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how to make perfect intersection of selections?

Nov 23, 2012 8:19 PM

So I have 2 alpha channels. When I try to make a new selection intersecting those 2 alpha channels, there is always about 1 px that is not selected...

 

In the image the black pixels are the new selection, and red pixels are one of the alpha channels. No matter what I do, I can't make the new selection completely "fill the red pixels".

 

Any ideas?

 

Screen Shot 2012-11-23 at 10.17.15 PM.png

 
Replies
  • Currently Being Moderated
    Nov 23, 2012 9:16 PM   in reply to pierBover

    That's just the anti-aliasing in your alphas. How are you making your alphas? There's various ways to have no anti-aliasing. But it will look very jaggy.

     
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  • Currently Being Moderated
    Nov 24, 2012 1:57 AM   in reply to pierBover

    black 50% transparent + black 50% transparent won't give a 100% black...

    And that is just at it should be.

    The assumption that a soft selection/transparency and its negative should result in a solid seems to be a popular misconception.

     

    The Blend Mode »Linear Dodge (Add)« either for Layers or in Image > Calculations should achieve what you are after (as you seem to have found out).

    Edit: That is for combining Alpha Channels, the transparency of Layers is naturally not affected by this.

     
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  • JJMack
    5,979 posts
    Jan 9, 2006
    Currently Being Moderated
    Nov 24, 2012 5:07 AM   in reply to pierBover

    pierBover wrote:

     

    I know it's antialiasing...

     

    So far what I have understood is that the problem is not in the selections,

    No the problem is the selections have anti-aliasing. Here is two Alpha channels that have no anti-aiasing combine them you get no anti-aliasing in the result.

    calu.jpg

     

    100%White visible over 100%Black hidden adding visibility

     
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