How to remove paper pattern from high-res watercolor scans (batch)

Question

I have about a hundred illustrations that I want to prepare for printing, and I am struggling to find the right pipeline to make the unpainted parts of the paper WHITE.

Using some form of edge detection is not enough because watercolors often have very soft gradients from 'no color' to 'color', so before I can clip off some level of gray to make it WHITE, I think, I need to remove the paper texture noise from the images while preserving as much of the color and the detail of the painting as possible.

The paintings' borders are often white, and therefore the image's 'white regions' also need to be WHITE or else when printing them on paper, they will appear greyish and horrible.

Yes I have found (2) related questions and have tried:

1. TTF (from gimp-gmic)
2. selective gaussian blur + levels (to crop off the white regions after they have been smoothed.

Main issue so far is that in low-contrast but still colored regions the detail is lost after the blurring, or appear blocky once I try to sharpen the image to regain some detail on the high-contrast areas. Also in order to blur away the largest paper shadows I have to set a high blur radius and that results some of the 'good' color details to be caught in the crossfire as you can see in the second image:

After cropping the white values I am left with:

which is not all that bad (the white regions are WHITE), but:

1. the colored regions have still some blurry paper pattern going on, which might look ugly once printed.
2. Some of the smaller color splotches are nice and sharp, some others are inexplicably blurred out.

So I guess the question is: what can I do to improve this process, or is there another I should try?

Answer 1

Since you already have G'MIC - might be an idea to look at the smoothing filters listed under the Repair section in the Plugin. This one is the Smooth (Bilateral) filter. Looks quite promising. However as a side note, I feel removing too much of the texture would detract from the look of a watercolour, making it look like it was painted on glass rather than paper.

Answer 2

I'm not an experienced GIMP user so I can't give you an application specific answer. This is perhaps more of a long comment. I do a lot of artwork scanning at work and want to share my experiences.

Watercolor is perhaps the hardest media to scan. Often it has many details in the lightest areas which disappear if you try to get a white background. And if the paper is heavily textured like this ... oh man it's hell to get a clean result.

That kind of paper just isn't good for reproduction. The artist should ideally use paper with as little texture as possible. Maybe even some kind of heavy board so it doesn't bulge too much when it gets wet. It takes some effort and experimentation from the artist's side really. They can't just expect good results with any kind of paper.

As @BillyKerr mentions in a comment: "An FFT (Fourier Transform) and edit won't work on a texture like this. It only works on repeating patterns", so don't waste time on that approach.

I would normally just try to soften the texture a little without ruining the painting too much and then make sure to print it to the edge of the paper. That way you at least don't see the difference between the blank paper and the printed paper texture. If needed to achieve this, I add bleed using content-aware fill. This obviously limits the possibilities to use the drawings as vignettes and you have to be careful that the paper gets the same tone on all scans.

Even if you found some way to remove texture you have the problem that the original seem to have been bulging in the scanner. So the texture isn't uniform. It's way sharper in some parts than in others:

Perhaps it's possible to get a better result if the originals are photographed. Some experienced artwork photographers know some lighting tricks which can diminish the surface texture so you get a cleaner original.

Answer 3

I'm afraid no program except a well trained AI application can decide which is paper texture and should be removed and which is something painted. Some detail separation and noise smoothing algorithms can get close, but seemingly there's already a good answer (by Mr.Kerr) of it.

I suggest you to remove all details on light areas and let the texture stay in darker areas by lifting the upper levels so much that clean or nearly clean paper areas are whitened. Use color adjustment curves to do it:

The texture stays only in darker areas. If the painter was you the change is OK, but if the painter was someone else he might have something to say. I know people who would boil over if I changed the look of their paintings without asking, no matter it's only a photo.

Sorry for having a screenshot from Krita, but the idea is the same in all RGB bitmap image editors.

In all scanners I have used the same result could be got with level settings in the scanner. But as my own opinion I say it's not generally a good idea to remove information in the beginning.

Answer 4

Have you tried scanning the picture twice? Before you start the second scan, turn the picture around by 180°. That way, the paper texture will cast its shadows in the opposite direction. Then use an image editor to align the two scans and average their pixel values together (e.g., by changing the opacity of the upper scan to 50%). You'll have to align the two scans very carefully, which might take quite a while. But with a bit of luck you'll be able to eliminate most of this shading effect.