[Techtalk] GIMP: Changing all set pixels to black

David Sumbler david at aeolia.co.uk
Tue Jan 19 09:38:24 UTC 2016

And thanks, Miriam, for another very useful tip; it is potentially very
useful.  I shall experiment with it, and then perhaps I shall remember
it the next time I actually need it.


On Tue, 2016-01-19 at 14:46 +1000, Miriam English wrote:
> What are the odds? I just had exactly the same problem occur in a 
> picture I was working on and I remembered that Akkana had answered your 
> question much more accurately than I had, so I looked again at her 
> answer and was going to attempt it to solve my problem when I stumbled 
> upon an even better solution.
> When going to Gimp's "Layer" menu and the "Transparency" submenu I was 
> going to choose the "Remove Alpha Channel" item when I suddenly noticed 
> the "Threshold Alpha..." item. Intrigued (I'd never used it before) I 
> tried it and found it did exactly what I wanted and allowed me to tune 
> exactly how much of the partly transparent black I wanted to make fully 
> opaque black.
> Thanks, David for the question and Akkana for your answer, without both 
> I would likely never have found this neat function. It made my day. :)
> Cheers,
> 	- Miriam
> On 08/01/16 09:21, 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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