# Extract particle age info as render layer

[.blend file here]

## The problem

My problem is related to node editor and multiple render layers, as far as I'm concerned. I have a simple particle emitter and an icosphere serving as a single particle object. Thanks to the Particle Info node I can easily get the info about particle age. But what I want is to render some kind of alpha channel map (or B&W bitmap) based on particle age info, where only particles of certain age are visible. The math part is not a problem for me. I can combine some math nodes to filter out the value I need. The only problem is how to extract this to the compositor (in parallel with the rest of the scene)?

My current node stack for the particle object looks like that:

## The "what I'm trying to achieve" part

My big goal is to render an animation of welding sparkles.

I watched a few videos of some real ones and noticed that most sparkles at the end of their life usually emit something like a small glare. Here's an example: YouTube video

If you pause the video you'll notice that some sparkles at the end of their life emit a 'subset' of smaller sparkles for a fraction of a second. I want to emulate this with a glare node but unfortunately it is only available in the compositor. What I'm missing is a separate render layer (aside from the whole scene) which consists only those sparkles which are about to die.

## Completely different approach

Maybe I'm doing everything wrong and maybe there are some other ways to achieve such effect? Please let me know if you have better ideas.

First create a dummy object and give it a new material and call it something like particle_age. Just plug the Particle Age output into an Emission shader and plug that into the output. Also make sure to give it a Fake User.