You need a white image (preferably a circle or an oval) for this technique - that will be your alpha mask.
Update - This is a much easier way to blur faces
Create a copy of your footage, blur it, place it on a lower track to overlap it onto the original, make a section of the area you want to be blurred then animate that section to follow the subject using keyframes.
The method is similar to the tilt shift. You make a duplicate of the footage and you place it on a lower track and then you place the alpha mask between the duplicate and the original. Then you apply the blur effect to the duplicate video on the lower track.
The effect will only transpose onto the original trough the alpha mask (which you can move an re-size as you would with any other object using keyframe animation).
Motion Tracking Solution with Magix
It seems there is an easier solution to this problem. What you can do is assign an object (or mask) to a particular element in the footage. Then that object will follow the element in the video - this is what motion tracking is (an alternative to manually following the element with keyframes).
In order for this to work properly, you need to have a high contrast between the element being followed (say your hear or your face) and the background (the sky, a white wall, etc.). Take this into consideration when you're shooting the video as well.
Here's the video tutorial from holivar TUTO:
Hope this helps. Drop questions in the comments if you have any. Cheers!
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