Lets you specify where in an image movement should be detected. Or at least it lets you shift the focus from the edge of an image to its centre.
Imagine a beam of light illuminating an entire image area. This would correspond to the setting “neutral”.
Now imagine a spotlight illuminating only the centre of the image area.
This would correspond to the setting “centre”.
Any movement in the area that is not illuminated would be ignored.
With the setting “border”, on the other hand, only the border area would be illuminated. Any movement in the centre of the image would be ignored. Are you wondering what this is for?
You might think that movement detection should always include all available information to achieve the best possible result. That is how many other stabilisation solutions work. Mercalli offers a small but significant difference.
Imagine that the filmed object you want the viewer to focus on is a duck, swimming in the centre of the screen. This duck would therefore be the object that needs to be stabilised. However, the waves around the duck also create movement, which would interfere with the stabilisation of the duck. If movement analysis focuses on the duck (and therefore ignores the waves), the stabilisation result will simply be much better!