Re: [Gimp-user] feature request: alpha level

[Ofnuts <ofnuts laposte net>]

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.
>
> Voilà!
+1

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.