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Joe Orange
compression limitations
on Nov 12, 2010 at 1:25:33 pm

Obviously putting an H264 clip through compression compromises quality, but after how many compressions does this become noticeable or even unacceptable for tv or cinema quality?

thanks


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Tom Wolsky
Re: compression limitations
on Nov 12, 2010 at 2:22:53 pm

H.264 is just a codec. Just one of the factors in compression which includes frame resolution, frame rate, and of course data rate. There is no way to give a simple answer with the information provided. Compressed for web delivery an expert could probably see the difference from one generation to the next.

All the best,

Tom

Class on Demand DVDs "Complete Training for FCP7," "Basic Training for FCS" and "Final Cut Express Made Easy"
Author: "Final Cut Pro 5 Editing Essentials" and "Final Cut Express 4 Editing Workshop"


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Joe Orange
Re: compression limitations
on Nov 12, 2010 at 2:46:55 pm

HD clip from a 5D, 1920x1080 shot at 25 fps converted to ProRes (via 5DtoRGB) slow motioned and rendered in AE then finally compressed again in Quicktime.
Expected final usage TV or cinema.

Would an expert notice differences between each compression?


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Tom Wolsky
Re: compression limitations
on Nov 12, 2010 at 2:52:41 pm

Your first message talks about H.264 and mow you're talking ProRes. No, there should be mo discernible loss in the process you describe.

All the best,

Tom

Class on Demand DVDs "Complete Training for FCP7," "Basic Training for FCS" and "Final Cut Express Made Easy"
Author: "Final Cut Pro 5 Editing Essentials" and "Final Cut Express 4 Editing Workshop"


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Joe Orange
Re: compression limitations
on Nov 12, 2010 at 4:09:56 pm

I thought changing a native H264 clip to ProRes as a process, if not via compression itself, would have some affect on those pixels and how they look.
Then to slow-mo them in AE followed by exporting (compression) them in QuickTime to remove audio (wrong way I know but done now) would have some impact on quality when intended for high end usage.


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Tom Wolsky
Re: compression limitations
on Nov 12, 2010 at 4:13:38 pm

You shouldn't have a problem with that process. That's what everyone does going from AVCHD and the Canon/Nikon stills camera formats.

All the best,

Tom

Class on Demand DVDs "Complete Training for FCP7," "Basic Training for FCS" and "Final Cut Express Made Easy"
Author: "Final Cut Pro 5 Editing Essentials" and "Final Cut Express 4 Editing Workshop"


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Thomas Worth
Re: compression limitations
on Nov 13, 2010 at 8:40:34 am

That's not really the right way to do it even though in practice you may not see any quality loss.

The correct way would be to render to uncompressed codecs. Example:

1. Transcode from H.264 to ProRes 4444 with 5DtoRGB. It's preferable to export to 4:4:4 because we know we will be working with this footage in AE, which only works in RGB. You can get away with 4:2:2 if you don't have the disk space.
2. Import into After Effects, do VFX. Export to uncompressed 4:2:2 to match the rest of your footage if it's also 4:2:2.
3. Export the final program to uncompressed 4:2:2. You can then color correct this or use it as a source for HDCAM SR output, Blu-ray or DVD.

Some will say this is overkill, and perhaps it is considering your final delivery. However, if you are worried about generation loss, the only way to avoid it is to render to lossless codecs.

And yes, uncompressed 4:2:2 is technically lossy because it discards chroma info, but this type of "compression" does not produce the type of DCT / blocky compression artifacts you are probably worried about.


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