reference will now be made in detail to the example embodiments of the present invention that are illustrated in the accompanying drawings . wherever possible , the same reference numbers will be used throughout the drawings to refer to the same or like parts . to begin with , before an embodiment of the present invention is described , in moving pictures having a scene change , a picture in which a scene change occurs entirely in the picture is defined as a scene cut picture and a picture in which a scene change occurs partially in the picture is defined as a partial scene change picture . fig3 a and 3b are flowcharts illustrating a method of coding a moving picture sequence in a moving picture coding system according to an example embodiment of the present invention . referring to fig3 a and 3b , pictures are sequentially input from a moving picture sequence ( s 111 ). kinds of pictures are determined ( s 114 ). in other words , it is determined whether the input picture is a p picture or a b picture . here , in this embodiment of the present invention , it is assumed that a coding with respect to an intra picture is completed in advance . if a picture is a p picture , it is determined whether or not a scene change occurs in the p picture ( s 117 ). here , the scene change is determined by comparing the p picture with a picture ( p picture or b picture ) displayed just before the p picture . if the scene is changed entirely among the p pictures , the p picture is a scene cut picture . if the p picture is determined as the scene cut picture , a coding is carried out with reference to a long - term reference picture ( s 120 ). if the p picture is not a scene cut picture , it is determined whether or not the p picture is a partial scene change picture ( s 123 ). if the p picture is a partial scene change picture , blocks contained in an area in which the scene is changed are coded with reference to the long - term reference picture as in step s 120 ( s 126 ). blocks contained in an area in which the scene is not changed are coded with reference to a short - term reference picture ( s 129 , s 132 ). here , the long - term reference picture is a picture stored in a long - term reference buffer , and the short - term reference picture is a picture stored in a short - term reference buffer . the short - term reference buffer is provided with a first - input , first - output ( fifo ) buffer in which a picture first input is output first , and the pictures coded a relatively short time ago are stored in the short - term reference buffer . the pictures coded a relatively long time ago are stored in the long - term reference buffer . and , pictures of respective scene sets , e . g ., an intra picture , the scene cut picture , the partial scene change picture and the like are stored in the long - term reference buffer . next , an example of scene sets and scene changes will be described to assist in understanding principles of the present invention . it should be understood that this is a non - limiting example . as shown in fig4 , an intra picture jo that is first scene cut picture of a scene set a1 , a first scene cut picture p50 of a scene set b1 and a first partial scene change picture p120 can be stored in the long - term reference buffer . here , a scene set is a set of similar pictures . for example , suppose the pictures represent a discussion program where an announcer appears , then a panel a appears , then the announcer appears again and then the panel a appears again . the scene where the announcer first appears is scene set a1 , and the scene where the panel a subsequently appears is scene set b1 . the scene where the announcer appears again is scene set a2 , and the scene where the panel a appears again is scene set b2 . as described above , when a scene change occurs , the p picture is coded in the inter mode with reference to a short - term reference or a long - term reference picture instead of being coded in the intra mode . this reduces the amount of the bits to enhance coding efficiency . description of the steps s 117 to s 132 in fig3 a will be made with reference fig4 . as shown in fig4 , if the p picture p200 to be coded now is the scene cut picture belonging to the scene set b2 , the short - term reference pictures stored in the short - term reference buffer are not used . the scene cut picture p200 is the first picture of the scene set b2 , and the scene set of the scene cut picture p200 is different from the short - term reference pictures such as p199 , p198 , p197 , etc ., belonging to the scene set a2 . the similarity of the scene cut picture p200 and the short - term reference pictures belonging to the scene set a2 is not great ( e . g ., p200 is part of the scene of panel a and p199 is part of the scene of the announcer in the above described example ), and precise coding cannot be achieved from such reference pictures . in this case , the p picture is coded in inter mode with reference to the reference pictures p50 and p120 belonging to a scene set b1 , which is a similar scene to the scene of scene set b2 ( e . g ., both are scenes of panel a in the above - described example ). on the other hand , if a partial scene change occurs in the p picture p250 , the coding is performed differently depending on two conditions . in other words , the blocks included in the area where a partial scene change occurs are coded in inter mode with reference to the long - term reference pictures p50 and p120 stored in the long - term reference buffer . the blocks included in the area where a partial scene change does not occur are coded in inter mode with reference to the short - term reference pictures p249 , p248 , p247 , etc ., stored in the short - term reference buffer . as described above , after one p picture is coded , if a next picture exists ( s 159 ), then the next picture is input ( s 111 ). returning to step s 114 , if the picture input in step s 111 is a b picture , then five prediction modes ( intra mode , forward mode , backward mode , bi - predictive mode and direct mode ) are tested and one of them is selected as an optimal coding mode ( s 135 , s 138 ). in this specification , the direct mode will be described mainly . first , one block of the b picture is read ( s 141 ). of course , the other blocks can be read subsequently . then , a kind or type of a reference buffer storing a specified picture is examined . namely , the type of reference picture ( e . g ., long or short ) is examined . the specified picture is determined of the earlier pictures than the b picture in the coding order regardless of the display order . in other words , the specified picture is one of the reference pictures used to code the b picture . therefore , the specified picture can be a short - term reference picture or a long - term reference picture . the reference pictures may be before or after the b picture in display order and they are stored in the short - term reference buffer and / or stored in the long - term reference buffer . if the specified picture is a long - term reference picture , the forward motion vector of direct mode for the b picture is set as a motion vector of the co - located block in the specified picture . the backward motion vector of direct mode for the b picture is determined to be zero ( s 150 ). however , if the specified picture is a short - term reference picture , the reference picture index and the motion vector calculated at the co - located block in the specified picture are read ( s 144 ). the reference picture index and the motion vector is calculated previously and stored in the system buffer . according to the reference picture index , it is determined whether the motion vector of the co - located block in the specified picture points to a long - term reference picture ( s 147 ). as described above , the short - term and long - term reference pictures are stored in the short - term reference buffer and the long - term reference buffer , respectively . if the motion vector of the co - located block in the specified picture points to the long - term reference picture , the b picture is coded using the following expressions 3 and 4 ( s 150 ): where mv is a motion vector of the co - located block in the specified picture , and mvf is a forward motion vector of direct mode for the b picture ; and where mv is a motion vector of the co - located block in the specified picture , and mvb is a backward motion vector of direct mode for the b picture in other words , if the motion vector of the co - located block in the specified picture points to the long - term reference picture , the forward motion vector in the direct mode for the b picture is the motion vector of the co - located block in the specified picture and the backward motion vector is zero . as shown in fig5 , in the step s 150 , if the motion vector of the co - located block in the specified picture p200 points to the long - term reference picture p50 , trd and trb are meaningless in the conventional expressions 1 and 2 . referring to fig5 , a more detailed description will be made . when inserting two b pictures b1 and b2 into a moving picture sequence and coding them , the p picture p200 that is earlier than the b1 and b2 pictures in coding order is coded first . here , since the p picture p200 is a scene cut picture in which a scene change occurs , the p picture p200 is coded in inter mode from the long - term reference picture p50 stored in the long - term reference buffer . according to the coding order , the next picture to be coded is the b1 picture . since the b1 picture belongs to the scene set a2 , most blocks thereof are coded in a forward mode from the short - term reference pictures belonging to the scene set a2 or in bi - predictive mode in which both of the two reference pictures belong to the scene set a2 . however , intra mode , backward mode or bi - predictive mode from the p picture p200 belonging to the other scene set b2 , and direct mode to obtain motion vectors of the direct mode from the co - located block in the p picture p200 are probably not used as the coding mode for the blocks in the b1 picture . differently , since not only the b2 picture but also the specified picture p200 used for motion vectors of the direct mode for the b2 picture belong to the same scene set b2 , the direct mode is selected as a coding mode for most blocks in the b2 picture . in other words , after obtaining the motion vector of each block in the specified picture p200 in the inter mode from the long - term reference picture p50 belonging to the same scene set b2 , the motion vectors of direct mode in the b2 picture are calculated from the motion vector of the co - located block in the specified picture p200 . since the b2 picture and the specified picture p200 belong to the scene set b2 , and the similarity between the scene set b1 to which the reference picture p50 belongs and the scene set b2 is very high , the direct mode can be selected as a coding mode for most blocks in the b2 picture . accordingly , the coding efficiency for the b2 picture is improved . on the other hand , if the motion vector of the co - located block in the specified picture points to a short - term reference picture , the b picture is coded using the conventional expressions 1 and 2 ( s 153 ). in this time , since the short - term reference picture stored in the short - term reference buffer belongs to the same scene set as the b picture and another scene set does not exist between the specified picture and the short - term reference picture , the forward motion vector and the backward motion vector of direct mode are determined using the conventional expressions 1 and 2 related to trd and trb representing time distance . after one block of a b picture is coded , the next block ( if it exists ) in the b picture is read and coded subsequently ( s 156 ). such processes are performed on the blocks in the b picture . after the b picture is coded , the next picture ( if it exists ) is input ( s 159 and s 111 ) and coded so that a moving picture coding is achieved . as described above , according to a moving picture coding method of the present invention , the forward motion vector and the backward motion vector in the direct mode for the b picture are determined differently based on the type of reference picture pointed to by the motion vector of the co - located block in the specified picture . when coding the b picture , the direct mode is mainly used as the coding mode to enhance coding efficiency . according to the moving picture coding method of the present invention , the p picture in which a scene change occurs is coded in inter mode using motion compensation from a long - term reference to reduce the amount of bits and enhance coding efficiency . it will be apparent to those skilled in the art that various modifications and variations can be made in the present invention . thus , it is intended that the present invention covers the modifications and variations of this invention .