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..::animating a static cloud skydome::..

There are several ways to animate a static cloud bitmap. Some are easy, some are not-so-easy. One harder way would be to overlay the bitmaps using the blue channel (hence the sky) of the bitmap as a transparent colour and creating an opacity map or alpha channel in a bitmap editing package such as Photoshop, overlay it on another cloud bitmap and offset the two over time, so there appears to be two layers of cloud travelling at different velocities.

One other way would be to use an amended cloud map as an opacity map for another animated shader effect such as smoke or noise. A more simpler and quicker way is to map the bitmap (preferably a panoramic cloud view) onto a sky-dome (a hemisphere with the normals flipped)

Using cylindrical mapping, select the top most vertex and using a selection method with intensity falloff, or similar, animate the vertex rotating a few degrees so the top of the sky-dome rotates and the translation fades off down the sides, ending in virtually no rotation at the 'horizon'.

Placing a camera the centre of the sky-dome viewing from ground level (or just slightly above), it will apprear that the clouds are all moving horizontally at the same speed, relative to their distance from the camera. However, this method will only be effective if the camera does not pan around too far to the sides or to the rear or the rotating effect will be visible and the illusion will be lost.

Enlarge ScreenshotA sky dome with a panoramic cloud texture applied
Enlarge ScreenshotWith selection falloff, animate the top vertex of the skydome rotating to give the illusion of moving clouds

Initially published: 3D World website, June 2000.

Copyright Pete Draper, June 2000. Reproduction without permission prohibited.

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