Creating a Drop Shadow

To make the robot really fit into the scene, it needs a drop shadow. You will make one by using a Brightness node to darken the background but limit the adjustment only to the area of a supplied shadow mask.

1.	Insert a Brightness node from the Color tab between the Resize1 and Over2 nodes. Do I really need to explain what the Brightness node does? I sure hope not.
2.	Drag a connecting line from the output of the shadow clip to the right side of the Brightness1 node. An M appears on the side of the node. This is the Mask input.
3.	In the Brightness1 node, click the + to the left of the Mask control to expand the Mask parameters. You can apply a mask to any operation by dragging the output of a node to the right side of a second node. That node will process only within the white areas of the channel you specify. When a node has a mask applied to it, new parameters will appear in the Parameters workspace, letting you activate the mask, invert the mask, and control the mask's strength. The other way to apply a mask to a node is through the Mask Create function. When you click the RotoShape pop-up menu next to the Mask Create button, you can choose from six mask-creation options that will automatically create a node and connect it to the Mask input of the node you are editing. Because the maskChannel parameter is set to use the alpha channel by default, and the shadow clip has values only in the RGB channels, you'll need to select a different channel.
4.	Click G for the maskChannel parameter. The Brightness1 node will use this channel when creating the drop shadow.
5.	Set the Brightness1 Value parameter to 0.25. The entire screen darkens except for the shadow area because, by default, the mask settings process only within the white areas of the channel you specify. The mask should be reversed.
6.	Activate invertMask. The shadow now looks correct.