Patent Publication Number: US-2015078436-A1

Title: Image encoding apparatus, image decoding apparatus and image transmission system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2013-193427, filed Sep. 18, 2013, the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments relate to image transmission. 
     BACKGROUND 
     When an image is transmitted to a remote location, a third person may intercept the communication to peek at or alter the contents of the transmitted image. To prevent this problem occurring, for example, encryption, scrambling, or the like has been utilized to ensure the communication is secure. 
     However, as the security ensured by encryption or scrambling relies on computational complexity, the security is likely to be undermined once the mechanism of the encryption or scrambling is revealed. That is, the third person may perform decryption or descrambling to intercept the communication and consequently peek at or alter the contents of the transmitted image. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram illustrating an image transmission system according to a first embodiment; 
         FIG. 2  is a block diagram illustrating a key image generation unit and a key information generation unit both shown in  FIG. 1 ; 
         FIG. 3  is a block diagram illustrating the key image generation unit and the key information generation unit both shown in  FIG. 1 ; 
         FIG. 4  is a block diagram illustrating an image transmission system according to a second embodiment; 
         FIG. 5  is a block diagram illustrating a key image generation unit and a key information generation unit both shown in  FIG. 4 ; 
         FIG. 6  is a block diagram illustrating an image transmission system according to a third embodiment; 
         FIG. 7  is a block diagram illustrating a key information generation unit in  FIG. 6 ; 
         FIG. 8  is a block diagram illustrating an image encoding unit included in the image transmission system according to the first embodiment; 
         FIG. 9  is a block diagram illustrating an image decoding unit included in the image transmission system according to the first embodiment; 
         FIG. 10  is a block diagram illustrating an image encoding unit included in an image transmission system according to a fourth embodiment; 
         FIG. 11  is a block diagram illustrating an image decoding unit included in the image transmission system according to the fourth embodiment; 
         FIG. 12  is a block diagram illustrating an image encoding unit included in an image transmission system according to a fifth embodiment; 
         FIG. 13  is a block diagram illustrating an image decoding unit included in the image transmission system according to the fifth embodiment; 
         FIG. 14  is a diagram illustrating hardware implementing an image encoding apparatus, an image decoding apparatus, and key information generation apparatus all included in the image transmission system according to each of the embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments will be described below with reference to the drawings. 
     According to an embodiment, an image encoding apparatus includes a key information generator, a key image generator and an image encoder. The key information generator generates key information. The key image generator generates a key image based on a base image and the key information. The base image includes one or more images. The image encoder encodes an input image using the key image and generates encoded data by encoding an input image using the key image. 
     In the description below, elements identical or similar to described elements are denoted by identical or similar reference numerals, and duplicate descriptions are basically omitted. 
     First Embodiment 
     As illustrated in  FIG. 1 , an image transmission system according to a first embodiment includes an image encoding apparatus  100  and an image decoding apparatus  200 . The image encoding apparatus  100  includes an image encoding unit  110 , a key image generation unit  120 , and a key information generation unit  130 . The image decoding apparatus  200  includes an image decoding unit  210  and a key image generation unit  220 . 
     The image encoding unit  110  acquires an input image  10  and receives a key image  14  from the key image generation unit  120 . As described below, the image encoding unit  110  generates encoded data  15  by encoding the input image  10  using the key image  14 . The image encoding unit  110  transmits the encoded data  15  to the image decoding apparatus  200 . The encoded data  15  may be transmitted to the image decoding apparatus  200  by wireless or wired communication or transmitted to the image decoding apparatus  200  via a recording medium. 
     For the encoded data  15 , security is ensured by encoding using the key image  14 . That is, not only the encoded data  15  but also the key image  14  is needed to substantially restore the input image  10 . Hence, even if a third person intercepts the communication to acquire the encoded data  15 , peeking at or altering the contents of the input image  10  is difficult unless the third person can acquire the key image  14 . 
     The key image generation unit  120  receives a base image  11  and also receives key information  13  from the key information generation unit  130 . The key image generation unit  120  generates a key image  14  based on the base image  11  and the key information  13 . The key image generation unit  120  outputs the key image  14  to the image encoding unit  110 . 
     Specifically, the base image  11  may be one or more still images or a moving picture including a plurality of image frames. In view of improvement of a coding rate, the base image  11  preferably contains an image likely to be similar to the input image  10  in terms of, for example, an imaging position and an imaging direction. For example, if the input image  10  is an image taken by a camera mounted on a first moving body (for example, a train or a bus) while the first moving body is traveling along a predetermined route, the base image  11  may be a past image taken by a camera mounted on a second moving body (which may be identical to or different from the first moving body) while the second moving body is traveling along the predetermined route. 
     The key information  13  includes identification information indicative of a part (selected image) of the base image  11  selected as a base for the key image  14 . The identification information may be, for example, a frame number when the base image  11  is a moving picture. The key information  13  may further include a correction parameter for correcting the selected image. 
     The key image  14  may be the selected image itself or the selected image on which image processing as described below has been performed. 
     When the selected image is different from the input image  10  in format, the key information generation unit  120  may generate a key image  14  by converting the format of the selected image into the same format as that of the input image  10 . The format as used herein refers to, for example, an image size, a pixel bit length, or a color format. 
     When the key information  13  includes the correction parameter, the key image generation unit  120  may generate a key image  14  by using the correction parameter to correct the selected image. The key image generation unit  120  may perform gamma correction on the selected image using a gamma value included in the correction parameter. The key image generation unit  120  may perform histogram conversion using histogram information on the input image  10  included in the correction parameter. The key image generation unit  120  may execute a Wiener filter process on the selected image using filter coefficients (which can be designed based on the selected image and the input image  10 ) for the Wiener filter included in the correction parameter. The key image generation unit  120  may perform weighted prediction on the selected image using a weighting factor and an offset value both included in the correction parameter, in order to correct, for example, brightness. The key image generation unit  120  may perform geometric conversion on the selected image using a parameter value for the geometric conversion included in the correction parameter, in order to correct, for example, the imaging position. The key image generation unit  120  may perform various image corrections not illustrated herein. 
     The key information generation unit  130  acquires the input image  10  and receives the base image  11  and supplemental information  12 . For example, based on the supplemental information  12 , the key information generation unit  130  searches the base image  11  for an image similar to the input image  10  to generate key information  13  including identification information indicative of the searched-for image (which is the same as the selected image). Moreover, the key information generation unit  130  may generate a correction parameter for correcting the selected image and include the correction parameter in the key information  13 . 
     The supplemental information  12  may include information on, for example, the imaging position, the imaging direction, an imaging date, and an imaging time. These pieces of information are useful for the key information generation unit  130  to search the base image  11  for an image similar to the input image  10 . For example, the information on the imaging position and the imaging direction is useful for searching the base image  11  for an image that is similar to the input image  10  in terms of a subject (particularly a background). The information on the imaging date and the imaging time is useful for searching the base image  11  for an image that is similar to the input image  10  in terms of sunshine conditions. 
     The key information generation unit  130  may search the base image  11  for an image similar to the input image  10  without utilizing the supplemental information  12 . Alternatively, the key information generation unit  130  may select any part of the base image  11  regardless of the input image  10 . However, in view of improvement of the coding rate, an image that is similar to the input image  10  is preferably searched for. Furthermore, in view of improvement of search efficiency, the supplemental information  12  is preferably utilized. 
     The key information generation unit  130  outputs the key information  13  to the key image generation unit  120 . Moreover, the key information generation unit  130  transmits the key information  13  to the image decoding apparatus  200 . The key information  13  may be transmitted to the image decoding apparatus  200  by wireless or wired communication or transmitted to the image decoding apparatus  200  via a recording medium. 
     The key information  13  may be transmitted separately from or along with the encoded data  15 . For example, the key information  13  and the encoded data  15  are transmitted after being multiplexed and packetized. For transmission of the key information  13 , it is possible that, for example, a framework for SEI (Supplemental Enhancement Information) in H.264/AVC or H.265/HEVC is utilized. 
     The key image generation unit  120  and the key information generation unit  130  may be designed as illustrated in  FIG. 2 . The key image generation unit  120  in  FIG. 2  includes an image selection unit  121 . The key information generation unit  130  includes a synchronous matching unit  131 . 
     The synchronous matching unit  131  acquires the input image  10  and receives the base image  11  and the supplemental information  12 . For example, based on the supplemental information  12 , the synchronous matching unit  131  searches the base image  11  for an image similar to the input image  10  to generate key information  13  including identification information indicative of the searched-for image (which is the same as the selected image). The selected image may be, for example, an image included in the base image  11  and which is most similar to the input image  10 . The synchronous matching unit  131  outputs the key information  13  to the image selection unit  121  shown in  FIG. 2 . Moreover, the synchronous matching unit  131  transmits the key information  13  to the image decoding apparatus  200 . 
     The image selection unit  121  in  FIG. 2  receives the base image  11  and also receives the key information  13  from the synchronous matching unit  131  shown in  FIG. 2 . The image selection unit  121  selects an image included in the base image  11  and which is indicated by the identification information included in the key information  13  (and which is the same as the selected image). The image selection unit  121  outputs the selected image to the image encoding unit  110  without any change, as the key image  14 . 
     The key image generation unit  120  and the key information generation unit may be designed as illustrated in  FIG. 3 . The key image generation unit  120  in  FIG. 3  includes the image selection unit  121  and an image correction unit  122 . The key information generation unit  130  in  FIG. 3  includes the synchronous matching unit  131  and an image correction parameter generation unit  132 . 
     The synchronous matching unit  131  in  FIG. 3  acquires the input image  10  and receives the base image  11  and the supplemental information  12 . For example, based on the supplemental information  12 , the synchronous matching unit  131  searches the base image  11  for an image similar to the input image  10  to generate identification information indicative of the searched-for image (which is the same as the selected image). The selected image may be, for example, an image included in the base image  11  and which is most similar to the input image  10 . The synchronous matching unit  131  outputs the identification information and the selected image to the image correction parameter generation unit  132 . 
     The image correction parameter generation unit  132  acquires the input image  10  and receives the identification information and the selected image from the synchronous matching unit  131 . The image correction parameter generation unit  132  generates a correction parameter for correcting the selected image based on the input image  10  and the selected image. The image correction parameter generation unit  132  generates key information including the identification information and the correction parameter, and outputs the key information to the image selection unit  121  in  FIG. 3 . Moreover, the image correction parameter generation unit  132  transmits the key information  13  to the image decoding apparatus  200 . 
     The image selection unit  121  in  FIG. 3  receives the base image  11  and also receives the key information  13  from the image correction parameter generation unit  132 . The image selection unit  121  selects an image included in the base image  11  and which is indicated by the identification information included in the key information  13  (and which is the same as the selected image). The image selection unit  121  outputs the selected image and the key information  13  to the image correction unit  122 . 
     The image correction unit  122  receives the selected image and the key information  13  from the image selection unit  121 . The image correction unit  122  generates a key image  14  by correcting the selected image using the correction parameter included in the key information  13 . The image correction unit  122  outputs the key image  14  to the image encoding unit  110 . 
     The image decoding unit  210  receives the encoded data  15  transmitted by the image encoding apparatus  100  and also receives a key image  17  from the key image generation unit  220 . The image decoding unit  210  decodes the encoded data  15  using the key image  17 , to generate an output image  18 , as described below. The key image  17  is identical to the key image  14  generated in the image encoding apparatus  100 . The output image  18  is obtained by substantially restoring the input image  10 . The image decoding unit  210  supplies the output image  18  to, for example, a display apparatus not shown in the drawings. 
     The key image generation unit  220  receives a base image  16  and receives the key information  13  transmitted by the image encoding apparatus  100 . The key image generation unit  220  may be identical or similar to the key image generation unit  120 . The base image  16  is identical to the base image  11  used in the image encoding apparatus  100 . The key image generation unit  220  generates a key image  17  based on the key information  13  and the base image  16 . The key image generation unit  220  outputs the key image  17  to the image decoding unit  210 . 
     As described above, the image encoding unit  110  generates encoded data  15  by encoding the input image  10 . For example, as shown in  FIG. 8 , the image encoding unit  110  includes a subtraction unit  601 , a transform and quantization unit  602 , de-quantization and inverse transform unit  603 , an addition unit  604 , a loop filter unit  605 , an image buffer unit  606 , a predicted image generation unit  607 , an entropy encoding unit  608 , and an encoding control unit  609 . 
     The subtraction unit  601  acquires the input image  10  and receives a predicted image from a predicted image generation unit  607 . The subtraction unit  601  generates a prediction error by subtracting the predicted image from the input image  10 . The subtraction unit  601  outputs the prediction error to the transform and quantization unit  602 . 
     The transform and quantization unit  602  generates a transform coefficient by transforming the prediction error and generates a quantized transform coefficient by quantizing the transform coefficient in accordance with a quantization parameter. For the transformation of the prediction error, orthogonal transformation, for example, DCT (Discrete Cosine Transform) is used. The transform and quantization unit  602  outputs the quantized transform coefficient to the de-quantization and inverse transform unit  603  and the entropy encoding unit  608 . 
     The de-quantization and inverse transform unit  603  receives the quantized transform coefficient from the transform and quantization unit  602 . The de-quantization and inverse transform unit  603  restores the transform coefficient by de-quantizing the quantized transform coefficient in accordance with the quantization parameter. The de-quantization and inverse transform unit  603  restores the prediction error by inversely transforming the transform coefficient (this corresponds to an inverse process to transformation performed by the transform and quantization unit  602 ). For the inverse transformation of the transform coefficient, for example, inverse orthogonal transformation, for example, IDCT (Inverse DCT). The de-quantization and inverse transform unit  603  outputs the prediction error to the addition unit  604 . 
     The addition unit  604  receives the prediction error from the de-quantization and inverse transform unit  603  and receives the predicted image from the predicted image generation unit  607 . The addition unit  604  generates a local decoded image by adding the prediction error and the predicted image together. The addition unit  604  outputs the local decoded image to the loop filter unit  605 . 
     The loop filter unit  605  receives the local decoded image from the addition unit  604 . The loop filter unit  605  generates a filtered image by applying a loop filter process to the local decoded image. The loop filter process may be, for example, a de-blocking filter process. The loop filter unit  605  outputs the filtered image to the image buffer unit  606 . 
     The image buffer unit  606  receives the filtered image from the loop filter unit  605  and the key image  14  from the key image generation unit  120 . The image buffer unit  606  stores the filtered image and the key image  14  as reference images. The image buffer unit  606  outputs the reference image (which may include the key image  14 ) to the predicted image generation unit  607  as necessary. 
     The predicted image generation unit  607  acquires the input image  10 , receives the reference image from the image buffer unit  606  and prediction control information from the encoding control unit  609 . In accordance with the prediction control information, the predicted image generation unit  607  generates a predicted image using the input image  10  and the reference image. For example, the predicted image generation unit  607  generates a predicted image by performing intra prediction or performing motion compensating prediction on the reference image. The predicted image generation unit  607  outputs the predicted image to the subtraction unit  601  and the addition unit  604 . Moreover, the predicted image generation unit  607  outputs prediction information (which includes, for example, motion vector information) on the predicted image to the entropy encoding unit  608 . 
     The entropy encoding unit  608  receives the quantized transform coefficient from the transform and quantization unit  602  and the prediction information from the predicted image generation unit  607 . The entropy encoding unit  608  generates encoded data  15  by performing entropy encoding on the quantized transform coefficient and the prediction information in accordance with syntax. The entropy encoding unit  608  outputs the encoded data  15  to the exterior of the image encoding unit  110 . The entropy encoding is, for example, Huffman coding or arithmetic coding. 
     The encoding control unit  609  generates and outputs prediction control information to the predicted image generation unit  607 . The encoding control unit  609  can control the operation of the predicted image generation unit  607  via the prediction control information. Specifically, the encoding control unit  609  can increase the frequency at which the key image  14  as a reference image is used to generate a predicted image. The dependence of decoding of the encoded data  15  on the key image  14  increases consistently with the frequency at which the key image  14  is used to generate a predicted image. That is, the third person, who is prevented from acquiring the key image  14 , fails to easily comprehend the contents of the input image  10  even if the third person acquired the encoded data  15  because the predicted image needed to decode the encoded data  15  is frequently unknown to the third person. 
     As described above, the image decoding unit  210  generates an output image  18  by decoding the encoded data  15  using the key image  17 . For example, as shown in  FIG. 9 , the image decoding unit  210  includes an entropy decoding unit  701 , a de-quantization and inverse transform unit  702 , an addition unit  703 , a loop filter unit  704 , an image buffer unit  705 , and a predicted image generation unit  706 . 
     The entropy decoding unit  701  receives the encoded data  15  transmitted by the image encoding apparatus  100 . The entropy decoding unit  701  restores the quantized transform coefficient and the prediction information (which includes, for example, motion vector information) by performing entropy decoding on the encoded data  15 . The entropy decoding unit  701  outputs the quantized transform coefficient to the de-quantization and inverse transform unit  702 , and outputs the prediction information to the predicted image generation unit  706 . 
     The de-quantization and inverse transform unit  702  receives the quantized transform coefficient from the entropy decoding unit  701 . The de-quantization and inverse transform unit  702  restores the transform coefficient by de-quantizing the quantized transform coefficient in accordance with the quantization parameter. The de-quantization and inverse transform unit  702  restores the prediction error by inversely transforming the transform coefficient (this corresponds to an inverse process to transformation performed in the image encoding unit  110 ). For the inverse transformation of the transform coefficient, for example, inverse orthogonal transformation, for example, IDCT is used. The de-quantization and inverse transform unit  702  outputs the prediction error to the addition unit  703 . 
     The addition unit  703  receives the prediction error from the de-quantization and inverse transform unit  702  and the predicted image from the predicted image generation unit  706 . The addition unit  703  generates a decoded image by adding the prediction error and the predicted image together. The addition unit  703  outputs the decoded image to the loop filter unit  704 . 
     The loop filter unit  704  receives the decoded image from the addition unit  703 . The loop filter unit  704  generates a filtered image by applying a loop filter process to the decoded image. The loop filter process may be, for example, a de-blocking filter process. The loop filter unit  704  outputs the filtered image to the image buffer unit  705 . 
     The image buffer unit  705  receives the filtered image from the loop filter unit  704  and also receives the key image  17  from the key image generation unit  220 . The image buffer unit  705  stores the filtered image and the key image  17  as reference images. The image buffer unit  705  outputs the reference images (which may include the key image  17 ) to the predicted image generation unit  706  as necessary. The image buffer unit  705  supplies the reference images (which does not include the key image  17 ) to, for example, a display apparatus not shown in the drawings, as the output image  18  in accordance with the order of display. 
     The predicted image generation unit  706  receives the reference images from the image buffer unit  705  and the prediction control information from the entropy decoding unit  701 . The predicted image generation unit  706  generates a predicted image using the prediction information and the reference images. For example, the predicted image generation unit  706  generates a predicted image by performing intra prediction or performing motion compensating prediction on the reference image. The predicted image generation unit  706  outputs the predicted image to the addition unit  703 . 
     As described above, in the image transmission system according to the first embodiment, the image encoding apparatus generates and transmits key information to the image decoding apparatus. Moreover, the image encoding apparatus encodes the input image using a key image generated based on the key information and transmits the encoded data to the image decoding apparatus. Although the key image is needed to substantially restore the input image from the encoded data, the third person has difficulty in acquiring the key image. Hence, the image transmission system allows the encoded data to be transmitted while preventing the third person from knowing the contents of the input image. That is, security of the transmitted image is ensured. The image decoding apparatus can substantially restore the input image by decoding the encoded data using the key image generated based on the transmitted key information. 
     Second Embodiment 
     As illustrated in  FIG. 4 , an image transmission system according to a second embodiment includes an image encoding apparatus  300  and an image decoding apparatus  400 . The image encoding apparatus  300  includes an image encoding unit  110  and a key image generation unit  120 . The image decoding apparatus  400  includes an image decoding unit  210 , a key image generation unit  220 , and a key information generation unit  230 . 
     The image encoding unit  110  in  FIG. 4  is different from the image encoding unit  110  in  FIG. 1  in that the image encoding unit  110  in  FIG. 4  transmits encoded data  15  to the image decoding apparatus  400 . The encoded data  15  may be transmitted to the image decoding apparatus  400  by wireless or wired communication or transmitted to the image decoding apparatus  400  via a recording medium. The key image generation unit  120  in  FIG. 4  is different from the key image generation unit  120  in  FIG. 1  in that the key image generation unit  120  in  FIG. 4  receives key information  13  transmitted by the image decoding apparatus  400 . 
     The image decoding unit  210  in  FIG. 4  is different from the image decoding unit  210  in  FIG. 1  in that the image decoding unit  210  in  FIG. 4  receives the encoded data  15  transmitted by the image encoding apparatus  300 . Moreover, the image decoding unit  210  supplies output image  18  not only to a display apparatus not shown in the drawings but also to the key information generation unit  230 . The key image generation unit  220  in  FIG. 4  is different from the key image generation unit  220  in  FIG. 1  in that the key image generation unit  220  in  FIG. 4  receives the key information  13  from the key information generation unit  230 . 
     The key information generation unit  230  receives a base image  16  and supplemental information  19  and also receives the output image  18  from the image decoding unit  210 . For example, based on the supplemental information  19 , the key information generation unit  230  searches the base image  16  for an image similar to the output image  18  to generate key information  13  including identification information indicative of the searched-for image (which is an image (selected image) selected as a base for the key image  17 ). Moreover, the key information generation unit  230  may generate a correction parameter for correcting the selected image and include the correction parameter in the key information  13 . 
     In short, the identification information described in the first embodiment is indicative of a selected image similar to the input image  10  to be encoded, whereas the identification information described in the present embodiment is indicative of a selected image similar to the output image  18  generated by decoding the previously encoded input image  10  instead of the input image  10  to be encoded. 
     The supplemental information  19  may include information on such as the imaging position, the imaging direction, the imaging date, and the imaging time. These pieces of information are useful for the key information generation unit  230  to search the base image  16  for an image similar to the output image  18 . For example, the information on the imaging position and the imaging direction is useful for searching the base image  16  for an image that is similar to the output image  18  in terms of the subject (particularly the background). The information on the imaging date and the imaging time is useful for searching the base image  16  for an image that is similar to the output image  18  in terms of sunshine conditions. 
     The key information generation unit  230  may search the base image  16  for an image similar to the output image  18  without utilizing the supplemental information  19 . Alternatively, the key information generation unit  230  may select any part of the base image  16  regardless of the output image  18 . However, in view of improvement of the coding rate, an image that is similar to the output image  18  is preferably searched for. Furthermore, in view of improvement of the search efficiency, the supplemental information  19  is preferably utilized. 
     The key information generation unit  230  outputs the key information  13  to the key image generation unit  220 . Moreover, the key information generation unit  230  transmits the key information  13  to the image encoding apparatus  300 . The key information  13  may be transmitted to the image encoding apparatus  300  by wireless or wired communication or transmitted to the image encoding apparatus  300  via a recording medium. 
     The key image generation unit  220  and the key information generation unit  230  may be designed as illustrated in  FIG. 5 . The key image generation unit  220  in  FIG. 5  includes an image selection unit  221  and an image correction unit  222 . The key information generation unit  230  in  FIG. 5  includes a predictive matching unit  231  and an image correction parameter generation unit  232 . 
     The predictive matching unit  231  receives the base image  16  and the supplemental information  19  and also receives the output image  18  from the image decoding unit  210 . The predictive matching unit  231  searches the base image  16  for an image similar to the output image  18  based on the supplemental information  19  to generate identification information indicative of the searched-for image (which is the same as the selected image). The selected image may be, for example, an image included in the base image  16  and which is most similar to the output image  18 . The predictive matching unit  231  outputs the identification information and the selected image to the image correction parameter generation unit  232 . 
     The image correction parameter generation unit  232  receives the output image  18  from the image decoding unit  210  and receives the identification information and the selected image from the predictive matching unit  231 . The image correction parameter generation unit  232  generates a correction parameter for correcting the selected image, based on the output image  18  and the selected image. The image correction parameter generation unit  232  then generates key information  13  including the identification information and the correction parameter, and outputs the key information  13  to the image selection unit  221 . The image correction parameter generation unit  232  further transmits the key information  13  to the image encoding apparatus  300 . 
     The image selection unit  221  receives the base image  16  and also receives the key information  13  from the image correction parameter generation unit  232 . The image selection unit  221  selects an image included in the base image  16  and which is indicated by the identification information included in the key information  13  (and which is the same as the selected image). The image selection unit  221  outputs the selected image and the key information  13  to the image correction unit  222 . 
     The image correction unit  222  receives the selected image and the key information  13  from the image selection unit  221 . The image correction unit  222  generates a key image  17  by correcting the selected image using the correction parameter included in the key information  13 . The image correction unit  222  outputs the key image  17  to the image decoding unit  210 . 
     As described above, in the image transmission system according to the second embodiment, the image decoding apparatus generates and transmits key information to the image encoding apparatus. The image encoding apparatus encodes the input image using a key image generated based on the key information and transmits the encoded data to the image decoding apparatus. Although the key image is needed to substantially restore the input image from the encoded data, the third person has difficulty in acquiring the key image. Hence, the image transmission system allows the encoded data to be transmitted while preventing the third person from knowing the contents of the input image. That is, security of the transmitted image is ensured. The image decoding apparatus can substantially restore the input image by decoding the encoded data using the key image generated based on the key information. 
     Third Embodiment 
     As illustrated in  FIG. 6 , an image transmission system according to a third embodiment includes an image encoding apparatus  300 , an image decoding apparatus  200 , and a key information generation apparatus  500 . The image encoding apparatus  300  includes an image encoding unit  110  and a key image generation unit  120 . The image decoding apparatus  200  includes an image decoding unit  210  and a key image generation unit  220 . The key information generation apparatus  500  includes a key information generation unit  510 . 
     The key image generation unit  120  in  FIG. 6  is different from the key image generation unit  120  in  FIG. 1  and  FIG. 4  in that the key image generation unit  120  in  FIG. 6  receives key information  13  transmitted by the key information generation apparatus  500 . The key image generation unit  220  is different from the key image generation unit  220  in  FIG. 1  and  FIG. 4  in that the key image generation unit  220  according to the third embodiment receives the key information  13  transmitted by the key information generation apparatus  500 . 
     The key information generation unit  510  receives a base image  20 . The key information generation unit  510  selects any part of the base image  20  to generate key information  13  including identification information indicative of a selected image. The base image  20  is identical to the base image  11  used in the image encoding apparatus  300  and the base image  16  used in the image decoding apparatus  200 . 
     The key information generation unit  510  transmits the key information  13  to the image encoding apparatus  300  and the image decoding apparatus  200 . The key information  13  may be transmitted to the image encoding apparatus  300  and the image decoding apparatus  200  by wireless or wired communication or transmitted to the image encoding apparatus  300  and the image decoding apparatus  200  via a recording medium. 
     The key image generation unit  510  may be designed as illustrated in  FIG. 7 . The key information generation unit  510  in  FIG. 7  includes an image selection unit  511 . The image selection unit  511  receives the base image  20 . The image selection unit  511  selects any part of the base image  20  to generate key information  13  including identification information indicative of a selected image. The image selection unit  511  transmits the key information  13  to the image encoding apparatus  300  and the image decoding apparatus  200 . 
     As described above, in the image transmission system according to the third embodiment, the key information generation unit generates and transmits key information to the image encoding apparatus and the image decoding apparatus. The image encoding apparatus encodes the input image using a key image generated based on the key information and transmits the encoded data to the image decoding apparatus. Although the key image is needed to substantially restore the input image from the encoded data, the third person has difficulty in acquiring the key image. Hence, the image transmission system allows the encoded data to be transmitted while preventing the third person from knowing the contents of the input image. That is, security of the transmitted image is ensured. The image decoding apparatus can substantially restore the input image by decoding the encoded data using the key image generated based on the transmitted key information. 
     Fourth Embodiment 
     The image encoding unit  110  and the image decoding unit  210  described using  FIG. 8  and  FIG. 9 , respectively, may be replaced with an image encoding unit  110  and an image decoding unit  210  described using  FIG. 10  and  FIG. 11 , respectively. 
     The image encoding unit  110  illustrated in  FIG. 10  includes a subtraction unit  611 , a transform and quantization unit  602 , de-quantization and inverse transform unit  603 , an addition unit  604 , a loop filter unit  605 , an image buffer unit  616 , a predicted image generation unit  617 , an entropy encoding unit  608 , an encoding control unit  619 , and a subtraction unit  610 . 
     The transform and quantization unit  602  in  FIG. 10  is different from the transform and quantization unit  602  in  FIG. 8  in that the transform and quantization unit  602  in  FIG. 10  receives a prediction error from the subtraction unit  611  instead of the subtraction unit  601 . The addition unit  604  in  FIG. 10  is different from the addition unit  604  in  FIG. 8  in that the addition unit  604  in  FIG. 10  receives the prediction error from the predicted image generation unit  617  instead of the predicted image generation unit  607 . The loop filter unit  605  in  FIG. 10  is different from the loop filter unit  605  in  FIG. 8  in that the loop filter unit  605  in  FIG. 10  outputs a filtered image to the image buffer unit  616  instead of the image buffer unit  606 . The entropy encoding unit  608  in  FIG. 10  is different from the entropy encoding unit  608  in  FIG. 8  in that the entropy encoding unit  608  in  FIG. 10  receives prediction information (which includes, for example, motion vector information) from the predicted image generation unit  617  instead of the predicted image generation unit  607 . 
     The subtraction unit  610  acquires an input image  10  and receives a key image  14  from the key image generation unit  120 . The subtraction unit  610  generates a differential image by subtracting the key image  14  from the input image  10 . The subtraction unit  610  outputs the differential image to the subtraction unit  611 . That is, the image encoding unit  110  predictively codes the differential image instead of the input image  10 . Therefore, the third person, who is prevented from acquiring the key image  14 , fails to easily know the contents of the input image  10  even if the third person restores encoded data  15  (that is, restores the differential image) because the key image  14  needed to restore the input image  10  is unknown to the third person. 
     The subtraction unit  611  receives the differential image from the subtraction unit  610  and a predicted image from the predicted image generation unit  617 . The subtraction unit  611  generates a prediction error by subtracting the predicted image from the differential image. The subtraction unit  611  outputs the prediction error to the transform and quantization unit  602 . 
     The image buffer unit  616  receives a filtered image from the loop filter unit  605 . The image buffer unit  616  stores the filtered image as a reference image. The image buffer unit  616  outputs the reference images to the predicted image generation unit  617  as necessary. 
     The predicted image generation unit  617  receives the differential image from the subtraction unit  610 , the reference images from the image buffer unit  616 , and prediction control information from the encoding control unit  619 . The predicted image generation unit  617  generates a predicted image using the differential image and the reference images, in accordance with the prediction control information. For example, the predicted image generation unit  617  generates a predicted image by performing intra prediction or performing motion compensating prediction on the reference images. The predicted image generation unit  617  outputs the predicted image to the subtraction unit  611  and the addition unit  604 . Moreover, the predicted image generation unit  617  outputs prediction information on the predicted image to the entropy encoding unit  608 . 
     The encoding control unit  619  generates and outputs prediction control information to the predicted image generation unit  617 . The encoding control unit  619  can control the operation of the predicted image generation unit  617  via the prediction control information. 
     The image decoding unit  210  illustrated in  FIG. 11  includes an entropy decoding unit  701 , a de-quantization and inverse transform unit  702 , an addition unit  703 , a loop filter unit  704 , an image buffer unit  715 , a predicted image generation unit  716 , and an addition unit  707 . 
     The entropy decoding unit  701  in  FIG. 11  is different from the entropy decoding unit  701  in  FIG. 9  in that the entropy decoding unit  701  in  FIG. 11  outputs the prediction information to the predicted image generation unit  716  instead of the predicted image generation unit  706 . The addition unit  703  in  FIG. 11  is different from the addition unit  703  in  FIG. 9  in that the addition unit  703  receives the predicted image from the predicted image generation unit  716  instead of the predicted image generation unit  706 . The loop filter unit  704  in  FIG. 11  is different from the loop filter unit  704  in  FIG. 9  in that the loop filter unit  704  in  FIG. 11  outputs the filtered image to the image buffer unit  715  instead of the image buffer unit  705 . 
     The image buffer unit  715  receives the filtered image from the loop filter unit  704 . The image buffer unit  715  stores the filtered image as a reference image. The image buffer unit  715  outputs reference images to the predicted image generation unit  716  as necessary. Moreover, the image buffer unit  715  outputs a reference image to the addition unit  707  as a differential image in accordance with the order of display. 
     The predicted image generation unit  716  receives the reference images from the image buffer unit  715  and the prediction information from the entropy decoding unit  701 . The predicted image generation unit  716  generates a predicted image using the prediction information and the reference images. For example, the predicted image generation unit  716  generates a predicted image by performing intra prediction or performing motion compensating prediction on the reference images. The predicted image generation unit  716  outputs the predicted image to the addition unit  703 . 
     The addition unit  707  receives a key image  17  from the key image generation unit  220  and also receives the differential image from the image buffer unit  715  in accordance with the order of display. The addition unit  707  generates an output image by adding the key image  17  and the differential image together. The addition unit  707  supplies the output image to, for example, a display apparatus not shown in the drawings. 
     Fifth Embodiment 
     The image encoding unit  110  and the image decoding unit  210  described using  FIG. 8  and  FIG. 9 , respectively, and  FIG. 10  and  FIG. 11 , respectively, may be replaced with an image encoding unit  110  and an image decoding unit  210  described using  FIG. 12  and  FIG. 13 , respectively. 
     The image encoding unit  110  illustrated in  FIG. 12  includes a subtraction unit  601 , a transform and quantization unit  602 , de-quantization and inverse transform unit  603 , an addition unit  604 , a loop filter unit  605 , an image buffer unit  616 , a predicted image generation unit  627 , an entropy encoding unit  608 , and an encoding control unit  629 . 
     The subtraction unit  601  in  FIG. 12  is different from the subtraction unit  601  in  FIG. 8  in that the subtraction unit  601  in  FIG. 12  receives a predicted image from the predicted image generation unit  627  instead of the predicted image generation unit  607 . The addition unit  604  in  FIG. 12  is different from the addition unit  604  in  FIG. 8  in that the addition unit  604  in  FIG. 12  receives the predicted image from the predicted image generation unit  627  instead of the predicted image generation unit  607 . The image buffer unit  616  in  FIG. 12  is different from the image buffer unit  616  in  FIG. 10  in that the image buffer unit  616  in  FIG. 12  outputs a reference image to the predicted image generation unit  627  instead of the predicted image generation unit  617 . The entropy encoding unit  608  in  FIG. 12  is different from the entropy encoding unit  608  in  FIG. 8  in that the entropy encoding unit  608  in  FIG. 12  receives prediction information (which includes, for example, motion vector information) from the predicted image generation unit  627  instead of the predicted image generation unit  607 . 
     The predicted image generation unit  627  acquires an input image  10  and receives a key image  14  from a key image generation unit  120 , the reference image from the image buffer unit  616 , and prediction control information from the encoding control unit  619 . The predicted image generation unit  627  generates a predicted image using the input image  10 , the key image  14 , and the reference image in accordance with the prediction control information. For example, the predicted image generation unit  627  generates a predicted image by performing intra prediction, performing motion compensating prediction on the reference image or performing prediction based on the key image  14 . The predicted image generation unit  627  outputs the predicted image to the subtraction unit  601  and the addition unit  604 . Moreover, the predicted image generation unit  627  outputs prediction information on the predicted image to the entropy encoding unit  608 . 
     The encoding control unit  629  generates and outputs prediction control information to the predicted image generation unit  627 . The encoding control unit  629  can control the operation of the predicted image generation unit  627  via the prediction control information. Specifically, the encoding control unit  629  can increase the frequency at which the predicted image generation unit  627  performs prediction based on the key image  14 . The dependence of decoding of encoded data  15  on the key image  14  increases consistently with the frequency at which the predicted image generation unit  627  performs prediction based on the key image  14 . That is, the third person, who is prevented from acquiring the key image  14 , fails to easily comprehend the contents of the input image  10  even if the third person acquired the encoded data  15 , because the predicted image needed to decode the encoded data  15  is frequently unknown to the third person. 
     The image decoding unit  210  illustrated in  FIG. 13  includes an entropy decoding unit  701 , a de-quantization and inverse transform unit  702 , an addition unit  703 , a loop filter unit  704 , an image buffer unit  725 , and a predicted image generation unit  726 . 
     The entropy decoding unit  701  in  FIG. 13  is different from the entropy decoding unit  701  in  FIG. 9  in that the entropy decoding unit  701  in  FIG. 13  outputs the prediction information to the predicted image generation unit  726  instead of the predicted image generation unit  706 . The addition unit  703  in  FIG. 13  is different from the addition unit  703  in  FIG. 9  in that the addition unit  703  in  FIG. 13  receives the predicted image from the predicted image generation unit  726  instead of the predicted image generation unit  706 . The loop filter unit  704  in  FIG. 13  is different from the loop filter unit  704  in  FIG. 9  in that the loop filter unit  704  in  FIG. 13  outputs a filtered image to the image buffer unit  725  instead of the image buffer unit  705 . 
     The image buffer unit  725  receives the filtered image from the loop filter unit  704 . The image buffer unit  725  stores the filtered image as a reference image. The image buffer unit  725  outputs reference images to the predicted image generation unit  726  as necessary. Moreover, the image buffer unit  725  supplies the reference images to, for example, the display apparatus not shown in the drawings, as an output image  18  in accordance with the order of display. 
     The predicted image generation unit  726  receives a key image  17  from the key image generation unit  220 , the reference images from the image buffer unit  725 , and prediction information from the entropy decoding unit  701 . The predicted image generation unit  726  generates a predicted image using the prediction image, the key image  17 , and the reference images. For example, the predicted image generation unit  726  generates a predicted image by performing intra prediction, performing motion compensating prediction on the reference images, or performing prediction based on the key image  17 . The predicted image generation unit  726  outputs the predicted image to the addition unit  703 . 
     The image encoding apparatus, image decoding apparatus, and key information generation apparatus all included in the image transmission system according to each of the embodiments may be implemented by hardware illustrated in  FIG. 14 . The hardware in  FIG. 14  includes a CPU (Central Processing Unit)  801 , ROM (Read Only Memory)  802 , RAM (Random Access Memory)  803 , a communication IF (Interface)  804 , and a bus  805 . 
     Among the CPU  801 , the ROM  802 , the RAM  803 , and the communication IF  804 , data transmissions and receptions are performed via the bus  805 . 
     The CPU  801  can operate as a functional unit that executes the above-described various processes. The RAM  803  may be utilized as any of the above-described various buffer units. The communication IF  804  may be utilized to transmit and receive, for example, the key information  13  and the encoded data  15 . 
     The processing in the above-described embodiments can be implemented using a general-purpose computer as basic hardware. A program implementing the processing in each of the above-described embodiments may be stored in a computer readable storage medium for provision. The program is stored in the storage medium as a file in an installable or executable format. The storage medium is a magnetic disk, an optical disc (CD-ROM, CD-R, DVD, or the like), a magnetooptic disc (MO or the like), a semiconductor memory, or the like. That is, the storage medium may be in any format provided that a program can be stored in the storage medium and that a computer can read the program from the storage medium. Furthermore, the program implementing the processing in each of the above-described embodiments may be stored on a computer (server) connected to a network such as the Internet so as to be downloaded into a computer (client) via the network. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel methods and systems described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the methods and systems described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.