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Compression Artifacts


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#1 Andre LeBlanc

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Posted 24 May 2008 - 12:17 PM

Hi all,

I've been doing some minor post production compositing on a short film we're finishing, and I'm noticing I'm getting some strange artifacts at one of the stages:

My workflow is editing in FCP, and doing any compositing work in Shake. The footage was originally shot on Varicam. What I do is bring the raw plate into Shake (which was originally shot on Varicam), do my comp work, then output the new version as '8-bit uncompressed 4:2:2'. If I re-import the new composited shot back into shake, and compare it frame by frame with the plate, they are identical. No artifacting, no increase in noise etc... 0 difference. But when I bring these same 2 shots into FCP and compare them, there's subtle differences. The new uncompressed 8 bit 4:2:2 movie has more defined noise, as if I ran a gamma curve on it, and some strange 'blocky' artifacts in dark areas. What is FCP doing to my footage? What step could I be missing in my workflow? Should I be bringing my new composited 'uncompressed' shot into Compressor and converting it back to DVCPRO format?

Currently, my project settings in FCP are set to read the footage as DVCPRO.

Any help or clarification is greatly appreciated. Thanks!

Edited by Andre LeBlanc, 24 May 2008 - 12:19 PM.

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#2 Andre LeBlanc

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Posted 24 May 2008 - 12:24 PM

As an update, I also notice these same subtle differences between my plate and comp when I view them side by side with quicktime. Is it possible that Shake is doing something to my footage?? That seems unlikely to me, but...
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#3 Andre LeBlanc

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Posted 24 May 2008 - 02:47 PM

Also, I am aware that fcp does its own gamma correction, while Shake expects you to provide your own lut, thus sometimes creating a brightness discrepancy between the two programs. This appears to be a different problem: what looks like some sort of compression going from shake to fcp. Could there be a problem from going between YUV space to RGB in quicktime?
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#4 Andre LeBlanc

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Posted 24 May 2008 - 10:32 PM

I've now managed to reply to my own post 3 times...

After hours of troubleshooting, I believe I've solved the problem. After scouring message boards, it appears that the problem resides in the conversion from YUV color space to RGB color space, and more specifically, the way quicktime handles this conversion. Most compositing/3d packages use an RGB color space, while most editing software uses YUV. Quicktime will do the file conversion for any file going into Shake, or coming out of fcp, and in this case, it was clipping some of my color values (it's not doing the most accurate YUV to RGB conversion). The workaround was to use After Effects S3 to do the conversion. After Effects has its own built in converter, and therefore does not rely on quicktime to do it for us. It was a bit ugly, but I end up converting the DVCPRO movie files to a targa sequence in AE, and then importing that targa sequence into Shake to do my compositing work. It's a bit ugly, but the only way to avoid quicktimes YUV to rgb conversion.

I guess that's it for this thread. Hope someone finds this useful! Or maybe someone else has a more elegant workaround for such a problem?

Edited by Andre LeBlanc, 24 May 2008 - 10:33 PM.

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#5 Phil Rhodes

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Posted 25 May 2008 - 07:13 AM

Well there's one for the "watch out for this" file.

I read the topic but wouldn't have had a clue. This strikes me as another reason to get compressed formats uncompressed ASAP and work on them that way. You don't want to be recompressing stuff back to DV100 all the time when doing compositing in any case.

P
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#6 Andre LeBlanc

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Posted 25 May 2008 - 07:59 PM

Agreed. Hopefully Quicktime will resolve this issue with their encoder at some point!
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