparatus that is configured by an apparatus alone like the vegetation inspection apparatus  11 . 
     As illustrated in  FIG. 7 , for example, a vegetation inspection system  51  has a configuration in which an image pickup apparatus  52  and a signal processing apparatus  53  for vegetation inspection are connected through a network  54 . 
     The image pickup apparatus  52  includes the spectral sensor  23  illustrated in  FIG. 1 . The image pickup apparatus  52  detects components in a plurality of different wavelength regions regarding reflected light in which the environmental light is reflected on the grass. Further, the image pickup apparatus  52  transmits a detection signal indicative of detection results to the signal processing apparatus  53  through the network  54 . 
     The signal processing apparatus  53  has the function similar to that of the signal processing block  24  illustrated in  FIG. 1 . The signal processing apparatus  53  receives the detection signal that is transmitted through the network  54  from the image pickup apparatus  52  for vegetation inspection. Further, the signal processing apparatus  53  performs the signal processing in which the relative NDVI image as described above is generated. Then, the signal processing apparatus  53  accumulates the relative NDVI images in an accumulation apparatus (not illustrated) connected to the network  54  to thereby observe a change in the growth conditions of the grass. 
     As described above, according to the present technology, the grass in a remote location can be inspected through the network  54  and a manager of the grass can perform a management of the grass everywhere. Although not illustrated in the figure, a plurality of the image pickup apparatuses  52  are connected to the network  54 , and thereby the manager can one-dimensionally manage the grass in a plurality of locations. Further, the manager can observe the growth conditions of the grass by using a multi-camera in which the plurality of the image pickup apparatuses  52  link up with one another. Further, the manager can observe the growth conditions of the grass while moving by using a UAV (Unmanned Aerial Vehicle) such as a so-called drone. 
     Note that, in the present embodiment, a vegetation index based on reflection of light by plants is described by using plants such as the grass as the inspection object. According to the present technology, an object other than plants may be used as the inspection object. The present technology can be applied to an inspection of various inspection objects by using indexes other than the vegetation index. Further, the vegetation index is an index that indicates the growth conditions of plants, and additionally, for example, can be used to make a contribution indirectly to the growth conditions and grasp plant conditions even if conditions are not directly regarded as the growth conditions like conditions of photosynthesis or the like. 
     Note that each processing described with reference to the flowchart described above need not always be sequentially executed in time series described in the flowchart. Thus, the processing includes the processes that are executed in parallel or discretely (parallel processes or a process based on objects, for example). Further, a program may be processed by a single CPU or by a plurality of CPUs in a distributed manner. Further, in the present specification, a system represents the entire apparatus configured by a plurality of apparatuses. 
     The above series of processing (signal processing method) may be executed by hardware or may be executed by software. When the series of processing is executed by software, programs constituting the software are installed to the computer from a program recording medium on which programs are recorded. In this case, the computer includes a computer embedded into dedicated hardware and a general-purpose personal computer capable of executing various kinds of functions by installing various kinds of programs. 
       FIG. 8  is a block diagram illustrating a configuration example of hardware of a computer which executes the series of processes described above under programs. 
     In the computer, a CPU (Central Processing Unit)  101 , a ROM (Read Only Memory)  102 , and a RAM (Random Access Memory)  103  are interconnected via a bus  104 . 
     An input/output interface  105  is further connected to the bus  104 . An input section  106 , an output section  107 , a storage section  108 , a communication section  109 , and a drive  110  are connected to the input/output interface  105 . The input section  106  is constituted by a keyboard, a mouse, a microphone, or the like. The output section  107  is constituted by a display, a speaker, or the like. The storage section  108  is constituted by a hard disk, a non-volatile memory, or the like. The communication section  109  is constituted by a network interface or the like. The drive  110  drives a removable medium  111  such as a magnetic disk, an optical disk, a magneto optical disk, or a semiconductor memory. 
     In the computer configured as described above, the CPU  101  loads, on the RAM  103 , a program stored in the storage section  108  through the input/output interface  105  and the bus  104  and executes the program to thereby execute the series of processes, for example. 
     The program executed by the computer (CPU  101 ) may be recorded, for example, in the removable medium  111  that is a package medium including a magnetic disk (including a flexible disk), an optical disk (CD-ROM (Compact Disc-Read Only Memory), DVD (Digital Versatile Disc), or the like), a magneto optical disk, a semiconductor memory, or the like and provided. Further, alternatively, the program may be provided via a wired or wireless transmission medium such as a local area network, the Internet, or digital satellite broadcasting. 
     In the computer, the program can be installed into the storage section  108  via the input/output interface  105  when the removable medium  111  is mounted on the drive  110 . Also, the program may be received by the communication section  109  via a wired or wireless transmission medium and be installed into the storage section  108 . In addition, the program may be installed beforehand into the ROM  102  or the storage section  108 . 
     Note that the present technology can also take the following configuration. 
     (1) 
     A signal processing apparatus including: 
     a relative value calculation section configured to calculate a relative value to an average of an index from the index indicative of a state of an inspection object, which is calculated on a basis of a sensing signal; and a display processing section configured to perform processing to allow an image indicative of the state of the inspection object on a basis of the relative value to be displayed. 
     (2) 
     The signal processing apparatus according to (1) above, further including: 
     an average calculation section configured to calculate the average of the index, which is obtained by averaging the index in an entire inspection object; and 
     a correlation coefficient calculation section configured to calculate a correlation coefficient that matches the average of the index calculated by the average calculation section with a predetermined specified value, in which 
     the relative value calculation section applies the correlation coefficient and calculates the relative value of the index relative to the average of the index in each measurement unit in which a measurement is performed on the inspection object. 
     (3) 
     The signal processing apparatus according to (2) above, in which 
     the sensing signal includes at least a detection value according to brightness of near-infrared light and red light, and 
     the relative value calculation section divides a value obtained by subtracting a value obtained by multiplying the detection value of the red light by the correlation coefficient from the detection value of the near-infrared light by a value obtained by adding a value obtained by multiplying the detection value of the red light by the correlation coefficient to the detection value of the near-infrared light to thereby calculate the relative value. 
     (4) 
     The signal processing apparatus according to any one of (1) to (3) above, further including: 
     a spectral sensor configured to detect components in a plurality of different wavelength regions regarding reflected light in which environmental light is reflected by the inspection object. 
     (5) 
     The signal processing apparatus according to (4) above, in which 
     the spectral sensor has a sensing device in which a plurality of pixels are arranged in a detector plane in a matrix pattern and a spectroscope in which a plurality of optical filters that transmit light in a predetermined wavelength region are arranged in each pixel of the sensing device. 
     (6) 
     The signal processing apparatus according to any one of (2) to (5) above, in which 
     the display processing section generates an image indicative of the state of the inspection object and allows a display section to display the image on a basis of the relative value of the index calculated in each measurement unit by the relative value calculation section. 
     (7) 
     The signal processing apparatus according to any one of (2) to (6) above, in which 
     the average calculation section specifies a predetermined area of the inspection object projected on a sensing image including the sensing signal as an area of interest and calculates the average of the index in the area of interest. 
     (8) 
     The signal processing apparatus according to (7) above, in which 
     the correlation coefficient calculation section obtains the correlation coefficient regarding the area of interest, and 
     the relative value calculation section calculates the relative value of the index in the area of interest. 
     (9) 
     The signal processing apparatus according to (7) above, in which 
     a sunny area in which sunlight is radiated on the inspection object and a shadow area in which sunlight is not radiated on the inspection object are specified as the area of interest. 
     (10) 
     The signal processing apparatus according to (7) above, in which 
     an area on which the inspection object in an entire sensing image is projected is specified as the area of interest. 
     (11) 
     The signal processing apparatus according to (7) above, in which 
     an arbitrary area in the entire sensing image is specified in accordance with setting of a user as the area of interest. 
     (12) 
     The signal processing apparatus according to any one of (1) to (11) above, in which 
     the inspection object is a plant, and 
     the state indicates growth conditions of the plant. 
     (13) 
     The signal processing apparatus according to (12) above, in which 
     the index is a vegetation index based on reflection of light by the plant. 
     (14) 
     The signal processing apparatus according to (13) above, in which 
     the vegetation index is an NDVI (Normalized Difference Vegetation Index). 
     (15) 
     A signal processing method including the steps of: 
     calculating a relative value to an average of a vegetation index from the vegetation index indicative of a state of an inspection object, which is calculated on a basis of a sensing signal; and 
     performing processing to allow an image indicative of the state of the inspection object on a basis of the relative value to be displayed. 
     (16) 
     A program for causing a computer to execute signal processing including the steps of: 
     calculating a relative value to an average of a vegetation index from the vegetation index indicative of a state of an inspection object, which is calculated on a basis of a sensing signal; and 
     performing processing to allow an image indicative of the state of the inspection object on a basis of the relative value to be displayed. 
     In addition, the embodiment of the present disclosure is not limited to the above described embodiment and can be variously modified without departing from the scope of the present disclosure. 
     REFERENCE SIGNS LIST 
       11  Vegetation inspection apparatus,  12  Display section,  21  Optical system,  22  Aperture diaphragm,  23  Spectral sensor,  24  Signal processing block,  25  Control block,  31  Spectroscope,  32  Sensing device,  41  NDVI average calculation section,  42  Correlation coefficient calculation section,  43  NDVI relative value calculation section,  44  Display processing section,  51  Vegetation inspection system,  52  Image pickup apparatus,  53  Signal processing apparatus,  54  Network