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Gaussian Blur Brings in Edges

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Daniel Schmidt
Gaussian Blur Brings in Edges
on Apr 22, 2011 at 8:10:43 pm

I have created a (luma) mask of the sky in my footage (using the channel mixer and curves) and use it as a track mask for a color correction adjustment layer. In order to fine tune the edges of the mask, I used gaussian blur. Here is the problem: Using gaussion blur brings in the edges of the image. Say I want to darken the sky. Using gaussian blur on the mask creates a bright edge at the top of the image where the sky meets the border of the image. How can I work around that?


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Darby Edelen
Re: Gaussian Blur Brings in Edges
on Apr 22, 2011 at 8:24:53 pm

I have two suggestions. First make sure your blur is set to repeat edge pixels. If that doesn't help, apply a levels effect after the blur and drop down to affect the alpha channel. Set the white input point on the alpha channel to 0.5 (or about 127 if you're using 8bpc numbers)

Darby Edelen


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Daniel Schmidt
Re: Gaussian Blur Brings in Edges
on Apr 23, 2011 at 8:56:57 am

"I have two suggestions. First make sure your blur is set to repeat edge pixels."

Where is that option? I can't see it in the gaussian blur controls.


"If that doesn't help, apply a levels effect after the blur and drop down to affect the alpha channel. Set the white input point on the alpha channel to 0.5 (or about 127 if you're using 8bpc numbers)"

If I would do that it would completely change my mask, making it harsher again.


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Erik Waluska
Re: Gaussian Blur Brings in Edges
on Apr 23, 2011 at 3:57:58 pm

"I have two suggestions. First make sure your blur is set to repeat edge pixels."


That option is only available in Fast Blur and Box Blur but not Gaussian, so maybe try using one of those blurs instead.

Here's a video about those blurs:
After Effects: Effects A-Z: Box Blur


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Daniel Schmidt
Re: Gaussian Blur Brings in Edges
on Apr 26, 2011 at 8:56:44 am

I used fast blur and that worked well. Thanks!


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