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[Solved] Layer inheriting unwanted transparency

Jul 5, 2012, 14:35 (This post was last modified: Jul 6, 2012 20:45 by gmp.)
Post: #1
[Solved] Layer inheriting unwanted transparency

I'm quite new to Gimp and despite I read several tutorials, I cannot figure out how is transparency (alpha) is transfered from one layer to a new one when a selection is active.


I'm trying to clip objects on a wooden table and make their borders smooth.
The idea is to use a Layer mask on a layer having alpha chanel (tranparency).
On this layer, the shape of the selected objects are filled in black (for transparency) and white is filled all around for opacity. A small gaussian blur is then applied to this layer to make a progressive transparency that smoothes the borders of objects.

The red layer on the screenshot is just to illustrate the problem : it seems that when creating a new layer with objects selected, some transparency was transfered. The area outside objets does not have pure color like it should.

After an initial selection of the wooden table with the Fuzzy Select tool Fuzzyselect, the selection was inverted and refined with the Free select tool Freeselect . The clipping path seemed clean when the new layer was created.

Any ideas what I did wrong and how to correct it ?

Many thanks.
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Jul 5, 2012, 21:10
Post: #2
Heart RE: Layer inheriting unwanted transparency
Thank you for your detailed answer.
I studied your file, which is a very comprehensive example; thanks.

When you speak about "50% selected", could you confirm that you really mean
a) 50% selected and then filled with pure black and 100% opacity
and not
b) filled medium grey and 100% opacity?

I did some tests that tends to prove "a". Typically, using the Free Select tool Freeselect , to explicitely remove the problematic areas from the selection solves the problem.

So, if I understand you correctly, each pixel can be selected to a free percentage between 0% and 100%, meaning that selection behaves more or less like a "selection channel" but --contrarily to true channels-- not attached to any particular layer.

In the case of my example, does the default threshold --15.0-- of the Fuzzy selection mean that the pixels extracted from the wooden area are those whose tone did not differ from more than 15% from the clicked color?
And also that they would have been partially selected according to some gaussian filter or equivalent?

Now, in my situation with the partially selected pixels, any idea how I could automatically remove them from my selection (without using Freeselect) ?
Given the powerful features of Gimp, I'm sure there is some way to filter with some threshold.

I tried using the "substract" selection mode and applying a Fuzzy selection again, as I hoped that this would remove the previouly wrongly selected pixels, but it did not work. Undecided
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Jul 6, 2012, 20:43 (This post was last modified: Jul 6, 2012 20:45 by gmp.)
Post: #3
[Solved] Layer inheriting unwanted transparency
Thank you very much for your very useful explanations.

Once you understand how it works it becomes much easier !

Using your tricks, here's how I solved my problem :

1) Converted selection to a chanel
2) Used the Bucket fill tool Bucketfill with a high threshold, to colorize in black all what should be opaque, including the areas that were previously partially selected
3) Used the Select by Color tool Bycolorselect with some threshold and applying Edit->Clear, to make sure that the the areas for "holes" were really transparent
4) Applied Filter->Blur->Gaussian blur
5) Created a new layer mask using the chanel created at step 1.
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