Help with a shadow effect

If you have a good understanding of AE's 3D space and the material options for 3D layers this is fairly simple. There are a few things you should know. First, perspective is controlled by camera position and framing by focal length. Second layers can cast shadows only and receive shadows only. Third, It's fairly easy to match the perspective on a shot if you can make a good guess at the focal length of the shot. And finally, if you want shadows to be cast on every surface of a building you have to find a shot where the sun is shining on all surfaces. Here's how I would set things up using an Adobe Stock image that I found here: 237405344

First I add the image to the timeline then create a new solid that is a little smaller than the layer. I'll start with a Red solid, add the Grid effect to it, make it 3D, set the blend mode to Overlay so I can see through it, then move the anchor point to the left edge and line it up with some prominent geometry close to the center of the image. I then rotate the image in Y so that it comes close to matching the perspective in the building. Now I add a Camera and estimate the focal length of the lens. I'm guessing this shot was taken with about an 80MM lens. A little manipulation of the camera position and the Y rotation of the layer will give you the first surface you are going to use to catch a shadow. Your comp would look something like this:

The next step is to start duplicating the solid layers, adjusting the rotation, moving the anchor point, and changing position until all of the surfaces that need a shadow are covered. That looks something like this:

The next step is to scale up the layers as needed and add masks if needed until you have everyplace on the building that needs to catch a shadow covered. Now your comp looks something like this:

Now you open up the layer settings from the menu and change the color to white. delete the Grid effect from each layer, select all the wall layers and press the A key twice to reveal the material options and set all layers to Receive Shadows Only. They will now disappear. 

The final step is to enable multiple views so you can arrange 3D layers. Create a spaceship layer (I used a simple shape layer) make it 3D, press 'aa' to reveal material options, and set the layer to Cast Shadows Only. Then position a spotlight if you want soft shadows or a parallel light at about the same angle you think the sun is coming from, animate the position size and scale of the space ship layer and make the final adjustments. You end up with a comp that looks like this:

You end up with something like this:

The project took about 15 minutes. The closer you get to matching the perspective of the building and the closer you get to matching the position of the sun, the more realistic your composite will look. Mine needs a few more minutes of tweaking. Feel free to download a sample of the Stock Image and play with this project file. 

One of these days I'll put together a really good tutorial on how to do this kind of composite using video footage with a moving camera.

Certainly a tougher one than just a basic shadow.

You'll need to consider that the shadow is going to land on different parts of your photo at different times depending on the angle of the light.  They're going to move across the ground plane differently than they will when they move up the sides of your building in your photo.

You can do this with isolated, masked areas or you could build some 3D layers to capture the shadows.

You might be able to create a gradient map to time out where the darker color correction will go.

Depending on your photo - you'll find one of these that works the best.

The key is: don't think of the building/shadow catcher as a planar surface.  It's got a bunch of sides (especially if you think about the details on the building).

You can post images with your questions! 😉