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

["Alex Vergara Gil" <alex cphr edu cu>]

From: "Ofnuts" <ofnuts laposte net>
Sent: Monday, July 22, 2013 3:45 PM
> 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.
> _______________________________________________

Great!
I don´t know this could be done so easily. Thanks!
I have adapted my workflow now.
1. Decompose original image in whatever you want RGB, LHS, LAB or even 
grayscale. Then pick some channel.
2. Copy the selected channel into an alpha mask of the original image.
3. Use level tool in the mask (threshold is not good because it creates 
sharp edges, but Curves and brightness/Contrast are good choices also).

With this workflow you can also set an alpha level on a specified channel!

Thank you all