Re: [Gimp-developer] Proposal for erasing background from an image

[Jay Smith <jay JaySmith com>]

On 03/02/2017 05:22 PM, Ofnuts wrote:
> On 02/03/17 10:05, Simon Budig wrote:
> > John Tapsell (johnflux gmail com) wrote:
> > > So to restate this again - I want to know how to change the top layer
> > > $latex src$ so that I can have the maximum possible alpha without
> > > changing
> > > the final visual image at all.  I.e. remove as much of the background as
> > > possible from our foreground+background image.
> > This is what color-to-alpha does. Except that it uses a constant color
> > for the whole image. It might be interesting to convert this to a gegl
> > op that takes two input drawable and picks the "background color" from
> > each corresponding pixel in the background image.
> >
> > Should be an easy fun hack. Who wants to take it?
> >
> > Bye,
> >          Simon
> Fun hack, but would it be useful in practice? Do we want the lion to be
> partially transparent because its hair is more or less the color of the
> dry grass (with some help from Darwin). Try the two pictures in
> difference mode, and check all the places that are near black.

If I understand this correctly, an example useful in practice, but the 
opposite direction (fill the background instead of clearing it is in a 
picture like this (postage stamp surrounded by a black background):

http://jsa.viewimage.net/jsa/web/Lists/Sweden/SpecStamps/sc0012a-f12g2_used-fvfplus_135668_r_l.jpg

(However, I selected this example image to show a problem -- that the 
background is "connected" to the dark areas of the postmark where the 
postmark reaches the edge of the stamp.)

For 15+ years I have been trying, without success, to solve this overall 
situation and automate (or have a useful manually operated tool) the 
process of filling the background with 100% black.  The scans are done 
on a black background (but it is impossible to scan it 100% black 
without distorting other aspects of the image); I have attempted using 
other contrasting/unique colors for the background during scanning with 
the idea of converting them to black, but there is always some sort of 
halo or bleeding at the edge of the stamp's perforations, even when 
there is not the "postmark problem".

Jay Smith