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Is a round trip through resolve with compressed quicktime lossless if video is unchanged?

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Nat Jencks
Is a round trip through resolve with compressed quicktime lossless if video is unchanged?
on Jun 2, 2014 at 7:26:39 pm
Last Edited By Nat Jencks on Jun 2, 2014 at 7:27:08 pm

If one takes a compressed quicktime into resolve and then exports it with the same compression settings, does it go through a decompress/recompress cycle, or does it remain untouched as it would in something like FCP?

Why would one ever want to do this you ask?

I occasionally export films by Reel as Prores4444 for temp screenings, and then bring the reels back into resolve or FCP to assemble them into a long play sequence with full audio.

Ive started occasionally doing this in resolve as opposed to FCP just to keep things simpleā€¦

But I'm concerned that doing this process in resolve might hit the image with an additional decompress/recompress cycle.

Anyone know the answer to this?

best-
-Nat


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Joseph Owens
Re: Is a round trip through resolve with compressed quicktime lossless if video is unchanged?
on Jun 2, 2014 at 10:54:39 pm

Are you rendering it or simply allowing the exported XML to re-link to original source?
That would answer your question. Rendering it takes it through the transform if it needs to converted or rescaled into linear 709/RGB.

jPo

"I always pass on free advice -- its never of any use to me" Oscar Wilde.


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Nat Jencks
Re: Is a round trip through resolve with compressed quicktime lossless if video is unchanged?
on Jun 3, 2014 at 5:10:54 pm

Rendering a new "Long Play" quicktime e.g. an assembly of the reels.

If this process is done in FCP and picture is untouched the software knows that there is no need to decompress/recompress and the new file is identical / lossless as it does not go through a decompress / recompress cycle.


best-
-Nat


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Marc Wielage
Re: Is a round trip through resolve with compressed quicktime lossless if video is unchanged?
on Jun 3, 2014 at 9:56:27 am

ProRes -- even 444 -- is still a lossy format, though the compression ratio is very low at 444. If you re-render the file, it will be compressed again. I'm not sure how many passes it would take before you started to notice artifacts. I can tell you with HDCam-SR, you'd have to go well above 20 generations to see any issues, assuming each pass didn't alter the signal (like a color correction or a composite).


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Nat Jencks
Re: Is a round trip through resolve with compressed quicktime lossless if video is unchanged?
on Jun 3, 2014 at 5:14:33 pm

Yes its true that a single decopress/recompress cycle for prores4444 isn't too bad, but if one does this process in FCP it is literally lossless since it doesn't go through any decompression decompression. I'll use DPX or do in joining in FCP.

Thanks folks!

-Nat


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Joakim Ziegler
Re: Is a round trip through resolve with compressed quicktime lossless if video is unchanged?
on Jun 14, 2014 at 12:50:08 am

Page 14 of the Apple ProRes white paper has a graph of Peak Signal to Noise Ratio of the ProRes codecs at different generations of recompression of the same material:

http://images.apple.com/final-cut-pro/docs/Apple_ProRes_White_Paper_Decembe...

In summary, there's a slight loss in the first generation or two, then basically none. The first generation loss is also lower in the higher quality codecs (HQ and 4444) than in the others, as can be expected.

--
Joakim Ziegler - Postproduction Supervisor


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Nat Jencks
Re: Is a round trip through resolve with compressed quicktime lossless if video is unchanged?
on Jun 19, 2014 at 9:20:13 am

Prores4444 is definitely not that bad, but have worked on VFX shots which had undergone many passes of Prores4444 compression, and you could definitely see the damage. Very subtle blocky patterns began to appear in subtle gradient areas like in mid tone sky.

The inability of Resolve to be able to export "Same as Source" in the way the avid or FCP can is a small feature that becomes bigger as it starts to become a tool that people if not editing are at least taking on some editorial tasks, such as joining a bunch or reels together to build an LP version of a film.

Perhaps a better example is if you rendered out a DCP in reels using easy DCP it would be nice to be able to join these together in a sequence and export an LP with audio without pushing the whole DCP thought a decode re-encode cycle. Clipster does this well.

Exporting "same as source" is a very useful feature for editorial tools and as Resolve becomes and editorial tool it should be one of the things on the list of things to implement. It actually comes up a fair amount with various editorial tasks.

best-
-Nat


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