Presentation at KGC2012 about the Architecture and Optimization techniques used to make the Relic FX System as used in the Company of Heroes and Dawn Of War Series, and many other Relic Games.

1. Relic’s FX System:
Making Big Battles Come Alive
Daniel Barrero, PhD
Senior Graphics Programmer
Relic Entertainment - THQ
Korea Game Conference – KGC 2012 – October 8 2012

2. Introduction
• Video/Screenshots of fx

3. Relic’s FX System Lineage
Direct Descendant
Influenced By Custom Port

4. FX System: More than a Few Good
Particles
• FX Systems are at the core particle systems, for our purposes we
require it to:
• Allow multiple particle types at the same time
• Be Fast and scale performance based on available resources
• Allow wildly different particle behaviors
• Multiple ways to generate particles
• Easy to use by content creators
• Able to interact easily with gameplay
• Be able to be used for wildly different types of game worlds

5. Relic’s FX System Features
• Broad variety of particle types
• Hierarchical Particle System
• Cascading FX (Trigger other FX and game actions)
• FX Batching ( just 1 to 2 render calls to render everything )
• Multiple Particle Behavior environments
• Lit particles
• Soft/Volumetric Particles
• Explicit draw particle order within a particle system
• Forward and Deferred renderer support.
• Automatic texture atlas generation on load
• Animated texture support
• Efficient memory management
• In camera and Post-Processing FX

6. System Architecture
• Object oriented style system
FX Type
FX Combo FX Particle FX Emitters
• FX Types hold the information for the effect, FX
Objects are instanced based on that information
when a FX gets triggered and hold the specific
instance mutable state information.

7. Hierarchical Particle System
• The FX Combo type is one of the core building
blocks of the hierarchical particle system.
– It can have up to 8 children of any FX Type, including
more combos.

8. Hierarchical Particle System
• The FX Combo type is one of the core building
blocks of the hierarchical particle system.
– It can have up to 8 children of any FX Type, including
more combos.
– It’s properties indicate how, when and where children
are created.

9. Hierarchical Particle System
• The FX Combo type is one of the core building
blocks of the hierarchical particle system.
– It can have up to 8 children of any FX Type, including
more combos.
– It’s properties indicate how, when and where children
are created.
– Children properties with the same name as the
combo are relative to it’s parent, similarly root combo
properties are relative to the action or object that
triggered the effect.

10. Hierarchical Particle System
• The second key building block of the
hierarchical particle system is the ability for
any FX Type other than a combo to trigger a
single child effect of any other FX type.

11. FX Particle Types
• Standard Geometric Particles:
• FX Ring: just a simple standard textured quad.
• The most common type of particle. E.g.
Smoke, Explosions, Clouds, Ground effects, UI…

12. FX Particle Types
• Standard Geometric Particles:
• FX Beam: a long stripped rectangle that goes from a point
of origin to a specific destination point following a curve.
• E.g. a Laser beam

13. FX Particle Types
• Standard Geometric Particles:
• FX Trail: A curved flat shape that follows the path created
by the movement of the fx emitter.
• E.g. A constantly growing ribbon like Homeworld engine trails

14. FX Particle Types
• Standard Geometric Particles:
• FX Streamer: A long stripped rectangle that goes from a
point of origin up in a given direction up to a given
distance.
• E.g. Laser sword or flame thrower.

15. FX Particle Types
• Standard Geometric Particles:
• FX Mesh: Generates the indicated mesh and attaches to a
particle that controls the look and position of the mesh.
• Useful for debris, rockets, 3D UI elements,...

16. Particle Types
• Post-Processing and non directly visible
particles:
• FX Distortion : An axis aligned quad that applies a post-
processing distortion effect mostly used to simulate heat
and pressure waves.
• FX Lens Flare : Generates a lens flare effect applied as a
post-processing effect in the screen area covered by the
effect.
• FX Light: Generates a dynamic point light in the world for
the duration of the effect.
• FX Audio: Creates an audio source with the indicated
sound at the particle position.

17. Particle Rendering: Blend Modes
• Having an explicit draw particle order within a
particle system allows having per particle
alpha blended or additive blending modes:
Particle Particle Background Particle Screen
Alpha Inv-Alpha
Additive: + =
Alpha
Blend: * + * =

18. FX Particle Types: Lighting
• Lighting style affects particle triangulation.
– E.g for a FX Ring:
Geometry Render
Unlit / Flat
Lit (Pyramid)

19. Particle Rendering: Orientation
• Orientation relative to world or parent.
• Billboarding:
– Spherical
– World Axis Constrained
• Follow Terrain

20. Particle Generation
• A single FX Type can be triggered either
directly or from other FX Types
– E.g. a single ring.
• Using the FX Spray type to generate sets of FX
Types with a common given behavior.
– E.g a column of smoke
– FX Sprays can be spawned
by any other FX Type

21. Particle Behavior
• FX Visuals are separate from specific FX
behavior.
FX Type FX Environment
Animation Curves Simple Dynamics Full Physics Simulation
• Each FX Type can have a different FX Environment
allowing particles in a single effect to react
differently to their environment.

22. Particle Behavior
• Animation Curves:
– Any particle parameter can be animated using key
framed animation.

23. Particle Behavior
• Simple Dynamics:
– We use Newtonian physics and Verlet integration on all
simulation environments to compute new particle positions.
– For ground collisions we calculate collisions directly against the
terrain height map.
– For collisions against objects first collisions are computed
against the animated bounding boxes then against the actual
object polygons if precise intersections are requested.
• Full Physics Simulation:
– FX Meshes being standard world objects, are able to use the
standard game physics (Havok) to compute precise collisions
against the environment if set up to do so.

24. Pipeline: FXEditor
• FX Editor is where individual effects get
created.

25. Pipeline: Object Editor
• FX get hooked up with game actions in this
tool.

26. Pipeline: Object Editor
• FX get hooked up with game actions in this
tool.
– Allows triggering FX from game data and gameplay
events using a Data-Command-Action pattern.
• Is an event based state machine system that is
completely data driven.
• Allows attaching FX to markers in objects and to trigger
other fx based on game states data.

27. Making it Fast
• Problem: There can be thousands of particles
of different types, blending modes and
textures that need to be created and updated
every frame.
• Straight forward approach to render each particle
separately produces too many render calls and
context switches.
• Updating and generating so many particles can
take a lot of time in the CPU

28. Making it Fast
• Solution to reduce CPU cost:
– Keep particle dynamics as simple as possible:
• That means fake complex physics behaviors as much as
possible with simple particle animations as much as
possible and only use physics simulation for particles
that really need it.
– Multithread the particle update
• Every frame the FX Manager spawns as many jobs as
there are root FX Objects (FX Objects with no other FX
Type as parent).

29. Making it Fast: Batching
• Solution to Render Cost: Batch Particles
• Add all particle triangles to a single dynamic fat vertex and
index buffers regardless of particle type.
• Dynamic vertex buffer works as a circular buffer.
• We do only 1 or 2 render calls per frame for all effects
combined! (max 21845 particle triangles per render call)

30. Making it Fast : Batching
• Solution to Render Cost: Batch Particle
• Pack FX textures in a texture atlas at load time to
use a single shader and avoid texture switches.
• Texture atlas is a cube map in d3d9 level HW

31. Making it Fast : Batching
• Supporting different modes in the same
shader pass and using a unique blend state:
– FX Textures are all pre-multiplied alpha.
– Blend state is set to a custom additive + blend
state:
– Screen Color = Particle Color + Screen Color * Particle Alpha
• Use separate alpha blend state:
– Screen Color = Particle Alpha * Inv Particle Alpha + Screen
Alpha

32. Making it Fast: Batching
• Supporting different modes in the same
shader pass and using a unique blend state:
– In shader code we compute the particle color and
alpha depending on blend mode requested:

33. Making it Fast : Deferred Rendering
• Render in a buffer the nearest and furthest
depth values during the depth pre-pass or the
GBuffer pass.
• Render the FX on a separate lower res buffer
using alpha blending using the near and far
values to determine if an effect should be
drawn in the buffer or not.
• Blend the FX buffer as full screen post-
processing pass.

34. Making it Fast : Weather FX
• Besides the hierarchical system there is a
specific separate System just for weather FX:
– Solve a very specific problem in a very efficient
way:
• Falling Rain and Snow
• Rain Splashes
– Can be implemented in the CPU or completely on
the GPU on recent hardware

35. Making it Fast : Weather FX

36. Making it Fast : Weather FX
Observations:
• Rain drops are only visible when close to the camera:
• Particles only need to be computed inside the view
frustum close to the camera:
• Easier to reuse particles thus simplifying memory
management by using a circular buffer for the particles.
• There is really only one type of particle needed
• Textured Quads

37. Making it Fast : Weather FX
Observations (cont):
• It is really hard to see what specific falling drop
produce a specific splash on an object at normal
speed:
• Means falling rain and rain splashes can use different
non interdependant techiques for particle generation.

38. Making it Fast : Weather FX
Falling Rain and Snow Overall algorithm:
• First Emit particles on an area in top of the view frustum
and close to the camera.

39. Making it Fast : Weather FX
Falling Rain and Snow Overall algorithm:
• First Emit particles on an area in top of the view frustum
and close to the camera.
• Calculate new positions for all particles

40. Making it Fast : Weather FX
Falling Rain and Snow Overall algorithm:
• First Emit particles on an area in top of the view frustum
and close to the camera.
• Calculate new positions for all particles
• If particle is outside of the view volume mark as dead and
reuse.
• Problem: moving the camera will show more
particles on one side until new particles are
generated on the side of the direction of the
movement.

41. Making it Fast : Weather FX
Falling Rain and Snow Overall algorithm:
• Solution: treat particles as an animated volumetric
texture
• If a particle goes out of the volume in one of the sides, just
change the position as if it wrapped around the other side
of the volume.

42. Making it Fast : Weather FX
• Volume particle tiling CPU code:
• Volume particle tiling GPU (Vertex Shader) code

43. Making it Fast : Weather FX
Falling Rain and Snow Overall algorithm:
• Solution: treat particles as an animated volumetric
texture
• If a particle goes out of the volume in one of the sides, just
change the position as if it wrapped around the other side
of the volume.
• If a particle goes out of the volume in the bottom then
reuse as a new particle.

44. Optimizations: Weather FX
• Rain Splashes algorithm
• Also treats splashes as an animated volumetric texture
that tiles in world space.

45. Optimizations: Weather FX
• Rain Splashes algorithm
• Generate random positions on the horizontal plane instead
of the top of the camera:

46. Optimizations: Weather FX
• Rain Splashes algorithm
• Generate random positions on the horizontal plane instead
of the top of the camera.
• Trace a ray at that particle point from a high altitude and
find first intersection with the terrain or objects allowed to
have splashes.
• Emit Splash particle
at that point.

47. Optimizations: Weather FX
• Problem: If camera moves then splashes would move
over the terrain.
• Solution: Let splash particles generated play their
animation while they are visible by the camera once
dead or out of the view frustum reuse for new
particle in another position.

48. Acknowledgements
• Thanks to all the great FX Artists that keep
pushing the limit of what the FX system can do
and keep us busy with more ideas of how to
make it better.
• Thanks to all the countless programmers that
have worked on the FX System its tools and
pipeline over the years.

49. Thank you
• Questions?
• Contact Information:
• daniel.barrero@gmail.com
• http://danielbarrero.com