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Dust in the desert - Keying out black

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Jeremy Haccoun
Dust in the desert - Keying out black
on Feb 17, 2015 at 8:18:09 am

Hello,

I have a question about keying out black in After Effects (CS6). I have to integrate layers of dust on top of sunny desert backgrounds.

If I use the add or screen blending mode, the result is very transparent and the shot becomes very overexposed. Also, even though my dust element contains some very dense areas, they hardly show up when using add/screen: the whole element appears transparent and the only way to have a denser dust is to duplicate the element a few times, which blows up the shot completely. Traditional keying doesn't give good results either (Keylight, Luma Key, etc.)

Is there a simple method (that does not involve switching to 32bit linear color space) that would work, such as combining different blending modes, playing with channels or filters, etc. to get a better result?

Thanks


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Rob White
Re: Dust in the desert - Keying out black
on Feb 17, 2015 at 9:15:03 am

I feel like you shouldn't have to use a different mode for adding dust.

I think this is an alpha channel transparency problem.

Do you not have a dust clip with an alpha channel/Black background?

This kid does a quick routine around 40 seconds in this video.







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Ken Teutsch
Re: Dust in the desert - Keying out black
on Feb 17, 2015 at 4:55:33 pm

I have had some success using the Shift Channels effect. You can use the luminance channel to make an alpha channel and use the whole thing to create a track matte. Depends on the footage, of course.


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Jeremy Haccoun
Re: Dust in the desert - Keying out black
on Feb 17, 2015 at 10:20:14 pm

Thanks guys, the Shift channels effect does give better result than blending modes. Yes, my dust element is on a black background.

I remember hearing of a method that involved using several instances of the same element with different blending modes, or adding solid layers in between.

Is there another method, one that would retain the exact density of the original element? So that it appears fully opaque where opaque and transparent where transparent, but without having to overexpose.


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Walter Soyka
Re: Dust in the desert - Keying out black
on Feb 18, 2015 at 12:58:58 am

[Jeremy Haccoun] "Is there another method, one that would retain the exact density of the original element? So that it appears fully opaque where opaque and transparent where transparent, but without having to overexpose."

Apply Unmult [link], then tweak a Levels effect, set to Alpha only, to taste.

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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Jeremy Haccoun
Re: Dust in the desert - Keying out black
on Feb 18, 2015 at 1:03:55 pm

Thanks Walter, that works well


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Zelina Madison
Re: Dust in the desert - Keying out black
on Feb 20, 2015 at 11:16:54 am

Hi! It is rather difficult question, sometimes I spend a lot of time to achieve my goal, and have to try different software and tools. Unfortunately I haven't ready answer to you question, but do you try http://ams-collage.com/? They say, it is good and suitable.


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