When FlightGear switched from PLIB to OSG the 3D clouds were temporarily lost. Currently efforts are going on to re-implement the clouds. As of October 2008 the clouds are back and committed to FlightGear CVS, and released with FlightGear 1.9.0 that December.
Cloud shapes and layers
Note: the cloud XML format in CVS is currently undergoing renovation to allow more control, and more natural looking clouds.
A 3D cloud layer is defined in the <layers> section. It contains of a number of randomly located <boxes>, containing a set of <clouds>.
Each cloud is generate with random size, based on the following tags:
<min-cloud-width-m> <max-cloud-width-m> <min-cloud-height-m> <max-cloud-height-m>
The cloud consists of <num-sprites> individual sprites. Each sprite is generated with random size, based on the following tags:
<min-sprite-width-m> <max-sprite-width-m> <min-sprite-height-m> <max-sprite-height-m>
The <texture> defines the texture file to be used for the sprite. The texture file may contain more than one texture in a grid, by setting the <num-textures-x> and <num-textures-y> tags.
Cloud are darker towards the bottom. bottom-shade indicates the shading factor of the bottom of the cloud compared with the top.
Finally, note that the clouds are defined as a set of properties, so can be modified within the simulator under /environment/cloud-layers. You can then see the effects of your changes by changing the clouds dialog, which reloads the clouds from properties.
Advice for those editing cloud textures:
- Change the transparency color to black in your image editing program. This makes it easier to see the white clouds, and in particular spot posterization. On GIMP this can be found under Preferences->Display
- Display a grid of the appropriate dimensions to ensure you don't draw clouds over the texture boundaries. Otherwise the clouds can bleed into each other and you can see nasty vertical and horizontal edges.
- Use a white airbrush and a smudge tool to make the cloud textures look random.