Having finished the tracking, press the Apply button to transfer the position, rotation and scale information to our Null Object. Dimensions should be both X and Y (Fig.38 & Fig.39).
Try playing the animation to check whether the Null is now fixed to the horizon (Fig.40).
The final piece of the tracking puzzle is to scrub to the frame where you aligned the sky horizon with the city horizon and, once there, parent the sky layers to the Null. Parenting in After Effects is similar to linking in 3ds Max, meaning the parented layers will follow their parent. To parent both sky layers to the Null, select them and drag the little spiral icon on one of the layers to the Null Object (Fig.41).
Play the animation again and make sure the sky stays within the frame at all times. If it doesn't - move the sky layers around in the viewer until they do. Congratulations! You have successfully tracked a shot in After Effects! You can now go ahead and animate the sky layers if you want.
Step 9: Blur, grain and pre-comps
We're definitely getting somewhere now and to round off the shot we'll add some depth of field, blur and a bit of grain. All three effects helps us to hide the fact that the buildings are a bit low-poly to be in the foreground and that the sky and city don't blend perfectly yet.
Step 9.1: Depth of field
As depth of field is an effect we want applied to the whole image, we'll first create a new Adjustment Layer and make it the top layer. I'll name it Depth of field. We'll also drag in a new Z-Depth layer and make it the bottom layer. Having done that, right-click the newly created Adjustment Layer, choose Effect, Blur & Sharpen and Lens Blur (Fig.42).
The image instantly blurs all over. In the Depth Map Layer select the bottom Z-Depth layer. Also check Repeat Edge Pixels to get rid of the dark border surrounding the frame (Fig.43).
Use Blur Focal Distance to focus and Iris Radius to control how much blur you apply. In this example I set Blur Focal Distance to 215 and Iris Radius to 25 (Fig.44).
This leaves us with one problem though, as the sky gets blurred way too much. To fix this, we have to tweak the Z-Depth layer we just added a little bit. Select the bottom Z-Depth layer and in the menu bar go to Layer and Pre-Compose (Fig.45).