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Some experiments with realtime water & erosion simulation on heightfield terrains, using OpenGL 4.3.

Technology, How stuff works

Basics

• C++11 (using mainly Visual Studio 2013)
• OpenGL 4.3

Dependencies

• trunk version of ezEngine - I also taking a minor part in its development (File IO, Memory Management, Basic Math, Window creation, Input, ...)
• GLEW (OpenGL function wrangler)
• AntTweakBar (DropIn solution for tweak variables)

Simulation

• The most of the sources were already GPU orientated, but almost none of them used the features of modern hardware. I rearranged nearly everything and make very often use of Compute Shader, Indirect Draw and other modern features.
• Simulation in fixed timesteps - each frame the simulation steps n times, number of steps per second is constant.
• Much FP32 :( - everything else limits the simulation or makes it explode

Water Simulation

Heightfield water: "Column base Water simulation" Water in column, each column connected to 4 neighbors with "virtual pipes". Outgoing flow in each of the 4 directions is saved per heightfield-pixel.

Idea relatively good explained and why the outgoing-flow thing is so important can be found here. While being not very detailed Theses slides can give a very good overview what this is all about.

• Outgoing flow RGBA32F
• Two passes atm: Compute outgoing flow, Apply flow
• Output flow map on the fly in second pass (RG16f)

Erosion Simulation

(todo: Short explanation, give some references)

Rendering

• Simple Forward renderer
• Gamma correct Rendering - it is really simple, try it if you haven't already!
• The definite Fog
• One directional light atm

Terrain Rendering

• Texture Blending
• Fresnel is great everywhere!
• Screenspace Tesselation inspired by this. Solved crack problem with triangle patches instead, no fine displacement map as described in the paper. Uses instancing, only 3 draw calls (one per Patch - full, one transition, two transisions)

Water Rendering

• Reuses grid from terrain. Almost the same geometry setup, but cull patches under terrain in Control shader
• tossed informations mainly from this article and from (this thread)[http://www.gamedev.net/topic/527145-rendering-natural-waters-coefficients-conversion/] around until I something came out I was confident with
• Flow from flowmap
• Some Foam for fast water (also influenced by flow map)

Sky Rendering

Cheap gradient rendered with fullscreen triangle after everything else (todo)

Notes on Code

• Using some fancy C++11 feature
• Renderer is based on some gl wrapper classes that are not meant to abstract OpenGL but just to make things easier

Todo/Planned Features

(List not complete, mostly a memory aid to my self)

Handling

• Add/Remove water with mouse

Rendering

• Athmospheric Scattering
• Compute Irradiance Spherical Harmonic from Skybox to light water and terrain
• Grass
• Culling
• Other Vegetation?
• Clouds
• Rain

Simulation

• Shared Memory Optimizations - there is much to gain!
• Fancy erosion stuff
• Wind
• Rain/Evaporation
• Water Spray?

Misc

• Moar sliders - make more stuff tweakable, still many magic numbers in code
• Less sliders - make parameters more interdependant, compute stuff; make things tweakable the EASY way
• More OpenGL wrapper stuff - especially these Samplers are annoying!