Frame blending
Also known as: frame mixing, blend frames
Mixing neighbouring frames together to fill a gap, used when footage is slowed down or retimed and there is nothing real to show.
Updated
What it means
Frame blending invents intermediate frames by averaging the ones either side. Slowing a clip needs more frames than were recorded, and blending is the cheapest way to produce them.
The result is a soft double image rather than real motion. Two positions of a moving subject appear at once faintly, which reads as a smear and is immediately recognisable once noticed.
It is the simplest of the retiming options. Optical flow and Frame interpolation both attempt to work out where the subject actually was, while blending merely fades between two guesses.
How it works in practice
It is acceptable for small adjustments and poor for large ones. Slowing a clip slightly blends almost invisibly, while halving the speed produces visible ghosting on anything that moves.
High frame rate footage removes the need entirely, which is the real answer. Material shot at a High frame rate (HFR) has real frames to show and needs nothing invented.
Where blending is unavoidable, choosing the stretches with least movement limits the damage, since a nearly static shot blends without any visible artefact. The faster the subject, the more obvious the ghosting becomes.
What to watch out for
Blended footage also hides genuine faults, so a clip containing a few Corrupted frames can be made to look acceptable while the underlying data is still wrong and still there.
It also interacts badly with stabilisation. Blending frames that Electronic image stabilization (EIS) has already warped produces a smear that moves independently of the subject, which is more distracting than either artefact alone.
Mixing blended and real slow motion in one sequence is the editorial mistake, because Slow motion shot properly and slow motion manufactured afterwards do not match in texture, and the difference is obvious the moment they sit next to each other.
How RawClip handles it
Clips are used at the speed they were recorded unless the footage genuinely supports slowing, which means shot at a high frame rate. Nothing is retimed by averaging frames together, because invented motion is visible to anybody watching the result with any attention at all.