Title: Real-time%20realistic%20illumination%20and%20shading%20of%20stratiform%20clouds
1Real-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
2Motivations
- Clouds are very complex
- Complex shape
- Lots of visual features
- Lots of details
- No one has reproduced everything yet
3Cloud facts
4Clouds types
5Multiple 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)
6Mie phase function
- Strongly anisotropic
- Computationally expensive
- Depends on droplet size, temperature, wavelength
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1
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7Droplet size distribution
- One common phase function for the whole cloud
8Visual features
9Visual features
- Anisotropic forward scattering
- Diffusive multiple scattering
10Visual features
11Visual features
- Clouds-ground inter-reflections
Water sky
Ice blink
12Previous workOffline approaches
Musgrave et. al.
Musgrave
13Previous 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
14Our Model
15Overview
- What we assume
- Stratiform ? locally equivalent to a slab
- Homogeneous density
- What we want
- Realistic
- Real-time (GPU-friendly)
- Allow animation
16Overview
- Complex behavior
- Rays with different orders of scattering
- Our idea a study of the contribution of each
order
17Overview
- 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
18Overview
- 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
19Overview
- 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
20Overview
- 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
21Overview
- 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
22Clouds representation
- Clouds are stored as a height field
- Well fitted for stratiform clouds
- Lot of details in a small space
- Procedural animatable
23Phase function
- Our Modified-Mie model
- Strong narrow (lt5) forward scattering moved
into extinction function - Error negligible with multiple scattering
24Phase function
- Modified-Mie model validation
Monte-Carlo bench generated reflection BRDFs
25Single scattering
- Slab -gt Local analytical function
26Double scattering
- Local approximation by convolution
273 scattering
- Convolution bad idea for high orders
- Assume slab ? 1D problem ? column
283 scattering
- Assuming diffusion ? simple interaction between
column cells - Given scattering behavior of a cell ? analytical
solutionfor the column
293 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
303 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
31Sky ground contributions
- Previous valid for directional L,V
- Sky ground Assumed diffuse source
- 1D model used
- ? gives diffuse reflectance transmittance
32Ground-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)
33Ground-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
34Validation
- Validation of our 1D model
35Validation
- Comparison of our model with a Monte-Carlo
integration
5m-thick slab
1
Monte-Carlo bench generated reflection BRDFs
2
3
100m-thick slab
3
2
1
36Implementation
- 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
37Results
38Lighting contributionsbottom view
1 and 2 scattering
3 scattering
Ground illumination (incl. radiosity)
Sky illumination
39Summing it all bottom view
40Lighting contributionstop view
1 and 2 scattering
3 scattering
Ground illumination
Sky illumination
41Summing it all top view
42FeaturesGlory and fogbow
43FeaturesGlory and fogbow
44FeaturesWater sky
45FeaturesPseudo-specular reflection
46FeaturesPseudo-specular reflection
47Videos
48Conclusion
- 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
49Questions ?