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Export process _adds_ stuttering to movies with frame-rate change

1826 Views 26 Replies Latest reply: Nov 28, 2012 5:56 PM by betaneptune RSS
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betaneptune Level 1 Level 1 (0 points)
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Nov 3, 2012 1:38 PM

SYNOPSIS: When I playback a certain movie project, called VFT, in iMovie 9.0.4, the motion is smooth. If I export it, and then view the export, the motion is jerky. I haven't found a way to view it where the motion is not jerky except to view it as a "project" in iMovie itself. There is a uniform stuttering exhibited in the exports. It looks like the film stops briefly several times a second. (For a very clear alternate example of this, get the Ren and Stimpy episodes "Black Hole" and "Space Madness". The title sequence background (with the moons, planets, etc. moving from right to left) is exactly the same, but appears to run smoothly in "Black Hole" and jerkily (5 jerks per second) in "Space Madness". Can anyone here explain this?) Note: The film VFT is characterized by a lot of moving scenery.


EXAMPLES OF THE STUTTERING: I picked the two parts of the movie (called "Visuals for Timesteps" (VFT)) that best exhibit the stuttering and put them in their own project. I exported the project from iMovie using "None" for compression and "Best" for quality. I then processed them through MPEG Streamclip according to's recommendations and posted it. You can see it at!i=219 2123682&k=mr9Vckn


THE FULL STORY: I made some Super 8 movies at 18 fps in the 70s and 80s. I had them transferred to digital. I received files from the transfer company in the form of QuickTime movie files. The format: Apple Motion JPEG A, 1440 x 1080, Millions. This was a high-definition scan. No interlacing. No blended frames. Each and every frame in the file is a digital copy of each and every frame in the film.


I imported these movie files into iMovie. Ideally one would edit them at 18 fps, but that's not an option with iMovie. So I created projects using 24 fps. To preserve the speed, iMovie doubles every third frame to "stretch" each 18 frames to 24. Why did I choose 24 fps? Easier editing, smaller files, and the hope of making 24 fps progressive DVDs or even BluRays one day. Furthermore, it was recommended by two independent sources, and it taxes the computer less. I was concerned that the 3:4 pulldown would give jerky motion, but playback in iMovie looked perfectly smooth. This surprised me, but it seemed fine, so I went with it.


One of the movies is called "Visuals for Timesteps" (VFT). This is a highly unusual film in that there is a lot of motion of the entire scene a lot of the time. While this looks fine when playback is done from the project browser, there is a uniform stuttering when playing exports. It looks like the movie stops several times a second, just what you might expect from the 3:4 "pulldown", so I assume it is. (My other films mostly look okay.) And it doesn't matter what app or hardware I use to view the export: QuickTime 7, QuickTime 10, MPEG Streamclip, iPod, iPad -- it matters not. (I'm talking about _local_ playback. The Net is not involved, though I get the same effect there, too, in Firefox and Safari.)


I tried different forms of compression: Cmd-E and "Export using Quicktime" with default settings, no compression, and AIC -- all to no avail, which didn't surprise me. I even imported the AIC back into iMovie just to see if there would still be stuttering. There was.


The original 18 fps files from the transfer company play back quite smoothly, with barely a hint of jitter in certain scenes, in QuickTime 7 and 10. The import in the project browser is just slightly more jittery, and again noticeable only upon careful scrutiny of certain shots. Also, the jittering doesn't make the film look like it stops several times a second. The motion appears quite uniform in time. This is far more acceptable (and I'm fine with it) than the stuttering in the exports.


What is going on? What "magic" is iMovie doing when viewing from the project browser that can't be done in exports?


And how can I fix this? I can redo the entire project at 30 fps, which would lessen the stuttering, I would think. But this would be a tremendous amount of work! There are 45 clips, each painstakingly timed, trimmed, and speed-adjusted to match the music. Many required  single-frame precision! Many had to be individually cropped to crop out conspicuous defects in the form of dust, splotches, splice gaps, and splice sprocket holes. Also, a few clips needed color correction.


Is there a way to eliminate or at least lessen the stuttering other than a redo at 30 fps? Or an easier way to convert to 30 fps? Or could I create a small virtual disk, add the raw footage and project files, modify file names and folders if needed, and post it for, or send it to, those individuals who have iMovie?  Any helpful comments and suggestions are welcome.


Running iMovie 9.0.4 on OS X 10.6.8.





iMovie '11, Mac OS X (10.6.8)
  • AppleMan1958 Level 7 Level 7 (27,335 points)

    The stuttering is not that noticable to me, but that is probably because I haven't seen the original, so I don't know what I'm missing.


    When you say you are exporting uncompressed, does that mean you are getting Motion JPEG as output? Or do you mean uncompressed 4:2:2 as output ?(very large files). At first I thought this could be an issue, but as I re-read your post, it appears that you have tried all the h.264 presets as well.


    My first thought would be to redo them in 30 FPS.

    Unfortunately, you can't change the frame rate without starting a whole new project. I don't think it would help you to switch the 24fps output to 30fps in another app. By the way, I don't think you can change the frame rate in the middle of the project in Final Cut Pro either.


    I did a project like this with 8MM and I used 30, and it was not quite as smooth as the 18fps, but still not bad.


    I just checked in Final Cut Pro X. FCPX gives you more control over how the frame rate is managed you have 4 options. As best I can tell, iMovie uses the first option, because iMovie does not do intermediate rendering.


    • Floor: The default setting. Final Cut Pro truncates down to the nearest integer during its calculation to match the clip’s frame rate to the project’s frame rate.
    • Nearest Neighbor: Final Cut Pro rounds to the nearest integer during its calculation to match the clip’s frame rate to the project’s frame rate. The Nearest Neighbor option reduces artifacts at the expense of visual stuttering. Rendering is required.
    • Frame Blending: Creates in-between frames by blending individual pixels of neighboring frames. Slow-motion clips created with Frame Blending appear to play back more smoothly than those created with the Floor or Nearest Neighbor setting. This setting provides better reduction of visual stuttering, but you may see some visual artifacts. Rendering is required.
    • Optical Flow: A type of frame blending that uses an optical flow algorithm to create new in-between frames. Final Cut Pro analyzes the clip to determine the directional movement of pixels, and then draws portions of the new frames based on the optical flow analysis. Choosing the Optical Flow option results in better reduction of visual stuttering, and Final Cut Pro spends a significant amount of time to fix visual artifacts.

    for more see here....


    I am doing a test on some of my own 8MM in FCP for the various options. Will post the results here.

  • AppleMan1958 Level 7 Level 7 (27,335 points)

    OK here are the results of my tests.

    My initial reaction is that for Floor and Nearest Neighbor, 30fps is better, but for Frame Blending and Optical Flow, 24 fps is fine.  I also have an opinion on which method produces the best results, but I would rather see what you think.


    Here is 18fps motion jpeg conformed to a 24fps timeline. Rendered in ProRes 422 and converted to h.264 in Compressor. In 1080P



    Here is 18fps motion jpeg conformed to a 30fps timeline. Rendered in ProRes 422 and converted to h.264 in Compressor. In 1080P


  • AppleMan1958 Level 7 Level 7 (27,335 points)

    I found a patent application that describes the 4 options in some detail.


    By the way, sorry for wearing you out on FCP. I know you probably don't want to buy it just for this.

  • AppleMan1958 Level 7 Level 7 (27,335 points)

    I think you are correct that Floor and Nearest Neighbor produce near identical results for 18 to 24, and for 18 to 30.


    Looking at my footage this morning, I can see why the 18 to 24 appears more stuttering. If you number the frames 1 to 18, it maps them to the 24 frames as 1 2 3 3 4 5 6 6 7 8 9 9 10 11 12 12 13 14 15 15 16 17 18 18. If you clap that out, it sounds like an engine starting up in first gear.


    If you map the 18 frames to a 30 frame timeline, you get something like this.

    1 1 2 2 3 4 4 5 5 6 7 7 8 8 9 10 10 11 11 12 13 13 14 14 15 16 16 17 17 18. If you clap that out, it sounds like the car is now in second gear.


    In both frame blending and optical flow, you don't have the stuttering. It creates extra frames interpolated between the existing frames to create continuous motion.


    As for the artifacts, if I look at the frame blending option slowly, the created frames look like they contain shadows. For example, when the baby (moi!) is waving his arms, one of the frames will have an arm that looks like a light shadow or motion artifact. But in optical flow, all pixels are clear. The artifacts are not noticable when you play it at speed for the same reason that you do not notice interlaced footage when played at speed on your TV. Your brain puts it together.


    So optical flow looks better, but at a cost of extra analytical time and rendering time. If you have time, it gives the best result, but for most applications, I would think that frame blending would be sufficient. 


    One application where optical flow would give superior results is if you retime a clip to produce slow motion, say 25% of normal.  In Floor, it would just repeat frames. In Frame Blending, it would create new frames, but you would see the shadowy artifacts for things in motion. In Optical flow, each frame would look like a photograph.


    One additional option I did not mention yesterday is to use Compressor (about $50). You would have to pre-process the clips in compressor and use frame blending or optical flow. I find the Compressor interface a lot harder to use than the FCP interface, but looking at the manual, it does appear to have this capability. I can't help you much beyond that with Compressor. You would basically process each 18fps clip to the highest level of rate conform that you want, and create AIC files that you could use in iMovie.

  • AppleMan1958 Level 7 Level 7 (27,335 points)

    Here is an example of an interpolated frame in Frame Blending. You can see the artifacts, but they are not very noticable at full speed.

    frame blending last.jpeg


    Here is the same frame in Optical Flow. There are no motion artifacts.

    optical flow.jpeg

  • AppleMan1958 Level 7 Level 7 (27,335 points)

    So you tried to export as uncompressed and it converted to h.264 anyway? I would think it would either export as Motion Jpeg or as uncompressed 4:2:2 (very large files). But it went to h.264 in standard definition size.

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