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Frame Interpolation in Compressor 2

Using 'Optical Flow' to retime video By Dave Nagel
I've never written much on the topic of Compressor, Apple's companion utility that ships with Final Cut Studio. Compressor is, as its name implies, a compression tool (for audio and video), and, as such, it's always been sort of a no-brainer. Drop in a file; pick your settings; and export the transcoded/compressed movie or sound file. Now, though, in version 2, it has a few extraordinary features worthy of explanation, including its new-found Optical Flow features, which use advanced algorithms for format conversions, including NTSC to PAL, PAL to NTSC, SD to HD, etc.

The new Optical Flow features are actually carried over from Apple's high-end Shake motion graphics software. The features don't appear by the name "Optical Flow" in Compressor 2, but they're present under the hood powering all of the image analysis that goes into rate conversion, resizing and deinterlacing. And one of the valuable aspects of these new features is Compressor's new ability to interpolate frames when retiming video.

What this means, in essence, is that Compressor will not only alter the frame rate of footage when converting between standards, but actually analyze the footage to the extent that it can add in new, unique frames automatically.

Applying rate conversion settings
To demonstrate this, we'll take a look at a piece of footage (wheat blowing in the wind) that starts out at 720 x 480 (DV/DVCPRO NTSC) at 24 FPS progressive. The footage is then converted to MPEG-2 at 29.97 FPS progressive. Now, obviously, when this happens, it means that you're going to wind up with more frames that you started with. And normally in a retiming process like this, it means that those extra frames are going to come in the form of repeated frames. That is, occasionally one frame will be copied to span two frames, this filling in the necessary "in between" frames.

However, in Compressor, in certain cases, you can use Optical Flow feature to render out new, unique in between frames that are derived from the surrounding frames. There are some limitations to this, as discussed below, but here's how it works when converting between major standard formats.

To begin, drag your footage into Compressor, and choose one of the presets that approximates the outpiut format you want to use for your footage. In my case, I'll choose the "MPEG-2 60min High Quality Encode Widescreen" preset.

In the Inspector palette, choose the various options for your output in the Encoder Settings tab, and set the Rate to 29.97 and the Field Dominance to "Progressive."

Then click on the Frame Controls button to access the frame control settings. By default, these controls are off.

So switch the controls to "Custom." This will allow you to adjust the resizing and retiming settings for the compression process. For resizing, I'm going to set the option to "Best (Statistical Prediction)" and the Output Fields to "Progressive." Then, down at the bottom, I'll set the Rate Conversion to the highest setting, which is called "Best (High quality motion compensated)."

I'll then save these settings by clicking the "Save As" button at the bottom of the Inspector palette and apply the new preset to my source footage in Compressor's main window.

These settings will now allow Compressor to use its Optical Flow features to retime my footage with advanced frame interpolation. The result you see below (recompressed and resized for the Web, but the essential details are there). If you scrub through the footage below, the first frame is part of the original move; the second frame is a new frame generated by Compressor; and the third frame is an original frame from the source movie.

If you'd like to see more examples of the results that can come out of Compressor's retiming feature, be sure to take a look at my review of DVD Studio Pro 4 (which includes Compressor 2) by clicking here.

'Kicking in' Optical Flow
Now, just so you know, this sort of frame interpolation doesn't happen magically in every situation. You can't, for example, take a 2 FPS hand-drawn animation and expect Compressor to be able to analyze it and fill in 22 extra frames per second. It doesn't work that way. (Animators need work too, after all.)

The Optical Flow features in Compressor are actually geared to kick in only when converting between major standards. This means that you can expect these sorts of results when converting between NTSC and PAL, between film frame rates and video frame rates and between various SD and HD formats. These major format conversions are the triggers to use the analysis that allows Compressor to generate new frames as a part of the retiming process. Even if you use the settings described above, you will not see the results that we've seen in our various examples unless the source and output frame rates fit into one of the major video standards.

We'll cover more aspects of Compressor 2's new feature in future articles, In the meantime, if you have any further questions, be sure to visit me in DMN's DVD Studio Pro forum by clicking here or our Motion forum by clicking here.

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Related Keywords:apple compressor, dvd studio pro, interpolate frames, retime pal, retime ntsc, standards conversion, optical flow, macintosh, convert pal, convert ntsc


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