Hello, I'm trying to render the lightning of clouds. I'm using a 2D method of ray tracing for a cloud texture in a fragment shader. First what is added int the sum ? Second when you have done the sum for the scattering effect thanks to the height map, how do you compute it in order to get the correct color ? Thanks for reading me. Sébastien.

I suggest you read:

Horng-Shyang Liao, Tan-Chi Ho, Jung-Hong Chuang, Cheng-Chung Lin. Fast rendering of dynamic clouds. In: Computers & Graphics 29 2005; 29-40

Dobashi Y, Kaneda K, Yamashita H, Okita T, Nishita
T. A simple, efficient method for realistic animation of
clouds. In: Proceedings of the SIG-GRAPH’00, 2000.
p. 19–28.

They are rather in-depth though but very useful.

(Edit: If you have trouble finding them, PM me. I might be able to get them somehow...)