A method of controlling an image device to perform forward play and reverse play of a video sequence, wherein the video sequence comprises a plurality of groups of pictures, and each GOP comprises a plurality of pictures, the method comprising: decoding compressed pictures; storing the decoded pictures in a buffer which is capable of storing a number of decoded pictures constituting more than one group of pictures; and controlling the buffer to discard or store a GOP, wherein storing a GOP comprises decoding and storing each picture of the GOP in order; wherein the controlling the buffer comprises:- if a forward play command is received, determining a first number which is the sum of a number of pictures which comprise previous pictures and a present picture stored in the buffer from among pictures of a present GOP (N th GOP), and a number of pictures of a previous GOP (N-1 th GOP) prior to the present GOP (N th GOP) which are stored in the buffer; and if the first number is equal to a second number of pictures that the buffer is capable of storing, controlling the buffer to discard the previous GOP (N-1 th GOP), wherein if a reverse play command is received while a picture is output for display according to the forward play, determining a third number which is the sum of a number of pictures which comprise previous pictures and the present picture stored in the buffer from among the pictures of a present GOP, and a number of pictures of the previous GOP prior to the present GOP which are stored in the buffer, and if the third number is greater than or equal to a fourth number of pictures constituting one GOP, controlling the buffer to output pictures for display in reverse order from the order of forward play, and if the third number is less than the fourth number, controlling the buffer to discard a reverse-played GOP (N+1 th GOP) and store a previous GOP (N-1 th GOP). decoding compressed pictures; storing the decoded pictures in a buffer which is capable of storing a number of decoded pictures constituting more than one group of pictures; and controlling the buffer to discard or store a GOP, wherein storing a GOP comprises decoding and storing each picture of the GOP in order; wherein the controlling the buffer comprises:- if a forward play command is received, determining a first number which is the sum of a number of pictures which comprise previous pictures and a present picture stored in the buffer from among pictures of a present GOP (N th GOP), and a number of pictures of a previous GOP (N-1 th GOP) prior to the present GOP (N th GOP) which are stored in the buffer; and if the first number is equal to a second number of pictures that the buffer is capable of storing, controlling the buffer to discard the previous GOP (N-1 th GOP), wherein if a reverse play command is received while a picture is output for display according to the forward play, determining a third number which is the sum of a number of pictures which comprise previous pictures and the present picture stored in the buffer from among the pictures of a present GOP, and a number of pictures of the previous GOP prior to the present GOP which are stored in the buffer, and if the third number is greater than or equal to a fourth number of pictures constituting one GOP, controlling the buffer to output pictures for display in reverse order from the order of forward play, and if the third number is less than the fourth number, controlling the buffer to discard a reverse-played GOP (N+1 th GOP) and store a previous GOP (N-1 th GOP).