Progressive to Interlaced via Optical Flow

Suppose you have original footage that is different to that of the required product.  For example you have progressive footage and require an interlaced product.  Or perhaps the given footage is interlaced, but at a different resolution to that product.

While it is naively possible to simply “bung whatever footage one shot into an NLE and render the requried format”, this will not in all cases provide the optimum quality.  Obtaining a quality interlaced product from progressive footage (e.g. as-shot or intermediate or an animation) requires some more “beyond the box” thinking and processes.

The following article extract (link and bullet-points) explains how to go from Progressive to Interlaced using a video-processing application such as After Effects.

  • The first stage is to derive double-rate progressive footage from the original, specifically via motion-compensated/estimated /optical-flow tools/techniques as opposed to simple frame-blending (which would give rise to unwanted motion-blur artefacts).  This can be achieved via various applications (e.g. as listed in the article).  For such processes, I have traditionally used AviSynth (e.g. QTGMC & MVTools, which I covered at, but I look forward to evaluating other applications in this regard.
    • For footage that is already interlaced but which is at a different resolution to the required product, I typically use AviSynth’s TDeint plugin, which use motion/optical methods via which one can derive complete progressive frames corresponding to each field of the given footage.  Then these frames can be resized to the required product resolution, prior to the second stage.
  • The second stage is to derive from this (double-rate progressive footage) the required interlaced footage, by extracting each required field (upper and lower alternating) from each frame in turn.  For this, I have traditionally used Sony Vegas, which does this well.  The article claims After Effects does it well, and better than (the erstwhile) Final Cut Pro, but no mention is made of Adobe Premiere (though it may well perform this task well).  Naturally, AviSynth could also be used for this, either by extending its script or as a separate script.
    • I queried whether Premiere could do it, on Adobe Premiere forum:
    • One reply said <<Premiere is pretty smart about such matters.  You should have no issues.>>
  • Note that it can be useful to preserve a double-rate intermediate file for other purposes (e.g. downscaling of HD to SD or maybe in future, double-the-current-normal-rate will become the new normal).


    • Interlacing Progressive Footage
    • {The following is slightly re-worded/paraphrased from the original}
    • Frame-Doubling:
      • The first step is to double up the literal frame count, resulting in one of the following:
        • Double the duration.
        • Double the frame-rate.
      • In order to do this properly, the new frames need to be interpolated by means of a vector-based pixel warping or morphing algorithm.
      • This can be accomplished by a variety of different applications, including:
        • Motion 3 (by use of the Optical Flow feature)
        • After Effects (by use of Layer > Frame Blending > Pixel Motion)
        • Shake
        • Twixtor plugin (which can be used in Final Cut Pro, After Effects and several other host applications)
        • Boris FX
      • You do NOT want to frame-blend this step.
      • The best way to tell if this step is working correctly is to look at the new frames that have been created. If they have an overlapping ghost look to them, then it’s frame-blending, which you do not want. If the new frames literally look like new frames with no ghosting or overlapping, then you’re on the right track.
    • Interlacing:
      • This can be done in After Effects, Final Cut Pro and pretty much any other video application
        • After Effects renders out a cleaner interlace (actually, a perfect interlace) than does Final Cut Pro
      • In Adobe After Effects:
        • Setup:
          • Select the rendered clip in the Project window and right-click it and select Interpret Footage > Main.
          • Suppose the original clip was “30p”, i.e. 29.97 fps, then the rendered clip will be “60p” i.e. 59.94 fps.
          • In the Frame Rate section, conform the frame-rate to the correct value, namely 59.94 fps, or “60p”.
          • Create a new Comp of “60i”
          • Place the 60p clip in that Comp’s timeline
          • (Even though your timeline is only 29.97 FPS and you can’t see the extra frames when scrubbing frame by frame, don’t fear; when you render the final clip, it will use the extra frames in the 60p clip to create the new fields.)
        • Render:
          • Render this by Menu:Composition > Make Movie].
          • This should open up the [Render Queue] window with a new comp in the queue. You’ll need to change the Render Settings either by selecting a pulldown option next to it or by clicking the name next to the pulldown option.
          • Ensure you render this clip with [Field Rendering] turned on. You’ll need to select either Upper Field First (UFF) or Lower Field First (LFF), depending on your editing hardware and format of choice.

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