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Using fractal noise with add mode

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David EellsUsing fractal noise with add mode
by on Dec 10, 2009 at 6:37:54 pm

First time I've encountered this and wondered if anyone had any insight.

Simply put, I'm trying to build an effect that involves adding fractal noise to a solid and then setting that layer to add mode. I want only the light parts of the noise layer to interact with the layer beneath. Instead, dark and light areas both interact with underlying layer. Precomposing doesn't affect the outcome, but rendering the layer out and reimporting it works as expected.

Is there an explanation? An easier fix than rendering and replacing the layer?



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Steve RenardRe: Using fractal noise with add mode
by on Dec 10, 2009 at 8:05:10 pm

I've tried this on my end and the best I've got is that maybe if you adjust the brightness/contrast settings of the Fractal Noise effect (or put a Curves effect on that layer below the Fractal Noise) you will get different results. It sounds like you need to raise the contrast and/or lower the brightness so that the dark areas of the noise are truly black (R0, G0, B0), and therefore will not interact at all with the layers underneath. Even R1 G1 B1 will interact with the layers underneath in Add mode.

It's not clear to me why the rendered video would be different than the noise effect on a layer, unless you're rendering to a color space that is crushing the blacks on the way out, giving you something that might look almost the same, but actually has more "true" black pixels than your noise layer. For confirmation, you could also render your noise to the Animation codec and try using that rendered video to replace your noise layer - if what I think is happening is the case, then you will see the same results.





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Dave LaRondeRe: Using fractal noise with add mode
by on Dec 10, 2009 at 10:07:52 pm

[David Eells] "I'm trying to build an effect that involves adding fractal noise to a solid and then setting that layer to add mode. I want only the light parts of the noise layer to interact with the layer beneath."

Your version of AE isn't broken: the Add mode will add anything over 100% black in that layer to the layers below it. I'd put the Fractal Noise layer in a precomp, add it to the main comp, and then play with the Levels or Curves effects to get the desired look. Tweaking opacity might help, too.

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


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David EellsRe: Using fractal noise with add mode
by on Dec 10, 2009 at 11:16:55 pm

Thanks Dave and Steve for responding. I tried the methods you describe - spent time playing with contrast and brightness, used other additive modes (lighten, screen, burn) and non of them behaved correctly. I also, added a Levels effect, and precomped. I changed the bit depth from 8 to 32 and back again.

Only rendering out the layer and reimporting worked for me.

For now I'm just assuming there's something exotic about Fractal Noise.



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