[Gimp-user] Opacity

[saulgoode at flashingtwelve.brickfilms.com]

Quoting Lap1994 <Lap1994 at itelefonica.com.br>:

> Hello. I want to draw a image with a 50% opacity border.
> When I change the opacity of the pencil and use it, the selected color and
> the color where I use the pencil, combine itself. I do not want it, I want
> draw a pixel with exact 50% of alpha and the color selected. Like a bitmap
> with a alpha channel.

I can't think of a direct way to do that but it can be accomplished  
with a few extra steps.

* Duplicate your original layer (this will let you see the image as  
you paint).
* Add a black layermask to the duplicate layer.
* Apply the layermask. (The layer will become transparent, but the  
original color data is still there.)
* Choose your paint tool and set the Opacity slider (in the Tool  
Options) to the level of _transparency_ which you desire. If you want  
the border to end up with 20% opacity then set the slider to 80%  
(fifty percent opacity means fifty percent transparency).
* Paint the duplicate layer. (You can toggle the visibility of the  
original layer to see how the semi-transparent result of your painting  
will appear; however, the unpainted regions will not be visible.)
* Add a layermask to the duplicate initialized to "Transfer the  
layer's alpha channel" with the "Invert" box checked.
* Hide or delete your original layer.