Long term reference
Long-term reference (LTR) - in terms of video encoding, it's a frame marked as a reference for future frames even on a long distance. The main difference between LTR and short-term reference (STR) - all manipulations with LTR (adding, removing) may be done according to explicit commands in bitstream only. STR can be removed automatically when DPB is full. There are several scenarios when LTR is useful:
- Resynchronization encoder and decoder for repair frames in case of data lost during transmission. In this scenario usually, the last I-frame is marked as a long reference and is used by the decoder instead of a lost short reference frame. This scenario has been used by many world-famous companies (like Cisco) for a long time already.
- Quality improvement in “static scene” low delay scenarios. In this scenario, LTR is encoding with a higher bitrate and used for the follow-up frames.
- LTR can be used for scene change encoding quality improvement in case of repeated scenes.