Repeated frames in video clips and Neat Video noise reduction
Repeated frames are sometimes present in clips produced by certain models of video cameras working in “slow shutter” or “night” mode. In such clips, each frame is repeated two or more times and those repeated frames are exact or almost exact copies of each other.
Video tutorial on using Slow Shutter mode in Neat Video
In the picture below, you can see a typical sequence of frames from a video clip containing
Here the frames 1 and 2 are almost identical. Frames 3 and 4 are almost identical. Frames 5 and 6 are almost identical or exactly identical.
At the same time, the frames 2 and 3 differ a lot (just like frames normally differ in a regular clip) because they depict objects at different points in time. The same applies to the frames 4 vs 5.
In this example, the frames 1 and 2 (also 3 and 4, 5 and 6) are what we call " repeated frames", because the frame 2 repeats the frame 1.
For a live illustration of repeated frames please play back this clip (it plays in slow
motion to make the issue easier to see):
This clip shows moving objects, noise and a frame counter. You can notice that in some moments of time, the frame counter changes while the frame content remains (almost) unchanged. That is the point where you see a repeated frame: a new frame follows the preceding one, their numbers are different but the content is the same (or almost the same).
This visually looks like a clip with decreased frame rate, because the actual frame content changes less frequently than the frames themselves. That is in fact correct: the effective frame rate of such a clip is indeed lower than the formal frame rate of the clip declared in the file metadata. Reduced effective frame rate is a natural consequence of presence of repeated frames in the clip.
Problem and solution
Repeated frames complicate the job of the noise filter because such frames do not carry useful information needed for noise reduction yet still require processing and potentially confuse the filter (since the filter looks for changing frames when trying to isolate the actual details from noise). Processing such a clip in a regular way may lead to insufficient and inaccurate noise reduction.
Neat Video includes a dedicated Slow Shutter mode allowing the filter to adjust its work to this special condition — presence of repeated frames — and to apply the noise reduction correctly to all frames. When using this mode, we also recommend to increase the temporal filter radius to let the filter gather enough information to accurately reduce noise and preserve actual details.
Additional notes about clips with repeated frames
There may be different reasons why repeated frames may be present in source clips:
- Cameras working in “slow shutter” or “night” mode can produce clips with repeated frames
- Increasing the frame rate of a "normal" clip is often done by duplicating some of the frames, which also creates repeated frames
- In-camera noise reduction used in some cameras can produce frames with repeating noise patterns: the objects in frames do move every frame but the noise remain static for several frames
In all these cases, the Slow Shutter mode of Neat Video 4 should be used to correctly process such video data.
See an example of such a clip with repeating noise (the objects in frames do move every
frame but the noise remain static for several frames):
(the clip plays in slow motion to make the issue easier to see).
When playing it back please focus on the noise elements. See how they remain almost static for 3 frames in a row, while the actual details (objects) move every frame.
When such a clip is processed by Neat Video in a regular way ( Slow
Shutter is not enabled), those static noise elements are not correctly detected as
noise and therefore are not fully reduced.
The Slow Shutter mode allows to adjust the filter to this special condition of repeated frames (repeated noise in this example) and to achieve much more accurate noise reduction.
The clip below shows the difference between processing without Slow Shutter and with Slow Shutter enabled: