VFR

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Variable frame rate (or VFR) is a term in video compression for a feature supported by some container formats which allows for the frame rate to change actively during video playback, or to drop the idea of frame rate completely and set individual timecode for each frame. VFR is especially useful for creating videos of slideshow presentations or when the video contains large amounts of completely static frames, as a means of improving compression rate, or if the video contains a combination of 24/25/30/50/60 FPS footages and the creator or editor of the video wishes to avoid artifacts arising from framerate-conversion. (source: Wikipedia).

Contents

AviSynth, variable framerate (vfr) video and hybrid video

There are two kinds of video when considering framerate, constant framerate (cfr) video and variable framerate (vfr) video. For cfr video the frames have a constant duration, and for vfr video the frames have a non-constant duration. Many editing programs (including VirtualDub and AviSynth) assume that the video is cfr, partly because avi doesn't support vfr. This won't change in the near future for various reasons. Although the avi container doesn't support vfr, there are several containers (mkv, mp4 and wmv/asf for example) which do support vfr.

Hybrid video is commonly defined as being a mix of pulled-down material and non-pulled-down material (where the pulldown can be of fields, as in standard 3:2 pulldown, or full frames). It's not relevant whether the pulldown is hard (the fields/frames are duplicated before the encoding) or soft (adding the appriopriate flags in the stream which indicate which fields/frames should be duplicated during playback). So, it can be either cfr or vfr. Thus hybrid video is simply video with different base framerates (for example 8, 12, and 16 fps at which anime is often drawn). The base framerate is the rate before any pulldown. What makes hybrids challenging is the need to decide what final framerate to use.


Variable framerate and hybrid video

It's important to understand that usually video is cfr. There is one example where converting to vfr can be very useful, which is hybrid video. Hybrid video is video with different base framerates (for example 8, 12, and 16 fps at which anime is often drawn). The most common example of hybrid video consists of parts that are interlaced/progressive NTSC (29.97 fps) and other parts which are FILM (telecined from 23.976 fps to 29.97 fps). For soft pulldown, the NTSC part (also called video part) is played back at 29.97 fps and the telecined part also by duplicating fields (to go from 23.976 fps to 29.97 fps). For hard pulldown, it is played back at 29.97 fps without adding any fields.

Examples of hybrid video include many of the modern anime TV Series, many of the Sci-Fi TV Series (such as Stargate: SG1, Star Trek: TNG, and Babylon 5), and many of the "Making Of" documentaries included on DVD.

The TIVTC package is designed to work with hybrid video losslessly, while the Decomb package has routines to convert to cfr via blending.

How to recognize vfr content (mkv/mp4)

Here are some ways to determine if the mkv/mp4 is vfr:

mpeg-2: DGIndex will report a Film/Video percentage, which can tell you much hybrid content a soft-pulldowned file has. It will not work with hard pulldown, and isn't always accurate if hard/soft are mixed.

mkv: get timecodes file using mkv2vfr to check this.

mp4: this can be found out by using mp4dump (from the MPEG4 tools by MPEG4ip package). Open a dos prompt and type (using appropriate paths)

mp4dump -verbose=2 holly_xvid.mp4 > log.txt

Open the log file, and look for output like this (look up the stts atom to figure out the length of each frame):

type stts
       version = 0 (0x00)
       flags = 0 (0x000000)
       entryCount = 41 (0x00000029)
        sampleCount = 3 (0x00000003)
        sampleDelta = 1000 (0x000003e8)
        sampleCount[1] = 1 (0x00000001)
        sampleDelta[1] = 2000 (0x000007d0)
        sampleCount[2] = 3 (0x00000003)
        sampleDelta[2] = 1000 (0x000003e8)
        sampleCount[3] = 1 (0x00000001)
        sampleDelta[3] = 2000 (0x000007d0)
        etc ...

sampleDelta indicates how long the frames get displayed and sampleCount tells how many frames. Thus on the example above:
3 frames are displayed with length 1000
1 frame are displayed with length 2000
3 frames with length 1000
1 frame with length 2000
etc ...

The time values are not seconds, but "ticks", which you have to calculate into seconds via the "timescale" value.  This "timescale" is stored in timescale atom for the video track (make sure that you look at the right timescale for your track, cause every track has its own timescale). Look for output like this:

type mdia
    type mdhd
...
     timeScale = 24976 (0x00006190)
     duration = 208000 (0x00032c80)
     language = 21956 (0x55c4)
     reserved = <2 bytes> 00 00 

In this example the timeScale is 24976. Most of the frames have a length of 1000. 1000/24976 = 0.04 which means each frame of the first 3 gets displayed with a length of 0.04 seconds, which is the equivalent to 25 fps (1/25 = 0.04). The next frame has a length of 2000. 2000/24976 = 0.08 which means that it is displayed with a length of 0.08, which is the equivalent to 12.5 fps (1/12.5 = 0.08). etc ...

The log file above comes from a video which is in fact hybrid.

Opening MPEG-2 hybrid video in AviSynth and re-encoding

Assuming you have hybrid video, there are several ways to encode it. They are listed below. The first method is to convert it to cfr video (either 23.976 or 29.97 fps). The second one is to encode it at 120 fps using avi and dropped frames (where duplicate frames are dropped upon playback). The third one is to create true vfr using the mkv or mp4 container.

encoding to cfr (23.976 fps or 29.97 fps)

If we choose the video rate, the video sequences will be OK, but the FILM sequences will not be decimated, appearing slightly jumpy (due to the duplicated frames). On the other hand, if we choose the FILM rate, the FILM sequences will be OK, but the video sequences will be decimated, appearing jumpy (due to the "missing" frames). Additionally, when encoding to 29.97 fps, you will get lower quality for the same file size, because of the 25% greater number of frames. It's a tough decision which to choose. If the clip is mostly FILM you might choose 23.976 fps, and if the clip is mostly video you might choose 29.97 fps. The source also is a factor. If the majority of the video portions are fairly static "talking heads", for example, you might be able to decimate them to 23.976 fps without any obvious stutter on playback.

When you create your d2v project file you will see whether the clip is mostly video (NTSC) or FILM (in the information box). However, many of these hybrids are encoded entirely as NTSC, with the film portions being "hard telecined" (the already telecined extra fields having also been encoded) so you'll have to examine the source carefully to determine what you have, and how you wish to treat it.

The AviSynth plugins Decomb and TIVTC provide two special decimation modes to better handle hybrid clips by blending. This will eat bitrate quickly, but it appears very smooth. Here is a typical script to enable this mode of operation:

Telecide(order=0, guide=1)
Decimate(mode=X) # tweak "threshold" for film/video detection

or

TFM(mode=1)
TDecimate(mode=0,hybrid=X) # tweak "vidThresh" for film/video detection

There are 2 factors that enable Decimate to treat the film and nonfilm portions appropriately. First, when Telecide declares guide=1, it is able to pass information to Decimate about which frames are derived from film and which from video. For this mechanism to work, Decimate must immediately follow Telecide. Clearly, the better job you do with pattern locking in Telecide (by tweaking parameters as required), the better job Decimate can do.

The second factor is the threshold. If a cycle of frames is seen that does not have a duplicate, then the cycle is treated as video. The threshold determines what percentage of frame difference is considered to be a duplicate. Note that threshold=0 disables the second factor.

Make sure to get the field order correct - DVDs are generally order=1, and captured video is generally order=0. The included DecombTutorial?.html explains how to determine the field order.

Mostly Film Clips (mode=3)

When the clip is mostly film, we want to decimate the film portions normally so they will be smooth. For the nonfilm portions, we want to reduce their frame rate by blend decimating each cycle of frames from 5 frames to 4 frames. Video sequences so rendered appear smoother than when they are decimated as film. Set Decimate to mode=3, or TDecimate to hybrid=1 for this behavior.

Another IVTC was developed specifically to handle hybrid material without blended frames: SmartDecimate. While you do get "clean" frames as a result, it also may play with slightly more stutter than does Decomb's result. A typical script might go:

B = TDeint(mode=1) # or KernelBob(order=1)
SmartDecimate(24, 60, B)

In order to keep the result as smooth playing as possible, it will insert the "Smart Bobbed" frames from time to time.

Mostly Video Clips (mode=1)

When the clip is mostly video, we want to avoid decimating the video portions in order to keep playback as smooth as possible. For the film portions, we want to leave them at the video rate but change the duplicated frames into frame blends so it is not so obvious. Set Decimate to mode=1, or TDecimate to hybrid=3 for this behavior.

In this case you may also consider leaving it interlaced and encoding as such, especially if you'll be watching on a TV later.

encoding to cfr - 120 fps

This is the most widely compatible option. For this you'll need TIVTC and avi_tc. Start by creating a decimated avi with timecodes.txt, but skip the muxing. Then open tc-gui's tc2cfr tab and add your files or use this command line:

tc2cfr 120000/1001 c:\video\video.avi c:\video\timecodes.txt c:\video\video-120.avi

Then mux with your audio. This works because tc2cfr creates an avi with drop frames filling in the extra space with drop frames to create a smooth 120fps avi.

encoding to vfr (mkv)

First download mkvtoolnix. We will use this to mux our video into the MKV container WITH a timecode adjustment file. Make sure that you have the latest version (1.6.0 as of this writing), as older ones read timecodes incorrectly.

There are several AviSynth plugins that you can use to generate the VFR video and required timecode file. An example is given below using the Decomb521VFR plugin. Another alternative is the TDecimate plugin contained in the TITVC package. See their respective documentations to learn more about tweaking them.

The DeDup plugin removes duplicate frames but does not change the framerate (leaving jerky video if not decimated first), so it won't be included. It can still be used after either method by using their timecodes as input to DeDup.

Decomb521VFR:

Add this to your script:

Decomb521VFR_Decimate(mode=4, threshold=1.0, progress=true, timecodes="timecodes.txt", vfrstats="stats.txt")

Open this script in VirtualDub, it will create the timecodes and stats files, then encode. It will seem to freeze at first, because it examines every frame on the first load.

TIVTC

This is a 2-pass mode. Add this to your script:

TFM(mode=1,output="tfm.txt")
TDecimate(mode=4,output="stats.txt")

Open this and play through it in VirtualDub. Then close it, comment those lines out (or start a second script) and add:

TFM(mode=1)
TDecimate(mode=5,hybrid=2,dupthresh=1.0,input="stats.txt",tfmin="tfm.txt",mkvout="timecodes.txt")

Load and encode.

framerate

If you're encoding to a specific size using a bitrate calculator, vfr decimation will mess up the calculations. To make them work again add these to your script:

Before decimation:

oldcount = framecount # this line must be before decimation

End of script:

oldfps = framerate
averagefps = (float(framecount)/float(oldcount))*oldfps
AssumeFPS(averagefps).Nicefps()

muxing

Now mux to MKV:

  1. Open mmg.exe (mkvmerge gui)
  2. Add your video stream file
  3. Add your audio stream file
  4. Click on the imported video track
  5. Browse for the "timecodes.txt" timecode file
  6. Click on the audio track
  7. If your audio already needs a delay, set one
  8. Start muxing

To play it you need a Matroska splitter. For AVC you will need Haali's Splitter, but for ASP you can use it or Gabest's Splitter.

encoding to vfr (mp4)

It's not possible to do this directly (using a timecodes file). If you create a 120 fps avi with drop-frames, however, the mp4 muxed from it will remove them along with any n-vops the encoder creates, leaving vfr. A more laborous way is to encode multiple cfr avi files (some with 23.976 fps film and some with 29.97 fps video) and join them directly into one vfr mp4 file with mp4box and the -cat option. Another option is to generate a NHML script for MP4Box.

summary of the methods

Summing up the advantages and disadvantages of the above mentioned methods. When encoding to 23.976 or 29.97 fps the clip will be cfr (which editors like AviSynth and Virtualdub need), but it may look jumpy on playback due to duplicated or missing frames. That can be avoided with blending, but encoders can't work as well with that. When encoding to 120 fps using drop frames, the clip is cfr, not jumpy on playback, and very compatible. Encoding to mkv using true vfr (using timecodes) neither loses nor duplicates frames, however it is not nearly as broadly supported as AVI.

Opening non MPEG-2 hybrid video in AviSynth and re-encoding

It is possible to open vfr video in AviSynth without losing sync: DirectShowSource. The most common formats that support hybrid video (vfr) are mkv, mp4, wmv, and rmvb, and the methods below work for all of them; however, if the source is mkv, you can also use mkv2vfr and AviSource.

opening non-avi vfr content in AviSynth

The best way to get all frames while keeping sync and timing is to convert to a common framerate, such as 120 fps for 24/30 (or rather 119.88). (Always use convertfps=true, which adds frames like ChangeFPS, or your audio will go out of sync.

DirectShowSource("F:\Hybrid\vfr.mp4", fps=119.88, convertfps=true)

You can also open it as 30p, which then has to be re-decimated but has less frames to deal with, or 24p, breaking any 30p sections:

Re-encoding to 23.976 or 29.97 fps:

DirectShowSource("F:\Hybrid\vfr.mkv", \
   fps=29.97, convertfps=true) # or fps=23.976

or

DirectShowSource("F:\Hybrid\vfr_startrek.mkv", \
   fps=119.88, convertfps=true)
FDecimate(29.97) # or FDecimate(23.976)

Another way is to find out the average framerate (by dividing the total number of frames by the duration in seconds) and use this rate in DirectShowSource. Depending on the duration of a frame, frames will be added or dropped to keep sync, and it's almost guaranteed to stutter. DirectShowSource will not telecine.

re-encoding 120 fps video

The easiest way to convert vfr sources back into vfr in AviSynth is by using DeDup:

1st pass:

DupMC(log="stats.txt")

2nd pass:

DeDup(threshold=.1,maxcopies=4,maxdrops=4,dec=true,log="stats.txt",times="timecodes.txt")

TIVTC can also do this:

1st pass:

TFM(mode=0,pp=0)
TDecimate(mode=4,input="stats.txt")

2nd pass:

TFM(mode=0,pp=0)
TDecimate(mode=6,hybrid=2,input="stats.txt",mkvout="timecodes.txt")

Once you've encoded your file, mux back to mkv or 120 fps avi.

This will chop out all the duplicate frames directshowsource inserts, while keeping framecount and timing nearly identical. But do not use the timecode file from the input video, use the new one. They may not be identical. (Of course you can play with parameters if you want to use more of the functionality of dedup.)

converting vfr to cfr avi for AviSynth

You can avoid analysing and decimating by using special tools to get a minimal constant-rate avi to feed avisynth. After processing and re-encoding, use tc2cfr or mmg on the output with the original timecodes to regain vfr and full sync. (If you perform any kind of decimation or frame-rate change you'll have to edit the timecode file yourself, although dedup does have a timesin parameter.)

avi

avi_tc will create a timecode and normal video, if the avi uses drop frames and not n-vops or fully encoded frames. It also requires that no audio or secondary tracks are present. To use it, open tc-gui and add your file, or use the following command line:

cfr2tc c:\video\video-120.avi c:\video\video.avi c:\video\timecodes.txt 1

mkv

mkv2vfr extracts all video frames from Matroska to a normal AVI file and a timecode file. This will only work if the mkv is in vfw-mode. The command-line to use it is:

mkv2vfr.exe input.mkv output.avi timecodes.txt

encoding to MPEG-2 vfr video

http://forum.doom9.org/showthread.php?t=93691

I didn't look at it yet, so i can't give any comments/hints.

Audio synchronization

Several methods are discussed to encode your video (at 23.976, 29.97 or vfr video). You might wonder why your audio stays in sync regardless of the method you used to encode your video. Prior to encoding, the video and audio have the same duration, so they start out in sync. The following two situations might occur:

  • you change the framerate of the stream by speeding it up or slowing it down (as is often done by PAL-FILM conversions). This implies that the duration of the video stream will change, and hence the audio stream will become out of sync.
  • you change the framerate of your the stream by adding or removing frames. This imlies that the duration of the video stream will remain the same, and hence the audio stream will be in sync.

If you encode the video stream at 23.976 or 29.97 fps (both cfr) by using Decimate(mode=3, threshold=1.0) or Decimate(mode=1, threshold=1.0), frames will be removed or added, and thus your audio stream will be in sync. By a similar reasoning the vfr encoding will be in sync.

Finally, suppose you open vfr video in AviSynth with DirectShowSource. Compare the following

DirectShowSource("F:\Hybrid\vfr_startrek.mkv", \
   fps=29.97) # or fps=23.976

and

DirectShowSource("F:\Hybrid\vfr_startrek.mkv", \
   fps=29.97, convertfps=true) # or fps=23.976

The former will be out of sync since 24p sections are speeded up, and the latter will be in sync since frames are added to convert it to cfr.

References

Waggoner, Ben (2009-11-16). Compression for Great Video and Audio: Master Tips and Common Sense. Taylor & Francis US.
Higgins, Jonathan (2000-04-24). Satellite Newsgathering. Taylor & Francis US.
Essential reading: Force Film, IVTC, and Deinterlacing and more (an article written by some people from at doom9).
Creating 120 fps video.
Documentation of Decomb521VFR.
About Decomb521VFR 1.0 mod for automated Matroska VFR.
Mkvextract GUI by DarkDudae.

Besides all people who contributed to the tools mentioned in this guide, the author of this tutorial (Wilbert) would like to thank bond, manono, tritical and foxyshadis for their useful suggestions and corrections of this tutorial.

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