Physically Plausible Scene Setup
While iray itself is capable to produce photorealistic pictures, one issue we are seeing in quite a lot of scenes that we get from customers, are weird material, light and tone mapper settings.
So as a result, you can have a strange looking shot of an otherwise nicely prepared scene by using a non-natural light and material setup, or choosing the wrong camera parameters (aka tone mapper).
To avoid this, here’s my personal step-by-step checklist (and actually you can also follow this link to get a much more detailed document on the topic: http://www.irayrender.com/#/tips/3ds-max-iray-educational):
- When using HDR based lighting through environment maps (aka IBL), make sure that the image is actually really high dynamic range. As an example, the un-occluded sun (or other bright light sources) is most of the time way too dark. As a workaround you can simply tweak this range in your existing HDR libraries within your favorite image processing program or write/use a shader that scales the sun values (as iray will bake all your custom environment shaders that you throw into mental ray or the iray Integration Framework, of course). Rule of thumb: The sun can be ~200.000 times as bright as the sky itself. As a result, you will get crispier shadows (in general more contrast in shading), less blueish looking objects (as the sun is much brighter than the rest of the sky), less noisy pictures (on the average) and it reduces the pain to tweak the tone mapper parameters to make the picture look less washed out.
Quick test with two different HDRs (both clear sky) upper picture: sun too dark, result: very blueish, misses some contrast in some regions. lower picture: better sun capture
- Which leads to the next point: “Correct” tone mapper settings. Of course setting this up is always biased towards own expectations, but there are some rules of thumb that are useful: Gamma correction should be used very carefully, as it originates from supporting different brightness curves on different display technologies (CRT vs. LCD vs. Projectors, etc). As such, “useful” gamma values range from 1.6 to 2.8 (or, if the default is 1, then your application is likely to take care of gamma). The same goes for Brightness settings (also hidden sometimes in parameters like camera shutter, ISO or f-number). These shouldn’t be used to severely increase/decrease the overall brightness of a rendering by a magnitude or even worse, but rather the light settings of the scene should be revisited: Are all lights setup with meaningful emission characteristics (I would even recommend using IES profiles on area lights and then model your scene in real world units, if feasible). This also avoids fiddling with the Saturation, Highlight and Shadow/Blackness options too much, as the material tints in general are much better preserved. Same as with correctly setup HDR environments, this will result in much crispier pictures and remove the washed out look.
- If you experience crappy looking anti-aliasing, especially on high contrast geometry like light sources, check if your image filter is set to Gauss filtering or something else that is not simply a 1x1 Box filter (Side note: If this is still not enough for your specific needs: Try rendering the picture at higher resolution (for example 2x xres x yres) and rescale afterwards). While this might not be directly related to physically correctness as such, it could be seen as a rough approximation to what the human eye is doing, too.
The noise is intentional, just to demonstrate that anti-aliasing of high contrast edges is not a matter of taking 100s of samples, but choosing a sane reconstruction filter
- Avoid reducing the path length. Some users try to override the automatic path length handling in iray by setting it to artificially low values like 5. While this can help performance in some cases, it can actually hurt the overall rendering time (due to increased noise issues in some scenes that require to run more iterations) or lead to missing indirect effects in complicated scenes.
Even very near to the light source, there can be indirect effects that contribute significantly to the final picture
- Reasonable material settings are a must: Nature only features very few materials that actually “fully” reflect or refract light, especially most common diffuse or glossy materials won’t reflect all incoming light, which means for your material settings: Avoid using perfect RGB values like (1,1,1) for a white material. Even mirrors are not perfect in real life, so you should always be careful with settings that result in too high reflectivity. Some rules of thumb to get a feeling for the settings: 95% reflectivity on a “perfect” mirror, 70% for a standard white paint. If your material otherwise looks too dull, rather crank up your light sources. This, again, leads to pictures with more contrast that avoid that washed out look or too much color bleeding like in the following picture (and as a bonus, your render times will go down, too).
Coder colors (1,1,1 1,0,0 0,1,0) vs. some more realistic color bleeding
- Transparent Objects ideally are not infinitely thin. Due to performance reasons (and easier modeling), it’s pretty common to create windows with only a single layer of geometry (and then setting the thin wall flag to not confuse the renderer). If you want to get subtle refraction effects and inter-reflections within the glass though, be sure to model these objects with double sided geometry and enough spacing in between. On a side note: Try to use the correct index of refraction on materials, there are tons of tabulated IOR values on the net.
- Avoid point and simple directed light sources. These also don’t exist in real life, but were introduced into CG as rendering them usually didn’t result in any noise and are very simple to implement. With iray this is not necessary anymore, as the sampling of area lights is very efficient, the renderer even offers a full texture pipeline on light sources and artificial emitter geometry (and of course IES profiles on area lights and geometry).
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Carsten