I see some very subtle use of blurs/blending modes or glow effects. That is, dupe the layer, knock the exposure down for the bottom layer a bit, blur the top layer and set it to add or screen (in a 32-bit color space, of course) and knock it’s opacity down. Or use something like Optical Glow from Red Giant or Deep Glow from aescripts + aeplugins or…whatever Sapphire’s glow is called from BorixFX – again, this would be after knocking the exposure back a bit and then lowering the power of the effect.
Personally, I use Magic Bullet Looks from Red Giant for a lot of stuff like this. You can emulate film stock, add subtle diffusion, add grain, and all kinds of handy things like that all in one GPU-accelerated effect. Most of it is stuff you could do with AE’s native tools, but it’s soooo much quicker to get to where I want to go with Looks.
Whatever way you’re doing it, you may need multiple versions of it. That is, use a couple of different diffusions at varying radii to really get it. Looking at your example, there looks to be a very subtle diffusion at a high radius and a somewhat more obvious (but still rather subtle) diffusion with a smaller radius.
There’s also work on the “shoulder” and “knee”. That is, nothing is pure black or pure white. Better to get this right in your 3d render (because you can’t really pull detail out of nothing), but you can enhance it even more with color grading.
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