A community member here recently sent in a question about painted surface properties when using Maxwell Render and I thought others might find it interesting as well. The Q&A comes down to understanding what the different settings in MXED are for. While my answer doesn't get into detail about each setting, it does talk about which ones we should be paying attention to depending on the type of material we are trying to create. A lot more information can be found in the user guide for Maxwell materials.
Here you go:
Here was my response:
Anisotrophy is only for metals, so that’s not it. Roughness is the actual surface of the object. Setting it to 0 is perfectly smooth like a mirror. 100 is totally rough (but not bumpy; that’s what Bump is for), like a matte paint or car tire rubber. I wouldn’t actually go to 100 though, 95 is the realistic top based on some things I’ve read and seen. So this setting is really what controls reflectivity, along with the color/texture you use in the reflectance 90 channel for objects that curve away from the view of the camera (the fresnel effect, where objects reflect their environment as the angle falls away from view), and the Neutral Density setting. For solid objects with a weak reflection (like concrete) ND should be 1 as it has no influence on surfaces with a very high roughness (95-100). You can read more about ND here.
Additionally, attenuation won’t do anything for you with concrete. It’s just for objects that have translucency/transparency like acrylic or glass with color throughout. It controls the depth that the color goes into the material from the outer surface in real world units. Also, as you might know Scattering is for materials that use subsurface scattering (SSS) like wax or skin, so you can leave that out of your testing as well.
It’s hard to tell from that chart you’ve provided if there would be a direct correlation to a setting in MXED or not. I’d want to see real world examples of the paint chips to know for sure, and even then it might be a bit of an equation to make it a direct correlation. I rarely use definitive RGB and reflection values (I’m more of an artist than a technician) because of color calibration and gamut between rendering (with variable lighting applied), computer screens and final output. In other words, rarely what I enter for color values are ever seen the same way by the client. I just make it look “right,” whatever that means. The good news is that Maxwell is in my opinion the best tool to be as accurate as possible, so you can definitely get a lot closer in it than in anything else.
After doing a bit more research, I found this article on Light Reflectance Values (the LRV column in the first image) and it points the way to attempt to find a correlation with Maxwell settings. Every light source (either natural or synthetic) has a specific output, color temperature, IES profiles and more. Combine that with variables in atmosphere and mixing of natural and synthetic and you can see how this gets tricky fast.
This is one of the reasons I render using Material ID's to be able to quickly make adjustments after rendering, which saves a ton of time if I need to slightly (our wholesale) adjust colors later on. You can see a couple of tutorials I did on the subject here and here.
If you have anything to add, please do so in the comments!
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