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Compressed vs. Uncompressed Alpha

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Roy Janke
Compressed vs. Uncompressed Alpha
on Sep 30, 2009 at 3:19:12 pm

This is my first post and I've been having difficulty searching for the answer to this. I am currently building Render and Output modules which will be passed around for others at my place of work to use. As such I need to make sure that they are correct and I am having difficulty determining what the better alpha setting to use is: Compressed or Uncompressed.

Obviously, I know the big difference is file size, but can someone please explain what the major differences are. The majority of my output settings will be using the Avid DNxHD codecs.

I am using AE CS4. I hope I have provided enough details.

Thanks,
Roy.


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alexandre braso
Re: Compressed vs. Uncompressed Alpha
on Sep 30, 2009 at 4:29:37 pm

You will export separate file with alpha?
if you thing export RGB+Alpha you have no options, need to be unconpressed.

Alexandre Braso
http://www.pixelsrender.com.br


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Roy Janke
Re: Compressed vs. Uncompressed Alpha
on Sep 30, 2009 at 4:43:56 pm

That is what I thought as I am rendering one QT RGB+alpha, so I built my templates using uncompressed. So, if I understand correctly, a compressed alpha is only usable when rendering as two separate files...one fill and one matte/alpha.

Thanks for the quick response, I apprecite it!

Roy.


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Dave LaRonde
Re: Compressed vs. Uncompressed Alpha
on Sep 30, 2009 at 5:21:06 pm

[Roy Janke] "...if I understand correctly, a compressed alpha is only usable when rendering as two separate files...one fill and one matte/alpha. "

Incorrect. Uncompressed/compressed has to do with the codec you use and any settings within that codec. It doesn't have any influence over the presence of an alpha channel. Certain codecs support embedded alpha channels, most don't.

The Animation codec set to RGB+Alpha and best settings is indeed compressed, but it is LOSSLESS. The same goes for the PNG codec I mentioned. You can use either to create a file with an embedded alpha channel, and they are both uncompressed.

Since it's almost always the best policy to render the best quality out of AE, I'm a firm believer in rendering in compressed but lossless codecs, but I only create uncompressed files when instructed to do so. I think they're wasteful.

Dave LaRonde
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA


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Roy Janke
Re: Compressed vs. Uncompressed Alpha
on Sep 30, 2009 at 5:31:36 pm

Thanks for the clarifications Dave, I appreciate the help. I had responded to your earlier post but there was an error after I hit Post Direct so I guess it never made it.

Anyway, all I had basically said was the since the work being done by the editors isn't all that intensive for the most part I wanted to keep their files in the same native codec as what they are using in the Avid. This is what lead to my original post, but I agree completely that you want the best possible quality coming out of your AE work. I just want to cut down on the transcoding time when bringing it back into the Avid. Also, the source footage sent from Avid to AE is exported "Same as Source" so using the Animation Codec (in this case) would unjustifiably bloat the file size.

Roy.


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Dave LaRonde
Re: Compressed vs. Uncompressed Alpha
on Sep 30, 2009 at 5:55:16 pm

Yup. If that Avid codec you mention supports alpha channels, use it. If you don't know whether to create Straight or Premultipled alpha channels, make two test alpha-channel movies using both kinds. Find out which kind works best for them, and you're good to go from then on.

Dave LaRonde
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA


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Roy Janke
Re: Compressed vs. Uncompressed Alpha
on Sep 30, 2009 at 6:13:54 pm

The Avid DNx codec does support alphas, however that hasn't always been the case. I can't remember when it changed but in the beginning it would render out a quicktime and claim it had an alpha. In reality it did...but it was a full screen alpha.

I personally like to use straight as it can get me as close to compositing entirely in AE when using soft or gradient alpha elements. However, I am building presets for both types so that the editors can choose which suits their particular needs best.


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Dave LaRonde
Re: Compressed vs. Uncompressed Alpha
on Sep 30, 2009 at 4:56:56 pm

Since I'm a Mac guy, I can offer two options when rendering Quicktime Movies:
1) Use the Animation codec set to best quality
2) Use the PNG codec. This is not a PNG sequence, but a QT using the PNG codec.

Both support alpha channels, both are compressed, but both are lossless.

The PNG option creates smaller file sizes if the picture contains a lot of detail. The Animation option creates smaller file sizes when there are large, empty areas in the picture, as in an animated lower-third. Animation can also be read by any Quicktime-equipped computer on the planet.

So: uncompressed or lossless? Which to use? Well, if you get a big word processor file compressed in Stuffit or Winzip, don't you expect to see every character and punctuation mark when you uncompress it? That's how lossless compression works: you don't lose anything.

Dave LaRonde
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA


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