Selections and Gaussian blur

Started Aug 27, 2018 | Discussions thread
Re: Selections and Gaussian blur
1

Mark Hollister wrote:

Here is a technique that'll work every time to prevent bleedover contamination.

In the red box example just shown, you could see the bleedover, but the box itself is solid white. If you blur solid white, you get solid white (no change there!). So let's keep the red box but instead of a white box beside it imagine you have something in it, something blurrable. A bird, a rock, a hand drawn X, who cares.

So, make your selection around the object you want to blur. You've got marching ants, right? Now hit the letter Q. Your marching ants disappear, now your selection is a solid color, which is also called a rubylith. You're now in Quickmask mode. Now hit Q again and the rubylith disappears and your marching ants return. What I'm getting at in this paragraph is this: hitting Q will tell you in an instant if you have an active selection, and where it is - even if you hid the marching ants with control-H. Q simply toggles back and forth from marching ants to quickmask mode.

I brought this up because more than once you said sometimes you got the result you wanted and other times you didn't. My guess is that sometimes you had an active selection and other times you didn't. So Q is a great way to quickly be sure that you have an active selection.

So getting back to the solution to your problem. You've got your red box and right beside it you've got something else that you want blurred, let's say a little bird. So select the bird. Control-J to pop that little bird off to its own layer. Now there's no way the red box can contaminate the bird.

But notice that the marching ants are not active. To contain the blurring as you described, Control-click the thumbnail of the bird-only layer. The marching ants should now be back. Now if you gaussian blur, it will be contained within the selection. You will not have bleedover from outside to inside, or inside to outside.

But some of the 'transparency' of those pixels outside the selection boundary, will bleed over to the pixels inside the selection. Unless you lock transparency to prevent this.

And if you've locked transparency, there is no need for a selection since pixels that are already transparent will stay that way necessarily. And those pixels that are fully transparent lack any visible colour, which might contaminate anything else.

Now all you have to do is merge your blurred bird back to the layer with the red box.

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