Cleaning up microfilm, microfiche and aperture cards

[kirbywan]

Howdy all,

I'm going to start fiddling with the best way to scan in and clean up aperture cards to make them as readable as possible. Has anyone solved this problem before? I expect the process would be similar for microfilm and microfiche if anyone has worked with those. 

As I sort out best practices I will share my findings here.

Thanks,

-Kirby

[kirbywan]

https://imgur.com/a/HbN2Zr1

So here's a typical image I'm looking at cleaning up. It's nice because it has very little actual information except at the right edge. I see three specific issues to deal with. The first is the how the image is hot in the middle and every one has a similar darkening in the middle compared with the edge. In this image it can only be seen by the splotchiness of the blank part of the image, which is the second problem. This causes some of the dark parts of the splotches to be darker than the lighter parts of the lines in the drawing at the edge, so a simple level curves didn't get the results I was looking for. I had some success just working with the brightness contrast control, but it ended up loosing some of the crispness of the drawing or having the lighter lines fade into oblivion. This is the third problem. I want as much detail as possible to bring out the original lines of the drawing.

Any suggestions on approaches to solve these problems would be welcome.

Thanks,

-Kirby

[Blighty]

There are many ways to tackle this. I have listed 4, but I'm sure others will come up with more ways.

1. Contrast (about 40)
2. Levels (right hand white triangle to about 220)
3. Threshold (black triangle to about 190)
4. G'MIC : Repair > Repair scanned document

I also suspect that there will not be a single best method that applies to all images.

[Ofnuts]

A good way to normalize the lighting of the image:

* Duplicate the layer
* Apply a very heavy gaussian blur. The drawoing should no longer be visible (I use something like 1/5th of the image diagonal)
* Put the top layer in Grain Extract mode.

This is more or less computing a 128-centered difference between the layer and a blurred copy of it. Then you can use Curves with mess impact on the fine detail.

Edit: some success with a different method, use the Wavelet decmpose plugin and decompose to 7 levels, then keep on the Level 4 and the residual. But time to go to work...