Problem with Nvidia render using dim lighting
Man Behind the Curtain
Posts: 61
I know I've seen this addressed before but I can't seem to find the thread. When I use a dim lighting level with Iray, my renders seem to look grainy and have small light colored speckles throughout the image. My Macbook Pro has 16 GB of RAM and an Nvidia GeForce 750M with 2GB of it's own Ram.
Thanks for any help, or point me to the thread.
Post edited by Man Behind the Curtain on
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Maybe what I do may help?
I have set up a simple scene, in an enclosed area, with a simple box/sphere. All have glossy/reflective surfaces. I have placed a single point light in scene, and set it to "Light geometry -> Sphere"
In "Render settings -> editor" from the default settings I first change:-
1 Tone mapping "Crush blacks" and "Burn Highlights" to 0(zero)
2 Progressive rendering "Rendering quality Enabled" to off, and "Max Samples" to 100
image01. First render, showing excessive noise/fireflies
Go back into render settings editor -> Filtering -> Fireflies". You will see that "Firefly Filter Enabled" is set to "on". Below that setting you will see that "Nominal Luminance" is set to zero(off). Enter a value. The value will depend on how bright your scene is. I first set the value to 1 (because the scene is low lit). If the value you enter is too high, then it will not help, if the the value is too low, it will remove highlighs(or even wash out the image). But the results can be seen quickly.
image02. "Nominal Luminance" set to 1. The image is a lot better, but the reflection of the light source in the sphere is too dim.
image03. "Nominal Luminance" set to 3. Better that original, but there is grain in the image.
To remove that grain (which may or not be an issue).
image04. Max samples increased to 300. Looks better, but still some grain in lower light areas. You can increase max samples or:-
image05. Tone mapping > "Crush blacks" set to 0.1. It does darken the image, but helps to remove grain for low light areas.
Add more light/lights, then to make the scene look darker increase the f/stop (or shutter speed) in the render panel. The default setting is f/8, and every doubling of the standard f/stop values halves the amount of light reaching the "film." The conventional sequence goes: 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, 45, 64, 90, and 128 (and beyond-- you can look it up on Wikipedia).
If you increase shutter speed every doubling of the speed causes half the light to reach the film. So if you want four times less light go from 1/128 to 1/500 (the numbers don't have to be exact doubling of values; it's okay to round off to standard camera shutter speeds).
But before you adjust the f/stop, try this: Light the scene so you get a normal exposure. See what artifacts you have to contend with. Work at getting a good render at normal exposure levels, then adjust for mood and artistic license.
Um.... what @Tobor said. I'm not too sure about the gymnastics that other fellow is going through, but... I wouldn't go that route.
Not sure as to why you think it is gymnastics, it is quite simple. I remove the default light clamping, set a 100 sample limit on the render, and render. As most noise in a render is fireflies(hot) and low level fireflies(usually secondary bounces from glossy/reflective surfaces), changing the fireflies setting would be the most obvious(for me anyway) way to go. Once I get a reasonable result, I increase the samples, usually no more than 1000 needed.
@stem_athome
The obvious way to go is what Tobor advised, i.e. get a good clean render then tone map for "feeling".
When you have sufficient experience with Iray, and understand how light works, you can combine the above into a single render step.
Adding more light into a scene with fireflies will just make the fireflies brighter. A self defeating process.
Have a search on info about iray to get a better understaning of how to use it.
Here is one link, http://blog.irayrender.com/post/87877693033/compositing-with-linear-image-output-vs-fireflies ; (Note: I only disable the tone mapper when compositing, or when using 3rd party tone mapper).
The firefires are an artifact caused by the rendering engine misinterpreting the luminance values of surfaces. This is why the article has you turn off tone mapping and then manually hint the luminance. Since crushing blacks and adjusting tone can alter the artistic goal of the scene, it's always better to avoid the fireflies in the first place rather than hide them. Specular objects, all-white objects, canceling the render before 100% (Iray processes fireflies progressively over iterations because of detecting luminance of nearest-neighbor pixels) and other elements can largely avoid the problem.
The OP did mention white noise, but without a picture it's hard to tell if these are true fireflies or unconverged pixels or artifacts in the early stages of rendering when the spectral filter is turned on.
I am not sure I would call it a misinterpretation. The bright spots (or as OP decribed "small light colored speckles") are the reflections from glossy/reflective surfaces.
I agree it is better to avoid them as much as possible, by taking time to set the light/materials so you do not see such an issue, but I find a lot of users of renderers do like glossy/relective surfaces. So when looking for a solution, it is better to find the quickest. Some renderers use a setting for "Filter Glossy" which can deal with those fireflies, while others there is still a need to use light clamps.
I will leave the thread. I hope the OP finds a solution to the isuue.
Oddly enough I don’t get fireflys in my renders so I guess I “understand” Iray well enough.
The misinterpretation comes from how Iray establishes an automatic nominal luminance for the scene. When you adjust it yourself, in order to reduce fireflies, you're taking over the role Iray should be doing internally. The fireflies themselves are an artifact and the problem is exacerbated when scene criteria is not properly identified pre-rendering. For those following the discussion this wikipedia page covers the basics pretty well:
https://en.wikipedia.org/wiki/Fireflies_(computer_graphics)
Note the wiki page also includes the "fix" to clamp levels and alter the nominal luminance which are the things you suggested (so are other things like reducing excessive specularity and not using 255/255/255 white on shaders), and as such they are perfectly fine as part of a more developed workflow. But since many scenes that exhibit the worst fireflies are not set up properly to begin with, my advice is to always normalize the lighting first, then tweak. When people know what "normal" is they can better deal with abnormal.
Depending on the scene adding more light (s) to a dim scene may indeed rid many/most/all of the fireflies. The dark scene van contribute to the errors made in establishing the baseline for nomnal luminance
For those interested this is from the Iray documentation for the Nominal Luminance setting:
iray nominal luminance - float The nominal luminance is a hint to iray on what is considered a reasonable luminance level when viewing the scene. This luminance level is used internally to tune the firefly filter and error estimate. When the nominal luminance value is set to 0, iray will estimate the nominal luminance value from the tonemapper settings. If a user application applies its own tonemapping without using the built-in tonemappers, it is strongly advised to provide a nominal luminance.
Edit: Removed extraneous HTML in documentation quoteRecommendations: For visualization, a reasonable nominal luminance would be the luminance value of a white color that maps to half the maximum brightness of the intended display device. For quantitative architectural daylight simulations (e.g. irradiance buffer), a reasonable nominal luminance could be the luminance of white paper under average day light. (default 0.0)Wow, ok. Thanks for all the tips. I'll have to devote more time to Iray and get a better understanding of how it handles and renders light.