I am making an animation that has two render layers RL1 and RL2. Frames 1-10 contains only RL1, Frames 11-20 contains a mixture of RL1 and RL2, Frames 21-30 contains only RL2.
In order to save time I have animated the check mark (see picture) of the RL1 and RL2 so that they are only checked in frames 1-20 and 11-30 respectively.
The compositing nodes are shown as the picture below. Frames 1-20 outputs what I want. However, between frames 21-30 the output becomes black as RL1 doesn't output an image.
Are there any neat ways to ignore a render layer with no output?
This link is the closest answer I've found: "How to stop rendering a renderlayer at any moment of the animation?" However, as gandalf3 admits, the method doesn't work if the render layers contain alpha values