below is the description of an embodiment of the present invention . fig1 is a block diagram of the image processing apparatus to realize the partial image expansion method of the first embodiment . image processing apparatus 1 contains input device 11 to input image data and user control requests , computational processing unit 20 to perform computational processing on input data by using the cpu , display device 12 to display processing results for images , etc ., main memory 40 to write to directly from cpu , and secondary memory 60 which consists of large capacity memory such as flash memory . alternatively , input device 11 can comprise imaging unit such as a camera , and an image captured by the imaging unit can be passed to computational processing unit 20 . this image processing apparatus 1 can be realized with a general computer such as a personal computer or a mobile phone with camera as long as the above - mentioned functionalities are satisfied . computational processing unit 20 contains input processing unit 21 to process input data passed from input device 11 , memory interface unit 22 to write to secondary memory 60 and read from secondary memory 60 ( hereinafter , called “ memory i / f unit ”), image data recording unit 23 to store image data in secondary memory 60 via memory interface unit 22 , control request input unit 24 to accept user control requests such as to display and to scroll , cache file creation control unit 25 to control cache file creation timing , cache file creation unit 26 to create cache file , shrunken image output unit 27 to select and display low - resolution shrunken image 53 or high - resolution shrunken image 52 according to a control request from a user , used block determination unit 28 to identify macroblocks to decode in the encoded image , partial decode unit 29 to decode macroblocks that have been identified for decoding , partially decoded image output unit 30 to output partially decoded image to display device , and display process unit 31 executes display process for display device 12 . in fig1 , encoded image data flows are indicated with broken line arrows , and the other data flows are indicated with solid line arrows . next , the operation of image processing apparatus 1 with the above structure is described . computational processing unit 20 of image processing apparatus 1 receives encoded image data from input device 11 via input process unit 21 , which is passed to image data registration unit 23 . image data registration unit stores image data as a file in encoded image database 61 of secondary memory 60 via memory interface unit 22 . id indicating input order ( hereinafter called “ image id ”) and input timestamp are attached to the image file . fig1 does not include description for creating encoded image from non - encoded image ; however , this feature can be included according to necessity . in this embodiment , the description is made under the assumption that non - encoded image will be input directly . next , cache file creation process steps performed by cache file creation control unit 25 and cache file creation unit 26 are described . cache file creation control unit 25 activates cache file creation unit 26 during cpu idle time when user is not operating the viewing software . cache file creation control unit 25 checks whether the image cache file exists in secondary memory 60 to display upon receiving user control request ( such as zoom in , zoom out , or scroll ) from control request input unit 24 , and if there is no cache file corresponding to the image to display , activate cache file creation unit 26 even when there is user control request . when cache file creation unit 26 is activated , shrunken image and map data 51 are created to make the image viewing experience smooth . two types of shrunken images are created , one is the relatively high - resolution shrunken image 52 for individually displaying an image , and another is the low - resolution shrunken image 53 ( also known as thumbnail image ) to be used for viewing indices of multiple images shown on a single screen . the size of high - resolution shrunken image 52 depends on the display size of display device 12 ( namely , screen resolution of the display device ). that is , high resolution shrunken image is created in a resolution ( such as 1024 by 768 pixels ) that matches input resolution information ( such as xga mode ) of the display device parameter of image processing apparatus 1 . it is preferable , since it is possible to do the prompt creation , that first the high - resolution shrunken image 52 is created from the source image , and then low - resolution shrunken image 53 is created using high - resolution shrunken image 52 , than high - resolution shrunken image 52 and low - resolution shrunken image 53 are not both created directly from the source image . creation of map data 51 requires scanning of the entire encoded image to extract scan positions and to perform cumulative calculation of dc components . on one hand , the entire encoded image must be scanned to create a high - resolution shrunken image . high - resolution shrunken image creation and map data creation require certain processing costs ; therefore , it is preferable to reduce this processing load by simultaneously creating both the high - resolution shrunken image 52 and map data 51 . cache file creation unit 26 creates each of map data 51 , high - resolution shrunken image 52 , and low - resolution shrunken image 53 , this cache data is stored in secondary memory 60 by associating it with the image id of relevant image file . further , when the image is output and displayed where cache data is not yet created , cache data creation unit 26 creates cache data 50 , displays the image using the cache data , then outputs the cache data 50 to cache file 62 of secondary memory 60 . this allows prompt image display , with user operability improved . creation process flow of cache data 50 and cache file 62 is indicated in fig9 . in fig9 , cache file creation unit 26 simultaneously creates ( in a single decode process ) both high - resolution shrunken image 52 and map data 51 from source image ( encoded image ) 61 by using high - resolution shrunken image / map data creation process ( s 101 ). further , low - resolution shrunken image creation process ( s 102 ) creates low - resolution shrunken image 53 from the high - resolution shrunken image . this map data 51 , high - resolution shrunken image 52 , and low - resolution shrunken image 53 are output to cache file 62 after these images are used for display output if a user requests the image to be displayed on display image 61 . next , cache file creation process is described using jpeg format image as an example . to take advantage of the jpeg format in high - resolution shrunken image creation , not all frequency bands are decoded , but only certain low frequency band data being lower than a predetermined frequency are used to speed up the processing . generally , jpeg data decoding is performed by steps indicated in fig1 . in this diagram , entropy coding composite ( s 201 ) is a process to restore data from an encoded bitstream . for jpeg , huffman coding is used for entropy encoding , and this process will be the decoding process for huffman coded data . afterwards , inverse quantization is performed ( s 202 ) to restore frequency data . frequency data becomes yuv color space data ( s 203 ) through inverse frequency conversion ( inverse dct for jpeg ). afterwards , color conversion is performed ( s 204 ) to meet the color space ( for example rgb ) of display device 12 , and decoding process is complete . the high - resolution shrunken image created is stored again in jpeg format . this reduces data volume . at this time , there is no need to perform inverse frequency conversion on all frequency bands . therefore , only performing inverse frequency conversion on the low frequency bands necessary depending on the shrunken image size enhances processing speed . also , map data 51 is created when entropy coding is composited . fig1 indicates creation steps for high - resolution shrunken image and map data . cache file creation unit 26 creates map data 51 when entropy coding is composited ( s 201 ) from encoded bitstream of encoded image . after step s 201 , inverse quantization ( s 202 ) is performed , and then inverse frequency conversion ( s 203 a ) is performed on low frequency bands . further , it is preferable in step s 203 a to prepare a table of relation values between display size and frequency band ( scan number ) where inverse frequency conversion is performed , and refer to the table to extract frequency bands that correspond to display size to perform inverse frequency conversion . after step s 203 a , shrinking process ( s 301 ), frequency conversion ( s 302 ), quantization ( s 303 ), entropy coding ( s 304 ) are performed to create high - resolution shrunken image 52 in jpeg format . cache file 62 creation process is performed during idle periods between user control requests as described above . at this time , to achieve a smoother image viewing , importance is placed on deciding which images to create cache file for , and the order of cache file creation . in this embodiment , cache file 62 is created in a different order depending on the user control request mode , namely the individual display mode where a selected image is displayed , and the thumbnail index mode where multiple images are displayed . below , fig1 is used to describe cache file creation control unit 25 . when cache file creation control unit 25 is activated periodically or by activation command from control request input unit 24 , the cache file creation control unit 25 first determines whether current control request mode is in individual display mode or not ( s 401 ). according to this result , if the current control request mode is the individual display mode (“ yes ” in s 401 ), then cache file creation control unit 25 first determines whether cache file 62 exists for the currently viewing image ( s 402 ). if there is no cache file 62 for the currently viewing image , then activate cache file creation unit 26 , and first create cache data 50 for the image ( s 403 ). this cache file creation unit 26 outputs the created cache data 50 to display device 12 , then writes the cache data 50 to cache file 62 of secondary memory 60 . next , cache file creation control unit 25 determines whether or not a user is in mid - operation through gui of input device 11 ( s 404 ). here , after user control request input is recognized , the time period where this control request is still processing is considered to be in mid - operation of device . if cache data 50 is created during device operation , then the response to control requests would be delayed , and it would end up interfering with smooth operations of viewing . therefore cache file creation control unit 25 terminates without creating cache data (“ yes ” in s 404 ) during device operation . on the other hand , while a user is not performing control requests , cache file creation control unit 25 determines whether cache file 62 exists for images right before and right after the current image id being viewed ( s 406 ), and if the corresponding cache file 62 does not exist , cache file creation unit 26 is activated , cache data 50 is created , and created cache data 50 is written to cache file 62 in secondary memory 60 ( s 407 ). next , if there is no user control request (“ no ” in s 408 ), cache file creation control unit 25 performs similar process ( s 406 - s 409 ) on images adjoining the images of which the cache files have been created . therefore , cache file 62 is created ( s 410 ) by repeating s 406 - s 409 processes while existence of user control request is confirmed each time , for each image within a certain range of image obtaining order from the currently viewing image . in thumbnail index mode , cache file creation order differs from that of individual display mode . this is due to the fact that if low - resolution shrunken image already exists it is just necessary to display the corresponding image , and therefore , its creation priority can be lowered . below , thumbnail index mode cache file creation control unit 25 process steps are described . in thumbnail index mode , cache file creation control unit 25 first determines whether user is currently operating the device ( s 411 ), if the user is currently operating , cache data creation is not performed . on the other hand , if user is not currently operating the device (“ no ” in s 411 ), then cache file creation control unit 25 determines whether cache file or low - resolution shrunken image is already stored ( s 412 , s 413 ). if cache file or low - resolution shrunken image is stored (“ yes ” in s 413 ), in this step cache file is not created . if neither the cache file nor the low - resolution shrunken image is stored (“ no ” in s 413 ) then cache file creation control unit 25 activates cache file creation unit 26 , cache file 50 is created and is written to cache file 60 ( s 414 ). next , if there are no user control requests (“ no ” in s 415 ), cache file creation control unit 25 performs the same processes ( s 413 , s 414 ) on images adjoining the images of which the cache files have been created . therefore , cache file 62 is created ( s 417 ) by repeating s 413 - s 416 processes while existence of user control request is confirmed each time , for each image within a certain range of image obtainment order from the currently viewing image . after step s 417 , cache file creation control unit 26 determines whether cache file of current image is stored , if the cache file is not stored , cache file creation unit 26 is activated to create cache file ( s 418 - s 420 ). at this step , even if low - resolution shrunken image exists , but if cache file does not exist , cache file is created . next , if there is no user control request (“ no ” in s 421 ), cache file creation control unit 25 performs the same processes ( s 419 - 420 ) on images adjoining the images of which the cache files have been created . therefore , cache file 62 is created ( s 423 ) by repeating s 419 - s 422 processes while existence of user control request is confirmed each time , for each image within a certain range of image obtainment order from the currently viewing image . cache file 62 creation steps are as described above ; however , creating cache file 62 for all images and storing them in secondary memory 60 uses significant amount of memory . therefore , cache file deletion unit 32 deletes unnecessary cache files if the number of cache files or cache files data volume total sum exceeds certain threshold value sc . this determination is made immediately after each cache file creation where cache file deletion unit 32 is activated by cache file creation unit 26 . the number of cache files increases only when cache data is created . once cache file deletion unit 32 determines to delete cache file , deletion continues until the number of cache files or cache total file volume sum is below predetermined threshold sc ′(& lt ; sc ). the cache file deletion order is indicated below ; however , in some cases the cache file created immediately before this cache file deletion may become the next cache file targeted to be deleted . in this case , the cache file is not deleted and cache file creation is terminated . however , in this case , when an image is displayed before or after the current image , the cache file creation status is reset to start the creating . the cache file deletion order of cache file deletion unit 32 first deletes images that are outside the specific range from the current image . the range can be defined arbitrarily , for example , it is possible to assign consecutive numbers when encoded images are registered , and to determine the numbers that are removed more than a certain number to be outside the range . another method is to assign categories to encoded images and if the currently displayed image belongs to the same category then the images of the same category are within the range , but if images belong to a different category , such images are outside of the range . cache file deletion unit 32 deletes files outside of the range , starting with the oldest creation date , and after all the images outside of the range are deleted , images within the range start being deleted . cache file deletion unit 32 deletes images that are the furthermost located from the current image . a cache file deletion order is described in fig1 . for example , priority of image deletion for images that are outside of the range from the current image and have old cache file creation timestamp are assigned higher deletion priority compared to other images . next , steps of decoding encoded image and output to display device 12 are described . when a user requests a screen control requests , control request input unit 24 notifies that there was a control request to cache file creation control unit 25 . cache file creation control unit 25 halts activation of cache file creation unit 26 until control request process is completed . also , control request input unit 24 activates both shrunken image output unit 27 and partially decoded image output unit 30 , and passes operation mode ( individual display mode , thumbnail index mode ), and control request information such as displayed image id , scroll direction , scroll speed , and zoom in / out to the shrunken image output unit 27 and partially decoded image output unit 30 . shrunken image output unit 27 is activated by control request input unit 24 and reads cache data 50 from cache file 62 for the image id of image to be displayed . the read cache data is stored in main memory 40 . next , shrunken image output unit 27 determines operation mode , and outputs low - resolution shrunken images of cache data 50 to the display if the operation mode is thumbnail index mode . on the other hand , in individual display mode , shrunken image output unit 27 determines display area of high - resolution shrunken image from the control request information , and outputs image data of the display area determined by display processing unit 31 to display device 12 . specifying the display area is achieved using existing technologies , for example if top left corner of high - resolution shrunken image is the coordinate origin ( 0 , 0 ), then the diagonally opposite corners of the rectangular display area ( hereinafter called “ diagonally opposite coordinate ”) are indicated as ( x1 , y1 ) ( x2 , y2 ), and the area data indicated in this style is passed to display device 12 . next , shrunken image output unit 27 activates used block determination unit 28 . when used block determination unit 28 is activated , ids of macroblocks included in the display area are calculated . one example of this calculation method is described using fig1 . fig1 ( a ) indicates high - resolution shrunken image and its display area , and fig1 ( b ) shows both the source image divided into macroblocks and the display area . for example , high - resolution shrunken image size is a by b pixels , and there are m by n source image blocks , then the macroblock id corresponding to the high - resolution shrunken image coordinate ( x , y ) can be determined using the following formula . here & lt ; v & gt ; is the largest integer being not less than v . above formula ( 2 ) is the formula when macroblock id has initial value of ‘ 0 ’ and the id is ordered consecutively from the top left to the bottom right . for example , if high - resolution shrunken image size is 1024 by 768 pixels , and the diagonally opposing coordinates of the rectangular display area are ( x1 , y1 )=( 500 , 200 ) and ( x2 , y2 )=( 750 , 500 ), then from the formula ( 2 ) above ( x1 , y1 ) macroblock id is ‘ 11 ’ and ( x2 , y2 ) macroblock id is ‘ 29 ’. similarly macroblock ids of other vertices are obtained , and macroblock id increases by 1 horizontally and by m vertically ; therefore all macroblock ids included in the rectangular display area can be determined . used block determination unit 28 stores macroblock ids belonging to rectangular display area to buffer 2 ( 42 ) of main memory 40 . afterwards , used block determination unit 28 activates partial decode unit 29 . after partial decode unit 29 is activated , encoded image 61 is partially decoded by utilizing map data of macroblocks having ids recorded in buffer 2 of encoded image map data 51 . partially decoded image data is stored in buffer 1 ( 41 ) of main memory 40 . partial decode unit 29 activates partially decoded image output unit 30 after decoding process of macroblocks having ids stored in buffer 2 ( 42 ) is completed . after partially decoded image output unit 30 is activated , image part included in display area of partially decoded image data stored in buffer 1 ( 41 ) is output to display device 12 . one example of a coordinate conversion is described in fig1 . for source image size of a by b pixels , left top vertex is the coordinate origin , then the relations between coordinate ( x , y ) and above high - resolution shrunken image coordinate ( x , y ) can be obtained with the following formulae . therefore , if the top left vertex coordinate of partially decoded image is ( α , β ), then the coordinate system with partially decoded image top left vertex as the coordinate origin is expressed as : also , ( α , β ) can be obtained by the following formulae if macroblock id is c . with the above formula ( 3 ), partially decoded image that corresponds to high - resolution shrunken image display area can be displayed by converting the coordinate system of the rectangular display area of high - resolution shrunken image to the partially decoded image coordinate system . with the above process , image data output from shrunken image output unit 27 or partially decoded image output unit 30 is output to display device 12 through display process unit 31 . according to this embodiment , after there is user control request input for individual display mode , high - resolution shrunken image is displayed first , then the encoded image is partially decoded and output to display device 12 ; therefore , partial decoding process is conducted smoothly without stressful user experience . also , the entire encoded image does not have to be decoded upon user control request ; therefore , memory resources are conserved . the above coordinate conversion using formulae ( 2 ) and ( 3 ) are merely examples , and this invention is not limited to these methods . next , the second embodiment of this invention is described . this embodiment effectively utilizes partially decoded image in main memory to quickly respond to user control requests . fig1 is the functional block diagram of the image processing apparatus to realize partial image expansion method of the second embodiment . the main difference from the image processing apparatus 1 of fig1 is the addition of partially decoded image deletion unit 33 to delete partially decoded image in main memory , and the change to high - resolution shrunken image output process of shrunken image output unit 27 to allow receiving termination request of display output process by receiving process termination request from partial decode unit 29 . this embodiment also includes buffer 1 to store in main memory 40 macroblock ids and their partially decoded image data that are currently being used , buffer 2 to store macroblock ids that correspond to the current display area of the high - resolution shrunken image , and buffer 3 to store macroblock ids of macroblocks adjoining the buffer 2 macroblocks but are outside of current display area . other features are the same as in fig1 , and the identical features are described using the same symbols so they will not be further described thereafter . what is described next is the operation of image processing apparatus 1 of the above system structure . when there is control request input , used block determination unit 28 activated by shrunken image output unit 27 stores macroblock ids corresponding to the current display area to buffer 2 of main memory 40 , and partial decode unit 29 is activated . then , after partial decoding process , macroblock addition routine described below is executed . after partial decode unit 29 is activated , macroblock ids stored in buffer 2 are extracted . next , whether all macroblock ids of buffer 2 exist is determined in buffer 1 , and if all macroblock ids exist , then termination command is output to terminate high - resolution shrunken image display output process in shrunken image output unit 27 . therefore , shrunken image output unit 27 terminates display output process for high - resolution shrunken image . on the other hand , if even a single macroblock id is missing amongst all the macroblock ids , partial decode unit 29 does not output termination command to shrunken image output unit 27 . the missing macroblock id blocks are partially decoded and added to buffer 1 . after decoding all missing macroblocks , partially decoded image output unit 30 is activated . operation of partially decoded image output unit 30 is the same as in the first embodiment . next , fig1 is used to describe the macroblock addition routine executed by used block determination unit 28 . after macroblock addition routine is activated , ids of macroblocks adjoining the macroblocks stored in buffer 2 are obtained ( s 501 ), and buffer 3 only stores macroblock ids that are not stored in buffer 2 ( s 502 ). then , macroblock ids stored in buffer 3 are extracted , and whether macroblocks of the extracted macroblock ids exist in buffer 1 is determined ( s 503 ), if the macroblocks do not exist , partial decode unit 29 is used to partially decode blocks belonging to those macroblock ids and they are added to buffer 1 ( s 504 ). after step s 504 is completed for all macroblock ids in buffer 3 , partially decoded image deletion unit 33 is activated . after partially decoded image deletion unit 33 is activated , the first macroblock id is extracted from buffer 1 ( s 601 ), whether the extracted macroblock id exists in buffer 2 is determined ( s 603 ), if such macroblock id does not exist , whether such macroblock id exists in buffer 3 is determined ( s 604 ), if such macroblock id does not exist , the corresponding macroblock id and the partially decoded image data for the macroblock id are deleted ( s 605 ). steps s 603 - s 605 for all macroblock ids extracted in step s 601 are repeated ( s 602 a , s 602 b ). for task processing , partially decoded image deletion unit 33 has lower priority than other units such as used block determination unit 28 , partial decode unit 29 , and partially decoded image output unit 30 . this allows for control requests to activate other units that take , precedence over and even interrupts partially decoded image deletion unit 33 even when it is active . according to this embodiment , since partially decoded image data that is being currently displayed is always stored in main memory , and adjoining blocks are prepared in advance , it is possible to realize quick displays of high - resolution shrunken images in response to control requests such as scrolling . also , since high - resolution shrunken images are displayed when processing becomes delayed , it is possible to realize more effective image display than that in the first embodiment . further , since partially decoded image data of macroblocks that are positioned far from the currently displayed macroblocks are consecutively deleted rather than deleting adjoining macroblocks , main memory can be effectively used . when adjoining images are decoded , for a zoom - out control request on an image , all adjoining images are decoded , and for scrolling control request only adjoining images that are in the scrolling direction can be decoded . this is a more effective use of main memory and allows for effective predictive decoding process . one example of predictive decoding steps in used block determination unit 28 is indicated in fig1 and fig2 . these example steps , as indicated in fig1 , are to decode adjoining macroblocks that are in the direction of vertex movement if the distance between each vertex and image boundary of macroblocks satisfies certain conditions . example of top left vertex ( x1 , y1 ) is further described here . used block determination unit 28 calculates macroblock id that belong to ( x1 , y1 ) using above formula ( 2 ) and substitutes the calculated id into variable i ( s 701 ). then according to the control request information passed by control request input unit 24 , if the vertex is moving in a negative x direction (“ yes ” in s 702 ), then whether distance between display area vertex coordinate ( x1 , y1 ) and image boundary in the negative x direction of macroblock where the vertex belongs is below a certain value is determined ( s 703 ). if such distance is below a certain value (“ yes ” in s 703 ), then whether the ‘ i - 1 ’ macroblock exists in the negative x direction is determined ( s 704 ). in other words , whether macroblock belonging to ( x1 , y1 ) is in the left edge is determined . this can be obtained from macroblock division condition ( m by n ) and current macroblock id , and can also be obtained by storing geometric array information as indicated in fig2 . with this determination result , if i − 1 macroblock exists in the negative x direction (“ yes ” in s 704 ), then partial decode unit 29 is activated and macroblock id i − 1 is decoded ( s 705 ). next , similar to above , used block determination unit 28 determines whether there is movement on the y axis in the negative direction ( s 706 ), and determines whether the distance between the current display area vertex coordinate ( x1 , y1 ) and the negative y direction boundary of macroblock containing the vertex is equal or less than a certain value ( s 707 ). if such distance is equal to or less than a certain value , whether i − m macroblock exists in the negative y direction is determined ( s 708 ). in other words , whether the macroblock containing the current ( x1 , y1 ) is the upper edge or not is determined . with this determination result , if i − m macroblock exists in the negative y direction (“ yes ” in s 708 ), partial decode unit 29 is activated , and macroblock id ‘ i − m − 1 ’ is decoded ( s 710 ). this is due to the vertex ( x1 , y1 ) moving in the upper left direction , and getting closer to a macroblock in a diagonal direction . if the determination is “ no ” in step s 702 , step s 703 , or step s 704 , then go to step s 711 and similar process as steps s 706 - s 709 is executed . above is the macroblock predictive decoding process according to the movement of the upper left vertex in display area , and predictive decoding can be processed for upper right ( x3 , y3 ), bottom left ( x4 , y4 ), and bottom right ( x2 , y2 ) vertices as well , as indicated by steps shown in fig1 and fig2 . with this predictive decoding process , since macroblocks that are outside the display area having high probability of being displayed are still decoded in advance , it is possible to improve response times in addition to the effects of the above embodiment . the present invention is not limited to the above embodiments , as it can be realized in various forms that are not beyond the scope of this summary . also , each feature ( unit ) described in each embodiment can be realized individually or in combination with various embodiments .