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I understand emission gives your objects a glow and if mapped, it can glow any color you want with any intensity you want.

Bloom seems to do the same thing to your base color except it's much easier. You don't need to create an emission map, just turn the bloom on and you seem to get the same results.

enter image description here

enter image description here

The one with the Bloom is a little brighter. I don't understand is this a stacked glow effect? Emission is already suppose to glow, but Bloom gives it even more glow. So I guess when you're making the Sun, you should have both on max?

Also is there a point in having a base color if you're using Emission?

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    $\begingroup$ Emission doesn't glow, only Bloom gives this effect (or Compositor) $\endgroup$ – moonboots Dec 31 '20 at 21:40
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Bloom and emission are two completely different things. Let's talk about Eevee, since that's what you're probably using (since implementing bloom in Cycles requires the use of compositing, with behavior that depends on the precise way that you implement the bloom.)

Emission just says, write this color to the screen. That's all. It doesn't get lighted/shadowed. It doesn't care about the surface normals.

Now, what happens to the color you tell it to write to the screen depends on your color management settings (and a few other things.) Let's say you're using a raw color transform. What happens when you write a color like 2.0, 1.0, 1.0? Your monitor is designed to only handle colors in the 0,1 range. So it clamps the output, and even though 2,1,1 is a pink hue, you still get white output. Because once you clamp the output, you just have 1,1,1 white.

Except, you're unlikely to be using a raw color transform. Standard, filmic, etc are designed to tone map colors outside of the 0,1 range into colors inside the 0,1 range. (This comes at the cost of some color fidelity.) So you're going to see a reduced impact of clamping overbright pixels. Still some loss of hue.

What's the point of having both emission and base color on a Principled BSDF? You're just adding the effect of the Principled, a combination of diffuse and specular on default settings, using the base color as the diffuse and the base color's value as the specular, to the effect of emission. Is that something you should ever do? You risk losing energy conservation, although that's not an issue with Eevee. Otherwise, sure. Think about a very dim lightbulb next to a very bright lightbulb. The dim light bulb may both be emitting light and reflecting light from its brighter neighbor.

Okay, so then what's bloom? It's clear that using plain emission, especially given the limits of our monitors, isn't enough to give us the perception of very bright light. Bloom is designed to give us that perception. It does it by first drawing the screen, subtracting a grayscale value from each pixel (the "threshold"), then blurring the output, and finally adding that darkened, blurred output back into the original screen. (Finally, of course, it runs the whole thing through the color transform, in order to get it back into the 0,1 range.)

This creates an aura around bright objects. Notice also how it interacts with the clamping of colors. 2,1,1 gets clamped to white. But if it's on a black background, some nearby pixel might get half that color from the blur: 1,0.5,0.5. So although the emitter gets clamped to white, the nearby pixels appear pink nearby.

It's not necessary to run bloom on your basic screen output, although that's how Eevee was designed to do it. A common alternative is to render an emission render layer and only bloom emission. This would make more sense to you once you start implementing bloom using compositing nodes instead of Eevee's out-of-the-box bloom.

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In Cycles Glare -> Fog Glow treats your scene as though there is an atmospheric volume, but acts differently than if you'd set it up manually. I'm not sure but it looks like Bloom attempts to do something similar.

Here's a cube where I duplicated it, scaled it up slightly, and gave it a volume of .2 density.

Cycles: enter image description here

And here's the Glare node by itself.

Cycles: enter image description here

You'll notice the effect terminates differently, and it's also decoupled from the cycles sample count, so it doesn't come out looking like a cloud of fireflies.

In both render engines you have to be a little careful with this effect. Any time any part of your scene exceeds the brightness threshold Bloom or the Glare effect will be invoked on those pixels, and it can drown out other parts of your image.

Eevee: enter image description here

To your last question, there is a slight effect on color that becomes more pronounced if the emission value is less than 1.

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Emission is the physical emitting of light. An object with an emitting material will light objects around it.

Bloom is an effect that tries to emulate what happens within a camera lense. Light is not just travelling through a lense, but actually also reflecting to a small degree off the bounderies between lenses and air aswell as lenses and other lenses. While this effect is not noteable with average lighted scenes, it becomes noticeable with high intensity lighting, as these bleed over the actual size of a lighted object.

While the final image might look similar using each of those, they are very different concepts and will look very different in different contexts.

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