trouble combining render layers

I am doing a project in which a rocket ship flies over an alien world with a lake filled with lava. The ship also has smoke coming out the back.

I have done divided my scene into four render layers; one for the rocket, one for the lava, one for the smoke, and one for the background. All other necessary compositing operations have been carried out. I have made the lava glow, the smoke look like smoke, and the rocket look blurred. All I need to do now is combine these layers together into a finished output. This is where the trouble started. I was able to combine the rocket and the background alright, but when I try to combine the lava with the background or the smoke with the background, nothing happens. I have tried the Alpha Over node, the Z Combine node, and even the Add node, all with no result. What could I be doing wrong?


You need to provide more information than that. Even example .blend file would be preferably.

Generally you should use Z combine only if the layers to be combined overlap in Z direction. Otherwise you can get away with alpha over or even mix. If you decide to use mix, feed the alpha mask of the image to be combined on some other to fac (factor). In this case the background image is provided to first image input and the image that’s supposed to become on top of it to latter.

It may be nicer to use alpha over though. You can find an example of how to use it at .

Sadly, my file is to large to upload. However, I will try to provide more info. In the below image you can see that I have rendered each of the layers separately, done post production on those layers, and then grouped the post pro for each layer. Now I simply need to combine them all and that is where the trouble is. Hope that was enough info. If you need more, please ask.


When using alpha over nodes, remember that the image attached to the top image input is the background and thus the image attached to the bottom input in the foreground object. You will have to keep this in mind as you cascade all of your layers together.