I simply want anything not inked to become transparent. I have gimp and I can also use some online tools, if available. I tried using gimp and Color To Alpha, but that leaves shadows and still some background (though gimp displays the background as transparent curiously). Is there some simple way for me to say make everything transparent that is not close enough to black color?
To remove shadows and grain from the background we can chose a threshold for values of transparency and of opacity when using the “Color to Alpha” tool in Gimp 2.10:
Simply drag the sliders until you are happy with the result. On a white background your image will then look like this:
Alternatively you can make a colour selection with the select by colour where you can also define a threshold. Remove the selection by pressing Del if you already have a transparency layer.