Submitted by , posted on 12 November 2004

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This is a screenshot of the visual system developed by Alsim for its flight trainers. I worked in this company eight months and one of the tasks was to design and implement some realistic volumetric clouds.

The method used is inspired from the Mark Harris and Niniane Wang papers. It consists of dividing a cloud into metaballs and represent them by billboards. In order to reduce the number of metaballs required to give the cloud a realistic appearance, the billboards are textured. A set of textures is generated thanks to Perlin Noise functions.

Also, to reduce the overdraw due to all those transparent billboards (125 per cloud), each cloud is rendered by an imposter (which itself uses a texture manager to avoid video memory fragmentation). On each frame, imposter's accuracy is tested in order to update it if needed. The number of imposters to update by frame rules the tolerance used in the test to keep this number low enough.

The color is computed by hardware light functions. Normal vectors depend both on the position of the metaball inside the cloud, the observer, the cloud, and the sun.

Finally, we can manage more than 200 clouds in real-time (at least 30 fps, on an Ati Radeon 9800 Pro powered PC). The user can fly around, and get in the clouds, resulting in realistic effects.

You can find more information on the Alsim website ( and on my personnal webpage (

Best regards,
Julien Guertault

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