Blob/MetaBall: Great effect but how does it work?

[joem11364421]

Hi all! I've discovered this great blob effect in Illustrator but I'm having trouble understanding how it actually works. Does anyone understand this?

Here is how you get the effect:

1. Create two (or more) shapes
2. Select them all and create compound path
3. Apply offset path effect (Effects > Path > Offset Path> Use a positive number, change "Joins" to Round)
4. Apply offset path effect again, all the same settings but with a negative number this time

I uploaded a quick video here as well. 

Thanks in advance!

[Kurt Gold · Accepted solution]

The first instance of the Offset Path effect forces the two circles of the compound path to unify.

The second instance of the Offset Path effect recognises the unified object to calculate the negative offset as long as it is below the first Offset Path effect in the Appearance palette. That's why it works.

[joem11364421]

Amazing! Thank you so much for your insight here! That really helps me understand this.

For anyone still scratching their head, this visual helped me understand it better:

Instead of applying two different instances of the Offset Path effect here, this was two overlapping circles that I unified into a single shape with the Unite command in the Pathfinder panel. The blue outline that you can see is the outline of this new shape. Then when you apply a single instance of the Offset Path effect with Round selected, it gives you the neat blob effect, smoothing out the sharp corners. 

Doing it with a compound path simply makes the effect sort of "live" with multiple shapes. Very cool.

Just for reference, here is what that same shape looks like with the other "Joins" options selected:

Miter:

Bevel:

Thanks again Kurt Gold!!