How to change background to white in other software than Adobe products?

Question

I have a bunch of black&white drawings (to be exact grayscale) and I took pictures of them. I want to change their backgrounds to white. I don't have a Photoshop license, so I was wondering if there are other software packages I can use. I have clip studio paint (formerly known as manga studio). They have a feature called Correction Layer -> Binarization, however, this gets rid of some gray pixels and the drawing looks a bit rough. Can this be done in gimp?

I found this message Change background color from brown to white (remove sepia) and I can do it on my mac but the background is still grayish. What I need, I think, is convert some the pixels that are closed to white to white.

Any ideas?

Thanks

PS. Just in case you were wondering, I did go to a store to scan them but high quality scanning is very expensive (I live in Vancouver) and I gave up that idea.

Answer 1

Gimp can definitely do this. use Color>Desaturate>Desaturate (yes, twice) to make sure you image is gray(otherwise some parts can take a color in the processing that follows).

Then you can try Colors>Auto>Stretch contrast or Colors>Auto>Stretch contrast HSV. If this is not sufficient, use the Levels tool. You will likely get a histogram that looks like this:

The bump in the histogram is the pixels of the background. In the input sliders, drag the rightmost handle to the left, to the left part of the bump. This will set the background to white. Drag the left handle to the right, to darken the black. You can adjust the contrast with the middle handle.

If you took photos, the lighting may be uneven and this may make adjusting levels difficult. In that case, see here

Answer 2

Krita (=freeware) can be used as well in Windows, Linux and Mac.

The problem is common. Typical case I have met is the following: A hobbyist has a drawing. He has taken a photo of it and asks me to help to make it clean. Here's the photo:

This is actually quite good photo. It's sharp. It has reasonably high resolution at least for onscreen watching. It's not full of nasty JPG noise. It isn't mangled unrecognizable by automatic noise reduction. It's taken nearly straight on the face and it's not rotated. There isn't blue outdoor light mixed with yellow indoor light, the color is generally same, only a little blueish.

The problem: The light non-uniform. The right side has already nearly white background, the left side is substantially darker. It isn't hopeless, because there's no full black nor overblown areas. But the lack of uniform light causes level adjustments to produce something like this:

One way to fix the original image is to duplicate the image layer and blur the copy to get light coverage profile:

When this layer is inverted and mixed with the original by having blending mode =ADD, the light differences are reduced:

I have seen the drawing and this was only a little lighter than the drawing. But the hobbyist demands higher contrast.

One can increase contrast by merging the layers and applying Curves:

Unfortunately drawings often have stray dots, which jump visible as one adds the contrast. It can be fixed by painting white on the most disturbing areas: