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Image Degradation: Rendering vs Codec changes vs compression

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Harold Eastman
Image Degradation: Rendering vs Codec changes vs compression
on Jan 29, 2009 at 5:53:40 pm

Can someone out there give me a quick overview on how image degradation, however slight, can happen during various steps in the post-production process prior to final compression?

I used to think that digital was digital was a digital, to a thousand generations, when working inside Final Cut Pro, but I now realize things aren't nearly that simple.

Any advice, cautions, tricks for maintaining image quality at the following steps...
• during capture (to transcode or not transcode...)
• when changing the codec settings of a sequence inside FCP for render or export
• when exporting footage as a movie, then reimporting movie for use in a sequence
• when rendering effects, titles, etc

Background: I'm working in FCS 1 (FCP 5.14), mainly with SD up to this point, but looking to move soon to FCS 2 and HDV, AVCHD or XDCAM footage.

Thanks.

H.


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Daniel Low
Re: Image Degradation: Rendering vs Codec changes vs compression
on Jan 29, 2009 at 7:27:17 pm

Simply put, most acquisition formats work to a specific datarate at a specific frame size. Some are constant bit rate and fewer are variable bitrates but all aim for a certain number of Mb/s per second.

To keep it simple let concentrate on DV

DV is 25Mb per second, no variation.

When you capture that into FCP it remains as a perfect digital version of what was on tape, but now as a file on your hard drive and still at 25Mb/s

As soon as you do anything to that clip like add a title or a transition or effect you will reduce the quality of the clip, as it still needs to fit inside 25Mb/s

If you don't do anything to that clip and export it out with the exact same sequence settings, then it too will remain exactly the same with no degradation.

Even a slight change in settings or indeed a change in codec will have an effect on quality.

The higher quality your source, the less you will notice doing any type of work on it.

There's obviously loads more to tell you, but that's a good start.

__________________________________________________________________
Please post back saying what solved your problem. It could help others, and saying 'thanks' is free!


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Harold Eastman
Re: Image Degradation: Rendering vs Codec changes vs compression
on Jan 29, 2009 at 9:01:31 pm

An excellent clear answer. Very helpful. Thanks.

And of course, the inevitable follow up questions...

• If I'm starting with a lower quality codec from the camera - say, SD DV or HDV - is there any point at all to transcoding my footage or changing the sequence/render setting to a better quality codec - say, ProRes - before adding titles and effect? I read somewhere that ProRes is more graphics friendly.

• Secondly, can you point me to a good book or web source that goes into detail on this stuff?

Again, thanks for taking the time Daniel.

H.



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Daniel Low
Re: Image Degradation: Rendering vs Codec changes vs compression
on Jan 29, 2009 at 10:50:11 pm

1. Yes, there are a number of good reasons to transcode to another codec like ProRes, if it fits with your workflow. Prores is a great codec all round.

2. Not really, this industry and the technologies are so fast-moving that books are out of date by they time they reach the printing press. There are a large number of great websites that cover a range of subjects and of course the COW is a fantastic resource both in terms of forums to ask questions and tutorials that are posted.

Take a look at Kenstone.net, for Mac/FCP friendly resources as will as the Provideocoalition.com



__________________________________________________________________
Please post back saying what solved your problem. It could help others, and saying 'thanks' is free!


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Harold Eastman
Re: Image Degradation: Rendering vs Codec changes vs compression
on Jan 30, 2009 at 1:19:48 am

Daniel, thanks again. Much appreciated.

H.


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