[Techtalk] GIMP: Changing all set pixels to black

David Sumbler david at aeolia.co.uk
Fri Jan 8 15:35:11 UTC 2016

Thanks you, both for your help in this.

Akkana's answer provided most of what I needed: my requirements are very
simple, because the images are simply contour lines (from a map) which I
want to fill and use in QGIS.  Absolute precision is not essential, so
it does not really matter whether the contour line itself is ultimately
shown as part of its fill or whether it appears as part of the next
layer down.  All I want to be able to do ultimately is to be able easily
to see summits, cols, watersheds and (where there are any) depressions.

Miriam's very detailed suggestions will be very useful in the future,
and I shall keep them carefully - that curve tool has long been a
mystery to me!  However, after some bewildered experimentation (which
seemed to achieve nothing), I discovered how to use it to change all the
blacks to any other colour I want, which is excellent.

So I can now achieve the result I wanted.  Thank you again.


Akkana Peck <akkana at shallowsky.com>On Thu, 2016-01-07 at 16:21 -0700,
Akkana Peck wrote:
> David Sumbler writes:
> > so the pixels in the outline are not all black, but various
> > shades of grey.
> > 
> > I want the outline to be entirely black ...
> > If I use the colour picker on one of these grey pixels, it actually
> > produces black, so I think that they are not really grey at all, but
> > black with some degree of transparency.
> > 
> > The answer would seem to be to remove all transparency from the layer at
> > the outset.
> Remove Alpha Channel (in the right-click context menu for the layer
> in the Layers dialog) should remove any transparency from the layer.
> That will probably make the grey areas grey even if they were
> transparent black before. But at that point, you should be able to
> use Colors->Threshold and drag the slider all the way to the right
> to make them black again.
> At least I hope so. :-) If it doesn't work, post again (and it might
> help if you could upload the image, or at least the layer).
>         ...Akkana
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