There were several problems in the old random calculation:
* Different result in the viewport and render.
* Noise "pop" in some frames.
* Random number was calculated every time the file was opened, so get different results.
Now, instead to calculate the random numbers when n number of frames changed, the random values are calculated using a unique seed by stroke.
Also, a new Seed parameter has been added and this adds more control in the noise generated. This value can be animated and get more variations.
Differential Revision: http://developer.blender.org/D5393
80123efc5f