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Transparency Alpha-to-Selection problem

Aug 20, 2011, 15:10
Post: #1
Transparency Alpha-to-Selection problem
For quite some time I've been trying to find the best way to remove a white background from images. I've checked out the sticky thread on this forum and as my image isn't of great quality and because it features quite a lot of white that won't apply.

Recently I found this tutorial which is supposed to work great (as seen in the comments). However, I think that because of a GIMP update that won't apply anymore somehow. I was hoping you guys might know what was causing it.

Everything works well until I get to step 7, where I have to select to Alpha. There it selects just the outlines, but my selection includes inner parts of the image, not just the background. And then after I invert and delete, the white that remains still has transparent parts that show up on the upper layer. So, am I doing something wrong with the Alpha to Selection, is that tutorial updated or is there a better way to remove the background from ie. a logo with just one background colour which isn't of great quality.

Thanks,
Mark
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Aug 20, 2011, 17:56
Post: #2
RE: Transparency Alpha-to-Selection problem
(Aug 20, 2011 17:42)ofnuts Wrote:  
(Aug 20, 2011 15:10)Markolie Wrote:  For quite some time I've been trying to find the best way to remove a white background from images. I've checked out the sticky thread on this forum and as my image isn't of great quality and because it features quite a lot of white that won't apply.

Recently I found this tutorial which is supposed to work great (as seen in the comments). However, I think that because of a GIMP update that won't apply anymore somehow. I was hoping you guys might know what was causing it.

Everything works well until I get to step 7, where I have to select to Alpha. There it selects just the outlines, but my selection includes inner parts of the image, not just the background. And then after I invert and delete, the white that remains still has transparent parts that show up on the upper layer. So, am I doing something wrong with the Alpha to Selection, is that tutorial updated or is there a better way to remove the background from ie. a logo with just one background colour which isn't of great quality.

Thanks,
Mark
Same here... doesn't work for my test image. Note that in step 10 he says "Any bits that are still transparent (sometimes happens) can be painted in quickly." WinkWinkWink

So, his technique relies on the idea that you can, if needed, manually provide a selection of the contour... in other words, if you can cut out your subject manually, his technique works...

Something that should have made you suspicious is that he starts with a copy of the layer because, quote, "In case you don't know, this is because you can't make original backgrounds transparent.", end quote. This person doesn't know about Layer/Transparency/Add alpha channel, and makes tutorials about transparency.

The method in the sticky thread can be summarized as follow:
- make a rough outline of the subject, slightly inside the subject
- make a selection that doesn't include most of the subject
- use color to alpha, that will be restricted to the edges of the subject.

If your subject has large parts that have a color close to the background color, and that are in contact with it, then it won't work... unless you are very imaginative in the ways you make the selection (but then it depends if you want the colors that are close to the background are so because they are partially transparent or because this is their color.

If you show us the picture we can make suggestions.

Thanks for the reply. This is the image I'm trying to apply transparency to. On the bottom side of the wheel there's a distinct white part that I pretty much always select, and if I choose my threshold too low it won't select anything, and thus the great outline-increase selection-color to alpha solution won't work.

Again, I really appreciate the help Smile.
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Aug 20, 2011, 21:22
Post: #3
RE: Transparency Alpha-to-Selection problem
(Aug 20, 2011 20:48)ofnuts Wrote:  Hmmm. Works for me. The standard threshold (15)) selects the inside of a tooth (same as you?) but low thresholds (1-3) work OK. Some artifact pixels (it's a JPEG) aren't selected by the selection but when it is grown by 2px it "eats" them.

Ah, a threshold of 1 does wonders. There are some small white dots inside the B, but they're unnoticable. Many thanks Smile!
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