Comparing Microfacet Multiple Scattering with Real-Life Measurements

Microfacet multiple scattering approximations change content authoring – both for good (energy conservation) and for not so good (saturation changes with roughness). I was curious how current microfacet multiple scattering techniques compare against a measured real-life reference. After all, real-life surfaces aren’t perfectly isotropic, they have small scratches causing diffraction which can’t be modeled by geometrical optics etc. So maybe color saturation changes shouldn’t be there?

Unfortunately there isn’t much data for measurements of the same materials with varying roughness. According to Wenzel Jakob they have some differently machined metals in queue for scanning and will include them in they awesome RGL material database, but for now I could only find data in studies made for the material manufacturing industry.

Let’s start with the state of the art approximation of microfacet multiple scattering from [Turquin]. We can see that with increasing roughness it conserves energy and color becomes more saturated:

microfacter_multiple_scattering

And here’s a photo of differently machined aluminium alloy from [Li 2018] which nicely fits multiple scattering energy conservation approach:

feed_rate

Surface roughness increases with increased feed rate

[Yonehara 2004] has some more detailed measurements of surface properties. Interestingly with increasing roughness color in real measurements is getting a bluish tint instead of simply gaining saturation as in microfacet multiple scattering models.

“In all specimens, as the Ra became smaller, a tendency was seen that the reflectance in the measured wavelength region became lower. In particular, the drop in reflectance in the long wavelength side was significant in comparison with that of the short wavelength side. (…) In other ways, for the case where the roughness plane is the same, in comparison with the short wavelength side, the light of the long wavelength side causes specular reflection more easily”

roughness_chromacity

References

[Turquin] – “Practical multiple scattering compensation for microfacet models”

[Li 2018] – Al6061 Surface roughness and optical reflectance when machined by single point diamond turning at a low feed rate”

 [Yonehara 2004] – “Experimental Relationships between Surface Roughness, Glossiness and Color of Chromatic Colored Metal”

This entry was posted in Graphics, Lighting. Bookmark the permalink.

2 Responses to Comparing Microfacet Multiple Scattering with Real-Life Measurements

  1. Hi, please guide me, what is micro flakes model?

    Like

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s