Rendering Slower with Sun / Sky vs HDRI

[dfcorona]

Hello,

I have noticed that rendering with Corona Sun/Sky is a lot slower than rendering with an HDRI. Why is that? I would of thought it would be the reverse.

[FrostKiwi]

Hello,

I have noticed that rendering with Corona Sun/Sky is a lot slower than rendering with an HDRI. Why is that? I would of thought it would be the reverse.

One of the release notes in very early corona updates was, that both Sky and HDRIs sample in the exact same way, with one not having the performance benefit over the other. In fact they wanted to release a scientific paper on this technique back in the day.
However what I believe you are seeing is, that the sun dumps way more energy into the scene, then the average HDRI. The first bounce in Corona is not cached, but brute forced with Pathtracing.
That's why a bright light source on it's first bounce creates more noise than one might expect, like the sun on the ceiling when it goes through a window and hits the floor.

If you have a HDRI with the exposure matching  the corona sun's, which you can create with the material editor by boosting the contrast or crushing the gamma, you can get the same effect of more perceived noise.

So I don't think sun + sky is inherently more noisy, but the high energy output of that bright and small light source creates hard to sample situations.

edit: Now I'm curious, I recall they did an improvement on Multi Importance Sampling (MIS) for sampling the environment, wonder what happened with that paper announcement...

[dfcorona]

I always noticed I have to put the VFB into a pretty high negative exposure when using the sun. So then is it better to lower the Suns brightness? Will the realism still be there but just with better speed?

[PROH]

Nope. For physical correctness set the Sun to 1 and the Sky as well. I don't think lowering the Sun intensity would give any speed improvement, but I haven't tested it.