Real-time volumetric effects

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Real-time volumetric effects

• Andrei Tatarinov

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Talk outline

• Introduction
• Part I Generating fire with Perlin noise
• Part II Generate smoke using 3D fluid
  simulation
• Part III Rendering volumetrics

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Why is this cool?

• Volumetrics represent such common effects as fog,
  smoke, fire, explosions
• Current approaches to rendering volumetrics in
  games have limitations
• Video textures
• Particle systems

4
What do we have now?

• Most volumetric effects are represented as sets
  of video textures
• Everything looks cool until effect intersects an
  obstacle

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Why now?

• Sheer number of operations needed can only be
  supported by modern high end GPUs
• New features in DirectX10
• Render to 3D texture
• Integer maths
• Geometry Shader
• Stream Out

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Part I - FireOutline

• Perlin noise overview
• Simplex noise overview
• Ways to generate procedural textures
• How to generate a flame using Perlin noise

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Perlin Fire
Fire in NVIDIAs DirectX10 SDK Sample
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Perlin noise

• Given an input point P
• Find its neighboring grid points Q
• For each grid point
• Pick pseudo-random gradient vector G
• Compute linear function G (P - Q)
• Interpolate between values in grid points

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Random Number generation on GPU

• Use a set of power and modulation operations,
  using some big prime number as a modulation base
• Use a permutation texture

R PermTex.Sample ( P.yw PermTex.Sample ( P.xz
) )
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Simplex Noise

• The same as Perlin noise, but using a simplex
  grid instead of hypercube grid
• For a space with N dimensions, simplex is the
  most compact shape that can be repeated to fill
  the entire space
• Using simplex grid can significantly reduce a
  number of interpolations

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Procedural Textures

• Use a weighted sum of several noise frequencies
  to create a texture

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Procedural Textures

• Try noise in different expressions

noise
sin (x sum 1/f( noise ))
sum 1/f(noise)
sum 1/f( noise )
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Volumetric Effects

• Use noise to animate turbulent flow
• Flame
• Clouds

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Flame

• Take a basic fire shape and revolve it around
  y-axis to create a fire unit
• Fire shape can be customized to achieve different
  look and feel

y
Basic shape
Fire unit
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Flame

• Perturb fire unit with 4D noise (4th component
  stands for time)

Fire unit
Perlin noise turbulence field
Flame
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Flame

• Use a weighted sum of several noise frequencies.
  Change weights to achieve different look

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Part II - SmokeOutline

• Why 3D fluid simulation is important
• Overview of process
• Fluid simulation basics
• Dynamic arbitrary boundaries

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Smoke
Smoke in NVIDIAs DirectX10 SDK Sample
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Overview
Composite on top of scene
Scene
Decide where to place the smoke
Render
Discretize space and simulate
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Fluid Simulation

• A fluid (with constant density and temperature)
  is described by a velocity and pressure field
• Navier-Stokes equations mathematically defines
  the evolution of these fields over time impose
  that the field conserves both mass and momentum
• To use these equations we discretize the space
  into a grid
• Define smoke density, velocity and pressure at
  the center of each grid cell
• At each time step, we use the equations to
  determine the new values of the fields

Pressure
Density
Velocity
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Fluid Simulation steps
Each time step
Initialize
Add Density
Add Velocity
Advect
Project
Advect
Density
Density
Density
Velocity
Velocity
Velocity
Velocity
Pressure
Iterate
Pressure
We skip the diffusion step
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Advect
Density
Velocity
Time Step t
Density
Time Step t 1
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Fluid Simulation on the GPU

• Velocity, Density, Pressure ? Textures
• Simulate one substep for entire grid ? Render a
  grid sized, screen aligned, quad
• Calculations for a grid cell ? Pixel Shader
• Output values ? using Render to Texture

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Advect on the GPU
O
O
Velocity in x
Velocity in y
M
M
Velocity Texture at timestep t
Density Texture at timestep t
Texture fetch
Texture fetch
O
O
PS_ADVECT calculate new density for this grid
cell using the density and velocity textures from
the previous time step
M
M
rasterized
rendered
Render Target Density for timestep t1
Quad
Pixel Shader
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Advect on the GPU in 3D
O
O
M
M
N
N
Velocity Texture at timestep t
Density Texture at timestep t
O
Texture fetch
Texture fetch
O
M
PS_ADVECT calculate new density for this grid
cell using the density and velocity textures from
the previous time step
M
N
rasterized
rendered
N Quads
Render Target Density for timestep t1
Pixel Shader
GS is used to rasterize each quad to proper layer
in output Render Target
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Obstacles

• Smoke interacting with
• obstacles and compositing with the scene

Smoke only compositing with the scene
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Obstacles

• Implicit shapes
• Like spheres, cylinders
• Voxelize objects
• Static Voxelize just once, offline
• MovingVoxelize objects per frame

Obstacle texture
Fluid cell inside obstacle
Fluid cell outside obstacle
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Dealing with Obstacles

• How should the fluid react to obstacles?
• The fluid should not enter obstacles cells
• If the obstacles are moving they should impart
  the correct velocity on the fluid
• How the fluid reacts to the obstacles ? Boundary
  Conditions

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Boundary Conditions for Density

• No density should be added to or advected into
  the interior of obstacles

Obstacles
Density
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Boundary Conditions for Pressure

• Derivative of the pressure across the boundary
  should be zero
• (Neumann boundary conditions - specifying
  derivative )

cell1
cell2
v
PressureCell1 PressureCell2 0
PressureCell1 PressureCell2
u
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Boundary Conditions for Velocity

• The velocity normal to the boundary of the
  obstacle should be equal for fluid and obstacle
• (Dirichlet boundary conditions specifying
  value)

v
u
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Voxelizing an object
Obstacle texture
Obstacle Velocity texture
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Use low res collision model for voxelization
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Voxelizing a simple object
?
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Voxelizing a simple object
Orthographic camera
Near plane
Stencil Buffer
Decrement on back faces Increment on front faces
Far plane
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Voxelization
Render model N times, each time with a different
near plane



2DArray of N stencil buffers
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Part III - RenderingOutline

• Ray marching
• Occluding the scene
• Artifacts and ways to avoid them

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Rendering
Render front faces of box
Composite into scene
Raycast into the 3D Density texture
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Raycasting into 3D texture
What we have
What we dont have
3D Density Texture
Transform from world to texture space
- Ray from eye to box
- Ray in texture space
Transform from world to texture space
- Ray entry point in the texture
- Ray box intersection
Transform from world to grid space
- Distance the eye ray traverses through the
box
- Number of voxels the ray traverses Number
of samples to take
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Raycasting blending
Trilinear samplefrom 3D texture
Render Fullscreen quad to frame buffer

• Density Texture

PositionInTexture TransformToTexSpace (RayDataT
exture.rgb)
MarchingVector TransformToTexSpace (eye -
RayDataTexture.rgb)
NumberOfSamples TransformToGridSpace (RayDataTe
xture.a)
RayDataTexture
FinalColor.rgb sampleColor.rgb SampleColor.a
(1.0
FinalColor.a) FinalColor.a SampleColor.a
(1.0 FinalColor.a)
FinalColor.rgba 0
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Occluding the scene

• Smoke correctly compositing with the scene

Smoke directly blended on top of the scene
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Integrating scene depth
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Artifacts

• During the ray-marching use jittering to avoid
  banding

Without jittering
With jittering
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Jittered sampling

• During the ray-marching use jittering to avoid
  banding

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Artifacts

• Increase sampling rate to reduce a noise caused
  by jittering
• Increasing sampling rate leads to performance
  loss

Low sampling rate
High sampling rate
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Artifacts

• Correctly using the depth by weighted sampling

Artifacts resulting from an integral number of
samples
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Correctly integrating scene depth by weighting
the last sample
d
sampleWidth
FinalColor.rgb d/sampleWidth SampleColor.rgb
SampleColor.a (1.0 FinalColor.a) FinalColor.
a d/sampleWidth SampleColor.a (1.0
FinalColor.a)
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Combining techniques
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Conclusion

• Interactive volumetric effect simulation at
  reasonable grid resolutions is feasible for games
• We presented here a brief overview of the entire
  process
• More information
• NVIDIA DirectX10 SDK code sample
• Upcoming GPU Gems3 article

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Questions?