Real-time%20realistic%20illumination%20and%20shading%20of%20stratiform%20clouds

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Real-time realistic illumination and shading of
stratiform clouds

• Antoine Bouthors, Fabrice Neyret, Sylvain
  Lefebvre
• Evasion-GRAVIR / IMAG-INRIA
• Grenoble, France
• Eurographics Workshop on Natural Phenomena

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Motivations

• Clouds are very complex
• Complex shape
• Lots of visual features
• Lots of details
• No one has reproduced everything yet

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Cloud facts
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Clouds types
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Multiple scattering

• No absorption, albedo 1
• Thick (up to several km)
• Dense (Mean free path several meters)
• ? Very high number of scattering events (up to
  several hundreds)

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Mie phase function

• Strongly anisotropic
• Computationally expensive

• Depends on droplet size, temperature, wavelength

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1
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Droplet size distribution

• One common phase function for the whole cloud

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Visual features

• Fogbow glory

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Visual features

• Anisotropic forward scattering
• Diffusive multiple scattering

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Visual features

• Pseudo-specular effect

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Visual features

• Clouds-ground inter-reflections

Water sky
Ice blink
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Previous workOffline approaches
Musgrave et. al.
Musgrave
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Previous workInteractive approaches

• Different approximations
• Low albedo / low density ? single scattering
• Simpler phase function ? not all features
• Diffusion approximation ? isotropic
• Volumetric models
• Very coarse
• Billboards or slices
• Lots of overdraw

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Our Model
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Overview

• What we assume
• Stratiform ? locally equivalent to a slab
• Homogeneous density
• What we want
• Realistic
• Real-time (GPU-friendly)
• Allow animation

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Overview

• Complex behavior
• Rays with different orders of scattering
• Our idea a study of the contribution of each
  order

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Overview

• Our analysis
• Narrow forward scattering only important on
  silhouettes (low orders)
• Low orders of scattering give anisotropic
  features (fogbow, glory, pseudo-specular)
• Higher orders become diffusive (i.e. isotropic)
• Sky and ground play a role
• Sky illuminance 25 of that of the sun
• Standard floor reflectance 20

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Overview

• Our analysis
• Narrow forward scattering only important on
  silhouettes (low orders)
• Low orders of scattering give anisotropic
  features (fogbow, glory, pseudo-specular)
• Higher orders become diffusive (i.e. isotropic)
• Sky and ground play a role
• Sky illuminance 25 of that of the sun
• Standard floor reflectance 20

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Overview

• Our analysis
• Narrow forward scattering only important on
  silhouettes (low orders)
• Low orders of scattering give anisotropic
  features (fogbow, glory, pseudo-specular)
• Higher orders become diffusive (i.e. isotropic)
• Sky and ground play a role
• Sky illuminance 25 of that of the sun
• Standard floor reflectance 20

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Overview

• Our analysis
• Narrow forward scattering only important on
  silhouettes (low orders)
• Low orders of scattering give anisotropic
  features (fogbow, glory, pseudo-specular)
• Higher orders become diffusive (i.e. isotropic)
• Sky and ground play a role
• Sky illuminance 25 of that of the sun
• Standard floor reflectance 20

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Overview

• Scattering
• Strong forward scattering special treatment
• 12 orders analytic
• Higher orders (3) considered isotropic
• Environment
• Take sky into account (diffuse source)
• Clouds-ground radiosity (plane parallel)
• Shape
• Height field

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Clouds representation

• Clouds are stored as a height field
• Well fitted for stratiform clouds
• Lot of details in a small space
• Procedural animatable

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Phase function

• Our Modified-Mie model
• Strong narrow (lt5) forward scattering moved
  into extinction function
• Error negligible with multiple scattering

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Phase function

• Modified-Mie model validation

Monte-Carlo bench generated reflection BRDFs
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Single scattering

• Slab -gt Local analytical function

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Double scattering

• Local approximation by convolution

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3 scattering

• Convolution bad idea for high orders
• Assume slab ? 1D problem ? column

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3 scattering

• Assuming diffusion ? simple interaction between
  column cells
• Given scattering behavior of a cell ? analytical
  solutionfor the column

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3 scattering

• How to know the multiple scattering behavior of
  one cell ?
• Characteristic of the  cloud material 
• Precompute Monte Carlo integration ? reflectance
  transmittance of a cell

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3 scattering

• Issue
• Diffuse hypothesis broken on top cellanisotropy
  still plays a role
• Solution
• 1D model correction term dependant on the sun
  incident angle
• Correction parameters fitted on Monte Carlo
  simulations

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Sky ground contributions

• Previous valid for directional L,V
• Sky ground Assumed diffuse source
• 1D model used
• ? gives diffuse reflectance transmittance

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Ground-clouds inter-reflections

• Radiosity between two facing parallel planes ?
  known form factors (analytic)
• Heterogeneous source (clear sky cloud bottom)
• Heterogenous reflectance (ground AND clouds)
• Ecloud (Sunsky)T Si(Rcloud_i FF Eground_i)
• Eground (Sunsky)R Si(Rground_i FF Ecloud_i)

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Ground-clouds inter-reflections

• Reflectances and radiosities in textures
• Plane-plane ? Form Factor (ring to dS)
• ? Using MIP-mapping
• GPU-enhanced iterative algorithm
• Render-to-texture
• Hardware MIP-mapping

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Validation

• Validation of our 1D model

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Validation

• Comparison of our model with a Monte-Carlo
  integration

5m-thick slab
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Monte-Carlo bench generated reflection BRDFs
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3
100m-thick slab
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2
1
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Implementation

• Height field advected textures Ney03
• 16km-wide landscape
• 2km-height, 500m-thick cloud layer
• Shaders radiosity on GPU
• 512x512 clouds textures
• 512x512 ground textures
• 512x512 shadows textures
• 16x16 radiosity textures
• 18 to 40 FPS on current hardware

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Results
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Lighting contributionsbottom view
1 and 2 scattering
3 scattering
Ground illumination (incl. radiosity)
Sky illumination
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Summing it all bottom view
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Lighting contributionstop view
1 and 2 scattering
3 scattering
Ground illumination
Sky illumination
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Summing it all top view
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FeaturesGlory and fogbow
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FeaturesGlory and fogbow
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FeaturesWater sky
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FeaturesPseudo-specular reflection
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FeaturesPseudo-specular reflection
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Videos

• Videos

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Conclusion

• Good
• Realistic
• Reproduce all clouds visual features
• Account for clouds-ground inter-reflections
• Real-time
• Animation-friendly
• Bad
• Poor lateral shading (clouds borders)
• 3 scattering can be improved
• Limited to stratiform clouds

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Questions ?