FDecimate Plugin for Avisynth


The FDecimate() filter provides extended decimation capabilities not available from Decimate(). It can remove frames from a clip to achieve the desired frame rate, while retaining audio/video synchronization. It preferentially removes duplicate frames where possible. ("FDecimate" stands for "Free Decimate", which implies that the output frame rate may be freely chosen, and is not limited to 1-in-N decimation).

Increasingly we are seeing applications where traditional 1-in-N decimation is not sufficient. Four of them in particular stand out.

First, High Definition (HD) digital television often delivers a doubled frame rate, e.g., 59.94 fps for ATSC. A progressive film with a native frame rate of 24 fps can be converted to HD rates by duplicating frames in a pattern. For example to get from 24 fps to 60 fps, we can use this pattern:

A A A B B C C C D D E E E F F ...

We can call this a 3232... frame duplication pattern. Another example is 25 fps to 60 fps conversion, which uses a 3232232322... pattern. Converting from 60 fps to the broadcast rate of 59.94 fps can be achieved by either slowing down the video and adjusting the audio to match it, or by excluding one of the duplicates every 1000 frames. In either case, our final stream has many duplicates and the traditional 1-in-N decimation approach can no longer be directly applied. Sometimes, successive 1-in-N decimations can be applied, but this is cumbersome. And while Decimate(mode=2) is helpful with multiple duplicates, it is not always successful in that regard.

Second, anime is often rendered at 12 fps and converted to 29.97 fps by duplicating frames in a manner similar to that described above. The result is the same: a stream with multiple duplicates that is not reliably addressed by 1-in-N decimation.

Third, many silent films are transferred to DVD by adding duplicates in unusual patterns, because the original frame rates are not 24 fps. It is not unusual to see clips requiring strange decimations such as 20 in 43. Sometimes these strange decimations can be attained, or nearly attained, through repeated application of Decimate() using different cycles, but that is a cumbersome approach that cannot always attain the exact decimation ratios required. FDecimate() allows specification of arbitrary frame rates and is therefore useful in recovering the original cranking rates of these silent movies.

Fourth, sometimes clips are rendered at 120 fps to properly present hybrids of film and video. if we wish to return to normal frame rates, we need to remove multiple duplicates.

The FDecimate() filter helps in these situations. An earlier filter, MultiDecimate(), also attempted to help but it is a two-pass implementation that requires an additional external program, and is therefore cumbersome. It also could produce audio desynchronization if not used very carefully. I consider FDecimate() to be a superior approach.

Please note that, while FDecimate() can be used for traditional 1-in-N decimation, it is probably preferable to stay with Decimate() for those applications, because it does not require the setting of a threshold for duplicate detection when used in mode=0.

This version supports YUY2 and YV12 for Avisynth 2.5.

FDecimate() is an advance over the internal filter ChangeFPS() because it can preferentially deliver unique frames, avoiding duplicates where possible.

Version 1.1.0 (includes source code under GPL)

Version 1.1.0 improves the avoidance of duplicate delivery and adds a ,i>protect option intended to help prevent delivery of "bad" frames after scene changes.

Version 1.0.2 (includes source code under GPL)

Version 1.0.2 fixes a theoretical problem with pitch handling (which was thus far never encountered in practice).

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