CITY FLOOD FX BREAKDOWN
FLIP and RBD
The FLIP setup itself is fairly standard with two sources on both sides obscured by the buildings, but the more interesting aspect is the RBD interaction. It took quite a while to figure out since there's not a lot of documentation on this subject.
So the gist of it is the static colliders are sourced using the static object DOP node and for volume collisions use a proxy volume SDF while for the bullet collisions, use concave geo representation. Also turned up feedback scale to 1 in the FLIP Solver.
vorticity attribute from FLIP Simulation
curvature attribute from FLIP SDF
Through the Whitewater Source node, there are four ways of sourcing whitewater; acceleration, vorticity, curvature and additional inputs. The acceleration method may be the easiest to grasp because it's usually apparent when the particles are moving faster or accelerating. The less obvious methods are vorticity and curvature. By visualizing the attributes and understanding how the node processes these values, we can get a better sense of what values we need to input for the whitewater source node. In this case, the acceleration method was not used because it didn't look very appealing. Particles moving fast leave behind unwanted trails and makes it seem less realistic.
Additional inputs were added as well to the leading edges of the fluid. It seemed a bit too thin by just using the curvature values calculated by the node, so by rebuilding how the curvature was calculated, I was able to exaggerate the curvature values where I wanted it to be and use that as additional inputs.
After getting the right amount of whitewater source. The whitewater sim is fairly straightforward as well. However to give a more interesting look, the erosion, repellants and clumping values were slightly tweaked add variety.
The wetmaps are generated by transferring a wetness attribute from the FLIP sim and the Whitewater to the colliding objects. The wetness would be subtracted over time.
However, to get that to work, points were scattered across the whole collider geometry and those points would have the UV attributes on them as well so that this wetness attribute can be remapped into the correct UV space. This is then rendered out as a black and white animated texture. It'll then be assigned as a mask to control the reflection, roughness and diffuse parameters of the collider's materials.
last bit of thing to say...
This project started as a simpler school project FLIP sim with static colliders. Then during a portfolio review, I was told that even though it's a cool shot, there are some things that are broken, one of which was the scale. So I went back and reworked the project.
This back and forth happened a few times over 3 years where either there was a critique to improve the shot or I was trying to experiment with something. Bit by bit I was adding in elements, whitewater, RBD interaction, wetmaps, textures, looking at reference and even learning to render with redshift!
It definitely was a long process, but in the end, I am still really proud of the result and the journey it took to get it here. There was a lot I learnt and I'd like to thank Jonathan Gilbert, Ilan Gabai, and Samuel Simanjuntak who all helped me realize what I had to improve in different stages of the project.