How to make a bokeh effect without photoshop?

Question

Design

Actually a subject.
Here is a link to the filter itself (although the designers, IMHO, already know what a miracle it is).

Answer 1

So, the blur technique, in general theses:
1. Make a depth mask (the so-called z-depth), in which the object in focus is painted over with black, and out of focus with white, how to do it? Gradients, local blurs, transparency changes. Very tedious.
2. Apply a mask to a blurred layer (for example, in Photoshop it allows you to apply a mask in LensBlur, that’s just it, I don’t know an analogue in Gimp).
Now I'll try to find the video, I remember, I saw it.
3. Refine, add round bokeh :) on bright details.
If it's a very simple idea, for example, a model's face and some background that needs to be blurred:
1. Select the face (make a mask / clip) and hide the layer with it.
2. Process the background without the model's face, as if it never existed.
3. Blur the background (here, if you follow step 2, there will be no traces of blurring the face).
4. Return the face of the model.

Answer 2

Very similar to motion blur, I think GIMP can do some magic with it.