DnxHD and alpha channel

dxdy wrote on 11/4/2011, 2:50 PM

Hi, I am struggling with a DnxHD render and the alpha channel.

I have some AVCHD clips that I used the Boris BCC chromakeyer on. I get a great looking key. Now I want to save the clips without the background, so I can insert other backgrounds in the future.

I turn off the background layer, render to QT DNXHD, set alpha to compressed (I have also tried uncompressed), RGB levels (I have also tried 709), 32 bpp color depth.

When I drag the output file into Vegas, it shows the character perfectly in front of a black background. I click on Properties, set the alpha channel from none to premultiplied (also tried straight).

All I see is the character on the black background. When I select a compositing mode other than straight alpha, the compositing mode button displays an "X". I suspect this means there is no alpha channel. Mediainfo says nothing about an alpha channel either.

I can make it work with the QT PNG template.

Can someone critique my DNXHD settings please?

Thanks,
Fred
QT is version 7.60.92.0, Vegas 11 64 bit.

Comments

Sidecar2 wrote on 3/28/2013, 5:09 PM

I have the exact same problem.

Imported 750 png images in a sequence to Vegas 10. Clip has a functioning alpha channel upon import.

If I render that sequence out to DNxHD with any combination of settings as you describe, I get no functioning alpha channel, even if I turn on alpha in the clip properties.

If I render out to uncompressed Quicktime instead of DNxHD and drop that render on the Vegas line, the alpha works fine.

Curious, eh wot?

videoITguy wrote on 3/28/2013, 5:12 PM

dxdy, you will not get the answer you were HOPING to get, but the following thread has some background that you need to read carefully and interpret for your situation.

http://www.sonycreativesoftware.com/forums/ShowMessage.asp?ForumID=4&MessageID=852551

JohnnyRoy wrote on 3/29/2013, 8:21 AM

> "Can someone critique my DNXHD settings please? "

There is nothing wrong with your DNxHD settings. As I understand it, the problem is with Avid. They handle alpha channels different than any other NLE in the industry and their DNxHD codec is no different. It will never work in an NLE outside of Avid unless that NLE specifically adds support for the way Avid hands alpha channels.

~jr

farss wrote on 3/29/2013, 9:29 AM

It seems Avid's DNxHD codec's alpha channel is the inverse of most others. It uses 0 (black) for transparent. Most systems have an Invert Alpha switch so it shouldn't be that much of a problem although as far as I can tell Vegas doesn't :(

I believe the reason Avid do it this way is because historically it was this way, superblack was used to denote transparency. Avid's systems seem to have maintained the convention from the days of analog.

Bob.

musicvid10 wrote on 3/29/2013, 9:54 AM

I have successfully tested alpha with DNxHD in Vegas 8.0c, going back to LE 2.3.2.
Is it all alpha that is a problem or just certain scenarios?
I'll test again using LE 2.3.8 to be sure.