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I want to spend a little bit of time explaining this concept of a shadow catcher, because it's pretty cool, but not entirely obvious. Now, I will go ahead and delete all my other layers in this scene and keep just my tracked footage. Press F3 to bring the Camera Tracker back forward again. If I created text in this scene--let's say it was supposed to be floating in space in front of that building-- ideally, we'd like that text to cast shadows onto other objects in the 2D scene. However, 3D layers in After Effects cannot cast shadows onto 2D layers, and this tracked footage is, after all, a 2D layer. This is where shadow catchers come in.
First, let's create some blank text. I will right-click, Create Text and Camera, and I will just get a dummy word Text there. As with solids, it's slightly askew. I will press R to get its rotation, unskew it a little bit from the building so it's kind of oriented right, and let's move it down the building a bit, to, say, right around there. Now, let's create a special layer that will catch the shadow projected by that text. I will select the tracked footage again, select the effect, choose a good triangle, this looks like it's on the face of the building, click to define the plane, right-click, and this time say Create Shadow Catcher and Light.
In addition to creating a light, this will create a solid, placed at this location of building, with some special settings. I will select that option, choose the Shadow Catcher, and type AA to reveal its Material Options. After Effects CS6 has tweaked some of these Material Options a little bit, and in particular, there is this Accepts Shadows parameter that can now be set to Only. Previously, this was Accept Shadows Off or On, and what you had to do to create a Shadow Catcher is make this all light layer, turn Accepts Lights off, and set it to Multiply mode to properly blend it into the building.
Well, in this case, with After Effects CS6 they now have this Only mode, which means it only will render a shadow; it won't render the rest of the layer. I am going to type R and straighten up this layer, just like I had to the text, and type S to reveal Scale, and scale it down so it's just the face of this building. Indeed, I think I need to scoot it over a little bit here in the X dimension along the face of the building so it only occupies that surface.
R for rotation, rotate and line up. Okay the Show Catcher is in place. The other tweak in CS6 is that whenever the 3D Camera Tracker creates a layer, it automatically turns on this Cast Shadows option. This normally defaults to off for 3D layers, but the 3D Camera Tracker is thinking ahead and thinking you might want shadows. Let's go ahead and switch this to 2 Views - Horizontal so we can see what we are doing. This is just for comp background colors, this is pink.
Now say View > Look at All Layers. There is my camera moving through this scene. Here is my text and Shadow Catcher. Here's my light. Let's move this light over to where it's more or less face-on with the text. Then select the text and pull it away from the face of the building. As I do so, now you will see the shadow from that text cast onto the face of the building just, where our Shadow Catcher layer is. If I have it misaligned, just see it cuts off the shadow. So you need to make sure it's properly positioned to catch that shadow, and I will carefully drag it up on Y so we don't lose the top of T.
Shadow parameters like Darkness and Diffusion are set by the Lights parameters. I will type AA. I can reduce the Show Darkness, increase it. It's already a pretty sharp shadow, but I could make it softer if I wanted to. If you need a sharper shadow, remember, that it's still underneath Composition Settings > Advanced, Classic 3D Renderer > Options, and set a higher value for the shadow buffer, and now you get a much sharper shadow.
Although this greatly automates the process of setting up a shadow catcher, you do need to do some work to make this scene completely realistic. For example, I should also set up a Shadow Catcher on this side of the building, and I should also set up a Shadow Catcher on this building back here, and on his pylon back here, et cetera, et cetera. However, you can see how this goes a long ways towards making this scene just a lot more realistic, and it's a nice touch that they put it in After Effects CS6.
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