On 07/22/2013 09:34 PM, Daniel Hornung wrote:
On Monday, 22. July 2013 08:38:00 Alex Vergara Gil wrote:
Hello GIMP developers:

I am working on a project where I need to set all pixels below a level as
alpha channel, is there a quick way to achieve this? The level can be both
set a priori or adjusted dinamically, the second way is prefered. Right now
my workflow includes: 1. duplicate layer.
2. use color level in duplicate layer, in menu COLOR -> LEVEL.
3. add alpha mask to original layer
4. copy duplicate layer into the alpha mask of the original.
5. delete duplicate layer.
6. If result is not ok then return to 1.
As you can see this workflow is a headache, specially point 6 which is often
achieved. Any thoughts
1. Create a channel mask from the grayscale value.
2. On the channel mask, use the threshold tool.

VoilĂ !

For step 2) you can also uses Levels, Curves or even Brightness/Contrast, to keep some intermediate values of opacity and achieve a smoother result.
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