I am working on military skyscraper thingy in the after effects and I am wondering do buildings cast shadows over them ?
Buildings cast shadows on clouds just like they cast shadows on the ground. If the building is between the light source (the sun) and anything, it will cast a shadow on that thing. Clouds are not a special exception.
Now, that being said, how it looks depend on the kind of lighting that's present and the thickness of the clouds. Here are some more reference images: Cities in the clouds - The Boston Globe Note how varied they are.
You have to make sure the light direction is correct. In your shot the light is coming from the upper right. I can't tell if it is supposed to be behind the building or in front of it. There is no reflection on the right side of the building so I am assuming the light is behind the plane of the building. This means that if there was any shadow on the clouds it would be extending down and to the left. It's going to look really fake unless the angles match.
Here's another consideration. It looks like the cloud image has a lot of diffused light and it's coming nearly directly from the right side. This tells me I want to see some highlights on the building. The diffused light would cast a very soft shadow and you'll have to take into consideration the shadows already in the clouds and the glow from the parts of the clouds that are receiving the light.
Depending on how the shot moves I might not worry as much about the shadow as I would about the highlights and reflections on the building. Look for some sample images. Here's one that I found that is close to what you are trying to do. There's not much shadow, but you can really see the light on the building.
With all of the haze in your cloud shot I don't think the lack of a shadow would give away the composite as much as the mis matched light on the building. If you do put one in it needs to be very subtle.
Thank you for replying , that was very helpful.
That was a great demonstration, It is very hard to get the right lighting with AE , but I will give it a shoot.
Thank you for sharing your knowledge.