Interactive Modelling and Rendering of Clouds Jesper Mosegaard , PhD - - PowerPoint PPT Presentation

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Interactive Modelling and Rendering of Clouds Jesper Mosegaard , PhD - - PowerPoint PPT Presentation

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Click to edit Master title style Interactive Modelling and Rendering of Clouds Jesper Mosegaard , PhD Head of Computer Graphics Lab Alexandra Institute Denmark Click to edit Master title style 19/03/15 Page 2 The Alexandra Institute Click

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Jesper Mosegaard, PhD Head of Computer Graphics Lab Alexandra Institute Denmark

Interactive Modelling and Rendering of Clouds

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19/03/15 Page 2

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  • Private not-for-profit company within IT

– Technology transfer from University research through GTS institutes (Danish model)

  • Application oriented research
  • Consultancy for companies

The Alexandra Institute

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Interdisciplinary collaboration

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Problem statement

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… even more difficult

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  • Simulating fluid dynamics is slow
  • Shaping through parameters is in-direct
  • Viewport does not represent final quality

Traditional workflow

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From https://vimeo.com/30189748

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External video

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  • User-guided procedural content generation with instant feedback to make even

the most advanced setups possible.

  • High-quality real-time rendering which allows for continuous visual quality

tweaking.

  • Less time spent waiting.

– More time for what really matters – being creative.

Taming the Elements

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This month tutorials featured in

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Elementacular.com

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Overview

Raymarch Voxelization Floodfilling Dilation & distance approximation Light Fields Raymarch User- controlled fractal evaluation Mesh Image Distance Field Noise Field Whispy Field

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Data Representation

Antoine Bouthors et. al.: ”Interactive multiple anisotropic scattering in clouds”

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Rasterization-based voxelization

"Octree-Based Sparse Voxelization Using The GPU Hardware Rasterizer" Cyril Crassin (NVIDIA), Simon Green (NVIDIA), in OpenGL Insights, July 2012

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Voxelization Results

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Voxelization Results

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Overview

Raymarch Voxelization Floodfilling Dilation & distance approximation Light Fields Raymarch User- controlled fractal evaluation Mesh Image Distance Field Noise Field Whispy Field

✔

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  • Voxelization produces voxels for each triangle fragment.

– In practice a lot of geometry is self-intersecting. – We need the outer voxels for distance field calculation

Why do we need to floodfill?

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  • Easy-to-use general-purpose compute.
  • OpenGL 4.3 feature.
  • Very powerful coupled with Image load/store.

– Read/write to 3D texture at arbitrary positions. – Perform atomic operations on storage.

Compute Shaders

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  • Two Compute Shaders

– tag initial outside voxels (if visible from all three axes) – Iteratively grow voxels outside.

Floodfilling on the GPU

Input mesh Thin voxelization Initial tagging Iterative growth

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My ¡Cloud ¡

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Thin ¡shell ¡voxeliza3on ¡

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Seed ¡floodfill ¡

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Itera3ve ¡floodfill ¡

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Itera3ve ¡floodfill ¡

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Itera3ve ¡floodfill ¡

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Itera3ve ¡floodfill ¡

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Itera3ve ¡floodfill ¡

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Removing ¡internal ¡voxels ¡

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Overview

Raymarch Voxelization Floodfilling Dilation & distance approximation Light Fields Raymarch User- controlled fractal evaluation Mesh Image Distance Field Noise Field Whispy Field

✔ ✔

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  • Single compute shader.
  • Propagate distance information from initial surface voxels:

– If voxel is tagged as outside and not part of the initial surface, pull minimum distance. – Combine using imageAtomicMin. – Complicated by the fact that atomic operations only work on 32bit integers.

Dilation and Distance Estimation

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Ini3al ¡distances ¡to ¡triangles ¡

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Distance ¡itera3on ¡1 ¡

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Distance ¡itera3on ¡2 ¡

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Map ¡to ¡densi3es ¡

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Overview

Raymarch Voxelization Floodfilling Dilation & distance approximation Light Fields Raymarch User- controlled fractal evaluation Mesh Image Distance Field Noise Field Whispy Field

✔ ✔ ✔

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  • Specialized compute shaders.
  • A major part of getting the look right.
  • Fractals based on GPU generated Simplex and Worley noise
  • Wispys based on gridless advection

Making Some Noise

Based on Siggraph 2011 Course “Production Volume Rendering Systems”.

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Making Some Noise

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Simplex ¡roughness ¡ Wispy ¡roughness ¡

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Overview

Raymarch Voxelization Floodfilling Dilation & distance approximation Light Fields Raymarch User- controlled fractal evaluation Mesh Image Distance Field Noise Field Whispy Field

✔ ✔ ✔ ✔

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Real-time single scattering

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  • Single- vs. multi-scatter of light in volumes

Lighting the Volumes

Multi-scatter (FLD) Combination Single-scatter

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Multi-scatter

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  • D. Koerner, J. Portsmouth, F. Sadlo, T. Ertl, B. Eberhardt:

Flux-Limited Diffusion for Multiple Scattering in Participating Media Computer Graphics Forum, vol. 33, no. 6, pp. 178-189, 2014.

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Overview

Raymarch Voxelization Floodfilling Dilation & distance approximation Light Fields Raymarch User- controlled fractal evaluation Mesh Image Whispy Field

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Distance Field Noise Field

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  • Naïve approach:
  • Prohibitively slow for real-time.

Real-time single scattering

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  • Exploit the fact that in-scattered irradiance can be separated from

primary ray absorption.

  • Update only when lights are dirtied.
  • March towards each light accumulating the irradiance into a separate

light buffer using compute shader.

  • When raymarching – Use light buffer instead of marching to light.

Real-time single scattering

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Overview

Raymarch Voxelization Floodfilling Dilation & distance approximation Light Fields Raymarch User- controlled fractal evaluation Mesh Image Distance Field Noise Field Whispy Field

✔ ✔ ✔ ✔ ✔ ✔

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  • Animation
  • Other plugins for real-time editing of natural effects
  • Integrate into later stages of the production pipeline

Future work

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Users are what makes this interesting

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Thank you for your attention

Mail: jesper.mosegaard@alexandra.dk Twitter: @mosegaard LinkedIn: linkedin.com/in/mosegaard Please complete the Presenter Evaluation sent to you by email or through the GTC Mobile App. Your feedback is important!