Animating Layer Order

When a group layer is created, you can access the Depth Sort panel in the Layer Settings dialog box by double-clicking the layer in the Layers palette. The Depth Sort panel, shown in Figure 23.5, includes options for specifying how the sublayers within the group are sorted. The options of sorting layers by depth and sorting layers by distance are described in Chapter 25, “Changing the View with Cameras.”

The third option, Enable Animated Layer Order, lets you change the layer order of the group’s sublayers as the animation progresses. The default layer behavior is that the layer at the top of the Layers palette obscures all overlapping layers beneath it, but when the Enable Animated Layer Order option is enabled in the Depth Sort panel of the group layer, you can set a key for changing layer order by dragging a group sublayer above or below another layer.

For example, imagine a character that is standing with its arms in front of it. If the arms are on a different layer, then having the character move its arms so they are positioned behind the character would be possible by changing the layer order as the arms move.

To animate the change in layer order, follow these steps:

1.	Open the Satellite orbiting Earth.anme file from the Chapter 23 folder on the CD. This file includes a large Earth object on one layer and a satellite object on another layer.
2.	In the Layers palette, click the New Layer button and select the Group option from the pop-up menu. Open the Layer Settings dialog box and select the Depth Sort panel. Then enable the Enable Animated Layer Order option and close the Layer Settings dialog box.
3.	Select and drag both layers onto the group layer. Position the Satellite layer so it is initially above the Earth layer.
4.	Set the current frame in the Timeline palette to frame 5; then use the Translate Layer (1) tool to drag the satellite until it is centered over the Earth. Select the Scale Layer (2) tool and drag slightly to increase the size of the satellite.
5.	Set the current frame in the Timeline palette to frame 10 and drag the satellite with the Translate Layer tool to the right of the Earth. Select the Scale Layer key located at frame 0 and copy the key and paste it to frame 10 using the Copy and Paste button at the top of the Timeline palette.
6.	With the satellite located to the right of the Earth, select and drag the Satellite sublayer in the Layers palette below the Earth layer to change the layer order.
7.	Set the current frame to frame 20 and then copy the Translate Layer key at frame 0 to frame 20.
8.	Set the end frame in the frame range to frame 20 and click the Play button to see the satellite orbiting the Earth, as shown in Figure 23.6. Figure 23.6. Satellite orbiting the Earth. [View full size image]