Re: [Gimp-user] Problems creating layer masks

[Akkana Peck <akkana shallowsky com>]

> > dereksalt1159 gmail com writes:
> > I hope that you can help me.  For a long time I have been trying to make
> > layer masks of fine details, such as leafless trees full of twigs, using
> > Gimp 2.8.  I can make what looks like a perfect black and white image for
> > the mask but when I apply it to the photo all the twigs come out with a
> > white halo around them.  I have tried selecting the image, growing,
> > shrinking, and feathering it, the copying and pasting but nothing works.
> > Working on solid areas such as a building gives no problems.

Richard writes:
> Not quite sure what you're trying to do based on this description but let's see ... is it safe to assume 
> that the object you're trying to mask was originally shot against some kind of white backdrop?  Then the 
> white "halos" you see are pixels that are originally some mix (think alpha blending) of foreground and 
> background, and GIMP has no way of automatically 'knowing' what the original, non-blended color of that 
> pixel was supposed to be.
>
> So you'll probably need to do something along the lines of a color-to-alpha transition.
[more suggestions snipped]

The problem may boil down to that phrase "a perfect black and white
image." If you really mean black and white, not greyscale, then
that's the problem. A layer mask for photographic detail almost
always needs to fade from black to white to look reasonable, for the
reason Richard gives: real-world photos don't have hard boundaries
where one pixel is the leaf and the one next to it is the sky.
There's always a bit of fade from one to the next.

So instead of using, for example, Threshold, try using Levels and
using the middle slider to get something that looks almost like a
black-and-white threshold, but includes a little bit of grey at the
boundaries. If you're using something other than Threshold to get
that black and white, adjust accordingly.

If you're already using greyscale and didn't really mean black and
white, then ignore that advice: but in that case, it might help to
post a sample image and mask, and details of how you're generating
the mask.

Good luck!

        ...Akkana