A different approach would be to assign a Pass Index to the engine objects in the Objects and Links panel of the Object buttons. Make sure to enable the Index pass in the RenderLayers panel. Then you pipe that Index pass through an ID Mask node, which can be found under converters. Set the ID in this node to match the Pass Index you set for the engines, and the node will output an Alpha channel that is just the engine objects!
You don’t have to mess around with duplicating objects and fooling with materials.
From there, I would make a blurred version of the mask, invert it, then subtract the original mask from it (this will give you a blurred area around the engine, but with the engine itself knocked out of it).
You can use that final mask as the Factor for an RGB curves node set to brighten, or however you had decided to “glow” things.
Bumped into some problems
I tried both ways of subtracting (original minus new, new minus original, although I don’t know which one is which one :D), but I don’t get the blurred are with the engine knocked out, as you describe it.
With your method, I will get the engines glowing, but to be honest (sorry for my lack of clarity) I wanted to make the yellow stuff insíde the engines glow. I did these with halo’s, which don’t seem to work with an index pass… Or am I missing something?
Thanks!
Edit: First image, top mask is the original mask, the 2nd & 3rd are the substractions.
Second image: The ship. I hope this makes clear what I want to glow.
Sorry for getting to you on this so late.
Here’s my advice, don’t put them on two diferent layers, just one.
You have to seperate the part of you enginve that you want to “glow” as a seperate object, not the engive it’s self, but the part that you want to emit the glow (in this case your yellow part).
Set the ID idex to 1 or 2(every object created is automatically set to 0).
In the node ditor, do as harkyman said, but the one thing he left out is that you will have to combine your output from your ID index with you original rendered image.
The way you do this is you add a Color Mix node and pipe your rendered image and your ID index output to this, it will mix the two, the setting that you probably want to give it is Screen, or Add, Mix doesn’t really help it much.
It will look dark when you do this, but turn up the factor to 1.00.
Hope that helps, if you want i can create a mini tutorial if you don’t understand.