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"Correct" way to use blending when working linear?

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Maria Smith
"Correct" way to use blending when working linear?
on Jan 15, 2018 at 6:24:37 pm

I know that you shouldn't use Screen to remove black backgrounds when working linear since Screen isn't physically correct anymore when working linearly.
Which blending-modes should I use when working linearly to...:
  • Remove a black background for compositing VFX?
  • Remove a white background for compositing VFX?




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Dave LaRonde
Re: "Correct" way to use blending when working linear?
on Jan 15, 2018 at 7:34:20 pm

My advice -- don't use ANY blend mode. You won't get guaranteed transparency.
Key if you can, mask if possible, rotobrush if you must.

Dave LaRonde
Promotion Producer
KGAN (CBS) & KFXA (Fox) Cedar Rapids, IA


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Kevin Camp
Re: "Correct" way to use blending when working linear?
on Jan 15, 2018 at 8:02:15 pm

removing white or black backgrounds is quite easy in Ae without using blending modes. it's so easy i wish i could do it in photoshop too.

to remove black, add the channel combiner effect and set the 'from' property to 'max rgb' and the 'to' property to 'alpha'. then add the remove color matting effect with the background set to black (default setting).

to remove white, do the same, but in the channel combiner effect, click the 'invert' option and then set the remove color matting color to white.

you can also save each of the setting off as animation presents to quickly recall as needed.

Kevin Camp
Art Director
KCPQ, KZJO & KRCW


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Walter Soyka
Re: "Correct" way to use blending when working linear?
on Jan 16, 2018 at 12:03:55 pm

If black is clamped to 0, Add will "remove" it. If White is clamped to 1, Multiply will "remove" it. In both cases, though, the remaining pixels will mix with the image accordingly, and that may not be what you intend.

If you want to eliminate black (or white) from footage and use Normal blending to composite it, you can look to Channel Combiner as Kevin suggests, but note that Channel Combiner is an 8bpc effect. I'd suggest instead using an UnMult effect; there are several free or low-cost options:

https://mekajiki.onfastspring.com/unmult
https://www.redgiant.com/downloads/legacy-versions/
https://www.redgiant.com/user-guide/universe/unmult/

Walter Soyka
Designer & Mad Scientist at Keen Live [link]
Motion Graphics, Widescreen Events, Presentation Design, and Consulting
@keenlive [twitter]   |   RenderBreak [blog]   |   Profile [LinkedIn]


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Daniel Waldron
Re: "Correct" way to use blending when working linear?
on Jan 16, 2018 at 3:11:32 pm

I usually use the free UnMult effect that Walter mentioned or the Extract effect. Extract will work with white as well.


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Walter Soyka
Re: "Correct" way to use blending when working linear?
on Jan 16, 2018 at 3:23:11 pm

[Daniel Waldron] "I usually use the free UnMult effect that Walter mentioned or the Extract effect. Extract will work with white as well."

To use UnMult to knock out a white background, make an UnMult sandwich. Use an Invert effect, then UnMult, then Invert again.

You can also use a Levels or Curves effect on the RGB channels before the UnMult to affect what will be removed, or a Levels or Curves effect on the alpha channel afterwards to modify the new transparency directly. Just watch the edges!

Walter Soyka
Designer & Mad Scientist at Keen Live [link]
Motion Graphics, Widescreen Events, Presentation Design, and Consulting
@keenlive [twitter]   |   RenderBreak [blog]   |   Profile [LinkedIn]


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Maria Smith
Re: "Correct" way to use blending when working linear?
on Jan 16, 2018 at 5:15:36 pm

But what if I want to composite a .mov-file containing fire against a black background? Is it a bad idea to work linearly and use "Add" to get rid of the black background? I prefer not to use third-party plug-ins, and when I use the "Extract"-effect I always end up with an something unrealistic (like a sharp edge around my object).



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Walter Soyka
Re: "Correct" way to use blending when working linear?
on Jan 17, 2018 at 5:06:06 pm

Add is a good way to capture the light from fire, but note that if there's also smoke, Add will not blend the smoke correctly.

Walter Soyka
Designer & Mad Scientist at Keen Live [link]
Motion Graphics, Widescreen Events, Presentation Design, and Consulting
@keenlive [twitter]   |   RenderBreak [blog]   |   Profile [LinkedIn]


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