Patent Publication Number: US-2022224918-A1

Title: Video transport system, video transmission device, video reception device, video distribution method, video transmission method, video reception method, and non-transitory computer readable recording medium

Description:
TECHNICAL FIELD 
     The present disclosure relates to a video transport system, a video transmission device, a video reception device, a video distribution method, a video transmission method, and a computer program. 
     The present application claims priority to Japanese Patent Application No. 2019-100291 filed on May 29, 2019, the entire contents of which are herein incorporated by reference. 
     BACKGROUND ART 
     For broadcasting and the like, a technique for transporting high-definition video data of ultra-high resolution such as 8K ultra high definition television (UHDTV) (which will be abbreviated as “8K” below) has been developed (see, for example, NPL 1). 
     Video images of ultra-high resolution, owing to expressive power thereof, are expected to increasingly be used in the future in every field such as surveillance applications, crime prevention applications, and applications for visual inspection of buildings and the like. Such expressive power, on the other hand, requires a high-rate communication path for transport of video data because a rate of transport therefor is, for example, several ten gigabits per second (Gbps) or higher. 
     CITATION LIST 
     Patent Literature 
     NPL 1: “Wikipedia”, [online], [searched on Apr. 8, 2019], the Internet &lt;URL:http://ja.wikipedia.org/wiki/H.265&gt; 
     NPL 2: “Shisen Kenshutsu Gijutsu Kihon Genri (basic principles of line-of-sight detection technique),” [online], Apr. 23, 2013, Fujitsu Laboratories, Ltd., [searched on Jan. 6, 2020], the Internet &lt;URL:
         https://www.fujitsu.com/jp/group/labs/resources/tech/techguide/list/eye-movements/p03.html&gt;       

     SUMMARY OF INVENTION 
     A video transport system according to one embodiment of the present disclosure includes a video transmission device that performs compression processing on video data and transmits the video data subjected to the compression processing and a video reception device that receives the video data subjected to the compression processing from the video transmission device and performs decompression processing on the received video data. Of a prescribed attention area within a frame of the video data and a prescribed non-attention area different from the attention area within the frame, the video transmission device performs prescribed compression processing within the frame on the non-attention area and does not perform the prescribed compression processing on the attention area. 
     A video transmission device according to another embodiment of the present disclosure includes a compression processing unit that performs, of a prescribed attention area within a frame of video data and a prescribed non-attention area different from the attention area within the frame, prescribed compression processing within the frame on the non-attention area and a transmitter that transmits the video data subjected to the prescribed compression processing to a video reception device. 
     A video reception device according to another embodiment of the present disclosure includes a receiver that receives video data from a video transmission device, the video data being video data resulting from prescribed compression processing within a frame of the video data on a prescribed non-attention area, of a prescribed attention area within the frame and the non-attention area different from the attention area within the frame, and a decompressor that decompresses the video data received by the receiver. 
     A video distribution method according to another embodiment of the present disclosure includes performing compression processing, by a video transmission device, on video data and transmitting, by the video transmission device, the video data subjected to the compression processing, and receiving, by a video reception device, the video data subjected to the compression processing from the video transmission device and performing, by the video reception device, decompression processing on the received video data. In the transmitting the video data, of a prescribed attention area within a frame of the video data and a prescribed non-attention area different from the attention area within the frame, the video transmission device performs prescribed compression processing within the frame on the non-attention area and does not perform the prescribed compression processing on the attention area. 
     A video transmission method according to another embodiment of the present disclosure includes performing, of a prescribed attention area within a frame of video data and a prescribed non-attention area different from the attention area within the frame, prescribed compression processing within the frame onto the non-attention area and transmitting the video data subjected to the prescribed compression processing to a video reception device. 
     A video reception method according to another embodiment of the present disclosure includes receiving video data from a video transmission device, the video data being video data resulting from prescribed compression processing within a frame of the video data on a prescribed non-attention area, of a prescribed attention area within the frame and the non-attention area different from the attention area within the frame, and decompressing the video data received by a receiver. 
     A computer program according to another embodiment of the present disclosure causes a computer to perform performing, of a prescribed attention area within a frame of video data and a prescribed non-attention area different from the attention area within the frame, prescribed compression processing within the frame on the non-attention area, and transmitting the video data subjected to the prescribed compression processing to a video reception device. 
     A computer program according to another embodiment of the present disclosure causes a computer to perform receiving video data from a video transmission device, the video data being video data resulting from prescribed compression processing within a frame of the video data on a prescribed non-attention area, of a prescribed attention area within the frame and the non-attention area different from the attention area within the frame, and decompressing the video data received by a receiver. 
     Naturally, the computer program can be distributed in a non-transitory computer readable recording medium such as a compact disc-read only memory (CD-ROM) or over a communication network such as the Internet. The present disclosure can also be implemented as a semiconductor integrated circuit that implements a part or the entirety of a video transmission device or a video reception device. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram showing an overall configuration of a video transport system according to a first embodiment of the present disclosure. 
         FIG. 2  is a block diagram showing a configuration of a video transmission device according to the first embodiment of the present disclosure. 
         FIG. 3  is a diagram showing exemplary image data. 
         FIG. 4  is a diagram showing exemplary image data after the image data shown in  FIG. 3  is divided into small blocks. 
         FIG. 5  is a diagram for illustrating an order of output of small blocks from a block divider to a subtractor and an area designation unit. 
         FIG. 6  is a diagram for illustrating subtraction processing. 
         FIG. 7  is a diagram showing block information for one frame (image data) determined by an area determination unit. 
         FIG. 8  is a diagram showing exemplary down-conversion processing. 
         FIG. 9  is a diagram for illustrating processing performed by the area designation unit, a down-converter unit, and a video image alignment unit. 
         FIG. 10  is a diagram for illustrating processing performed by the area designation unit, the down-converter unit, and the video image alignment unit. 
         FIG. 11  is a block diagram showing a configuration of a video reception device according to the first embodiment of the present disclosure. 
         FIG. 12  is a diagram showing exemplary compressed video data. 
         FIG. 13  is a sequence diagram showing an exemplary procedure of processing by the video transport system. 
         FIG. 14  is a flowchart showing details of compression processing (step S 2  in  FIG. 13 ). 
         FIG. 15  is a flowchart showing details of decompression processing (step S 6  in  FIG. 13 ). 
         FIG. 16  is a flowchart showing details of compression processing (step S 2  in  FIG. 13 ) performed by the video transmission device. 
         FIG. 17  is a diagram for illustrating processing performed by the area designation unit, the down-converter unit, and the video image alignment unit. 
         FIG. 18  is a flowchart showing details of compression processing (step S 2  in  FIG. 13 ) performed by the video transmission device. 
         FIG. 19  is a block diagram showing a configuration of a video transmission device according to a fourth embodiment of the present disclosure. 
         FIG. 20  is a block diagram showing a configuration of a video reception device according to the fourth embodiment of the present disclosure. 
         FIG. 21  is a sequence diagram showing an exemplary procedure of processing by the video transport system. 
         FIG. 22  is a flowchart showing details of compression processing (step S 2  in  FIG. 21 ). 
         FIG. 23  is a diagram showing exemplary compressed video data. 
         FIG. 24  is a diagram showing an overall configuration of a video transport system according to a fifth embodiment of the present disclosure. 
         FIG. 25  is a diagram showing an exemplary display and an exemplary camera. 
         FIG. 26  is a block diagram showing a configuration of a video reception device according to the fifth embodiment of the present disclosure. 
         FIG. 27  is a diagram for illustrating a method of determining an attention area. 
         FIG. 28  is a sequence diagram showing an exemplary procedure of processing by the video transport system. 
         FIG. 29  is a flowchart showing details of attention area determination processing (step S 52  in  FIG. 28 ). 
         FIG. 30  is a diagram schematically showing video shooting by a drone. 
         FIG. 31  is a diagram schematically showing a controller for operating a drone and a user who operates the controller. 
         FIG. 32  is a diagram schematically showing the controller for operating the drone and the user who operates the controller. 
         FIG. 33  is a flowchart showing details of attention area determination processing (step S 52  in  FIG. 28 ) according to a sixth embodiment of the present disclosure. 
         FIG. 34  is a diagram showing an exemplary display and an exemplary camera. 
         FIG. 35  is a diagram for illustrating a method of determining an attention area. 
         FIG. 36  is a diagram for illustrating a method of determining an attention area and a non-attention area. 
     
    
    
     DETAILED DESCRIPTION 
     [Problem to be Solved by the Present Disclosure] 
     For example, usage such as transmission of video data taken by a camera (which is referred to as a “8K camera” below) that is provided in a mobile body such as heavy equipment (a crane or a crawler dozer), a drone, or a robot and is capable of taking 8K video data from a video transmission device to a video reception device and checking of the video data at a remote location is also envisioned. 
     Transport capability of wireless communication under “the fifth generation mobile communication system” (which is abbreviated as “5G” (5th generation) below), however, is about several Gbps. Transport of video data adapted to an 8K dual green system requires transport capability around 24 Gbps. Therefore, it is difficult to transmit 8K video data in a format as it is through 5G wireless communication. A similar problem occurs also in transmission of 8K video data by using a network protocol of 10-gigabit Ethernet™. 
     Video data may be transported as being compressed with such a system as H.265 (ISO/IEC 23008-2 HEVC) used in broadcast and the like. A time period of approximately several seconds, however, is required for compression processing and decompression processing, and hence video images may be delayed. 
     Transported video data is used, for example, for surveillance applications for checking a suspicious individual, a flow of people, or attendees. Specifically, by subjecting video data to image recognition processing, an object to be recognized such as a suspicious individual is extracted. An important area in the video data in the surveillance applications is an area around the object to be recognized, and a resolution of an area other than that may be lowered. In an application other than the above as well, a resolution of an area other than an area to which attention should be paid may be lowered. 
     The present disclosure was made in view of such circumstances, and an object thereof is to provide a video transport system, a video transmission device, a video reception device, a video distribution method, a video transmission method, a video reception method, and a computer program that allow low-latency distribution of video data, identicalness of which with original video images is held in an attention area. 
     [Advantageous Effect of the Present Disclosure] 
     According to the present disclosure, low-latency distribution of video data, identicalness of which with original video images is held in an attention area, can be achieved. 
     Overview of Embodiments of the Present Disclosure 
     Overview of embodiments of the present disclosure will initially be listed and described. 
     (1) A video transport system according to one embodiment of the present disclosure includes a video transmission device that performs compression processing on video data and transmits the video data subjected to the compression processing and a video reception device that receives the video data subjected to the compression processing from the video transmission device and performs decompression processing on the received video data. Of a prescribed attention area within a frame of the video data and a prescribed non-attention area different from the attention area within the frame, the video transmission device performs prescribed compression processing within the frame on the non-attention area and does not perform the prescribed compression processing on the attention area. 
     According to this configuration, prescribed compression processing is not performed on an attention area within a frame of video data but prescribed compression processing is performed on a non-attention area, and then compressed video data can be transmitted. Therefore, identicalness with original video images is held in the attention area. The prescribed compression processing is compression processing within the frame. Therefore, delay of video images caused in H.265 in which compression processing is performed between frames is less likely. Therefore, low-latency distribution of video data can be achieved. 
     (2) Preferably, the attention area is determined based on a line-of-sight position of a user within the frame. 
     According to this configuration, for example, an area in the vicinity of a line-of-sight position of a user within the frame is defined as the attention area, and an area other than that is defined as the non-attention area. Therefore, identicalness with original video images is held in an area within the frame watched by a user, and prescribed compression processing is performed on an area not watched by the user. Therefore, compression and low-latency distribution of video data can be achieved without giving uncomfortable feeling to the user who watches the frame. 
     (3) Further preferably, the attention area is fixed for a prescribed time period based on a time period for which the line-of-sight position is maintained within a prescribed area. 
     According to this configuration, as the user gazes a prescribed position within the frame or a position in the vicinity of the prescribed position, the attention area can be fixed for a prescribed time period. The position in the vicinity of the prescribed position means, for example, a position within a prescribed distance from the prescribed position. The attention area thus remains fixed even when the user gazes the position and thereafter momentarily averts his/her eyes. Therefore, when the user moves his/her line of sight back to the original position, the user can immediately watch the video images identicalness of which with original video images is held. 
     (4) There may be a plurality of users, and the attention area may be determined for each user. 
     According to this configuration, the attention area is determined for each user based on the line-of-sight position of the user. Therefore, even when a plurality of users are watching different positions on the same frame, areas in the vicinity of the line-of-sight positions of the users are defined as the attention areas, respectively, and identicalness with original video images is held in each attention area. Therefore, the plurality of users do not feel uncomfortable. 
     (5) The video transmission device may change a size of the attention area in accordance with transmission condition information representing a condition of transmission of the video data subjected to the compression processing. 
     According to this configuration, for example, when a rate of transport of video data is lowered, a size of video data can be reduced by reducing a size of the attention area. Low-latency distribution of video data can thus be achieved. 
     (6) The video data may be generated by a camera mounted on a mobile body, and the attention area may be determined based on a direction of travel of the mobile body. 
     According to this configuration, low-latency distribution of video data identicalness of which with original video images is held in the attention area determined based on a direction of travel of the mobile body can be achieved. Thus, for example, the mobile body can fly in a stable manner. 
     (7) The video data may include an image of an object of visual inspection, and the attention area may be an area including a portion to be inspected of the object. 
     According to this configuration, low-latency distribution of video data identicalness of which with original video images is held at a portion to be inspected of an object of visual inspection can be achieved. Therefore, visual inspection of an object can be conducted with less delay. 
     (8) The attention area may be determined based on an amount of variation in luminance value between frames of the video data. 
     According to this configuration, for example, a portion where an amount of variation in luminance value between frames is large can preferentially be set as the attention area. Thus, for example, when video data is used in surveillance applications, an area including a suspicious individual can be set as the attention area and image recognition processing can efficiently be performed. 
     (9) The video reception device may transmit information for designating the attention area to the video transmission device. 
     According to this configuration, low-latency distribution of video data identicalness of which with original video images is held in a designated area can be achieved. For example, when video data is used in surveillance applications in which an area to be monitored is known in advance, surveillance processing can efficiently be performed as the user designates the area to be monitored as the attention area. 
     (10) The prescribed compression processing may be processing for reducing a color depth of each pixel within the non-attention area. 
     According to this configuration, since a color depth of each pixel in the non-attention area can be lowered, low-latency distribution of video data can be achieved. Since the non-attention area corresponds to a periphery of a field of view of a user, the user is less likely to notice reduction in color depth even though it occurs. 
     (11) The frame may be divided into a plurality of blocks, and the attention area and the non-attention area may be designated in a unit of a block. 
     According to this configuration, prescribed compression processing can be performed in a unit of a block. Compression processing can thus quickly be performed. 
     (12) The prescribed compression processing may be down-conversion processing for each block within the non-attention area. 
     According to this configuration, since a resolution within the non-attention area can be lowered, low-latency distribution of video data can be achieved. 
     (13) The non-attention area may include a plurality of areas different in compression rate in the prescribed compression processing, and an area adjacent to the attention area among the plurality of areas may be lowest in compression rate. 
     According to this configuration, compression processing can be performed with a compression rate in an area in the non-attention area closer to a center of the field of view of a user being lower and with the compression rate in an area farther from the center being higher. Therefore, low-latency distribution of video data can be achieved while sudden change in how video images look at a portion of boundary between the attention area and the non-attention area is prevented. 
     (14) A video transmission device according to another embodiment of the present disclosure includes a compression processing unit that performs, of a prescribed attention area within a frame of video data and a prescribed non-attention area different from the attention area within the frame, prescribed compression processing within the frame onto the non-attention area and a transmitter that transmits the video data subjected to the prescribed compression processing to a video reception device. 
     According to this configuration, prescribed compression processing is not performed on an attention area within a frame of video data but prescribed compression processing is performed on a non-attention area, and then compressed video data can be transmitted. Therefore, identicalness with original video images is held in the attention area. The prescribed compression processing is compression processing within the frame. Therefore, delay of video images caused in H.265 in which compression processing is performed between frames is less likely. Therefore, low-latency distribution of video data can be achieved. 
     (15) A video reception device according to another embodiment of the present disclosure includes a receiver that receives video data from a video transmission device, the video data being video data resulting from prescribed compression processing within a frame of video data on a prescribed non-attention area, of a prescribed attention area within the frame and the non-attention area different from the attention area within the frame, and a decompressor that decompresses the video data received by the receiver. 
     According to this configuration, compressed video data for which prescribed compression processing is not performed on an attention area within a frame of video data but prescribed compression processing has been performed on a non-attention area can be received. Therefore, identicalness with original video images is held in the attention area. Prescribed compression processing within the frame is performed on the non-attention area. Therefore, delay of video images caused in H.265 in which compression processing is performed between frames is less likely. Therefore, low-latency distribution of video data can be achieved. 
     (16) A video distribution method according to another embodiment of the present disclosure includes performing compression processing, by a video transmission device, on video data and transmitting, by the video transmission device, the video data subjected to the compression processing and receiving, by a video reception device, video data subjected to the compression processing from the video transmission device and performing, by the video reception device, decompression processing on the received video data. In the transmitting the video data, of a prescribed attention area within a frame of the video data and a prescribed non-attention area different from the attention area within the frame, the video transmission device performs prescribed compression processing within the frame on the non-attention area and does not perform the prescribed compression processing on the attention area. 
     This configuration includes the step corresponding to a characteristic processing unit included in the video transport system described above. Therefore, according to this configuration, functions and effects similar to those of the video transport system described above can be achieved. 
     (17) A video transmission method according to another embodiment of the present disclosure includes performing, of a prescribed attention area within a frame of video data and a prescribed non-attention area different from the attention area within the frame, prescribed compression processing within the frame on the non-attention area and transmitting the video data subjected to the prescribed compression processing to a video reception device. 
     This configuration includes the step corresponding to the characteristic processing unit included in the video transmission device described above. Therefore, according to this configuration, functions and effects similar to those of the video transmission device described above can be achieved. 
     (18) A video reception method according to another embodiment of the present disclosure includes receiving video data from a video transmission device, the video data being video data resulting from prescribed compression processing within a frame of the video data on a prescribed non-attention area, of a prescribed attention area within the frame and the non-attention area different from the attention area within the frame, and decompressing the received video data. 
     This configuration includes the step corresponding to the characteristic processing unit included in the video reception device described above. Therefore, according to this configuration, functions and effects similar to those of the video reception device described above can be achieved. 
     (19) A computer program according to another embodiment of the present disclosure causes a computer to perform performing, of a prescribed attention area within a frame of video data and a prescribed non-attention area different from the attention area within the frame, prescribed compression processing within the frame on the non-attention area, and transmitting the video data subjected to the prescribed compression processing to a video reception device. 
     According to this configuration, the computer can function as the video transmission device described above. Therefore, functions and effects similar to those of the video transmission device described above can be achieved. 
     (20) A computer program according to another embodiment of the present disclosure causes a computer to perform receiving video data from a video transmission device, the video data being video data resulting from prescribed compression processing within a frame of the video data on a prescribed non-attention area, of a prescribed attention area within the frame and the non-attention area different from the attention area within the frame, and decompressing the received video data. 
     According to this configuration, the computer can function as the video reception device described above. Therefore, functions and effects similar to those of the video reception device described above can be achieved. 
     Details of Embodiments of the Present Disclosure 
     Embodiments of the present disclosure will be described below with reference to the drawings. The embodiments which will be described below each show one preferred specific example of the present disclosure. A numeric value, a shape, a material, a component, a position of arrangement and a form of connection of a component, a step, and an order of steps shown in the embodiments below are merely by way of example, and do not intend to limit the present disclosure. A component not described in an independent claim describing a most generic concept of the present disclosure among components in the embodiments below is described as any component that implements a more preferred form. 
     The same components have the same reference characters allotted. Since functions and labels thereof are also the same, description thereof will not be provided as appropriate. 
     First Embodiment 
     &lt;Overall Configuration of Video Transport System&gt; 
       FIG. 1  is a diagram showing an overall configuration of a video transport system according to a first embodiment of the present disclosure. 
     Referring to  FIG. 1 , a video transport system  100  includes a camera  1 , a video transmission device  2 , a video reception device  4 , and a display  5 . 
     Camera  1  captures an image of a prescribed object. Camera  1  is, for example, a surveillance camera provided in a facility or the like. Camera  1  may be attached to a mobile body such as heavy equipment or a drone. 
     Camera  1  takes high-definition video images of an object. Video data includes a plurality of frames. For example, video data of 60 frames per second (fps) include sixty frames per one second. 
     More specifically, camera  1  generates video data of an object in a resolution of 8K UHDTV, for example, in conformity with the dual green system or the 4:2:2 system. This video data includes image data for each frame. 
     The rate of transport of video data of 60 fps generated in conformity with the dual green system is, for example, 23.89 Gbps or 19.91 Gbps. The rate of transport of video data generated in conformity with the 4:2:2 system is, for example, 47.78 Gbps or 39.81 Gbps. 
     Video transmission device  2  transmits video data resulting from shooting by camera  1  to video reception device  4  over a network  3 . 
     Video reception device  4  receives video data from video transmission device  2  and shows the received video data on display  5 . 
     &lt;Configuration of Video Transmission Device  2 &gt; 
       FIG. 2  is a block diagram showing a configuration of video transmission device  2  according to the first embodiment of the present disclosure. 
     Referring to  FIG. 2 , video transmission device  2  includes a block divider  21 , a buffer  22 , a subtractor  23 , an area determination unit  24 , an area designation unit  25 , a down-converter unit  26 , a video image alignment unit  27 , a video image compressor  28 , a compressed video image alignment unit  29 , and a transmitter  30 . 
     A part or the entirety of video transmission device  2  is implemented by hardware including an integrated circuit such as an application specific integrated circuit (ASIC) or a field-programmable gate array (FPGA). 
     Video transmission device  2  can also be implemented by a computer including a central processing unit (CPU), a random access memory (RAM), and a read only memory (ROM). Each processing unit is implemented as a functional component by execution of a computer program on a computing processing device such as a CPU. 
     Block divider  21  includes a communication interface, and it is a processing unit that receives a frame (which is also referred to as “image data” below) that composes video data resulting from image capture by camera  1  and divides the image data into small blocks of a prescribed size. 
       FIG. 3  is a diagram showing exemplary image data. Image data  10  shows, for example, an image of an airplane  11  that flies in the air. 
       FIG. 4  is a diagram showing exemplary image data  10  after image data  10  shown in  FIG. 3  is divided into small blocks. As shown in  FIG. 4 , image data  10  is divided into small blocks  12  regularly arranged laterally and vertically. The number of small blocks  12  is not limited as illustrated. 
     Block divider  21  has video data received from camera  1  temporarily stored in buffer  22 . 
     Block divider  21  provides small blocks  12  to subtractor  23  and area designation unit  25  in accordance with a prescribed order. 
       FIG. 5  is a diagram for illustrating an order of output of small blocks  12  from block divider  21  to subtractor  23  and area designation unit  25 . As shown in  FIG. 5 , image data  10  is divided into large blocks  14  regularly arranged laterally and vertically. Each large block  14  is composed of 2×2 small blocks  12 . Block divider  21  scans large block  14  to be processed in image data  10  from an upper left large block  14 A to a lower right large block  14 Z in the order of raster. Block divider  21  scans small blocks  12  to be processed in each large block  14  from an upper left small block to a lower right small block in the order of raster and provides small blocks  12 . Block divider  21  scans small blocks in large block  14 Z, for example, in the order of a small block  12 A a small block  12 B a small block  12 C a small block  12 D, and provides each small block  12  to subtractor  23  and area designation unit  25 . The number of small blocks  12  that compose large block  14  is not limited as described above. For example, large block  14  may be composed of 3×3 small blocks  12 . 
     Subtractor  23  sequentially receives small blocks  12  from block divider  21  and performs subtraction processing between small blocks  12  in the order of reception. Specifically, subtractor  23  performs subtraction processing between small block  12  of image data to be compressed that is received from block divider  21  and small block  12  at the same block position of image data that precedes the image data by a prescribed number of frames (for example, one frame). 
       FIG. 6  is a diagram for illustrating subtraction processing.  FIG. 6  shows a time sequence of image data  10  that composes video data, and shows a sequence of three temporally continuous pieces of image data  10  from a frame  1  to a frame  3 . Image data  10  in frame  1  is oldest and image data  10  in frame  3  is newest. Subtractor  23  is assumed to have received from block divider  21 , small block  12  of image data  10  in frame  3  to be compressed. Subtractor  23  reads small block  12  at a position the same as the small block received from block divider  21 , from image data  10  in frame  2  that precedes by one frame and is stored in buffer  22 . Subtractor  23  calculates a difference in luminance value for each pixel between two small blocks  12  identical in position and different in frame. 
     For example, it is assumed that small block  12  has a size of m×n pixels and a luminance value of each pixel in small block  12  is expressed as I(t, i, j), where t represents a frame number, (i, j) represents a coordinate within small block  12 , and relation of 1≤i≤m and 1≤j≤n is satisfied. 
     A difference sub between small block  12  having a frame number t and small block  12  having a frame number t−1 is expressed in an Expression 1 below. In the expression, t represents a number of a frame to be compressed. L represents the number of levels of luminance ( 256  when the luminance value is expressed in eight bits). 
     
       
         
           
             
               
                 
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     Subtraction processing is not limited to subtraction processing performed between adjacent frames. For example, subtraction processing may be performed between image data  10  in frame  1  and image data  10  in frame  3  distant by two frames from frame  1 . 
     Area determination unit  24  receives difference sub between small blocks  12  from subtractor  23 , and determines whether to set small block  12  to which attention is paid as an attention area or a non-attention area based on comparison between difference sub and a prescribed threshold value Tsub. Specifically, when an Expression 2 below is satisfied, small block  12  is determined as the attention area, and when the expression is not satisfied, it is determined as the non-attention area. In other words, area determination unit  24  determines small block  12  large in variation in luminance value between frames as the attention area, and determines small block  12  small in variation in luminance value as the non-attention area. 
     
       
         
           
             
               
                 
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     Area determination unit  24  sets as block information, a result of determination as the attention area or the non-attention area, and provides the block information to area designation unit  25  and video image alignment unit  27 , together with position information of small block  12  of image data  10 . The order of output of small block  12  provided from block divider  21  to subtractor  23  is determined in advance as described above. Therefore, position information of small block  12  is determined based on the order of output. The position of small block  12  is not limited so long as the position of small block  12  of image data  10  can be identified based on the information, and for example, a coordinate at an upper left corner of small block  12  of image data  10  or the order of output of small block  12  may be applicable. 
     Area designation unit  25  provides small block  12  received from block divider  21  to down-converter unit  26  or video image alignment unit  27  based on the block information of small block  12  received from area determination unit  24 . 
     Processing for output of small block  12  by area designation unit  25  will now be described.  FIG. 7  is a diagram showing block information for one frame (image data  10 ) determined by area determination unit  24 . 
     Large block  14  (for example, large blocks  14 B to  14 D) labeled only as B indicates that all (four) small blocks  12  included in large block  14  are each a non-attention area (which is also referred to as a “block B” below). 
     Large block  14  (for example, large blocks  14 E to  14 I) divided into four small blocks  12  refers to large block  14  composed only of the attention area (which is also referred to as a “block A” below) or large block  14  in which blocks A and B are mixed. For example, large block  14 E is composed of four small blocks  12 P to  12 S where blocks A and B are mixed. Large block  14 F is composed of four small blocks  12  consisting of blocks A. 
     When large block  14  includes even a single block A, area designation unit  25  provides all small blocks  12  included in that large block  14  to video image alignment unit  27 . For example, small block  12 S shown in  FIG. 7  is determined as block A. Therefore, area designation unit  25  provides all small blocks  12 P to  12 S included in large block  14 E to which small block  12 S belongs to video image alignment unit  27 . 
     When all small blocks  12  in large block  14  are blocks B, area designation unit  25  provides all small blocks  12  included in that large block  14  to down-converter unit  26 . For example, small blocks  12  in large block  14 B shown in  FIG. 7  are all blocks B. Therefore, area designation unit  25  provides all small blocks  12  included in large block  14 B to down-converter unit  26 . 
     Down-converter unit  26  functions as a compression processing unit that performs compression processing as prescribed preprocessing, and performs down-conversion processing for reducing the size of small block  12  received from area designation unit  25  as preprocessing compression processing. 
       FIG. 8  is a diagram showing exemplary down-conversion processing. For example, down-converter unit  26  receives four small blocks  12  included in large block  14  to be processed from area designation unit  25 . Down-converter unit  26  performs down-conversion processing for vertically and laterally reducing large block  14  to ½ to generate a downsized block  16  (which is also referred to as a “block C” below). Down-converter unit  26  provides generated downsized block  16  to video image alignment unit  27 . 
     Video image alignment unit  27  receives small block  12  or downsized block  16  from area designation unit  25  or down-converter unit  26 , and provides small block  12  or downsized block  16  as being arranged in the order corresponding to output from area designation unit  25 . Video image alignment unit  27  provides position information and block information of small block  12  or downsized block  16  to compressed video image alignment unit  29 . 
     Processing performed by area designation unit  25 , down-converter unit  26 , and video image alignment unit  27  will be described below.  FIG. 9  is a diagram for illustrating processing performed on large blocks  14 B to  14 D in image data  10  in  FIG. 7  by way of example.  FIG. 10  is a diagram for illustrating processing performed on large blocks  14 E to  14 G in image data  10  in  FIG. 7  by way of example. 
     The upper part of  FIG. 9  shows the order of small blocks  12  provided from area designation unit  25  and the lower part of  FIG. 9  shows the order of small blocks  12  provided to video image alignment unit  27 . This is also applicable to  FIG. 10 . 
     Referring to  FIG. 9 , area designation unit  25  successively receives four small blocks  12  included in large block  14 B from block divider  21  in the order of raster. Area designation unit  25  receives block information of four small blocks  12  from area determination unit  24  in the order of raster. Area designation unit  25  determines that four small blocks  12  are all blocks B based on the block information. Therefore, area determination unit  25  provides four blocks B to down-converter unit  26 . Down-converter unit  26  receives four blocks B from area designation unit  25  and performs down-conversion processing on these blocks B to generate downsized block  16  (block C). Down-converter unit  26  provides generated block C to video image alignment unit  27 . 
     Video image alignment unit  27  provides downsized block  16  received from down-converter unit  26  to video image compressor  28  in the order of reception. Video image alignment unit  27  provides position information and block information of downsized block  16  to compressed video image alignment unit  29 . Position information of downsized block  16  is position information of any small block  12  (for example, at the upper left corner) included in large block  14 B from which downsized block  16  is generated. Block information of downsized block  16  is information indicating that downsized block  16  is generated by down-conversion processing of small block  12  (for example, information indicating block C). 
     Area designation unit  25 , down-converter unit  26 , and video image alignment unit  27  successively perform similar processing also for large block  14 C and large block  14 D. 
     Referring to  FIG. 10 , area designation unit  25  successively receives four small blocks  12 P to  12 S included in large block  14 E from block divider  21  in the order of raster. Area designation unit  25  receives block information of four small blocks  12 P to  12 S from area determination unit  24  in the order of raster. Area designation unit  25  determines that small block  12 S which is block A is included in four small blocks  12  based on the block information. Therefore, area designation unit  25  provides four small blocks  12 P to  12 S to video image alignment unit  27  in the order of raster. 
     Video image alignment unit  27  provides small blocks  12 P to  12 S received from area designation unit  25  to video image compressor  28  in the order of reception. Video image alignment unit  27  provides position information and block information of small blocks  12 P to  12 S to compressed video image alignment unit  29 . Position information and block information of small blocks  12 P to  12 S are the same as those received from area determination unit  24 . 
     Area designation unit  25  successively performs similar processing also on large blocks  14 F and  14 G. 
     Video image compressor  28  receives small block  12  (block A or B) or downsized block  16  (block C) from video image alignment unit  27 . Video image compressor  28  performs video image compression processing for video images as a whole on the blocks, in the order of received blocks, and provides compressed blocks to compressed video image alignment unit  29 . Video image compression processing is reversible compression processing or lossy compression processing. Reversible compression processing refers to processing for compression such that a compressed block can return to an uncompressed block, and it is generally low in compression rate and the compression rate greatly varies depending on an image. Specifically, a compression rate of an image close to noise is low, whereas the compression rate of a sharp image is high. Lossy compression processing refers to processing for compression such that a compressed block cannot return to an uncompressed block. Lossy compression processing by using an algorithm called visually lossless compression or visually reversible compression, however, is a method of compression with visual reversibility. Therefore, in the present embodiment, for example, video image compressor  28  performs lossy compression processing based on visually reversible compression. 
     Compressed video image alignment unit  29  receives a compressed block from video image compressor  28 . Compressed video image alignment unit  29  adds position information and block information obtained from video image alignment unit  27  to a block in the order of received blocks, and provides a resultant block to transmitter  30 . 
     Transmitter  30  includes a communication interface, and encodes a compressed block to which position information and block information are added to transmit the compressed block to video reception device  4  as compressed video data. 
     &lt;Configuration of Video Reception Device  4 &gt; 
       FIG. 11  is a block diagram showing a configuration of video reception device  4  according to the first embodiment of the present disclosure. 
     Referring to  FIG. 11 , video reception device  4  includes a receiver  41 , an information extractor  42 , a video image decompressor  44 , a video image alignment unit  45 , an up-converter unit  46 , and a video image composite unit  47 . 
     A part or the entirety of video reception device  4  is implemented by hardware including an integrated circuit such as an ASIC or an FPGA. 
     Video reception device  4  can also be implemented by a computer including a CPU, a RAM, and a ROM. Each processing unit is implemented as a functional component by execution of a computer program on a computing processing device such as a CPU. 
     Receiver  41  includes a communication interface. Receiver  41  receives compressed video data for one frame from video transmission device  2  and decodes received data. The decoded data includes a compressed block to which position information and block information are added. Receiver  41  successively provides compressed blocks to information extractor  42  and video image decompressor  44 . 
     Information extractor  42  receives the compressed blocks from receiver  41 . Information extractor  42  extracts position information and block information from the blocks and provides them to video image alignment unit  45  and video image composite unit  47 . 
     Video image decompressor  44  successively receives the compressed blocks from receiver  41 . Video image decompressor  44  performs video image decompression processing on the compressed blocks in the order of reception and provides decompressed blocks to video image alignment unit  45 . Video image decompression processing is reversible decompression processing or lossy decompression processing. Video image decompressor  44  performs decompression processing corresponding to compression processing by video image compressor  28  of video transmission device  2 . Specifically, when video image compressor  28  performs reversible compression processing, video image decompressor  44  performs reversible decompression processing corresponding to that processing, and when video image compressor  28  performs lossy compression processing, video image decompressor  44  performs lossy decompression processing corresponding to that processing. 
     Video image alignment unit  45  successively receives decompressed blocks from video image decompressor  44 . Video image alignment unit  45  receives position information and block information of the decompressed blocks from information extractor  42 . Video image alignment unit  45  aligns decompressed blocks based on the position information. In other words, video image alignment unit  45  aligns decompressed blocks in the order of raster. Video image alignment unit  45  determines a type of the decompressed block based on the block information. When the decompressed block is block A or block B, video image alignment unit  45  provides that block to video image composite unit  47 . When the decompressed block is block C, video image alignment unit  45  provides that block to up-converter unit  46 . 
     Up-converter unit  46  receives block C from video image alignment unit  45  and performs up-conversion processing for vertically and laterally enlarging block C by two times. In other words, up-converter unit  46  performs processing for enhancing a resolution of block C. Up-converter unit  46  provides the generated up-converted block to video image composite unit  47 . 
     Video image composite unit  47  receives blocks from video image alignment unit  45  or up-converter unit  46  and receives position information of the blocks from information extractor  42 . Video image composite unit  47  composites image data by arranging each block at a position indicated in position information. Video image composite unit  47  provides video data to display  5  by successively providing video data to display  5 . 
     Processing performed by video image alignment unit  45 , up-converter unit  46 , and video image composite unit  47  will now be described with reference to specific examples.  FIG. 12  is a diagram showing exemplary compressed video data.  FIG. 12  shows data for one frame resulting from compression of image data  10  shown in  FIG. 7 . 
     Large blocks  14  in the first row of image data  10  in  FIG. 7  are totally composed of B blocks. Therefore, the first row of compressed video data shown in  FIG. 12  is totally composed of C blocks. This is also applicable to fourth and fifth rows of image data  10 . 
     First three large blocks  14  in the second row of image data  10  are totally composed of B blocks. Therefore, first three pieces in the second row of compressed video data are composed of C blocks. Fourth large block  14 H and fifth large block  14 I in the second row of image data  10  each include one or more A blocks. Therefore, a fourth piece to an eleventh piece in the second row of compressed video data are the same as small blocks  12  included in large blocks  14 H and  14 I. Sixth to eighth large blocks  14  in the second row of image data  10  are totally composed of B blocks. Therefore, last three pieces in the second row of compressed video data are composed of C blocks. 
     The third row of compressed video data is also similarly composed in correspondence with the third row of image data  10 . 
     Video image alignment unit  45  receives blocks that compose video data resulting from decompression of compressed video data shown in  FIG. 12  in the order of position shown in  FIG. 12 . Specifically, video image alignment unit  45  receives blocks from block C at the upper left to block C at the lower right in the order of raster. 
     When video image alignment unit  45  receives block C, it provides block C to up-converter unit  46 . Up-converter unit  46  up-converts block C and provides the up-converted block to video image composite unit  47 . When video image alignment unit  45  receives block A or B, it provides the block to video image composite unit  47 . 
     Video image composite unit  47  generates image data  10  in which blocks are aligned as shown in  FIG. 7  by compositing block A or B received from video image alignment unit  45  and up-converted block C received from up-converter unit  46  with each other. 
     &lt;Flow of Processing by Video Transport System  100 &gt; 
       FIG. 13  is a sequence diagram showing an exemplary procedure of processing by video transport system  100 . 
     Referring to  FIG. 13 , video transmission device  2  obtains video data from camera  1  (S 1 ). 
     Video transmission device  2  performs compression processing on the obtained video data for each piece of image data that composes the video data (S 2 ). Details of compression processing will be described later. 
     Video transmission device  2  encodes compressed video data (S 3 ). 
     Video transmission device  2  transmits encoded compressed video data to video reception device  4  and video reception device  4  receives the data (S 4 ). 
     Video reception device  4  decodes received compressed video data (S 5 ). 
     Video reception device  4  performs decompression processing on the compressed video data for each frame (S 6 ). Details of decompression processing will be described later. 
     Video reception device  4  provides decompressed video data to display  5  (S 7 ). 
     Compression processing (step S 2  in  FIG. 13 ) will now be described.  FIG. 14  is a flowchart showing details of compression processing (step S 2  in  FIG. 13 ). 
     Block divider  21  divides image data into small blocks  12  of a prescribed size (S 11 ). Image data  10  is thus divided into small blocks  12  as shown in  FIG. 4 . 
     Video transmission device  2  repeatedly performs a loop B which will be described later and steps S 17  to S 21  in a unit of large block  14  in the order of raster shown in  FIG. 5  (a loop A). 
     Video transmission device  2  repeatedly performs on each large block  14 , steps S 12  to S 16  which will be described later in a unit of small block  12  in the order of raster (loop B). 
     In other words, subtractor  23  calculates difference sub between small blocks  12  in frames in accordance with the Expression 1 (S 12 ). 
     Area determination unit  24  compares difference sub with threshold value Tsub in accordance with the Expression 2 (S 13 ). 
     When the Expression 2 is satisfied (YES in S 13 ), area determination unit  24  determines small block  12  as block A and provides block information and position information of small block  12  to area designation unit  25  and video image alignment unit  27  (S 14 ). When the Expression 2 is not satisfied (NO in S 13 ), area determination unit  24  determines small block  12  as block B and provides block information and position information of small block  12  to area designation unit  25  and video image alignment unit  27  (S 15 ). 
     Area designation unit  25  has small block  12 , a type of which has been determined, buffered in buffer  22  (S 16 ). 
     After processing in loop B, area designation unit  25  determines whether or not block A is included in large block  14  based on block information of small block  12  received from area determination unit  24  (S 17 ). When block A is included (YES in S 17 ), area designation unit  25  provides four small blocks  12  included in large block  14  buffered in buffer  22  to video image alignment unit  27  (S 18 ). 
     When block A is not included (NO in S 17 ), area designation unit  25  provides four small blocks  12  included in large block  14  buffered in buffer  22  to down-converter unit  26 . Down-converter unit  26  down-converts four small blocks  12  and provides downsized block  16  to video image alignment unit  27  (S 19 ). 
     Video image alignment unit  27  provides small block  12  or downsized block  16  received from area designation unit  25  or down-converter unit  26  to video image compressor  28 , and video image compressor  28  performs video image compression processing on that block (S 20 ). 
     Compressed video image alignment unit  29  adds position information and block information to a compressed block and provides the resultant block to transmitter  30  (S 21 ). 
     Decompression processing (step S 6  in  FIG. 13 ) will now be described.  FIG. 15  is a flowchart showing details of decompression processing (step S 6  in  FIG. 13 ). 
     Video reception device  4  repeatedly performs steps S 42  to S 48  below in a unit of a compressed block that composes compressed video data (a loop C). 
     Information extractor  42  extracts position information and block information from the compressed block and provides the extracted information to video image alignment unit  45  and video image composite unit  47  (S 42 ). 
     Video image decompressor  44  performs video image decompression processing on the compressed block and provides the decompressed block to video image alignment unit  45  (S 44 ). 
     Video image alignment unit  45  determines whether or not the decompressed block is block C (S 45 ). When the decompressed block is block C (YES in S 45 ), video image alignment unit  45  provides that block to up-converter unit  46 . Up-converter unit  46  up-converts block C and provides the up-converted block to video image composite unit  47  (S 46 ). 
     When the decompressed block is block A or B (NO in S 45 ), video image alignment unit  45  provides that block to video image composite unit  47  (S 47 ). 
     Video image composite unit  47  receives the block from video image alignment unit  45  or up-converter unit  46 , and composites image data by arranging each block at a position indicated in position information (S 48 ). 
     &lt;Effect etc. in First Embodiment&gt; 
     As described above, according to the first embodiment of the present disclosure, down-conversion processing is not performed on the attention area within the frame of video data but down-conversion processing is performed on the non-attention area, and then compressed video data can be transmitted. Therefore, identicalness with original video images is held in the attention area. The non-attention area is subjected to down-conversion processing within the frame. Therefore, delay in video images caused in H.265 in which compression processing is performed between frames is less likely. Therefore, low-latency distribution of video data can be achieved. 
     The attention area and the non-attention area are designated in a unit of a block. Therefore, down-conversion processing can be performed in a unit of a block. Compression processing can thus quickly be performed. 
     In the first embodiment, when all small blocks  12  in large block  14  are B blocks, large block  14  is down-converted. When large block  14  includes even a single B block, however, large block  14  may be down-converted. 
     Second Embodiment 
     In the first embodiment, when A block and B block are mixed in a single large block  14 , small blocks  12  in large block  14  are not down-converted. In contrast, in a second embodiment, video transport system  100  that generates, for such a large block  14 , compressed video data including an A block that is not down-converted and a C block resulting from down-conversion of large block  14  will be described. 
     Video transport system  100  is similar in configuration to that in the first embodiment. 
     A procedure of processing by video transport system  100  is similar to that in the first embodiment. Compression processing (step S 2  in  FIG. 13 ), however, is different from that in the first embodiment. 
       FIG. 16  is a flowchart showing details of compression processing (step S 2  in  FIG. 13 ) performed by video transmission device  2 . Processing similar to that in the flowchart shown in  FIG. 14  has the same step number allotted. 
     After processing in step S 14 , area designation unit  25  provides block A to video image alignment unit  27  (S 31 ). 
     After processing in loop B, area designation unit  25  determines whether or not large block  14  includes block B based on block information of small block  12  received from area determination unit  24  (S 32 ). When block B is included (YES in S 32 ), area designation unit  25  provides four small blocks  12  included in large block  14  buffered in buffer  22  to down-converter unit  26 . Down-converter unit  26  down-converts four small blocks  12  and provides downsized block  16  to video image alignment unit  27  (S 33 ). 
     Processing performed by area designation unit  25 , down-converter unit  26 , and video image alignment unit  27  will be described below.  FIG. 17  is a diagram for illustrating processing for large blocks  14 E to  14 G in image data  10  in  FIG. 7  by way of example. The upper part in  FIG. 17  shows the order of small blocks  12  provided from area designation unit  25  and the lower part of  FIG. 17  shows the order of small blocks  12  provided to video image alignment unit  27 . 
     Referring to  FIG. 17 , area designation unit  25  successively receives four small blocks  12 P to  12 S included in large block  14 E from block divider  21  in the order of raster. Area designation unit  25  receives block information of four small blocks  12 P to  12 S from area determination unit  24  in the order of raster. Area designation unit  25  determines that small block  12 S that falls under block A is included and provides small block  12 S to video image alignment unit  27 . Area designation unit  25  determines that block B is included in four small blocks  12 . Therefore, area designation unit  25  provides small blocks  12 P to  12 S to down-converter unit  26  in the order of raster. Down-converter unit  26  receives small blocks  12 P to  12 S and performs down-conversion processing onto these small blocks to generate downsized block  16  (block C). Down-converter unit  26  provides generated block C to video image alignment unit  27 . 
     Video image alignment unit  27  provides downsized block  16  received from area designation unit  25  to video image compressor  28  in the order of reception. Video image alignment unit  27  provides position information and block information of downsized block  16  to compressed video image alignment unit  29 . The position information and the block information of downsized block  16  are the same as those received from area determination unit  24 . 
     Area designation unit  25  successively performs similar processing also on large blocks  14 F and  14 G. 
     A flow of decompression processing (step S 6  in  FIG. 13 ) is similar to that shown in  FIG. 15 . A part of image data composite processing (step S 48  in  FIG. 15 ), however, is different. Specifically, as shown in  FIG. 17 , in the second embodiment, an A block and a C block may be generated for one large block  14 E. Therefore, an area of the A block and an area of a block resulting from up-conversion of a C block overlap with each other. Therefore, when an up-converted block is arranged at a position of the A block after the A block is arranged, video image composite unit  47  leaves the A block and arranges the up-converted block at a position except for the area of the A block. Overwriting of the A block with the up-converted block is thus prevented. 
     Third Embodiment 
     In the first or second embodiment, threshold value Tsub of difference sub for determining whether small block  12  is defined as the attention area or the non-attention area is fixed. Threshold value Tsub, however, can also be variable. In a third embodiment, an example in which threshold value Tsub is varied depending on a condition of transport of compressed video data will be described. Specifically, the number of attention areas is decreased by increasing threshold value Tsub when the condition of transport becomes poor, so that a data size of compressed video data is reduced. 
     Video transport system  100  is similar in configuration to that in the first embodiment. 
     A procedure of processing by video transport system  100  is similar to that in the first embodiment. Compression processing (step S 2  in  FIG. 13 ), however, is different from that in the first embodiment. 
       FIG. 18  is a flowchart showing details of compression processing (step S 2  in  FIG. 13 ) performed by video transmission device  2 . Processing similar to that in the flowchart shown in  FIG. 14  has the same step number allotted. 
     After processing in step S 12 , area determination unit  24  determines whether or not an amount of unprocessed buffered data accumulated in buffer  22  is larger than a threshold value Tdata 1  (S 33 ). Block divider  21  has video data received from camera  1  successively stored in buffer  22 . When transport of compressed video data from video transmission device  2  to video reception device  4  is delayed, however, the amount of unprocessed buffered data in buffer  22  increases. In other words, the amount of unprocessed buffered data serves as transmission condition information indicating a condition of transmission of video data. 
     When the amount of unprocessed buffered data is larger than Tdata 1  (YES in S 33 ), area determination unit  24  increases threshold value Tsub by a (a positive constant). Generation of the attention area is thus less likely. 
     When the amount of unprocessed buffered data is equal to or smaller than Tdata 1  (NO in S 33 ), area determination unit  24  determines whether or not the amount of unprocessed buffered data is equal to or smaller than a threshold value Tdata 2  (S 35 ). Tdata 2  represents a value smaller than Tdata 1 . When the amount of unprocessed buffered data is equal to or smaller than threshold value Tdata 2  (YES in S 35 ), block divider  21  decreases threshold value Tsub by β (a positive constant). The attention area can thus more readily be generated. 
     α may be equal to or different from β. 
     When the amount of unprocessed buffered data is larger than threshold value Tdata 2  (NO in S 35 ) or after processing in S 34  or S 36 , processing in step S 13  or later is performed. 
     According to the third embodiment, when the amount of unprocessed buffered data increases, the number of small blocks  12  determined as the attention area can be reduced. As the rate of transport of video data is lowered, the amount of unprocessed buffer data increases. In other words, according to the second embodiment, the size of video data to be transported can be reduced by reducing the size of the attention area when the rate of transport of video data is lowered. Low-latency distribution of video data can thus be achieved. 
     Fourth Embodiment 
     The attention area is determined based on difference sub between small blocks  12  in the first to third embodiments. In a fourth embodiment, a user designates the attention area. 
     Video transport system  100  is similar in configuration to that in the first embodiment. The configuration of video transmission device  2  and video reception device  4  is partially different from that in the first embodiment. 
       FIG. 19  is a block diagram showing a configuration of video transmission device  2  according to the fourth embodiment of the present disclosure. 
     Referring to  FIG. 19 , video transmission device  2  according to the fourth embodiment includes block divider  21 , buffer  22 , area designation unit  25 , down-converter unit  26 , video image alignment unit  27 , video image compressor  28 , compressed video image alignment unit  29 , transmitter  30 , and a receiver  31 . Processing units  21 ,  22 , and  25  to  30  are similar to those shown in  FIG. 2 . 
     Receiver  31  receives attention area information from video reception device  4 . Attention area information is information indicating a position of the attention area in the frame of video data. Attention area information may include, for example, a coordinate of an upper left corner of the attention area or may be a number brought in correspondence with a position of small block  12 . Attention area information may include position information of the non-attention area instead of position information of the attention area. Attention area information may include both of position information of the attention area and position information of the non-attention area. 
     Area designation unit  25  provides small blocks  12  resulting from division by block divider  21  to down-converter unit  26  or video image alignment unit  27  based on attention area information received by receiver  31 . In other words, area designation unit  25  provides small blocks  12  in the attention area to video image alignment unit  27  and provides small blocks  12  in the non-attention area to down-converter unit  26 . 
       FIG. 20  is a block diagram showing a configuration of video reception device  4  according to the fourth embodiment of the present disclosure. 
     Referring to  FIG. 20 , video reception device  4  according to the fourth embodiment includes receiver  41 , information extractor  42 , video image decompressor  44 , video image alignment unit  45 , up-converter unit  46 , video image composite unit  47 , a position information obtaining unit  48 , an attention area determination unit  49 , and a transmitter  50 . Processing units  41  to  47  are similar to those shown in  FIG. 11 . 
     Position information obtaining unit  48  obtains position information of the attention area entered by a user by operating such input means as a mouse or a keyboard and provides obtained position information to attention area determination unit  49 . Position information obtaining unit  48  may obtain position information of the attention area from a processing device connected to video reception device  4 . For example, the processing device receives video data from video reception device  4  and determines position information of the attention area by performing image processing based on the video data or by using artificial intelligence. The processing device provides the determined position information of the attention area to video reception device  4  so that position information obtaining unit  48  of video reception device  4  obtains the position information. 
     Attention area determination unit  49  receives position information from position information obtaining unit  48  and generates attention area information for designating the attention area. For example, attention area determination unit  49  generates attention area information including the coordinate of the upper left corner of small block  12  in the attention area or a number brought in correspondence with the position of small block  12  in the attention area. Attention area determination unit  49  provides the generated attention area information to transmitter  50 . 
     Transmitter  50  receives attention area information from attention area determination unit  49  and transmits the attention area information to video transmission device  2 . 
     A flow of processing by video transport system  100  will now be described. 
       FIG. 21  is a sequence diagram showing an exemplary procedure of processing by video transport system  100 . 
     Referring to  FIG. 21 , video reception device  4  transmits attention area information generated based on a user input to video transmission device  2  and video transmission device  2  receives the attention area information (S 8 ). 
     After processing in step S 8 , processing in steps S 1  to S 7  similar to that shown in  FIG. 13  is performed. Some of contents of compression processing (step S 2 ) are different. 
       FIG. 22  is a flowchart showing details of compression processing (step S 2  in  FIG. 21 ). The flowchart shown in  FIG. 22  is the same as the flowchart showing details of compression processing shown in  FIG. 14  except for processing for determining which of block A and block B small block  12  falls under (steps S 12  to S 15  in  FIG. 14 ). 
     Specifically, video transmission device  2  can determine which of block A and block B small block  12  falls under based on attention area information received from video reception device  4 . Therefore, processing in steps S 12  to S 15  in  FIG. 14  does not have to be performed. 
     According to the fourth embodiment, low-latency distribution of video data identicalness of which with original video images is held in an area designated by the user can be achieved. For example, when video data is used in surveillance applications in which an area to be monitored is known in advance, surveillance processing can efficiently be performed as the user designates the area to be monitored as the attention area. 
     Fifth Embodiment 
     In a fifth embodiment, an example in which an attention area is determined based on a line of sight of a user will be described. 
       FIG. 24  is a diagram showing an overall configuration of a video transport system according to the fifth embodiment of the present disclosure. 
     Referring to  FIG. 24 , a video transport system  100 A includes camera  1 , video transmission device  2 , a video reception device  4 A, and display  5  and a camera  6 . 
     Camera  1  and display  5  are similar in configuration to those shown in the first embodiment. 
     Video transmission device  2  is similar in configuration to that shown in the fourth embodiment. 
     Similarly to video reception device  4  described in the fourth embodiment, video reception device  4 A receives video data from video transmission device  2  and shows received video data on display  5 . Video reception device  4 A, however, is partially different in configuration from video reception device  4 . The configuration of video reception device  4 A will be described later. 
       FIG. 25  is a diagram showing exemplary display  5  and exemplary camera  6 . 
     Display  5  is a device for showing video images on a frame of a liquid crystal display, an organic electroluminescence (EL) display, or the like. 
     Camera  6  is contained in a bezel of display  5 . Camera  6  may be provided separately from display  5 . For example, camera  6  may be used as being attached to display  5 . Positional relation between the frame of display  5  and camera  6  is assumed as being known in advance. Camera  6  is provided at a position where it can shoot a face of a user  61 A who looks at the frame of display  5 . In particular, camera  6  is provided at a position where it can capture an image of eyes of user  61 A. 
       FIG. 26  is a block diagram showing a configuration of video reception device  4 A according to the fifth embodiment of the present disclosure. 
     Referring to  FIG. 26 , video reception device  4 A according to the fifth embodiment includes a video data obtaining unit  51  and an attention area determination unit  49 A instead of position information obtaining unit  48  and attention area determination unit  49  in the configuration of video reception device  4  according to the fourth embodiment shown in  FIG. 20 . 
     Video data obtaining unit  51  receives video data resulting from image capture by camera  6  from camera  6  and provides received video data to attention area determination unit  49 A. 
     Attention area determination unit  49 A receives video data from video data obtaining unit  51 , and determines a line-of-sight position of a user on the frame of display  5  based on the video data. For example, it is assumed that user  61 A turns his/her eyes in a line-of-sight direction  71 A and looks at a motorcycle  81  shown on the frame of display  5  as shown in  FIG. 25 . A known technique is available for detection of line-of-sight direction  71 A. For example, attention area determination unit  49 A detects from video data of user  61 A, a part of eyes that does not move (a reference point) and a part of the eyes that moves (a moving point). An inner corner of the eye of user  61 A is defined as the reference point and an iris of user  61 A is defined as the moving point. Attention area determination unit  49 A detects an orientation of the line of sight of user  61 A with an orientation of an optical axis of camera  6  being defined as the reference, based on the position of the moving point with respect to the reference point (see, for example, NPL 2). Attention area determination unit  49 A determines an intersection between line-of-sight direction  71 A and the frame as a line-of-sight position  72 A. Line-of-sight position  72 A is expressed, for example, by a coordinate of video data shown on the frame. 
     Attention area determination unit  49 A determines the attention area within video data shown on display  5  based on a determined line-of-sight position  72 A. 
       FIG. 27  is a diagram for illustrating a method of determining an attention area.  FIG. 27  shows an example in which image data  10  shown on the frame of display  5  is divided into a plurality of small blocks  12 . User  61 A is assumed to look, for example, at the inside of small block  12 E. In other words, line-of-sight position  72 A of user  61 A is assumed to be within small block  12 E. Attention area determination unit  49 A determines that line-of-sight position  72 A is included in small block  12 E based on the coordinate of line-of-sight position  72 A. Attention area determination unit  49 A determines an area composed of a plurality of small blocks  12  including small block  12  as an attention area  91 A. For example, attention area determination unit  49 A determines an area composed of small block  12 E and eight proximate small blocks  12  adjacent to small block  12 E as attention area  91 A. 
     The size of attention area  91 A is merely by way of example, and not limited as illustrated. A shape or a color of an object can accurately be observed by the sense of sight of humans in a range called a central vision within an angle of approximately one to two degrees from the line-of-sight direction. Therefore, when an approximate distance from user  61 A to display  5  is known, the central vision on the frame can also be defined. Therefore, the central vision around line-of-sight position  72 A may be determined as attention area  91 A. 
     Similarly to attention area determination unit  49 , attention area determination unit  49 A generates attention area information for designating the attention area and provides generated attention area information to transmitter  50 . 
     A flow of processing by video transport system  100 A will now be described. 
       FIG. 28  is a sequence diagram showing an exemplary procedure of processing by video transport system  100 A. 
     Referring to  FIG. 28 , video reception device  4 A obtains video data including an image of eyes of user  61 A who looks at the frame of display  5  from camera  6  (S 51 ). 
     Video reception device  4 A determines the attention area of user  61 A in video data shown on display  5  based on the obtained video data. 
     Video reception device  4  transmits attention area information indicating the determined attention area to video transmission device  2  and video transmission device  2  receives the attention area information (S 8 ). 
     After processing in step S 8 , processing in steps S 1  to S 7  as shown in  FIG. 13  is performed. 
       FIG. 29  is a flowchart showing details of attention area determination processing (step S 52  in  FIG. 28 ). 
     Referring to  FIG. 29 , attention area determination unit  49 A of video reception device  4  determines line-of-sight position  72 A of user  61 A on the frame based on the video data obtained in step S 51  (S 61 ). 
     Attention area determination unit  49 A determines a prescribed area including line-of-sight position  72 A as attention area  91 A (S 62 ). 
     An exemplary manner of use of video transport system  100 A will now be described. An example in which camera  1  is attached to a mobile body (for example, a drone) will be described below. 
       FIG. 30  is a diagram schematically showing video shooting by a drone. Referring to  FIG. 30 , camera  1  for taking video images of surroundings is mounted on a drone  110 . Drone  110  takes video images with camera  1  while it flies as being remotely controlled by a user. For example, drone  110  takes video images over an image capture range  120 A and thereafter moves to another position by an operation by the user to take video images over an image capture range  120 B. 
       FIGS. 31 and 32  are diagrams schematically showing a controller for operating drone  110  and a user who operates the controller. 
     Referring to  FIG. 31 , a controller  111  is assumed to contain video reception device  4 A. Controller  111  includes a frame  112  for showing video images, a joystick  113  for maneuvering drone  110 , and camera  6  that shoots a user  61 C who operates controller  111 . As user  61 C operates joystick  113 , a direction of travel or a speed of drone  110  can be changed. 
     It is assumed that video images over image capture range  120 A are shown on frame  112 . It is assumed that user  61 C turns his/her eyes into a line-of-sight direction  71 C to watch a ship  83  shown on frame  112 , and a line of sight of user  61 C falls on a line-of-sight position  72 C. In this case, video reception device  4 A determines a prescribed area including line-of-sight position  72 C as an attention area  91 C. Ship  83  watched by the user thus holds identicalness with original video images. An area other than attention area  91 C on frame  112  is defined as the non-attention area, which is subjected to down-conversion processing. 
     Referring to  FIG. 32 , it is assumed that user  61 C changes his/her line-of-sight direction  71 C to watch a ship  84  shown on frame  112  and the line of sight of user  61 C falls on line-of-sight position  72 C. In this case, video reception device  4 A determines a prescribed area including line-of-sight position  72 C as attention area  91 C. Ship  84  watched by the user thus holds identicalness with original video images. An area other than attention area  91 C on frame  112  is defined as the non-attention area, which is subjected to down-conversion processing. 
     According to the fifth embodiment, for example, an area in the vicinity of the line-of-sight position of the user within the frame of display  5  is defined as the attention area, and an area other than that is defined as the non-attention area. Therefore, in an area within the frame watched by the user, identicalness with original video images is held, whereas an area not watched by the user is subjected to prescribed compression processing. Therefore, compression and low-latency distribution of video data can be achieved without giving uncomfortable feeling to a user who looks at the frame. 
     Sixth Embodiment 
     In the fifth embodiment, an example in which the attention area is determined depending on a line-of-sight position of a user is described. In a sixth embodiment, an example in which the attention area is fixed based on a time period for which a line-of-sight position is maintained. When a user is gazing the same position on the frame for a long period of time, the user may be highly interested in that position. Therefore, even though the user averts his/her eyes from the position of gaze, the user may be highly likely to look at that position again. Therefore, when the same position is gazed for a long period of time, the attention area is fixed. 
     The video transport system according to the sixth embodiment is configured as in the fifth embodiment. Processing by attention area determination unit  49 A of video reception device  4  is different from that in the fifth embodiment. 
       FIG. 33  is a flowchart showing details of attention area determination processing (step S 52  in  FIG. 28 ) according to the sixth embodiment of the present disclosure. 
     Referring to  FIGS. 27 and 33 , attention area determination unit  49 A of video reception device  4  determines whether or not attention area  91 A has been fixed (S 71 ). When attention area  91 A has been fixed (YES in S 71 ), attention area determination processing (step S 52  in  FIG. 28 ) ends. 
     When attention area  91 A has not been fixed (NO in S 71 ), attention area determination unit  49 A performs processing in steps S 61  and S 62 . This processing is similar to that shown in  FIG. 29 . 
     Attention area determination unit  49 A has information on the line-of-sight position detected in step S 61  recorded in a not-shown storage, together with information on time of detection of the line-of-sight position (S 72 ). 
     Attention area determination unit  49 A determines whether or not the line-of-sight position has remained in the same small block for a certain period of time or longer based on information on the line-of-sight position and time of detection recorded in the storage (S 73 ). For example, attention area determination unit  49 A determines whether or not a state that the line-of-sight position is present within small block  12 E lasts for a certain period of time or longer. 
     When a result of determination is true (YES in S 73 ), attention area determination unit  49 A thereafter fixes attention area  91 A for a prescribed time period (S 74 ). 
     When the result of determination is false (NO in S 73 ), attention area determination unit  49 A quits attention area determination processing (step S 52  in  FIG. 28 ). 
     According to the sixth embodiment, as the user gazes a prescribed position in the frame or a position in the vicinity of the prescribed position, the attention area can be fixed for a prescribed period of time. A position in the vicinity of the prescribed position refers, for example, to a position belonging to a small block where the prescribed position is located. Thus, even when the user momentarily averts his/her line of sight after gazing, the attention area remains fixed. Therefore, when the user thereafter moves the line of sight back to the original position, the user can immediately watch video images identicalness of which with original video images is held. Definition of the vicinity of the prescribed position is not limited as above. 
     Seventh Embodiment 
     In the fifth and sixth embodiments, an example in which there is a single user is described. In a seventh embodiment, an example in which there are a plurality of users will be described. 
     The video transport system according to the seventh embodiment is configured as in the fifth embodiment. The seventh embodiment is different from the fifth embodiment in that a plurality of attention areas are determined by attention area determination unit  49 A of video reception device  4 A. 
       FIG. 34  is a diagram showing exemplary display  5  and exemplary camera  6 . Display  5  and camera  6  shown in  FIG. 34  are similar to those shown in  FIG. 25 . In the seventh embodiment, unlike the fifth embodiment, a plurality of users are assumed to look at the frame of display  5 . For example, user  61 A and a user  61 B are assumed to look at the frame of display  5 . For example, user  61 A is assumed to turn his/her eyes in line-of-sight direction  71 A and to watch motorcycle  81  shown on the frame. User  61 B is assumed to turn his/her eyes in a line-of-sight direction  71 B and to watch a car  82  shown on the frame. 
     Attention area determination unit  49 A of video reception device  4 A receives video data from video image data obtaining unit  51  and determines a line-of-sight position of the user on the frame of display  5  based on the video data. How to determine the line-of-sight position is the same as in the fifth embodiment. In the example in  FIG. 34 , attention area determination unit  49 A determines an intersection between line-of-sight direction  71 A and the frame as a line-of-sight position  72 A of user  61 A. Attention area determination unit  49 A determines an intersection between line-of-sight direction  71 B and the frame as line-of-sight position  72 B of user  61 B. Line-of-sight position  72 A and line-of-sight position  72 B are expressed, for example, by coordinates of video data shown on the frame. 
     Attention area determination unit  49 A determines the attention area within video data shown on display  5  based on determined line-of-sight position  72 A and line-of-sight position  72 B. 
       FIG. 35  is a diagram for illustrating a method of determining an attention area.  FIG. 35  shows an example in which image data  10  shown on the frame of display  5  is divided into a plurality of small blocks  12 . User  61 A is assumed to look, for example, at the inside of small block  12 E. In other words, line-of-sight position  72 A of user  61 A is assumed to be present within small block  12 E. Attention area determination unit  49 A determines that line-of-sight position  72 A is included in small block  12 E based on the coordinate of line-of-sight position  72 A. Attention area determination unit  49 A determines an area composed of a plurality of small blocks  12  including small block  12 E as attention area  91 A. For example, attention area determination unit  49 A determines an area composed of small block  12 E and eight proximate small blocks  12  adjacent to small block  12 E as attention area  91 A. 
     User  61 B is assumed to look, for example, at the inside of small block  12 F. In other words, line-of-sight position  72 B of user  61 B is assumed to be present within small block  12 F. Attention area determination unit  49 A determines that line-of-sight position  72 B is included in small block  12 F based on the coordinate of line-of-sight position  72 B. Attention area determination unit  49 A determines an area composed of a plurality of small blocks  12  including small block  12 F as an attention area  91 B. For example, attention area determination unit  49 A determines an area composed of small block  12 F and eight proximate small blocks  12  adjacent to small block  12 F as attention area  91 B. 
     The size of attention area  91 A and the size of attention area  91 B are by way of example, and not limited as illustrated. A shape or a color of an object can accurately be observed by the sense of sight of humans in a range called a central vision within an angle of approximately one to two degrees from the line-of-sight direction. Therefore, when an approximate distance from user  61 A or  61 B to display  5  is known, the central vision on the frame can also be defined. Therefore, the central visions around line-of-sight positions  72 A and  72 B may be determined as attention areas  91 A and  91 B, respectively. 
     Thus, in video reception device  4 A, an area other than attention area  91 A and attention area  91 B on the frame is defined as the non-attention area, which is subjected to down-conversion processing. 
     According to the fourth embodiment, the attention area is determined for each user based on the line-of-sight position of the user. Therefore, even though a plurality of users are looking at different positions on the same frame, an area in the vicinity of the line-of-sight position of each user is defined as the attention area and identicalness with original video images is held in each attention area. Therefore, uncomfortable feeling is not given to the plurality of users. 
     Though line-of-sight positions of the plurality of users are determined based on video data resulting from image capture by camera  6  in the fourth embodiment, camera  6  may be provided for each user. For example, in the example shown in  FIG. 34 , camera  6  for image capture of user  61 A and camera  6  for image capture of user  61 B may be provided. Attention area determination unit  49 A determines the attention area based on video data resulting from image capture by each camera  6 . 
     Eighth Embodiment 
     In the embodiments described above, a frame of video data is divided into the attention area and the non-attention area. In an eighth embodiment, an example in which the non-attention area is further divided into two types of non-attention areas will be described. 
     The video transport system according to the seventh embodiment is similar in configuration to that in the fifth embodiment. The eighth embodiment is different from the fifth embodiment in that attention area determination unit  49 A of video reception device  4 A determines two types of non-attention areas. 
       FIG. 36  is a diagram for illustrating a method of determining an attention area and a non-attention area.  FIG. 36  shows an example in which image data  10  shown on the frame of display  5  is divided into a plurality of small blocks  12 . 
     As in the fifth embodiment, attention area determination unit  49 A determines attention area  91 A based on line-of-sight position  72 A of user  61 A. Then, attention area determination unit  49 A determines an area adjacent to attention area  91 A as a non-attention area  92 A. For example, attention area determination unit  49 A determines sixteen small blocks  12  arranged around attention area  91 A as non-attention area  92 A. Furthermore, attention area determination unit  49 A determines an area other than attention area  91 A and non-attention area  92 A of image data  10  as a non-attention area  92 B. 
     Attention area determination unit  49 A generates attention area information for designating attention area  91 A, non-attention area  92 A, and non-attention area  92 B and provides generated attention area information to transmitter  50 . Transmitter  50  transmits the attention area information to video transmission device  2 . 
     Receiver  31  of video transmission device  2  receives attention area information from video reception device  4 A and provides the attention area information to area designation unit  25 . 
     Area designation unit  25  provides small blocks  12  in attention area  91 A to video image alignment unit  27  based on the attention area information received by receiver  31  and provides small blocks  12  in non-attention area  92 A and non-attention area  92 B to down-converter unit  26 . At this time, area designation unit  25  provides identification information (information for identifying non-attention area  92 A and non-attention area  92 B) of the non-attention area to down-converter unit  26 . 
     Down-converter unit  26  performs down-conversion processing on small blocks  12  with a compression rate of small blocks  12  being varied based on the identification information of the non-attention area. In other words, down-converter unit  26  determines the compression rate such that small block  12  corresponding to non-attention area  92 A is lower in compression rate than small block  12  corresponding to non-attention area  92 B and performs down-conversion processing on small blocks  12  based on the determined compression rates. Relation of the compression rate with non-attention area  92 A and non-attention area  92 B may be set in advance. 
     According to the eighth embodiment, compression processing can be performed in such a manner that the compression rate is lower for non-attention area  92 A closer to the center of the field of view of the user in the non-attention area and the compression rate is higher for non-attention area  92 B more distant from the center. Therefore, low-latency distribution of video data can be achieved while sudden change in how video images look at a portion of boundary between the attention area and the non-attention area is prevented. 
     The type of the non-attention area is not limited to the two types, and three or more types of non-attention areas may be provided. In this case, the compression rate is desirably lower for the non-attention area closer to attention area  91 A. 
     Area determination unit  24  of video transmission device  2  shown in  FIG. 2  may determine a plurality of types of non-attention areas similarly to attention area determination unit  49 A. 
     Attention area determination unit  49  of video reception device  4  shown in  FIG. 20  may determine a plurality of types of non-attention areas similarly to attention area determination unit  49 A. 
     [First Modification] 
     Though a difference between small blocks  12  is calculated in accordance with the Expression 1 in the embodiments described above, the method of calculating a difference is not limited as such. For example, a peak signal-to-noise ratio (PSNR) between small blocks  12  may be adopted as a difference between small blocks  12 . In this case, as the PSNR is higher, two small blocks  12  are more similar to each other, whereas as the PSNR is lower, two small blocks  12  are less similar to each other. Therefore, video transmission device  2  determines small block  12  as block A (attention area) when the PSNR is lower than a prescribed threshold value, and determines small block  12  as block B (non-attention area) when the PSNR is higher than the prescribed threshold value. 
     [Second Modification] 
     Though the attention area is determined based on the difference between small blocks  12  in the first to third embodiments, the method of determining the attention area is not limited as such. For example, when camera  1  is attached to a drone, video transmission device  2  may determine the attention area based on a direction of travel of the drone. For example, small block  12  where surroundings in a direction of travel of the drone are shown may be determined as the attention area. The direction of travel of the drone may be received from a control device of the drone. Alternatively, the direction of travel of the drone may be found based on movement of a subject in an image. For example, when the subject in the image moves to the left with camera  1  being attached to a front surface of the drone, the drone can be determined as traveling to the right. Movement of the subject can be found, for example, by calculating an optical flow by image processing. 
     According to the second modification, low-latency distribution of video data identicalness of which with original video images is held in the attention area determined based on the direction of travel of the drone can be achieved. Thus, for example, the drone can fly in a stable manner. An object to which camera  1  is attached is not limited to the drone, and the camera may be attached to another mobile body such as heavy equipment. 
     [Third Modification] 
     When video data includes an image of an object of visual inspection, an area including a portion to be inspected of the object may be defined as the attention area. The attention area may be designated by a user in accordance with the method shown in the fourth embodiment or by a processing device connected to video reception device  4 . 
     According to a third modification, low-latency distribution of video data identicalness of which with original video images is held in a portion to be inspected of an object of visual inspection can be achieved. Therefore, visual inspection of the object can be conducted with less delay. 
     [Fourth Modification] 
     Though small blocks  12  are categorized into any of the attention area and the non-attention area in the embodiments and the modifications, areas into which small blocks  12  are categorized are not limited to these two types of areas. 
     For example, small blocks  12  may be categorized into any of the attention area, a peripheral area, and a non-transport area. The peripheral area refers to an area located around the attention area (for example, an area adjacent to the attention area). The non-transport area refers to an area other than the attention area and the peripheral area in the area within a frame. 
     The peripheral area is an area around the attention area, although it is out of the range of the attention area. Therefore, though detailed video image information is not required for the peripheral area, video image information to such an extent that a user can recognize an object is required. Therefore, video transmission device  2  is controlled to perform down-conversion processing on the peripheral area as the non-attention area described above. Thus, an amount of data transport from video transmission device  2  to video reception device  4  can be reduced while certain visual recognizability is secured for the non-attention area. Video transmission device  2  does not perform down-conversion processing on the attention area as in the embodiments described above. 
     Video transmission device  2  does not transport small blocks  12  belonging to the non-transport area to video reception device  4 . Therefore, an amount of data transport from video transmission device  2  to video reception device  4  can be reduced. As small blocks  12  belonging to the non-transport area are not transported, video transmission device  2  does not have to perform down-conversion processing on the non-transport area. Therefore, an amount of processing in video transmission device  2  can be reduced. 
     Small block  12  may be categorized into four or more areas. 
     [Fifth Modification] 
     In the first embodiment described above, when all small blocks  12  included in a single large block  14  are B blocks, that large block  14  is subjected to down-conversion processing to generate a C block ( FIG. 9 ). When one large block  14  includes even a single A block, that large block  14  is not subjected to down-conversion processing ( FIG. 10 ). 
     In contrast, all large blocks  14  included in each piece of image data that composes video data may be subjected to down-conversion processing and thereafter small block  12  not to be subjected to down-conversion processing may be determined. 
     Specifically, referring to  FIG. 2 , block divider  21  successively divides image data into large blocks  14  and provides large blocks  14  to area designation unit  25 . 
     Area designation unit  25  receives large blocks  14  from block divider  21  and provides large blocks  14  to down-converter unit  26 . 
     Down-converter unit  26  performs down-conversion processing on large blocks  14  received from block divider  21  to generate C blocks. 
     Video image alignment unit  27  provides the C blocks to video image compressor  28  and provides position information and block information of the C blocks to compressed video image alignment unit  29 . 
     Video image compressor  28  performs video image compression processing on the C blocks received from video image alignment unit  27  and provides the resultant C blocks to compressed video image alignment unit  29 . 
     Compressed video image alignment unit  29  receives compressed blocks from video image compressor  28 . Compressed video image alignment unit  29  adds the position information and the block information obtained from video image alignment unit  27  to the compressed blocks in the order of reception of the compressed blocks and provides the resultant blocks to transmitter  30 . 
     Transmitter  30  includes a communication interface, and encodes the compressed blocks to which the position information and the block information are added and transmits them as compressed video data to video reception device  4 . 
     Through processing so far, compressed video data resulting from down-conversion processing of large blocks  14  that compose image data is transmitted to video reception device  4 . 
     Thereafter, video transmission device  2  performs processing as in the first embodiment onto the same image data. When small blocks  12  that compose large block  14  are all B blocks, processing for that large block  14  is not performed. Thus, redundant generation of a C block can be prevented, and only A blocks or B blocks can be transmitted to video reception device  4 . 
       FIG. 23  is a diagram showing exemplary compressed video data.  FIG. 23  shows data for one frame resulting from compression of image data  10  shown in  FIG. 7 . In other words, since large blocks  14  included in image data  10  are all converted to C blocks, compressed video data from the first row to the fifth row are all composed of C blocks. 
     The sixth row of compressed video data is composed of small blocks  12  included in large blocks  14 H and  14 I of image data  10 . 
     Furthermore, the seventh row of compressed video data is composed of small blocks  12  included in large blocks  14 E to  14 G of image data  10 . 
     [Sixth Modification] 
     Down-converter unit  26  of video transmission device  2  may perform processing for reducing a color depth of each pixel within the non-attention area as prescribed compression processing. For example, the color depth of each pixel in original video data is assumed as full color of 24 bits per pixel (bpp). In other words, luminance of each of RGB of each pixel is expressed by eight bits. Down-converter unit  26  converts luminance of each color into pixel data of 12 bpp expressed by four of eight bits. 
     Up-converter unit  46  of video reception device  4  converts pixel data in which each color is expressed by four bits into pixel data of 24 bpp in which each color is expressed by eight bits, with higher-order four bits corresponding to four bits of each color and four lower-order bits being padded with 0. 
     According to the sixth modification, since the color depth of each pixel within the non-attention area can be reduced, low-latency distribution of video data can be achieved. Since the non-attention area corresponds to a periphery of the field of view of the user, the user is less likely to notice reduction in color depth even when it occurs. 
     [Additional Aspects] 
     At least a part of the embodiments and the modifications may be combined in any manner. 
     It should be understood that the embodiments disclosed herein are illustrative and non-restrictive in every respect. The scope of the present disclosure is defined by the terms of the claims rather than the meaning above and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims. 
     REFERENCE SIGNS LIST 
     
         
           1  camera 
           2  video transmission device 
           3  network 
           4  video reception device 
           4 A video reception device 
           5  display 
           6  camera 
           10  image data 
           11  airplane 
           12  small block 
           12 A small block 
           12 B small block 
           12 C small block 
           12 D small block 
           12 P small block 
           12 Q small block 
           12 R small block 
           12 S small block 
           14  large block 
           14 A large block 
           14 B large block 
           14 C large block 
           14 D large block 
           14 E large block 
           14 F large block 
           14 G large block 
           14 H large block 
           14 I large block 
           14 Z large block 
           16  downsized block 
           21  block divider 
           22  buffer 
           23  subtractor 
           24  area determination unit 
           25  area designation unit 
           26  down-converter unit (compression processing unit) 
           27  video image alignment unit 
           28  video image compressor 
           29  compressed video image alignment unit 
           30  transmitter 
           31  receiver 
           41  receiver 
           42  information extractor 
           44  video image decompressor (decompressor) 
           45  video image alignment unit 
           46  up-converter unit 
           47  video image composite unit 
           48  position information obtaining unit 
           49  attention area determination unit 
           49 A attention area determination unit 
           50  transmitter 
           51  video image data obtaining unit 
           61 A user 
           61 B user 
           61 C user 
           71 A line-of-sight direction 
           71 B line-of-sight direction 
           71 C line-of-sight direction 
           72 A line-of-sight position 
           72 B line-of-sight position 
           72 C line-of-sight position 
           81  motorcycle 
           82  car 
           83  ship 
           84  ship 
           91 A attention area 
           91 B attention area 
           91 C attention area 
           92 A non-attention area 
           92 B non-attention area 
           100  video transport system 
           100 A video transport system 
           110  drone 
           111  controller 
           112  frame 
           113  joystick 
           120 A image capture range 
           120 B image capture range