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| If you want to do it mathematically then it’s more complex than you think, as you need to consider the gamma of the workspace you’re using. Making something glow by stacking up multiple layers with different blurs is faking it, but probably the best way to do it in 8 or 16 bit modes. It’s the way i do it too, although I use the screen mode (not add), and I usually have a desaturated layer near the top set to classic colour dodge to adjust the “hotness”. From a mathematical perspective, you’re not just looking at the blur profile but also the gamma of the workspace so this multiple layer approach is going to be difficult to calculate. The alternative approach is to comp in 32 bit linear mode, where you don’t need to stack up multiple layers. You just have your source layer and make the colour super hot. Then you blur it and the blur naturally spreads out the light. The falloff will depend on the type of blur you use. I did ask a while ago (for exactly the same reason as you) if there’s a blur with an adjustable profile including inverse square, which would be very useful for comping glows in 32 bit float mode. I can’t remember the answer, but there’s something for you to look at. So basically - multiple layers using add/screen and blur is an 8/16 bit fudge for a gamma working space like sRGB/HDR. A single, super hot layer with a single blur in 32 bit linear float is the more natural approach. Or you can just use a plugin that does it, as lloyd suggests :-) -Chris
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