Compositing render layers causes emission to become black

It’s as the title says. I figured out when compositing my render layers that to get the alpha to work, I had to separate RGBA and plug the alpha into the “alpha over” node, but now I have this problem. The materials which are emitters are black, more or less. I have pretty pictures to explain.

In the image above, the right hand side “hoverpack” is what it is supposed to look like. I’ve bypassed all the color add/multiply nodes and directly connected the combined channel to the alpha over mix.

Here, I’ve composited all the render layers together, and then plugged it in to the alpha over, along with the separated alpha channel for the factor. All the emission (the blue glowy stuff) has gone black.

Has anybody seen this before? I’m sure it’s not too hard to fix; it might be because this is my first time using the compositor that I don’t know how to fix this. I searched for the problem here, but maybe my keyword usage isn’t that great either. Let me know if I haven’t made something clear.

Could you post a .blend for us to fiddle with? Maybe you can simplify your project to just one emit surface and one non-emit surface that shows the same problems. It’s easier to work it that way with blenderartists, in my opinion.

As you suggested, I am currently making a simple scene to work with. However, I just noticed in the “passes” section, there is a checkbox for emission. Do you think this might the the fix? I didn’t check it earlier, so I guess there is no emission pass.

Yep, that will probably do it. Use a screen or add mode to lay it in.

So, I got it, thanks to you. I added the emission pass and plugged it in, and it worked. If you hadn’t made me go look at the passes for that simple scene, I wouldn’t have found the emission pass. Thanks so much!