2. The apparatus of claim 1 , wherein the key-frame selector is configured to select the key frame according to a probability of occurrence within the plurality of unit frames, wherein the probability of occurrence indicates a number of unit frames among the plurality of unit frames having a similar parameter vector to a key parameter vector of the key frame.
3. The apparatus of claim 2 , wherein the key-frame selector is configured to select a unit frame with a higher probability of occurrence than a predetermined fraction of the plurality of unit frames as the key frame.