How to remove a scratch from an image using matlab

Question

Let's say I have this image this :

With a black scratch and I want to remove it from my image. I know it is noise. I have tried neighbourhood filter and also gaussian filter but no success.

Answer 1

If you know the location of the scratch, this problem is known as inpainting , and there are very sophisticated algorithms for that. So one approach would be to detect the scratch as good as you can, then use a standard inpainting algorithm on it. I've played with your image in Mathematica a little:

First I applied a median filter to the image. As you found out yourself, this removes the scratch, but also removes a lot of detail. The difference between median and original image is a good indicator for your scratch, though:

When I binarize this image with a manually selected threshold, I get a quick&dirty scratch detector:

If you have more knowledge about what your scratches look like, you can improve this detector a lot. eg are the scratches always dark? Do they always have high contrast? Are they always smooth curves, ie is their curvature always low? - Each of these properties can be measured somehow, so you'd combine these measurements to a single image and binarize that.

One small improvement is to remove small components:

This is still not perfect, but the result is good enough to use it as an inpainting mask:

This will remove some detail, too, but the differences are harder to spot.

Full Mathematica code:

difference = ImageDifference[sourceImage, MedianFilter[sourceImage, 2]];
mask = DeleteSmallComponents[Binarize[difference, 0.15], 15];
Inpaint[sourceImage, mask]

EDIT:

If you're don't have access to a standard inpainting algorithm (like Navier Stokes or Telea), a poor man's algorithm would be to use the median filtered image in those regions where the mask is 1 (probably something like mask*sourceImage + (1-mask)*medialFilteredImage in Matlab). Depending on the image data, the difference might not be worth the extra effort of a "real" inpainting algorithm:

Answer 2

My result using median filter with ImageJ

Answer 3

A filter for Avisynth and a plugin for VirtualDub (my two favourite video editing tools). It will hardly get better than these two (You can learn from them if you really need to implement it yourself).