The main VFX challenge was designing the clouds. One of the key challenges was figuring out the scale as everyone has an idea of what clouds look like, but generally the clouds we see are massive. When they're seen close up they're much more misty. We focused on trying to capture the sense of "softness" that you get when you look at a nice white fluffy cloud in the sky.

We also introduced a cloud-ball concept. The idea was to make it feel like the clouds were a collection of balls of cloud. All the animation is based on small spheres that move around and interact with the hero. This sort of motion helped add to that feeling of a soft fabric made of clouds, rather than a misty cloud.

The interaction is done by a mixture of simulation techniques and some hand animation to get the cloud-balls moving around.