Patent Publication Number: US-2011064276-A1

Title: Image transmission device and image reception device

Description:
FIELD OF THE INVENTION 
     The present invention relates to an image transmission device and an image reception device used in a system for transmitting and receiving a moving image to and from apparatuses connected to a data communication line having a limited data transmission capacity such as network. 
     BACKGROUND OF THE INVENTION 
     A disclosed conventional system encodes image data with a high resolution in a particular region of an image and a low resolution in any other region of the image to transmit the image data via a network having a limited bandwidth, thereby ensuring an intended image quality while reducing an information volume of the image data (for example, see the Patent Document 1). 
     Prior Art Document 
     Patent Document 
     Patent Document 1: Unexamined Japanese Patent Applications Laid-Open No. 07-288806   
     SUMMARY OF THE INVENTION 
     Problem to be Solved by the Invention 
     In recent years, the development of image pickup devices is advancing to achieve a higher number of pixels, all the more increasing an image data volume to be handled. An advanced technique demanded under the circumstances for an apparatus which transmits image data to a network such as a network camera is to minimize the deterioration of an image quality when a moving image is transmitted to the network. When the resolution of a particular region is increased in the system disclosed in the Patent Document 1, the resolution of any other region is far lower than expected. This possibly makes the image quality too poor, and contents of the image reproduced in a reception apparatus may not be visually determined in some regions. 
     Means for Solving the Problem 
     An image transmission device according to the present invention comprises:
     an image obtaining unit for obtaining an image data inputted from outside;   an encoding unit for selecting, as a color component to be encoded, a part of a plurality of color components constituting a color information of the image data obtained by the image obtaining unit while periodically changing the color component to be encoded to encode the image data of the selected color component to be encoded; and   a data transfer unit for transferring the image data of the selected color component encoded by the encoding unit to a data communication line.   

     An image reception device according to the present invention comprises:
     a data receiving unit for receiving an encoded image data of a color component to be encoded via a data communication line, the color component to be encoded being a part of a plurality of color components constituting a color information of the image data and selected therefrom through periodical changes;   a decoding unit for decoding the encoded image data of the color component to be encoded received by the data receiving unit;   a storage unit for storing therein the encoded image data of the color component to be encoded decoded by the decoding unit; and   a combining unit for combining the image data of the current color component to be encoded decoded by the decoding unit with an image data of a past color component to be encoded representing a color different to the current color component to be encoded and decoded earlier and already stored in the storage unit.   

     According to the present invention, the encoding unit of the data transmission device selects the color component to be encoded from the plurality of color components in the inputted image data while periodically changing the color component to be encoded, and encodes the image data of the selected color component using a predefined encoding method and sends the encoded image data to the data transfer unit. The data transfer unit transfers the encoded image data of the color component to be encoded to the image reception device via the data communication line. The encoding unit of the image reception device repeatedly encodes the image data while periodically changing the color component to be encoded. According to the present invention, only a part of the color components of the inputted image data are chosen as the color component to be encoded by the encoding unit and the color component to be transferred by the data transfer unit. Therefore, an information volume of the image data to be processed is lessened, and an encoding rate is accordingly controlled. As a result, the present invention can transmit the image data with less deterioration of its image quality via the data communication line having a limited data transmission capacity. 
     Another technical characteristic of the image reception device according to the present invention is for the combining unit to combine the image data of the current color component to be encoded decoded by the decoding unit with the image data of the past color component to be encoded different to the current color component and decoded earlier and already stored in the storage unit, so that the image is reproduced. According to these technical features of the storage unit and the combining unit, the image reception device can reproduce the original image data with less deterioration of its image quality even if the image data transmitted through the data communication line represents only a part of a plurality of color components in the inputted image data. 
     According to an exemplary mode of the present invention, the encoding unit of the image transmission device encodes the image data while periodically changing the color component to be encoded in the order of Y component, UV component, Y component, UV component per frame, or in the order of Y component, UV component, Y component, UV component per frame. 
     According to another preferred mode of the present invention, the image transmission device further comprises a division pattern setting unit for setting a color component division pattern indicating setting of change of the color component to be encoded, wherein
     the encoding unit selects the color component to be encoded from the plurality of color components while periodically changing the color component to be encoded in accordance with the color component division pattern set by the division pattern setting unit and then encodes the image data of the selected color component to be encoded, and   the data transfer unit appends the color component division pattern set by the division pattern setting unit to the encoded image data of the selected color component to be encoded and transfers the resulting encoded image data to the data communication line.   

     It is preferable in the another preferred mode that the data transfer unit measure a communication load of the data communication line, and the division pattern setting unit set the color component division pattern depending on the communication load measured by the data transfer unit. 
     According to the another preferred mode, the division pattern setting unit is able to arbitrarily set the color component division pattern depending on changing congestion situation of the data communication line. Thus, the image data can be transmitted with less deterioration of its image quality irrespective of how busy the data communication line is. 
     According to still another preferred mode of the present invention, the image transmission device further comprises a motion detecting unit for detecting a quantity of image motion in the inputted image data, wherein
     the encoding unit encodes all of the color components constituting the image data when the encoding unit determines that the quantity of image motion detected by the motion detecting unit is greater than a given threshold value.   

     Possible degradation of the image quality due to lower resolution is visually not a noticeable disadvantage in any image moving very fast. Therefore, the image data of all of the color components are encoded in place of selecting the color component to be encoded when the quantity of image motion is greater than the threshold value. As a result, an image processing speed can be improved. 
     According to still another preferred mode of the present invention, the image transmission device further comprises a motion detecting unit for detecting a quantity of image motion in the inputted image data, wherein
     the encoding unit changes the color component to be encoded per frame,   the motion detecting unit detects a quantity of motion of a color component different to the color component to be encoded, and   the data transfer unit further transfers the quantity of motion of the color component different to the color component to be encoded to the data communication line.   

     When the data transfer unit thus transfers the encoded data of the color component to be encoded and the quantity of motion of the color component different to the color component to be encoded to the data communication line, the image reception device can perform a motion compensation to the image data of the past color component to be encoded depending on the quantity of image motion. 
     According to still another preferred mode of the present invention, the image reception device comprises, to deal with the image transmission device according to the mode described earlier, a motion compensating unit, a smoothening unit, and a combining unit, wherein
     the data receiving unit further receives a quantity of motion of a color component different to the color component to be encoded,   the motion compensating unit performs a motion compensation to the past image data stored in the storage unit using the quantity of motion of the color component newly received,   the smoothening unit smoothens the past image data stored in the storage unit and the image data of the color component to be encoded newly received and decoded by the decoding unit depending on the quantity of motion of the color component newly received, and   the combining unit combines the image data of the color component to be encoded newly received by the data receiving unit and smoothened by the smoothening unit and then decoded by the decoding unit with the past image data motion-compensated by the motion compensating unit.   

     Due to a time lag generated between the image data of the current color component to be encoded and the image data of the past color component to be encoded, there may be color drift in a moving photographic subject. The motion compensating unit and the smoothening unit are provided to alleviate the color drift. 
     Effect of the Invention 
     According to the present invention wherein only a part of the color components are chosen from the image data to be encoded and transmitted when the image signal is transmitted through the data communication line such as network, the volume of information to be encoded can be lessened, and the encoding rate is thereby controlled. This technical advantage can transmit an image with less deterioration of its image quality. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram illustrating an overall structure of an image communication apparatus according to an exemplary embodiment 1 of the present invention. 
         FIG. 2  is an illustration of an operation according to the exemplary embodiment 1 in which image data is divided into Y components, U components, and V components, transmitted and received in the order of Y, U, V. 
         FIG. 3  is an illustration of an operation according to the exemplary embodiment 1 in which image data is divided into Y components, U components, and V components, and transmitted and received in the order of Y, U, Y, V. 
         FIG. 4  is an illustration of an operation according to the exemplary embodiment 1 in which image data is divided into Y components and UV components, and transmitted and received in the order of Y, UV. 
         FIG. 5  is an illustration of an operation according to the exemplary embodiment 1 in which image data is divided into R components, G components, and B components, and transmitted and received in the order of R, G, B. 
         FIG. 6  is a block diagram illustrating an overall structure of an image communication apparatus according to an exemplary embodiment 2 of the present invention. 
         FIG. 7  is a block diagram illustrating an overall structure of an image communication apparatus according to an exemplary embodiment 3 of the present invention. 
         FIG. 8  is a block diagram illustrating an overall structure of an image communication apparatus according to an exemplary embodiment 4 of the present invention. 
         FIG. 9  illustrates a motion vector and a predicted error according to the exemplary embodiment 4. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Hereinafter, exemplary embodiments of the present invention are described in detail referring to the drawings. The exemplary embodiments described below are just a few examples and can be variously modified. 
     Exemplary Embodiment 1 
       FIG. 1  is a block diagram illustrating an overall structure of an image communication apparatus A 1  according to an exemplary embodiment 1 of the present invention. The present exemplary embodiment provides a system for transmitting an image having a sizable data volume via a data communication line having a limited data transmission capacity such as network. The present exemplary embodiment is not necessarily limited to the network but can be applied to a variety of data communication lines which interconnects an image transmission device and an image reception device. 
     The present exemplary embodiment is technically characterized in that an image data of a part of a plurality of color components constituting a color information of the image data is transmitted in one frame as a color component to be encoded, so that the current color component to be encoded received on reception side is combined with an image data of a past color component to be encoded (representing a color different to the current color component to be encoded). This technical advantage can reduce an information volume of the image data and controls an image encoding rate, thereby transmitting an image with less deterioration of its image quality. 
     The image communication apparatus Al has an image transmission device  10 A, an image reception device  20 A, and a network  30 . The network  30  transmits data to and from the mage transmission device  10 A and the image reception device  20 A. An image pickup device  40  is connected to the image transmission device  10 A, and a display device  50  is connected to the image reception device  20 A. Apart from the image pickup device  40 , a device which outputs image data can be connected to the image transmission device  10 A. 
     The image transmission device  10 A has an image obtaining unit  11 , an encoding unit  12 , and a data transfer unit  13 . The image obtaining unit  11  fetches an image data inputted from the image pickup device  40  into a memory. The image obtaining unit  11  can handle the inputted image data in the format of YUV or RGB. YUV represents a luminance/color difference multiplex signal, Y represents a luminance signal, U represents a red color difference signal, and V represents a blue color difference signal. 
     The encoding unit  12 ;
     reads the image data from the memory of the image obtaining unit  11 ,   selects whenever necessary a part of a plurality of color components constituting a color information of the read image data as a color component to be encoded,   encodes the image data of the selected color component to be encoded using a conventional image compression algorithm such as prediction encoding, orthogonal encoding or vector quantization, and   outputs the resulting image data of the color component to be encoded to the data transfer unit  13 .   

     The selection of the color component to be encoded follows a color component division pattern. The information of color component division pattern indicates which of a plurality of color component division patterns including, for example, a color component division pattern illustrated in  FIG. 2  and a color component division pattern illustrated in  FIG. 3  is used to transmit and receive the image data. The color component division pattern is set in advance in a storage device such as ROM. In the technical structure described so far, the color component to be encoded is periodically changed in accordance with the color component division pattern. 
     The data transfer unit  13  transmits the image data of the color component to be encoded to the network  30 . 
     The image reception device  20 A has a data receiving unit  21 , a decoding unit  22 , a storage unit  23 , and a combining unit  24 . The data reception unit  21  receives the encoded data of the color component to be encoded from the network  30  and outputs the received encoded data to the decoding unit  22 . The decoding unit  22  decodes the encoded data of the color component to be encoded and outputs the decoded data to the storage unit  23 . The combining unit  24  combines the image data of the current color component to be encoded decoded by the decoding unit  22  with the image data of the past color component to be encoded representing a color different to the current image data and read from the storage unit  23  to thereby restore an image. The combining unit  24  implements the image decoding process described so far in accordance with the preset color component division pattern. 
     The image decoding process carried out by the combining unit  24  is described referring to  FIG. 2  based on an example in which the inputted image data has YUV format, and the color components of the inputted image data are divided into Y components, U components, and V components and transmitted in the order of Y, U, V, Y, U, V. 
     In the description given below, for example, the image obtaining unit  11  of the image transmission device  10 A obtains the image data of nth frame. 
     [Processing in nth Frame] 
     Upon the reception of the image data of nth frame, the encoding unit  12  encodes a Y component Y n  of the inputted image data in nth frame as the color component to be encoded and outputs an encoding result thereby obtained to the data transfer unit  13 . The data transfer unit  13  transmits the encoded data of the color component to be encoded to the network  30 . The decoding unit  22  of the image reception device  20 A receives the encoded data of the Y component Y n  of the inputted image data in nth frame and decodes the received encoded data, and then stores a decoding result thereby obtained in the storage unit  23 . 
     The combining unit  24  combines;
     Y component Y n  which is the color component to be encoded of current nth frame received and stored in the storage unit  23 ,   V component V n−1  which is the color component to be encoded of (n−1)th frame received immediately before the current frame, and   U component V n−2  which is a color component to be encoded of (n−2)th frame received two frames before the current frame.   

     Then, the combining unit  24  reproduces the image data thus combined as a reproduction image P n  of nth frame. 
     [Processing in (n+1)th Frame] 
     Upon the reception of the image data of (n−1)th frame, the combining unit  24  combines;
     V component V n−1  which is the color component to be encoded of (n−1)th frame,   Y component Y n  which is the color component to be encoded of nth frame   U component U n+1  which is the color component to be encoded of (n+1)th frame.   

     Then, the combining unit  24  reproduces the image data thus combined as a reproduction image P n+1  of (n+1)th frame. 
     The image transmission device  10 A and the image reception device  20 A repeatedly process the image data as described above while changing the encoding color component to be updated. 
     To transmit the image data, the color components of the image data may be divided into different groups as illustrated in  FIGS. 3 and 4 . In the example illustrated in  FIG. 3 , the color components of the image data are divided into Y components, U components, and V components, and the color components to be encoded are changed in the order of Y, U, Y, V, Y, U, Y, V during the image data transmission. 
     In (n−2)th frame, the encoded data of U component U n−2  in the inputted image data is transmitted as the color component to be encoded. In (n−1)th frame, the encoded data of Y component Y n−1  in the inputted image data is transmitted as the color component to be encoded. 
     The decoding unit  22  decodes;
     encoded data of U component U n−2  which is the color component to be encoded of (n−2)th frame stored in the storage unit  23 , and   encoded data of U component Y n−1  which is the color component to be encoded of (n−1)th frame.   

     The combining unit combines;
     U component U n−2  and Y component Y n−1  which are the image data of the color components to be encoded in (n−2)th frame and (n−1)th frame decoded by the decoding unit  22 ,   reproduced image data of nth frame, and   V component V n  of the inputted image data received in the current frame.   

     The combining unit  24  then outputs the image data thus combined to the display device  50 , so that the image data is reproduced thereon as a reproduction image P n .
     In (n+1)th frame, the encoded data of Y component Y n+1  in the inputted image data is updated by the decoding unit  22 .   

     The combining unit  24  combines,
     Y component Y n+1  of the inputted image data in (n+1)th frame,   V component V n  of the inputted image data in nth frame, and   U component U n+2  of the inputted image data in (n−2)th frame.   

     The combining unit  24  then outputs the image data thus combined to the display device  50 , so that the image data is reproduced thereon. 
     In the example illustrated in  FIG. 4 , the color components of the image data are divided into Y components and UV components, and these components are transmitted in turn. 
     [Processing in nth Frame] 
     The data transfer unit  13  of the image transmission device  10 A transmits the encoded data of UV component UV n  of the inputted image data (color component to be encoded in nth frame) to the network  30 . The combining unit  24  of the image reception device  20 A combines Y component Y n−1  of the image data received in (n−1)th frame and stored in the storage unit  23  (color component to be encoded in (n−1)th frame) with UV component UV n  of the image data received in the current frame (color component to be encoded in nth frame), and reproduces an image obtained from the combined image data. 
     [Processing in (n+1)th Frame] 
     The data transfer unit  13  of the image transmission device  10 A transmits the encoded data of Y component Y n+1  of the inputted image data (color component to be encoded in (n+1)th frame) to the network  30 . The combining unit  24  of the image reception device  20 A combines UV component UV n  of the image data received in nth frame (color component to be encoded in nth frame) with Y component Y n+1  of the image data received in the current frame (color component to be encoded in (n+1)th frame), and reproduces an image obtained from the combined image data. 
     The combining unit  24  reproduces the image data, and outputs the post-reproduction image data to the display device  50 . The combining unit  24  thereafter repeatedly updates the Y components and the UV components in turn. 
     There are many other different methods for dividing and combining the color components other than the example described so far, for example, the YU components and YV components of the image data are transmitted in turn every other frame. The signal of YUV format is not necessarily limited to YUV444. Such formats as YUV422, YUV420, and YUV 411 can further lessen the information volume. An image of RGB format can be inputted to the image obtaining unit  11 , and the image obtaining unit  11  may be equipped with a function to convert YUV format into RGB format so that the image data can be divided into R, G, and B components and then transmitted as illustrated in  FIG. 5 . 
     As described so far, according to the present exemplary embodiment, the encoding unit  12  fetches a part of the plurality of color components in the inputted image data in one frame as the color component to be encoded and encodes the image data of the fetched color component to be encoded, and the data transfer unit  13  transfers the encoded data to the image reception device  20 A. As a result, the information volume of the image data to be processed is lessened, and the encoding rate is thereby controlled. This technical advantage can transmit the image data with less deterioration of its image quality in the case where the network  30  has a limited transmission capacity. Further, the image reception device  20 A according to the present exemplary embodiment can combine the image data of the current color component to be encoded newly received with the image data of the past color component to be encoded. The image reception device  20 A thus technically advantageous can reproduce the image data with less deterioration of its image quality. 
     Exemplary Embodiment 2 
       FIG. 6  is a block diagram illustrating an overall structure of an image communication apparatus A 2  according to an exemplary embodiment 2 of the present invention. In  FIG. 6 , the same reference symbols as those illustrated in  FIG. 1  according to the exemplary embodiment 1 denote the same structural elements, and description of these structural elements will be omitted. In the image communication apparatus A 2 , an image transmission device  10 B is further provided with a division pattern setting unit  14 . The division pattern setting unit  14  has a changeable storage device such as register, and the storage device stores therein an inputted image data format and information of color component division pattern and. In a manner similar to the exemplary embodiment 1, the information of color component division pattern indicates which of a plurality of color component division patterns including the color component division pattern illustrated in  FIG. 2  and the color component division pattern illustrated in  FIG. 3  is used to transmit and receive the image data. 
     Similarly to the exemplary embodiment 1, the encoding unit  12  of the image transmission device  10 B;
     reads the image data from the memory of the image obtaining unit  11 ,   selects whenever necessary a part of a plurality of color components constituting a color information of the read image data as a color component to be encoded,   encodes the image data of the selected color component to be encoded using a conventional image compression algorithm such as prediction encoding, orthogonal encoding or vector quantization, and   outputs the resulting image data of the color component to be encoded to the data transfer unit  13 .   

     The data transfer unit  13  appends the information of color component division pattern to the inputted encoded data and transmits the resulting encoded data to the network  30 . 
     In an image reception device  20 B, the data receiving unit  21  receives the encoded data of the color component to be encoded from the network  30 , and outputs the received encoded data to the decoding unit  22 . The decoding unit  22  decodes the encoded data of the color component to be encoded and outputs the decoded data to the storage unit  23 . The combining unit  24  reads the image data of the current color component to be encoded decoded by the decoding unit  22  and the image data of the past color component to be encoded previously stored in the storage unit  23  (representing a color different to the current color component to be encoded) from the storage unit  23  and combines these image data to restore an image. The combining unit  24  implements the image decoding process described so far in accordance with the information of color component division pattern transmitted from the image transmission device  10 B along with the image data. The transmission and reproduction of the image data are carried out in a manner similar to the exemplary embodiment 1, therefore, description of these operations is omitted. 
     As described so far, according to the present exemplary embodiment, the division pattern setting unit  14  can set the color component division pattern depending on changing congestion situation of the network  30 . This technical advantage can transmit the image data with less deterioration of its image quality irrespective of how busy the network  30  is. 
     The data transfer unit  13  may be equipped with a function to measure the traffic of the network  30 , wherein the division pattern setting unit  14  can automatically change the color component division pattern depending on the communication load of the network  30 . In the suggested structure, all of the color components of the image data in one frame are transmitted when the communication load of the network  30  is not particularly heavy, and a part of the color components is selected as the color component to be encoded and the image data of the selected color component to be encoded alone is transmitted when the network  30  is very busy. As a result, the data volume of the image data to be transmitted can be increased or decreased depending on how busy the network  30  is. 
     Exemplary Embodiment 3 
       FIG. 7  is a block diagram illustrating an overall structure of an image communication apparatus A 3  according to an exemplary embodiment 3 of the present invention. In  FIG. 7 , the same reference symbols as those illustrated in  FIG. 1  according to the exemplary embodiment 1 denote the same structural elements, and description of these structural elements will be omitted. In the image communication apparatus A 3 , an image transmission device  10 C is further provided with a motion detecting unit  15 . The inputted image data of a frame previous to the current frame is stored in the memory provided in the image obtaining unit  11 , and the motion detecting unit  15  then detects a quantity of image motion is detected from a difference between the inputted image data in the current frame and the inputted image data in the previous frame. A storage device of the motion detecting unit  15  stores therein a threshold value based on which the color component division pattern of the image data is selected. In a storage device of the encoding unit  12  such as ROM, the color component division pattern is stored in advance. 
     The encoding unit  12  compares the quantity of image motion detected by the motion detecting unit  15  to the threshold value. When a comparison result thereby obtained says that the motion quantity is at most the threshold value, the encoding unit  12 , in a manner similar to the exemplary embodiment 1;
     selects whenever necessary a part of a plurality of color components constituting a color information of the read image data as a color component to be encoded in accordance with the color component division pattern previously set,   encodes the image data of the selected color component to be encoded using a conventional image compression algorithm such as prediction encoding, orthogonal encoding or vector quantization, and   outputs the encoded image data to the data transfer unit  13 .   

     When it is known from the comparison result that the motion quantity is greater than the threshold value, the encoding unit  12  does not divide the color components of the image data but encodes the image data of all of the color components to be encoded in one frame. 
     The data transfer unit  13  appends the information of color component division pattern to the encoded data and transmits the resulting encoded data to the network  30 . 
     In an image reception device  20 C, the data receiving unit  21  receives the encoded data of the color component to be encoded from the network  30 , and outputs the received encoded data to the decoding unit  22 . The decoding unit  22  decodes the encoded data of the color component to be encoded and outputs the decoded data to the storage unit  23 . The combining unit  24  determines whether or not the color components were divided in the encoded data received from the network  30 . When the combining unit  24  determines that the color components were divided in the received encoded data, the combining unit  24 , in a manner similar to the exemplary embodiment 1, reads the image data of the current color component to be encoded decoded by the decoding unit  22  and the image data of the past color component to be encoded previously stored in the storage unit  23  (representing a color different to the current color component to be encoded) from the storage unit  23  and combines these image data to restore an image. The combining unit  24  implements the image decoding process described so far in accordance with the information of color component division pattern defined in advance. When it is determined that the encoded data received from the network  30  does not include the divided color components but includes all of the color components, the combining unit  24  directly outputs the image data decoded by the decoding unit  22  to the display device  50 . 
     The image transmission device  10 C may be provided with a division pattern setting unit configured similarly to that of the exemplary embodiment 2 to preset two color component division patterns, so that one of the two color component division patterns is selected based on a threshold value given to select the color component division patterns. 
     As described so far, according to the present exemplary embodiment, the motion detecting unit  15  detects the speed of the image motion, and the image data of all of the color components is encoded in place of selecting the color component to be encoded when the detected quantity of image motion is greater than the threshold value. This technical feature can be effective because any image moving fast has a poor image quality due to low resolution but the poor image quality is visually not a noticeable disadvantage. As a result, an image processing speed can be improved. 
     Exemplary Embodiment 4 
       FIG. 8  is a block diagram illustrating an overall structure of an image communication apparatus A 4  according to an exemplary embodiment 4 of the present invention. In  FIG. 8 , the same reference symbols as those illustrated in  FIG. 7  according to the exemplary embodiment 3 denote the same structural elements, and description of these structural elements will be omitted. In an image reception device  20 D of the image communication apparatus A 4 , the combining unit  24  reads the image data of the current color component to be encoded decoded by the decoding unit  22  and stored in the storage unit  23  and the image data of the past color component to be encoded decoded earlier and stored in the storage unit  23  (representing a color different to the current color component to be encoded) from the storage unit  23  in accordance with the preset color component division pattern, and combines these image data to restore an image. When the image data are thus combined, the image data of a moving photographic subject possibly undergoes color drift due to a time lag generated between the image data of the color component to be encoded in the current frame and the image data of the color component to be encoded in the past frame. The present exemplary embodiment alleviates the color drift. 
     The image reception device  20 D of the image communication apparatus A 4  is further equipped with a motion compensating unit  25  and a smoothening unit  26 . The motion compensating unit  25  performs a motion compensation to the image data of the color component to be encoded in the past frame stored in the storage unit  23  using a predicted error and a motion vector transmitted from the image transmission device  10 D along with the image data of the color component to be encoded in the current frame, and then outputs the motion-compensated image data to the smoothening unit  26 . The predicted error and the motion vector are calculated by the motion detecting unit  15  in the image transmission device  10 D, and transmitted by the data transfer unit  13  to the image reception device  20 D via the network  30 . The smoothening unit  26  blurs the image data depending on the quantity of image motion transmitted along with the image data of the color component to be encoded. The smoothening unit  26  uses a smoothening filter and moving average method to blur the image data. 
     The motion detecting unit  15  of the image transmission device  10 D detects the motion vector between the inputted image data and the image data in a frame previous to the current frame, and creates the predicted image based on the motion vector of the image data in the previous frame. Then, the motion detecting unit  15  appends the predicted image of the color component which is not transmitted in the current frame, predicted error which is a difference between the inputted images, and motion vector to the image data of the color component to be encoded in the current frame along with the quantity of image motion, and transmits the resulting image data. 
     The motion vector is more specifically described. When the inputted image data is divided into m×n blocks, and the block which is most similar (hereinafter, called similar block) to the block to be encoded (hereinafter, called block to be encoded) is detected from the predicted image, the motion vector is generated between the block to be encoded and the similar block.  FIG. 9  is a schematic drawing of the predicted error and the motion vector. 
     The data transfer unit  13  appends the predicted error, motion vector, and quantity of image motion, which are calculated by the motion detecting unit  15  when the encoded image data is transmitted to the network  30 , to the image data, and then transmits the resulting image data. 
     The operation is more specifically described based on an example in which the image data is divided into Y components, U components, and V components, and Y component Y n  of the image data in nth frame is transmitted. In the given example, the motion detecting unit  15  calculates;
     the predicted error and the motion vector from V component V n  of the image data in nth frame and V component V n−1  of the image data in (n−1)th frame, and   the predicted error and the motion vector from the U component U n  of the image data in nth frame and a predicted image U n−1 ′ of U component in (n−1)th frame.   

     Then, the data transfer unit  13  appends the predicted errors and the motion vectors calculated by the motion detecting unit  15  to the image data to be transmitted. The predicted image U n−1 ′ is created from U component U n−1  of the image data in (n−1)th frame and U component U n−2  of the image data in (n−2)th frame. 
     The motion compensating unit  25  of the image reception device  20 D which received these image data performs the motion compensation to V component V n−1  which is the image data of the color component to be encoded in (n−1)th frame and U component U n−2  which is the image data of the color component to be encoded in (n−2)th frame using the predicted errors and the motion vectors transmitted along with the image data of the color component to be encoded in the current frame (nth frame). 
     The smoothening unit  26  smoothens the following components depending on the quantity of image motion received along with the image data;
     V component V n−1  which is the image data of the motion-compensated color component to be encoded in (n−1)th frame,   U component U n−2  which is the image data of the motion-compensated color component to be encoded in (n−2)th frame, and   Y component Y n  which is the image data of the color component to be encoded in the current frame (nth frame).   

     The combining unit  24  combines the following components and reproduces an image from the combined image data;
     Y component Y n  which is the image data of the smoothened color component to be encoded in the current frame (nth frame),   V component V n−1  which is the image data of the motion-compensated/smoothened color component to be encoded in (n−1)th frame, and   U component U n−2  which is the image data of the motion-compensated/smoothened color component to be encoded in (n−2)th frame.   

     The human eyesight is relatively poor for any moving object. Therefore, when the image data of the color components to be encoded in the past and current frames are blurred and then combined by the combining unit  24 , the color drift due to any time lag between the color components to be encoded can be lessened. 
     Another way to smoothen the color components is to divide image data into m×n blocks using the motion detecting unit  15  to detect the quantity of motion by each of the blocks, and transmit the image data with the detected quantity of motion appended thereto so that the color components are smoothened by the smoothening unit  26  by each of the m×n blocks depending on the quantity of motion. The suggested technique can reproduce an image more natural to the eye as far as a suitable number of blocks having a right size are determined because the information volume is increased as the blocks are smaller. 
     To prevent such an unfavorable event that only a part of the image is blurred when the image data is smoothened depending on the quantity of motion by each block, the whole image data may be smoothened based on the block having a largest quantity of motion. It is also effective to reduce colorfulness of the color components in the case of a large quantity of motion so that the color drift is inconspicuous. 
     The image transmission device  10 D may be provided with the division pattern setting unit, wherein the image data is encoded in accordance with the preset color component division pattern, and the information of color component division pattern is transmitted from the data transfer unit  13  along with the image data and the quantity of motion. 
     INDUSTRIAL APPLICABILITY 
     The technology provided by the present invention is advantageously utilized in an image transmission device which transmits image data with less deterioration of its image quality and an image reception device which reproduces the original image data with less deterioration of its image quality in a system for transmitting and receiving a moving image via a data communication line having a limited transmission capacity such as network. 
     DESCRIPTION OF REFERENCE SYMBOLS 
     
         
         A 1 -A 4  image communication apparatus 
           10 A- 10 D image transmission device 
           11  image obtaining unit 
           12  encoding unit 
           13  data transfer unit 
           14  division pattern setting unit 
           15  motion detecting unit 
           20 A- 20 D image reception device 
           21  data receiving unit 
           22  decoding unit 
           23  storage unit 
           24  combining unit 
           25  motion compensating unit 
           26  smoothening unit 
           30  network 
           40  image pickup device 
           50  display device