Variable framerate is the practice of encoding video with different framerates for different parts of the movie. This practice was nearly unheard of until the high-definition (HD) movies came to prominence. Some common HD video formats will use a variable framerate to enhance compression and reduce file sizes. The basic method of this practice is increasing rate during high movement and decreasing rate during static scenes. While this practice in videos is still uncommon, using variable bitrates in audio files is a very similar practice and has been around for many years.
To understand variable framerate, it is important to understand a few basic terms. A frame is a single image, like a slice of a movie. A framerate is the number of frames that displays on the screen per second, basically making a movie work like a complex flipbook. Higher framerates will result in a smother picture and lower rates will jerk and skip. It is important to note that framerate is independent of output resolution, as a low resolution but high framerate video will still look blurry even though the movement will flow nicely.
Videos with a fixed framerate may not always appear to have the same smoothness. In lower movement scenes, such as people sitting and talking or movement shot from a distance, the actual change from frame to frame is very minor. In higher motion portions, like a fight scene or a shot of detailed movement, the change from frame to frame might be very large. Even though the frames are moving at the same speed, the movement will look jerky in the fast motion due to the difference between frames.
By using variable framerate, it is possible to both eliminate that problem and reduce the size of the video. In general, with all other things being equal, the higher the framerate, the larger the end file. In the example above, the encoder would have the choice of making a very large file to accommodate the action scenes, even if that only accounts for a small part of the file. On the other hand, the file could be much smaller, but the action scene will look poorly encoded.
A variable framerate system will lower the framerate during slow parts and increase the rate during fast parts. This will allow views to have a constant appearance without vastly increasing the file size. This is done by creating a constant that relates to frame change. In a slow part with low difference between frames, the speed keeps the amount of change constant by slowing the framerate. In fast parts, the amount of motion from frame to frame is reduced by speeding up the framerate.
