CROSS REFERENCE TO RELATED APPLICATION 
     The present application is based on and claims the benefit of priority of Japanese Patent Application No. 2007-241134 filed on Sep. 18, 2007, the disclosure of which is incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The present disclosure generally relates to a display apparatus for clearly notifying a user of an obstacle such as a human, a vehicle or the like by using a headup display apparatus or the like. 
     BACKGROUND INFORMATION 
     Conventionally, an obstacle around a vehicle is notified for a driver of the vehicle by using a display system. For example, a night vision system uses a far-infrared ray camera disposed on the vehicle to detect a position and/or a movement of a detecting object that emits an infrared ray, and recognizes that the obstacle is existing on a road ahead of the vehicle, or crossing a road. Further, the night vision system determines whether or not the object is a walker based on a size and shape of the object. Then, an image of a vehicle front field is displayed on a display unit for notifying the driver of the vehicle of the walker on an expected travel route of the vehicle or of the walker crossing the street. 
     However, the above system displays the image of the detecting object on the display unit that is disposed on an instrument panel or the like, thereby making it difficult for the driver to watch the display unit due to a requirement of a move of a look of the driver away from a windshield to the display unit. 
     Therefore, a display system that projects an image light to be reflected on the windshield to generate in a vehicle front space a virtual image in a view of the driver for information notification is proposed. For example, when a walker  101  that causes unsafeness of the subject vehicle is seen through a windshield  17  as shown in  FIG. 7 , the proposed system such as the one described on page 73 of a non-patent document of Automotive Technology 2006 Vol. 60 No. 2 can display a frame image  105  as a virtual image for surrounding the walker  101  for the purpose of notifying the driver of a position of the unsafe obstacle. 
     However, due to the vibration of the vehicle during driving, the frame image  105  is also vibrated, thereby making the positions of the images  105   a ,  105   b ,  105   c  at times T, T+1, T+2 respectively different from each other in the view of the driver as shown in  FIG. 8 . The respectively different position of the image  105  annoys the driver of the vehicle in a great deal. 
     SUMMARY OF THE INVENTION 
     In view of the above and other problems, the present disclosure provides a display apparatus that displays a virtual image without letting a driver of a vehicle to recognizing a vibration of the image. 
     The display apparatus of the present disclosure uses an image display unit to display the virtual image in front of the vehicle, and the virtual image is controlled to circle around an obstacle on a path or an orbit in a view of the driver. Therefore, the position of the virtual image at a certain point of time is different from the position of the virtual image at a different point of time, thereby making it difficult for the driver to recognize the vibration of the virtual image. That is, the display unit of the present invention does not let the driver to recognize the vibration of the vehicle in the course of displaying the virtual image. Further, by controlling the position of the virtual image to move along an obit that surrounds the obstacle, the existence of the obstacle is clearly notified for the driver. Furthermore, the obstacle is marked by the virtual image that circles around the obstacle only after determining that the obstacle is unsafe for the driver. Therefore, the display apparatus prevents the driver from being annoyed by impartially displaying unnecessary virtual images for all of the detected objects that are not necessarily unsafe for the driver. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects, features and advantages of the present invention will become more apparent from the following detailed description made with reference to the accompanying drawings, in which: 
         FIG. 1  is an illustration expressing a configuration of a display unit in an embodiment of the present invention; 
         FIG. 2  is a flow chart expressing a processing performed in the display unit; 
         FIGS. 3A to 3D  are illustrations expressing display coordinates of a liquid crystal panel, a virtually displayed liquid crystal panel on a windshield, a conversion table between obstacle directions and the display coordinates, and X and Y angles of the obstacle directions; 
         FIG. 4  is an illustration expressing a virtual image displayed by the display unit in a view of a driver looking a front field of the vehicle; 
         FIG. 5  is another illustration expressing a virtual image displayed by the display unit in a view of a driver looking a front field of the vehicle; 
         FIG. 6  is yet another illustration expressing a virtual image displayed by the display unit in a view of a driver looking a front field of the vehicle; 
         FIG. 7  is still yet another illustration expressing a virtual image displayed by the display unit in a view of a driver looking a front field of the vehicle in a prior art; and 
         FIG. 8  is still yet another illustration expressing a virtual image displayed by the display unit in a view of a driver looking a front field of the vehicle in a prior art. 
     
    
    
     DETAILED DESCRIPTION 
     An embodiment of the present invention is explained with reference to  FIGS. 1 ,  4 ,  5 . 
     1. Configuration of a Display Unit 
     A display unit  1  is a device disposed on a vehicle  2 , and includes an image output circuit  3 , a liquid crystal panel  5 , a camera  7 , an obstacle detection circuit  9 , a danger detection circuit  11 , a vehicle speed sensor  13  and a speaker  14  as shown in  FIG. 1 . The image output circuit  3  outputs image data on the liquid crystal panel  5 . The liquid crystal panel  5  is disposed in an upper part of an instrumental panel  15 , and displays an image output from the image output circuit  3 . The light to display an image output from the liquid crystal panel  5  reflects on a windshield  17 , and enters a viewpoint  19  of a driver. As a result, in a view of the driver, a virtual image  21  of an image displayed on the liquid crystal panel  5  is displayed in a space in front of the vehicle. The image output circuit  3  and the liquid crystal panel  5  can control a display position of the virtual image  21  so that the virtual image  21  moves sequentially over an orbit  27  surrounding an obstacle (e.g., another vehicle  23  in  FIG. 4 , a walker  25  in  FIG. 5 ) in the circumference of the vehicle (e.g., in particular, in the front field of the vehicle) in a view of the driver as shown in  FIGS. 4 and 5 . In addition, a position, a size, and a shape of the orbit  27  vary depending on determination results of the danger detection circuit  11 . The orbit  27  will be explained later in detail in the description. 
     The camera  7  is an imaging device to photograph a picture of a front field (designated as a “front picture” in the following) of the vehicle  2 . The camera  7  is disposed inside of the vehicle  2  at a position that is close to the viewpoint  19  of the driver. The obstacle detection circuit  9  determines whether there is the obstacle (e.g., a walker or other vehicles) in the front picture photographed by the camera  7 , and calculates an obstacle direction (i.e., a direction to the obstacle from the viewpoint  19 ), an obstacle distance (i.e., the distance to the obstacle from the vehicle  2 ) and the size of the obstacle when there is the obstacle in the front picture. The calculation can be performed by a well-known method from the shape, the movement and the brightness of the obstacle. The danger detection circuit  11  is a device to determine whether or not the obstacle recognized by the obstacle detection circuit  9  is unsafe. The method of how the unsafeness is determined is mentioned later. The vehicle speed sensor  13  detects a vehicle speed of the vehicle  2 , and the result is output to the danger detection circuit  11 . The speaker  14  is installed inside of the vehicle  2 , and, depending on a determination result of the danger detection circuit  11 , a beep sound is output therefrom. 
     2. The Processing Performed by the Display Unit 
     The processing performed by the display unit  1  is explained with reference to a flow chart in  FIG. 2  and illustrations in  FIGS. 3 to 6 . 
     The processing in  FIG. 2  is started when an engine of the vehicle  2  is started. In step S 100 , the front picture of the vehicle  2  is photographed by the camera  7 . 
     In step S 110 , the obstacle detection circuit  9  extracts an obstacle from the front picture acquired in step S 100 . The extraction of the obstacle can be performed by using a well-known image recognition technology. For example, an element in the front picture can be extracted as an obstacle when the element is similar to the shape, size or other features of various obstacles (e.g., a walker, a vehicle or the like) memorized in advance. 
     Then, in step S 120 , whether an obstacle has been extracted from the front picture by the obstacle detection circuit  9  in step S 110  is determined. When the obstacle has been extracted, the process proceeds to step S 130 . When no obstacle is extracted, the process returns to step S 100 . 
     In step S 130 , the obstacle direction (a direction to the obstacle seen from the viewpoint  19 ) of an extracted obstacle, the obstacle distance (the distance to the obstacle from the vehicle  2 ) and the size of the obstacle are calculated by the obstacle detection circuit  9 . 
     Based on the vehicle speed of the vehicle  2  derived from the vehicle speed sensor  13  and the obstacle distance, a time-to-reach (TTR) for the vehicle  2  to reach the obstacle is calculated by the danger detection circuit  11  in step S 140 . 
     Then, in step S 150 , whether or not the calculated time-to-reach (TTR) is within a certain range (e.g., equal to or smaller than 10 seconds in this case) is determined by the danger detection circuit  11 . In addition, whether or not the obstacle direction is within a certain scope (e.g., on a road) is determined in step S 150 . 
     When the time-to-reach (TTR) is determined to be equal to or smaller than 10 seconds and the obstacle is determined to be in the certain scope, the obstacle is determined as being unsafe, and the process proceeds to step S 160 . On the other hand, when the time-to-reach (TTR) is determined to be greater than 10 seconds or the obstacle is not in the certain scope, the process returns to step S 100 . 
     Then, in step S 160 , whether or not the time-to-reach (TTR) calculated in step S 140  is within a predetermined range (e.g., 5 to 10 seconds in this case) is determined by the danger detection circuit  11 . When TTR is within the predetermined range, the process proceeds to step S 170 . When TTR is not within the predetermined range, the process proceeds to step S 220 . 
     In step S 170 , a radius R of the orbit  27  of the virtual image  21  is determined by the danger detection circuit  11  based on an equation 1.
 
 R=K ×( S/D )  (Equation 1)
 
     In the above equation 1, K is a constant, S is a size of the obstacle in a unit of meter, and D is the obstacle distance in a unit of meter. 
     In step S 180 , it is determined by the danger detection circuit  11  whether the obstacle is a human or another vehicle. This determination can be conducted by using a well-known image recognition technology. For example, the shape, size, and other features are memorized in advance respectively for the human and vehicle, and the obstacle is determined as one of the human or the vehicle by the comparison of the shape, size, and other features. When the obstacle is determined as human, the process proceeds to step S 190 . When the obstacle is determined as the vehicle, the process proceeds to step S 200 . 
     In step S 190 , the shape of the orbit  27  is determined as a vertically elongated ellipse by the image output circuit  3 . In step S 200 , the shape of the orbit  27  is determined as a horizontally elongated ellipse by the image output circuit  3 . 
     In step S 210 , the image data is output to the liquid crystal panel  5  from the image output circuit  3 . The virtual image displayed by the image data, that is, the virtual image  21  in a circular shape, moves on the orbit  27  arranged in a surrounding manner relative to the obstacle in a view of the driver as shown in  FIG. 4  and  FIG. 5  (e.g., the vehicle  23  in  FIG. 4 , the walker  25  in  FIG. 5 ). The position, the radius R, and the shape (a vertically elongated ellipse/a horizontally elongated ellipse) of the orbit  27  in the image data are determined in the following manner. 
     The direction of the obstacle is calculated by the obstacle detection circuit  9 , as stated above, in step S 130 . The obstacle direction of an obstacle  33  is represented as a combination of X direction (a horizontal direction) and Y direction (a vertical direction) as shown in  FIG. 3D . In a memory unit of the display unit  1  (not illustrated), a conversion table that converts the obstacle direction of the obstacle  33  calculated by the obstacle detection circuit  9  to display coordinates in the liquid crystal panel  5 , as shown in  FIG. 3C , is memorized. When a pixel  31  in the liquid crystal panel  5  is lit as shown in  FIG. 3A  according to the display coordinates converted by the conversion table, a virtual image  21  (in  FIG. 3B ) generated by the pixel  31  is positioned at a position of the obstacle  33  as shown in  FIGS. 3B and 3D  in a view of the driver. 
     The image output circuit  3  calculates, based on the obstacle direction calculated by the obstacle detection circuit  9  and the conversion table stated above, the display coordinates corresponding to the obstacle  33  in the liquid crystal panel  5 , and the display coordinates are used as a center of the orbit  27 . Further, the radius of the orbit  27  is set by using the radius R calculated in step S 170 . That is, the radius of the orbit  27  along the longer axis is set to the radius R, and the radius along the shorter axis is set to a value that is calculated by multiplying a fixed coefficient of, for example, 0.8 to the radius R. 
     Furthermore, the shape of the orbit  27  (a vertical ellipse/a horizontal ellipse) is set to the shape that is determined in step S 190  or step S 200 . On the other hand, when the time-to-reach (TTR) is determined to be smaller than 5 seconds in step S 160 , the process proceeds to step S 220 , and a beep sound is output from the speaker  14 . 
     3. The Advantageous Effects of the Display Unit 
     The display unit  1  provides the following effects. 
     (i) The virtual image displayed by the display unit  1  is, as shown in  FIGS. 4 and 5 , the virtual image  21  partially occupying the orbit  27  and sequentially moving around the obstacle. In this case, the vibration of the virtual image  21  is hardly recognized by the driver because of the continuous movement of the virtual image  21 , that is, because a position of the virtual image  21  at a certain point in time is different from a position of the virtual image  21  at a next point in time. For example, suppose that the virtual image  21  stands at the position as shown in  FIG. 4  and  FIG. 5  at a time t 1 , and the virtual image  21  circles around the orbit  27  to return to the same position at a time t 2 . Even if the position of the virtual image  21  at the time t 2  is different from the position at the time t 1  due to the vibration of the vehicle  2 , the difference is hardly recognized by the driver because of the travel of the virtual image  21  away from the above position during the time between t 1  and t 2 . In other words, the display unit  1  can display the virtual image without allowing the driver to recognize the vibration. 
     (ii) Because the display unit  1  displays an obstacle (e.g., the vehicle  23 , the walker  25 ) as a virtual image, the display unit  1  allows a driver to easily recognize the obstacle in an intelligible manner. 
     (iii) The display unit  1  sets the radius of the orbit  27  to be small when the obstacle is small in size and when the obstacle is distant from the vehicle. Therefore, the display of the virtual image on the orbit  27  is provided in a fitting manner that naturally represents a sense of size and distance. 
     (iv) The display unit  1  uses different orbit shapes for the obstacle when the obstacle is human or when the obstacle is vehicle. That is, in other words, though the shape of the orbit  27  is a horizontally elongated oval in correspondence to the shape of the vehicle  23  as shown in  FIG. 4 , the shape of the orbit  27  is a vertically elongated oval in correspondence to the shape of the human  25  as shown in  FIG. 5 . As a result, even when the driver hardly distinguish in his/her direct view whether the obstacle is the vehicle  23  or the human  25 , the obstacle can be instantaneously recognized as the vehicle  23  or the human  25  based on the shape of the orbit  27 . 
     (v) When the display unit  1  determines whether or not an obstacle is dangerous, and a virtual image to move around an obstacle is displayed only when it is determined to be dangerous (step S 150  in  FIG. 2 ). Therefore, a virtual image is not displayed for the obstacle which is not dangerous, thereby preventing annoying feeling of the driver. 
     (vi) The display unit  1  only outputs the beep warning sound without displaying the virtual image circling around the obstacle when the time-to-reach (TTR) is very short, because there is no time for the driver to confirm the virtual image in that case. Therefore, the unsafe condition can be conveyed to the driver in an appropriate manner. 
     Although the present invention has been fully described in connection with the preferred embodiment thereof with reference to the accompanying drawings, it is to be noted that various changes and modifications will become apparent to those skilled in the art. 
     For example, the speed of the vehicle may be received from an engine ECU, or from other devices, instead of the speed sensor  13 . 
     Further, the unsafeness of the obstacle may be determined by using a prediction of the movement of the obstacle in a later time based on the movement of the obstacle at the time of the determination. 
     Furthermore, though the obstacle is recognized by using the sensing machinery such as the camera  7  on the vehicle  2  in the above embodiment, positional information of the obstacle may be obtained by a method externally from outside of the vehicle  2 , that is, from an infrastructure such as a vehicle-to-vehicle communication or the like, and the virtual image may be displayed based on the positional information from outside. 
     Furthermore, the extraction and the recognition of the obstacle may be performed by a laser radar, a millimeter wave radar, a stereo camera, or by a combination of these sensors, instead of using the camera  7 . 
     Furthermore, the TTR (i.e., 10 seconds and 5 seconds) used in steps S 150  and S 160  in  FIG. 2  may be changed to other values. That is, the TTR may be determined based on the time to stop according to the speed of the vehicle  2 , or may be determined based on the time to travel a certain distance (e.g., a distance ‘too far to watch’ for the driver) such as 200 meters. 
     Furthermore, as shown in  FIG. 6 , the orbit  27  may always be a perfect circle that circles around the obstacle (i.e., the walker  25  in  FIG. 6 ). In this case, the processing in the display unit  1  can be simplified because it is not necessary to change the shape of the orbit  27  depending on the obstacle. The radius of the perfect circle may be set to the radius R. 
     Such changes and modifications are to be understood as being within the scope of the present invention as defined by the appended claims.
A display apparatus has an image display unit that displays a virtual image by projecting an image light to a windshield, and an obstacle detector that detects an obstacle existing around the vehicle as well as a safety determination unit that determines safety of the vehicle relative to the detected obstacle based at least on one of a distance from the obstacle and a relative speed of the vehicle relative to the obstacle. The display apparatus also has an image control unit that controls a position of the virtual image to circle around the obstacle in a view of a driver when the obstacle is determined to be unsafe by the safety determination unit.