Iray noise + HDRI

joseft

Hi All,

So last night i was playing around with an outside night-time scene, and i figured the best way to get good lighting would be to use a nightsky HDRI to light it. The problem was, i was getting some seriously ugly noise on my character, and only on my character. It would not clear up, no matter what i did. I let it render to 15000 samples, and it looked no different to 6000 samples. It was frustrating as hell, i figured it was something to do with my character, since the noise was only present on the character, and not the rest of the scene.

I spent ages playing with different things, but could not figure it out so i saved the scene and figured i would do something else and come back to it. Then i was setting up a skin shader on a different character, and i had the same noise come back, in a scene that had nothing in it except the character. so after initially getting angry again, i reset all the render settings to default, and the noise was gone. After playing around a little i figured out it was the HDRI map causing the noise problems. Confirmed after going back to the night scene i was doing, deleting the sky HDRI and using the environment light.

So what is it with certain HDRI maps that cause this noise? low resolution? or is it in conjunction with something else?

happy that i know what was causing it now, but also curious to know specifically why this happens

Havos

Low light is what causes noise in any render, and a night HDR will have low light, hence the issue. You could always render the scene with a daytime HDR but then use the tone mapping to make the scene darker, or alternatively darken the image in post work. This is a trick regularly used by movie makers (or so I have been told), to film a night scene in daylight and then make it dark during editing.

joseft

I understand that concept, however when i was doing that second scene which was just setting up a skin shader, and i was getting noise (which had too much light if anything), thats when i looked deeper. One of the many things i tried to fix the issue in the night scene originally was to crank the environment intensity way up so that it was way too light for a night scene, and then use ISO and fstop adjustments to dampen that light significantly. That had no affect on the noise at all, it was still there. I also tried the opposite - environment intensity right down, and crank the ISO to some obscene high amounts to get more light, and that didnt work either

 

Plant

Hi I've been having somthing  simler today and i found if i turned the translucency wieght to 0 no more speckles

Tobor

The pepper-looking noise (as opposed to light-colored speckles) is the result of not enough "photons" for Iray to do adequate convergence estimates. Each pixel in your scene needs multiple hits from light rays before it can declare that pixel as "done." The fewer of these rays, the harder it is for Iray to judge convergence, and the longer the render will take.

The usual corrective action is to add light *intensity*, and compensate by lowering one or two of the camera tone mapping settings. It doesn't matter which one you adjust, as long as each one stops down the exposure to reduce the light getting to the Iray camera. For example, if you double the environment intensity, you could stop down the f/stop from the usual f/8 to f/16, or the shutter speed from 128/s to 256/s. Likely, to make ad effect, you'll need to increase intensity and offset exposure by maybe 4-8 stops worth, if your scene is that dark. Your trial of lowering intensity and increasing ISO was the inverse of what you need to do.

In short, what you are trying to achieve is the same as the age-old method of "day-for-night" used in film and TV. Night HDRis typically supply too-little light for good exposure. They're better used as backdrops that provide ambience, and you actually light the scene with separate scene lights.

If the noise s light-colored speckles, those are fireflies, and they're not caused by the same thing. A typical cause of fireflies is a surface with an overly bright specular setting. Fireflies also commonly appear in eyes, where the multiple shaders that make up the eye surfaces conflict with one another, causing excessive internal reflections.

mjc1016

Tobor said:

 

If the noise s light-colored speckles, those are fireflies, and they're not caused by the same thing. A typical cause of fireflies is a surface with an overly bright specular setting. Fireflies also commonly appear in eyes, where the multiple shaders that make up the eye surfaces conflict with one another, causing excessive internal reflections.

And sometimes, if the caustic sampler is off, those 'fireflies' would resolve as caustics, if the sampler were on, instead (especially with eyes/glass/liquids).