blue screen

[nejko]

Hello, I have 2 questions. first, I brought a video of couple mountains to do a fly over, behind the mountains I use a blue scree, so my first question is, when I switch the blue for the sky, I dont know what kind of motion should I give the sky to make it look real, I imagine it should be slower than the forground.
2. Well, as wi know it is video (2D) if I want to apply some depth to it, how can I do it, what would be the best way, cause the 3d fog only works when having a comp. on the z axis.

thanks

Fabio