Variable frame rate for very low bit-rate video coding

Motion description of objects time evolution often wastes bit-rate even if computed at a low fixed frame rate. Bit-rate reduction can be achieved taking advantage of the inactivity periods: in such time intervals there is no need for motion coding, as an accurate description of the scene can be obtained only considering the frame difference information. We propose to estimate the motion between couples of frames of the sequence only when the detected activity is high. The possibility of achieving very low bit rate values resides in the ability of dealing with precise motion estimators. To this end we propose here use of a modified block matching algorithm (BMA), the sliced-BMA, applied on separate sub-sets of each macroblock. Experimental results with test video-sequences show a noteworthy gain together with object segmentation and tracking capabilities.