splotchy hdri

[LucentDreams]

its generally a good idea to used a slightly blurred HDRI when doing HDRI image based lighting, the contrast between dark and light is sooooo extreme that you can have an extremely bright smaple right nest to an extremely dark sample on your surface which will look splotchy in any renderer.

[DynamicRealism]

here is with 25% blur, i dont understand?!

[nutriman]

You can try using 2 skies, one for crisp reflections and the other for the lighting. (via rendertag)
The latter you can blur even more.
The old approach was to use big spheres surrounding the scene but that isn't very advisable since it spreads the GI samples to that large scene so you would need more samples.
It won't happen with the sky object.
Also you can try to use AO additionally and a subtle lightsource.

[paulselhi]

increasing the stochiastic samples will often reduce splotchiness

[ThePriest]

here is with 25% blur, i dont understand?!

It doesn't look like the hdr is having much of an affect on the scene either way,
you'd do better with a regular sky and just hdr for reflections.

[Per-Anders]

The reason that HDRI can potentially result in splotchy or grainy GI is very simple once you understand how most GI algorithms work.

A HDRI as you know is a bitmap that contains image information that's actually brighter than your monitor can display, i.e. brighter than as far as the computer is concerned pure white. This means it's extremely contrasty, i.e. if you were to mathematically sample the brightness at any two points there could be a very big difference between the two brightness values, much more so than with traditional bitmaps (where the difference could never be greater than 100%, i.e. a black sample = 0%, pure white = 100% bright, the difference therefore is 100%).

So why is that important to GI? Because most GI works by for every sample point it cases out a bunch of rays that sample the surrounding area, and the result is then averaged. So if the sky around your scene is all the same color the average will more or less be the same wherever you are (apart from right up close to your scene geometry), because it'll cast out a bunch of rays and they'll allr eturn this same sky color, i.e. you'll get a smooth result.

Now if you have an HDRI there, you can start to guess what will happen. You would need to send out many more rays to get a smooth result, many many more rays, because as the HDRI brightness for a single point might be in the hundereds of thousands or percent brightness at one point, but at nearly black at another each ray that hits those super bright areas is going to result in a big jump up in the resulting average brightness/color. But because it's always random for every sample point the result is that you're going to get splothcyness and grainyness unless you use insanely high samples.

To help you can do two things, you could either pre-average out some of the hdri, by blurring it somewhat, or you could help the gi samples out by blurring them, fR allows you to blend the samples by a certain ammount, increase the GI blending and you will get a smoother result, at the cost of clarity and precision. The third option is to reduce the contrast of your HDRI that you're using for lighting and later on increase the contrast of your GI pass to compensate.

In most situations if your HDRI is causing splotchyness then chances are it's just far too contrasty to be useable under most circumstances for lighting and probably isn't refecting a real world lighting scenario very well anyway, if it is then try to fake thigns more using actual lights, in generally remember that GI is the bounce light, it's better to use actual lights than to try to use GI for the full lighting as GI on it's own will be much harder to control, much slower and far more artifact ridden.

[DynamicRealism]

very grateful for all of your responses! thanks a ton

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