I used a layer mask to separate the object of interest from
the background. I did this by using a technique that I learnt
a long time ago from GIMP experts. It has worked well in
most cases. It is as follows:

1. Desaturate a copy of the original.
2. Use Curves to increase contrast so that the subject is
well separated from the background.
3. Use Levels to create a binary mask.

I can then finetune the mask by hand, if needed.

I did this to a rather simple image with a sharp white-black
edge in some places. The final mask was good in most
places but not at the sharp black white edge. As I had to
keep some fine detail in other places, the settings I used
for the Levels and Curves let a fine (one or two pixel) part of the
white background unmasked. It looks ugly (unless I use a very
pale new background, in which case the white edge does not
show much). But, in any mixed colour background, there is
a thin halo around the subject. I would like to "grow" the mask
in this troublesome area and eliminate the halo.

Here I ran into a problem. I thought that I could use the
"wand" tool in a preselected area to select the black
(i.e. transparent) part of the mask *within that preselected
area". I just used the lasso tool to preselect the edge that
I need to work on. But, on using the "wand" tool, the
contiguous black portion in the entire transparent part
of the mask, and not just in the selected area, gets
selected.

I would like to know if there is a techinque where I could
select the black area in the layer mask in just a portion
of the layer. Finetuing by had is an option, but it is a lot
of work!

Thanks.

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