Video-encoding method, video-decoding method, and apparatus implementing same

A non-transitory decoder-readable storage medium storing a bitstream outputted by performing the steps of:

• encoding a random access point picture, wherein the random access point picture is an instantaneous decoding refresh (IDR) picture or a clean random access (CRA) picture;
• encoding leading pictures and normal pictures associated with the random access point picture;
• outputting a bitstream comprising information on the random access point picture, the leading pictures and the normal pictures based on the encoding;
• wherein the leading picture precedes the normal picture in decoding order and the normal picture follow the random access point picture in decoding order;
• wherein the leading picture is not used as a reference picture for the normal picture, wherein the leading picture associated with the CRA picture is not output when the random access point picture is the CRA picture and a random access happens to the CRA picture, and wherein the leading picture associated with the IDR picture is decoded by using at least one of the IDR picture and another leading picture as a reference picture when the random access point picture is the IDR picture and the random access happens to the IDR picture.