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MPEG4 Compression

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Mark Nightingale
MPEG4 Compression
on Oct 29, 2012 at 4:48:09 pm

Hi I have small sequence shot on a Nikon D7000 24p, converted to ProRes422 25p for FCP, I've speeded it up by approx 800%, its basically following someone into venue. Its rendered absolutely fine in FCP and plays perfectly, however when exported to MPEG4 its compressing heavily compared to the rest of the footage. Given a heavy interlaced look, and once in YouTube doesn't help much more.

The MPEG4 Settings
30fps (advised by YouTube)

Any ideas?

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Jeff Meyer
Re: MPEG4 Compression
on Oct 29, 2012 at 10:10:09 pm

I think you're just running into a barrier that inherently exists with video compression.

Part of video compression depends on the fact that under normal circumstances the entire background of a shot doesn't completely change from frame to frame. There are instances where this isn't the case (concerts, sports, fast motion) but for a lot of the footage out there this holds true - interviews, dialogue, product demonstrations, screen captures, etc.

Compression algorithms take advantage of this by sending a "keyframe" or a complete frame, then the next several frames note what has changed since the last keyframe. The frames that are all children ofof the parent "keyframe" are referred to as a "group of pictures." Codecs that use this kind of scheme are referred to as "Long GOP" codecs - long group of pictures codecs. I would recommend a Google search on the topic if you aren't familiar.

With video at 800% the background changes much more than normal, so the MPEG4 algorithm can't keep up. There are few things you can try:

• Deliver in a codec that isn't long-GOP - ProRes, Animation, Uncompressed. You'll pay a severe file size penalty this way, but the target will get a near perfect (or perfect) copy of your video.
• Turn your bitrate up and keep your keyframe interval at automatic. Youtube/Vimeo still have their set bitrate they're going to use no matter what you deliver, but their output will never be better than your input.
• You might play around with some blurs on the fast motion. If the algorithm has fewer edges to worry about it might handle things a bit better. Consequently, you might see macroblocking where there should be smooth gradients. If I had to guess, it won't be much different, but it might be worth a try if nothing else is working.

I work in sports. When we compress quick pans against a noisy stadium background the limits of compression fiend us all the time.

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