SIlverLining is a C++ SDK for OpenGL and DirectX developers for rendering the sky and everything in it, with support for popular game engines and scene graphs built in.

Real-Time 3D Clouds

SilverLining™ provides visual simulation of the following cloud types:

• Cumulus Congestus
• Cumulus Mediocris
• Cumulonimbus (with lightning)
• Stratus (solid and broken)
• Cirrus
• Cirrocumulus
• Stratocumulus

With SilverLining™ , you can add realistic 3D clouds to your outdoor scenes - while still maintaining real-time framerates. These are truly 3D clouds that may be flown around and through with accurate effects and lighting.

Typical scenes featuring dense cloud decks of cumulus clouds will run at 60 frames per second on commodity hardware. For the most performance-conscious applications, SilverLining™ is highly configurable to allow you to make your own decisions about how to best trade off rendering time and quality.

Unlike other 3D cloud rendering solutions, our clouds are procedurally generated. That means that instead of seeing copies of the same clouds over and over again, every cloud is different, every time you generate a scene. This leads to enhanced realism.

When initializing a scene, a cloud deck is "grown" from scratch using physical principles that govern cloud development in the real world.

Not only are individual clouds formed in this manner, but the size and distribution of the clouds is also governed by a model based on a survey of observations of the properties of cumulus clouds at various locations around the world.

Lightning and lighting effects

Our clouds are integrated with SilverLining™ 's outdoor lighting simulation in order to provide cloud lighting with a physical realism that has never before been seen in real-time rendering applications.

Starting with the actual spectrum and luminance of the sun itself, SilverLining™ models the passage of the sun's rays through Earth's atmosphere for any given simulated environment.

Then, using concepts from prior research at the Jet Propulsion Laboratory, we model the scattering of this light through and within each cloud as it finds its way to the scene's camera location - using real empirical data about the actual size and density of the droplets that make up a given cloud type.

The result is stunningly realistic clouds at dawn, daytime, dusk, night, or any time in between.

Cumulonimbus clouds also incorporate lightning effects. As with the clouds themselves, the lightning is procedurally generated for added realism. Lightning illuminates the parent cloud and have animation effects to simulate the flickering due to return strokes.

Stratus cloud decks have thickness and a configurable "scud" layer above and below them. They accurately influence lighting when the viewpoint is below the cloud deck based on our own extension of the CIE overcast sky model.

Stratus decks may also be configured to be "broken" - this provides a visual approximation to scattered clouds with extremely high performance characteristics.

Fog effects inside a stratus cloud will honor the "holes" of a broken cloud deck - that is, you can fly right through a clear patch with no visibility impact.

Simulation of the Sky

SilverLining™ also provides a visual simulation of the sky itself. This is no simple gradient of blue drawn in the background - this is based on a real-time physical simulation that starts with NASA data of the solar spectrum, astronomical computations of the solar position relative to the local horizon, and simulates the scattering of this light through the Earth's atmosphere as it makes its way through the molecules and particles of the air.

This is done using a highly accurate clear-sky model developed by the National Renewable Energy Laboratory adapted for real-time use, in conjuction with previously published work on sky luminance distribution.

The result: accurate skies for any time of day, for any location on the planet. Sunsets and sunrises with red horizons. Illuminated full-moon nights. Long shadows in the winter. It all adds up to increased realism for your outdoor scenes.

How do we do all this computation in real-time? Much of the calculation is offloaded to your 3D graphics card using custom shaders written to compute the sky color at any given direction in the sky. This allows SilverLining™ to continue its calculations while the sky is simulated in parallel using the processor on your video card.

There are more things in the sky than just the sun. SilverLining™ also accurately computes positions of visible stars and planets for nighttime scenes, and renders them accurately according to their stellar magnitude.

The moon also acts as a light source in SilverLining™, and its position and phase is simulated and affects your outdoor scenes as well. On dark nights, especially bright planets or stars are also subjected to a physically-based glare simulation, which is also offloaded to your graphics card as a shader.