00:00Peggy 16
00:30Particle System and how we used it to create some of the key VFX features for Returnal.
00:35This includes things such as some of the vegetation,
00:39character effects, like tentacles,
00:49and
00:50volumetric fog, driven by fluid simulations.
00:59Let's start off with character effects, and tentacles in particular.
01:09Tentacles, in Returnal, are constructed from chains of particles.
01:13Our VFX engine allows particles to have parent-to-child relationships.
01:16That means each child particle can read its parent's data, including its position.
01:21We're also allowed to write custom behavior per particle.
01:23Here we give the first couple of particles some velocity, and tell the rest to follow them.
01:28With the ability to customize the particle behavior and data,
01:30we can tell each particle to store its rotation as well.
01:34With position and rotation data available,
01:36we have enough information to create a smooth 3D surface between parent and a child particle.
01:43Let's make some of these particle chains follow a moving object.
01:47As the renderable surface is constructed per particle at runtime, we can easily modify their appearance in-game.
01:54Here, we make the particles switch to a different material, and grow the chains longer and surface thicker.
02:01We can use signed distance fields to make the particles collide with in-game geometry.
02:06And from these collisions, we can spawn new particles or create events, for example sounds.
02:13In-game, we use the tentacles to signal enemy states by changing the tentacle colors and behavior.
02:19Here, you can see the emissive color getting brighter before and during the enemy attack.
02:24We can access the character bone matrices from our particle system, and bind the tentacles to bones or vertices of
02:32the enemy mesh.
02:32Besides telegraphing enemy intentions, the tentacles give the enemies dynamic, secondary motion procedurally,
02:40without the need for keyframe animation.
02:43And when the enemies are defeated, we can detach the tentacles from them to provide a satisfying feedback to the
02:50player.
02:51But the use of tentacles wasn't limited to just enemies.
02:55We made some of the interactive vegetation on Atropos with them as well.
02:59In the next segment, we will have a closer look at our fluid simulation tech.
03:04We use fluid simulations extensively in Returnal.
03:07One of the most important use cases is to simulate airflow around the player.
03:11For that purpose, we use a fluid simulation grid that follows Selene as she moves around the levels.
03:16We refer to this simulation as global fluid simulation.
03:20Game events can apply impulses to that fluid grid.
03:23Here, we can see a visualization of the fluid velocities appear when Selene fires her weapon.
03:28Other player actions like dashing and melee also have an effect on the simulation.
03:36Every visual effect system in the game can sample the global fluid simulation.
03:41Here we can see the vegetation reacting to bullets fired by Selene.
03:47We can also do similar things with almost any actor in the game.
03:51Here we can see the enemy bullets affecting the fluid simulation.
04:06In Returnal, we use a lot of volumetric fog in our levels.
04:10The fog density can be affected with the global fluid simulation, making the fog more reactive.
04:20Similarly, we can use the fluid velocities that infect regular particles like the Xeno Archive holograms.
04:27And tentacles.
04:32In some cases, we wanted to use more detailed fluid simulations.
04:36Like here, Freiki has its own fluid simulation for the fog that is emitted from its skeletal mesh.
04:43Also, there's a separate simulation for the fog on the ground, as we wanted to have some extra detail there
04:48as well.
04:49To emit fog from the skeletal mesh, we made a custom component that converts the mesh into voxels.
04:54Each of those voxels can emit fog density, and VFX artists have full control of the emission logic on a
05:00per-voxel basis.
05:02To make the fog look more interesting, we can ramp up the density amount and add turbulence into the simulation.
05:09Volumetric fog isn't the only way to visualize the density field though.
05:13We can do isosurface extraction from the simulation data and give the effect a more solid appearance.
05:19This technique was used to bring some of the alien technology and cosmic horror elements on Atropos to life.
05:27In the last segment of this presentation, we're going to have a look at how we did some lightweight destruction
05:33visuals with our particle system.
05:35We do pre-facturing of the destructible assets in Houdini.
05:39Before the asset is fractured, we do a pass where we separate the interior of the object from its surface.
05:44Here, the object on the left is the source mesh, the object in the middle is the fractured interior, and
05:49the object on the right is the fractured exterior.
05:52We export both the interior on exterior meshes from Houdini to Unreal Engine using a custom format that allows us
05:58to retain information such as bounds and pivots.
06:01To keep things lightweight, we don't do RBD at all. Instead, everything is done via particle simulation.
06:07By using particle simulation only, we can keep the performance costs low enough to afford a thousand pieces per destructible
06:14object and several destructibles on screen simultaneously.
06:17When a destructible object receives damage, we start detaching the exterior pieces from it first.
06:23When its health reaches zero, the whole thing collapses.
06:26During collapsing, we take advantage of our particle hierarchies and group particles together.
06:30This allows us to move groups of particles together and make them appear as if they were a single bigger
06:34piece.
06:35When the particle group collides with SDFs, we can detach child particles from it.
06:39Naturally, these objects can be destroyed by enemies as well.
06:49And we can use the tech to destroy some of our mechanical enemies.
06:55The body of the Nemesis boss was rendered and simulated entirely using the particle system.
07:01Per-particle control of the pieces allowed us to animate them based on where the player bullets hit.
07:06We also used a sparse fluid simulation and voxelizer for the Nemesis' head as well.
07:13In the later stages, we made the pieces move around by themselves to support the dreamlike atmosphere of the encounter.
07:26That concludes our visual effects breakdown of Returnal.
07:29We hope you enjoyed it, and thank you for watching.
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