Cycles Render: How Do I Describe Porcelain?

Rendering
,
1

I have some actual physical data, for a somewhat rougher porcelain than I’m modeling. (It’s a toilet. I do not recommend toilets as modeling projects.) Prof. J. Alstan Jakubiec’s Spectral Materials Database gives RGB reflectance and Radiance material properties (“Ward shader”) for a white porcelain. Anyone know how to translate this into properties I can use with Rhino’s version of Cycles Render?

2

Cycles is not a spectral renderer, so these values are not all that useful.

Here is a material that could work as a white porcelain: Porcelain.rmtl (33.7 KB)

image
image1884×1516 224 KB

Play with the roughness of both the clearcoat and the main roughness to get less glossy result.

3

Thanks. I do wish that the simple Ward shader properties would just work, though.

4

There is no Ward shader in Cycles. Besides, the linked page gives a Radiance material definition. After some reseach on the internet I found the Radiance Github page. According Radiance/doc/notes/materials at master · LBNL-ETA/Radiance · GitHub the material you linked to is defining a Radiance plastic material. I don’t know what the first three numbers (0, 0, 5) mean in the definition, but I assume that the following three are the color, then specularity, then roughness (inverted). The color 0.805, 0.805, 0.785 is a rather gray color. The specularity comes out as 61% and the roughness as 75% for a Rhino Physically Based Material. That is the closest to the values given as far as I can tell:

image
image1288×966 110 KB

1 Like
5

Thanks. “0 0 5” are parameter counts; Radiance is old.

The color values are reflectances on a 0-1 scale; that’s a slightly yellowish white. (Anything neutral over about 50% reflectance generally appears white.) How did you translate the roughness and specularity values to Cycles values? (I can point to the mathematics, but I don’t claim to understand it.)

6

In the link I gave it describes plastic with these values:

Plastic:
	color
	[0:1],[0:1],[0:1]
	black - white

	specularity
	[0:.07>
	matte - satin

	roughness
	[0:.2>
	polished - low gloss

This describes the ranges used for specularity and roughness. Basically I just divided the numbers from your link by the maximums given here. Rereading this after a nights sleep I probably shouldn’t have inverted the roughness here, it should be 25% (0.05 / 0.2 = 0.25).

7

Thanks. They aren’t physical maximums; they’re just ranges of physically plausible values; the allowed values range run from 0 to 1.

Roughness is specified as the rms slope of surface facets. A value of 0 corresponds to a perfectly smooth surface, and a value of 1 would be a very rough surface. Specularity fractions greater than 0.1 and roughness values greater than 0.2 are not very realistic. – Radiance Reference Manual

There’s more mathematics but, as I said, I don’t claim to understand it.

8

I know they aren’t physical maximums, but based on the documentation I assumed that the plausible values would be good for maximums to translate to PBR in this particular case.

9

My thought was that, since both systems are physically-based, there ought to be an algorithm for translating one to the other; Jakubiec’s site is based on measurements, and it would be useful for lighting calculations and rendering realism to be able to use that data with Cycles. There probably could be such an algorithm, but writing it would require a lot more knowledge of the calculations than I have. Maybe I can ask the Cycles authors.

Anyhow, thanks for the advice!