Adding transparency to ink drawing

Question

I would like to add transparency to the following image

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?

Answer 1

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.

Answer 2

You can clean the picture first, for instance using Levels:

• the big peak on the right is the original background. This shows that the background is not pure white. If you use C2A without preprocessing, you have to aim for this color.
• The white handle is dragged to the left (left edge of the background peak), so that most of the background becomes pure white.
• The gamma handle (middle) is adjusted so the the dark circles remain dark
• This is best done after making sure that the image is monochrome. You can use Color>Desaturate, but if you want to get rid of the light blue dots, just work on a copy of the Blue channel: open the Channels list, and drag the Blue channel on the canvas to create a layer that is a copy of it. If you want to keep them, do the same but copy the red or green channels. After the channel copy, click again on the channel to make it selected like the other two otherwise you will have color shifts later.

Once you have done this you can use color-to-alpha with all defaults: