White noise / spots in rough materials

[guest86847]

Hi there,

I have a scene with different kind of materials and some area lights that illuminate the scene from the right side and left side in relation to the render camera. Every time I rendered that scene out I observed strange white dots especially in the regions where the area lights are visible in the reflections of surfaces. I tried out every tip I found here in the forums like making sure the scene is in the right dimensions (it is), using IES, using emissive mats, changing materials, playing with GI, higher roughness samples, and many more. NOTHING helped.

In the end I broke the scene down into three objects: 1 large diffuse area light, 1 steel bar with steel rough preset, 1 wall with advanced plastics preset. The plastics preset is meant to mimik a special kind of cloudy acrylic glass so I gave it a little bit of roughness.

Here's the scene without the steel bar. Everything is (relatively) nice and clean. Though very low sample rates everywhere no white dots or spots appear.



Here's the scene with the steel bare made visible. Strange white dots in many parts of the picture.



Since i broke my scene down I can say: These white spots are NOT avoidable!!! They always occur when area lights are reflected between objects which have first roughness > 0 and second refractive index > 1. ALL other settings made NO effect in my scene, even switching from product to basic render settings.

Since I have to rely on a higher refractive index to have a properly looking acrlic plastic material I played with the samples. First of all: roughness settings of the different materials made NO effect, too! Only the light samples of the area light diffuse as well as the global samples in the advanced render settings (realtime renderer is even worse with these spots, so I use the advanced renderer) make an effect:

Global samples 16, light samples 256


Global samples 256, light samples 16


Not much of a difference, is it! I guess we have to crank up all the samples:

Global samples 256, light samples 256


Though we put both sample settings to very high rates we stell get some (minor) spots here and there. In a similar test scene I put both sample settings to 1024 each (!!), and even then there were some spots which is kinda disappointing. Although I love Keyshot (I'm using 6.2) for its simplicity, I find its behavior with in-scene lights (like spot and area) a little bit disappointing. Conclusion: I guess Keyshot isn't that great with handling area lights. When rendering with 256/256 (fifth pic) the renders take FOREVER.

So, in the end I only can ask you: Is there any setting I might have missed? Or is this a bug, or just the normal behavior?

Thanks for reading my long post

[bdesign]

Hello Armill-

Can you share this scene?

Eric

[guest86847]

Well, I can't provide the original file due to confidential reasons but I set up a scene which is very similar. Download it here: http://www71.zippyshare.com/v/l7XEr8nJ/file.html

[bdesign]

Hey Armill-

I've been doing some tests with your scene, and I've concluded that KeyShot's Interior Mode can definitely be beneficial for more than its intended purpose of rendering enclosed scenes. But before I get into that, I'd first point out that the light color of the Area Lights in your scene was set to 100% pure white; it is advisable to always keep light values less than 100%. I changed it to 90%, and this alone made a noticeable difference. And I changed the color of the acrylic material from 0% to 5%. Pure white and pure black don't exist in reality. Then, I changed the rendering mode of your scene from the default to Interior Mode, and the Area Light reflection noise cleared up almost immediately upon rendering, on both the acrylic and metal surfaces. Being that Interior Mode uses a different lighting algorithm, the overall look of the lighting differed. Most notably, the Area Light reflections were brighter, and the environment reflections were less pronounced on the acrylic wall surface. So I experimented with making some material and lighting adjustments to try and get a relatively close match to the "look" of the default render mode. I increased the Refraction Index of the acrylic from 1.45 to 1.85, and reduced the Area Light intensity value from 2 watts to .816 watts. This made the overall lighting look fairly close to the default look. The main difference was that, due to the decrease in light intensity, the specular highlights on the steel bars were not as hot, which to my eye, is a good thing. The default look of the specular highlights could be retained by either: a) adjusting only the acrylic RI and leaving the light intensity at 2 (which still left the Area Light reflections on the acrylic brighter than default), or b) adjusting the acrylic RI, adjusting the light intensity, and increasing the steel specular value from 76% to 94% (which increased the overall brightness of the steel). I think with these various possible options, you should be able to find a balance that is pleasing to your eye. And as an added bonus, it renders almost 3x faster   I'm attaching a .ksp of my revisions to your scene, and some renders, below. All were set to Maximum Time = 5 minutes in the render dialogue box. They are:

Image 1: Default Mode
Image 2: Interior Mode (No Adjustments)
Image 3: Interior Mode (acrylic RI=1.85)
Image 4: Interior Mode (acrylic RI=1.85, Area Light intensity=.816 Watts)
Image 5: Interior Mode (acrylic RI=1.85, Area Light intensity=.816 Watts, steel Specular value=94%)

I hope this is helpful. I discovered some new things myself...

Cheers,
Eric

[guest86847]

Hi bdesign

Full white, full black - that was it! I must have missed that since its logical that these shouldn't bet set to maximum. So I lowered the whites a bit and pushed the blacks a bit, man, this helps A LOT. Well, the spots are not completely gone, but they are heavily reduced. Now I can render with 64 samples for lights and global which is so much faster. Not perfect renders (still some spots), but totally sufficient for the moment. I will deal with interior lighting later on when this project is finished.

Thank you SO MUCH!!! Really appreciate that you took time and dealt with my scene to improve the rendering quality.

[bdesign]

Hi bdesign

Full white, full black - that was it! I must have missed that since its logical that these shouldn't bet set to maximum. So I lowered the whites a bit and pushed the blacks a bit, man, this helps A LOT. Well, the spots are not completely gone, but they are heavily reduced. Now I can render with 64 samples for lights and global which is so much faster. Not perfect renders (still some spots), but totally sufficient for the moment. I will deal with interior lighting later on when this project is finished.

Thank you SO MUCH!!! Really appreciate that you took time and dealt with my scene to improve the rendering quality.

You're very welcome, Armill. Really glad to hear that it helped. Did you try out the Interior Mode at all? I'm sure you're probably aware, but in the scene tree of the revised scene I uploaded, I put separate revisions of the acrylic wall and lights, for default mode (DM) and Interior mode (IM). Just turn off the DM wall and lights, turn on the IM wall and lights, enable Interior Mode in the Lighting tab, and give it a render. . It cleans up very fast...and I only used 16 samples per material and 16 samples for the Area Lights

Cheers,
Eric