Full CG except the background image, rendered with Redshift in Houdini
I fixed some small issues and adjusted the attribute values from the last scene: bigger raindrops, nicer droplet trail path, more falling droplets and new interaction between the rain and window.
There are four sets of interactions: initial rainfall from sky colliding on surface, splash from colliding particles, new particles spawn on collision point, trail of moving particles on surface
Interactions when particles travels:
snapping on surface, merging, particle inter-attraction force , randomize directional force, surface normal/friction force based on surface normal/friction attribute,
deformation by gravity to get a droplet shape- deform the shape of water droplet based on the location of points, the lower it is the more deformed it is
Wet map using solver and attribute transfer and hold the value based of time. Then it's mapped to the roughness of the window glass shader.