Cineversity

I am trying to create so called PBR-materials in Cinema 4D. I render with the Physical render with a Reflectance only workflow. So I don’t use the Color Channel but instead a Lambertian diffuse layer in the reflectance channel. On top of that I use Reflective layer (GGX)

If you read about PBR-workflow you will notice that it is not a real standard cut in stone. More some guidelines to create a more realistic and simplified workflow. Because of this it is not always easy to now if you find the correct information.

I have tried to learn from this article:

https://www.artstation.com/blogs/luismesquita/jGXd/everything-about-pbr-textures-and-a-little-more-part-2

Under the headline ”Specular map” you learn that the strength of the reflectivity vary on different materials but that the range for non-metallic metals are surprisingly small.

We read

”For parts that represent non-metals: use only shades of gray, as the specular color is always white. The brightness of the gray tones will indicate the F0. Darker tones means less reflective and lighter tones, more reflective. In general, non-metals have very low luminosity, close to black. On an sRGB scale, the ideal brightness values would be between 40-75 sRGB, or between 0.02-0.05 on a linear scale, representing a reflectivity between 2-5%.”

My typical non-metal material uses two layers in the Reflectance channel. The first is a lambertain diffuse and the other is a GGX layer. In the GGX layer I set a IOR to maybe 1.5. I also use a roughness map to control the blurriness of the reflections. (I also use Normal maps, and bump maps)

Now to my question. How do I control the reflection strength. This my sound like a stupid question at first (maybe it is) but if you read the quote above It is not obvious to me how to translate the values to Cinema 4d. I can control the strength of the reflection in many ways in the GGX layer. With the overall layer opacity, with ”Reflections Strenght”, with layer color and with Layer Mask (that is connected to the layer opacity).

Should the Layer Color (for GGX layer) always be set to 100% white?

Should the Reflection Strenght be used to put in the values in quote above? If so does the ”40-75 sRBG” correspond to 40-75% in this slider? Or should we use the linear scale 0,02-0,05 (2-5%?)

I hope my question (questions make sense). Maybe someone here has the answer or can point me in the right direction.

I also hope that I have posted this in the right place. It is my first post here so I may have missed some important guideline.

Thanks
Henrik

Hi Henrik,

Specular highlights are only white based on limitations of the format one uses to store those. (Assuming that the Specular highlight is brighter than 100% for all channels) If an image or footage format can’t hold anything above 1.0 (or 100%), anything brighter will be clipped. As a rule, when all channels are equal (RGB), your get black, gray, or white. White would result from clipped channels if the value were more considerable otherwise.

Having a 100% white specular with a format that can hold a brightness above 100%, the light must be white (whatever that is), and the camera or app needs to agree with the given color temperature [Kelvin]. The color space in which you work can provide RGB enough “room” to be not clipped.

Besides, any plausible system uses an energy conservation system.
This means that reflected light can’t be collectively brighter than the summary of the source. So, 100% reflectivity is not a given in nature but possible in CG. Since we are artists, we might dial that over the top for specific expressions, which should never be forgotten. We work mainly in the visual communication area here, not the physics department.

If you stay in sRGB, you have a limited Color Gamut equal to BT. REC 709. So, this will limit your result already to using this small Gamut. Typically, Gamut encompasses most of the reflective colors (Pointer’s Gamut), but with Metal, the surface becomes a light source, and that is closer to nearly fully saturated colors. (CIE 1931 diagram, Spectrum Locus)

The metalness or specular map should never be gamma-based. If so, it needs to be set to become raw or linear.

https://help.maxon.net/c4d/s26/en-us/Default.htm#html/MMATERIAL-ID_MATERIALGROUP_REFLECTION.html#REFLECTION_LAYER_MAIN_DISTRIBUTION

More about this here, and perhaps the best short answer.
https://youtu.be/2Gm07weuBdA?t=2479

If you like to have a good source about this theme, as it is deep and should be presented in an easy to read way:
The Complete Guide to Photorealism for Visual Effects, Visualization and Games 1st Edition, by Eran Dinur

The most scientific resource that I have enjoyed reading is deep, complex, and has 700+ pages.
Colour Reproduction in Electronic Imaging Systems: Photography, Television, Cinematography 1st Edition
by Michael S. Tooms

All the best

P.S.:

To showcase the color/white theme, I have created a tiny scene
https://www.amazon.com/clouddrive/share/G0NJrSXoO1bsultgcFm2wsW3hooKq9xEOujVqvRvhqx

Open it, render it to the Picture Viewer, and get a white reflection.
Then go in the Picture Viewer to the Filter and enable it, exposure to -8, for example. The dot on the sphere gets its saturation back; it was clipped before based on screen settings.

This test will not work the same in HDRI mode while having a screen that provides proper HDR results.

All the best

Thanks Dr Sassi for your response.

I don’t fully grasp your answer. I will ponder it for a while and see if I can connect the dots and return later with a follow up question.

/Henrik

Hi Henrik,

Yes, please let me know if there is question.

All the best