This filter reduces the framerate of a video by 1/2, by blending odd and even frames together. However, it does this in a smart way such that in case of duplicate frames, it will do the smart thing to remove unnecessary blurring in the output video.
Source: https://github.com/arkeet/fpsdown
Binary download: https://github.com/arkeet/fpsdown/releases
Demonstration: http://www.youtube.com/watch?v=XULTJAWCDWk
FpsDown(clip clip [, bool maxnorm, int threshold, bool debug,
bool adjust3xslowdown, float gamma])
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clip: This is the clip to reduce the framerate of. Note that only clips in RGB32 format are accepted.
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maxnorm: Selects which norm to use when determining how close two consecutive frames have to be to be considered duplicates. If false, uses an L^1 norm (sum over all pixels of absolute values of differences in each channel). If true, uses a L^infinity (max) norm (the largest absolute value of differences over all pixels). Default value is true.
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threshold: Two consecutive frames are considered duplicates if the norm (as determined by the maxnorm argument) is smaller than this value. Default value is 8 if maxnorm = true, and 2000000 if maxnorm = false.
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debug: If true, this will show a small square in the bottom left which is normally white, but black when duplicate frames which should not be blended are detected.
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adjust3xslowdown: If true, this will blend frames differently in situations with 3 consecutive duplicate frames, in an attempt to make the result smoother and less jittery. Default value is false.
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gamma: Set the gamma value for gamma correction when blending frames together. Default value is 2.2.
To load the plugin, simply place fpsdown.dll in the Avisynth plugins directory. Then, to do the framerate conversion, for default settings it suffices to just have this line:
FpsDown()
It is good to use the debug option when testing, however. If two frames which are supposed to be duplicates are not actually detected as such (so that the debug square remains white), try increasing the threshold.
Important note! Because I'm lazy, this function only accepts clips in RGB32 format. If necessary, use the ConvertToRGB32 function first.