If you take a picture of - for example - a car and you get the license plate number in it; how do you blur out the numbers?
(I would also like to know how to do this with a video if possible).
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What I usually do (in GIMP), but is maybe overkill, is to duplicate the layer, apply a gaussian blur and then use transparency masks to limit the blur to the interested area. You can use the same trick with any other filter, like the mosaic; or you can also get creative and use some other effect, like white noise.
You can achieve more or less the same effect by using selection tools like the lazo.
Image taken from Wikimedia.
I think that in Photoshop you can directly create a filter layer.
Of course, you can also paint it with the background color of the plate, if it's uniform enough it will look realistic:
Slightly more complicated, but can be funny: replace the car plate with a text of your choice, paying attention to colors, perspective, sharpness and so on.
Super-quick option: with the smudge tool of GIMP, you can make a couple of passes and get this: