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Alpha - inserting new footage behind presenter

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Chris PulleynAlpha - inserting new footage behind presenter
by on Sep 20, 2009 at 11:32:09 pm

Hi All
I have recieved a .mov, of a presenter moving and talking, green-keyed. He is now on white apparently with an alpha including him and his shadow.

My question is; how do get the presenter's layer to support passing other footage behind him. I have tried various layer mode changes and obvious Alpha commades but keep loosing his density/transparency or luminosity. If I re-key the white I loose most of him and his shirt!

Using: AE-CS3 on pc. Editing with Edius Pro 4

I have looked all over and everyone explains keying but not alpha manipulation


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Laurie TurnerRe: Alpha - inserting new footage behind presenter
by on Sep 21, 2009 at 1:19:02 pm

I am a little confused. Are you saying you have him keyed but with a white background in one .mov file? You don't have him on a seperate layer and the white background on the another layer? If that's the case then all you can really do is roto scope him out. You can try to create a matte like andrew kramer's sky replacement video on videocopilot.net


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Dave LaRondeRe: Alpha - inserting new footage behind presenter
by on Sep 21, 2009 at 2:54:50 pm

You no longer have an alpha channel on the footage which would have allowed you to easily put a new background behind the subject -- the white background took care of that.

Now your options are:
  • Get another copy of the footage with an alpha channel -- no white.
  • Get the green screen footage and key it yourself.
  • Work around the fact that you don't have an alpha channel. Don't expect to insert a new background, do everything over the white, and watch out that you don't place stuff over the subject.

You'll note I didn't way anything about trying to pull a key off the white. That's because about 80% of the time, it's impossible. Are you prepared to try it when the odds are 4-1 against you?

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


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Chris PulleynRe: Alpha - inserting new footage behind presenter
by on Sep 21, 2009 at 3:14:17 pm

Thanks Dave & Laurie,
I have done alot more reading since the post and came to the conclusion i was in trouble.

I didn't want to go back to the producer with a silly question like "did that footage NOT have an Alpha?" but I got an updated version this morning and this one worked straight away.... so it seems that it wasn't there.

BTW Can I fine tune the Alpha though or is it WYSIWYG?
There is still a bit of a white shimmer around him.
It is a low level web-edit but as I'm at it I'd like to get it as right as i can.


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Dave LaRondeRe: Alpha - inserting new footage behind presenter
by on Sep 21, 2009 at 4:09:35 pm

[Chris Pulleyn] "...There is still a bit of a white shimmer around him...."

Is it because it's a Premultiplied (rather than Straight) Alpha Channel? Check out the difference here:
http://library.creativecow.net/articles/rabinowitz_aharon/straight_vs_premu...

If it's premultiplied -- and if it doesn't work to set the footage to the proper Alpha Channel interpretation in the Interpret Footage settings -- check this out, too:

http://library.creativecow.net/articles/rabinowitz_aharon/CCQT_6_Remove_Col...

But if the footage is intended to go over a white background anyway, it's not a big deal.

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


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