Corona Physical Material - C4D


What is it?

Introduced in Corona Renderer 7 for 3ds Max and Cinema 4D is the new Physical Material. This material has been designed from the ground up and is intended to replace the previously default Corona Material, which in version 7 is called Corona Legacy Material. 

Some of the benefits of using the Physical Material are its ease of use and its ability to achieve realistic results, ensuring you can't accidentally create unrealistic "fake" materials that break energy conservation and other laws of physics regardless of the settings you use. The result will always be (and look) realistic. 


The Physical Material also includes presets that you can easily select from a dropdown menu. These include materials such as Aluminium, Brass, Chrome, Copper, Diamond, Glass, Gold, Iron, Mirror, Plastic, Plexiglass, Satin, and even Velvet.


Why was it added? 

The Corona Physical Material was added as the replacement of the old Corona Material. Some of the benefits of the Physical Material include:

  • The ability to get more realistic and physically plausible results.
  • Better and easier layering system without the need to set up complex Layered Material networks (clearcoat, sheen).
  • Compatibility with other software following the physically-based (PBR) guidelines. 


How is it better than the old Corona Material (now called Legacy Material)? 

There are many benefits over the old Corona Material. For starters, Corona Physical Material offers a natural way to set up realistic materials, making various workflows much more intuitive and simpler in the long run. Its diffuse calculations have been switched from Lambertian to the Oren-Nayar model, so even the simplest materials will now look better and be more physically correct.


In addition, fake and non-physically plausible material properties are not possible anymore; the current material parameters are designed in a way to prevent such cases.


When to use Corona Physical Material?

The Physical Material should primarily be used as the new default for any newly created materials, unless it is absolutely necessary to use the Corona Legacy Material (e.g., in case of re-rendering older scenes in Corona Renderer 7 or newer).


CoronaPhysical Material Basics


Metalness: Metal & Non-Metal(Dielectric) and how are they controlled by base color.

Starting with the basic parameters, the CoronaPhysical Material is set as a Non-Metal by default, essentially a dielectric material where its base color can define its reflectivity and diffusion. In this mode, various types of Dielectric materials can be made in a physically plausible manner. Non-metal materials (dielectrics) can also be transparent and consist of various glass, crystals, polymers, or other organic materials.


(Wooden body and glass pearls are considered as dielectric, non-metal materials, 

while the instrument's strings and black metallic tuning keys are set as Metal.)


In the case of a metallic base layer, Metals are opaque and defined only by their reflection color, which is set by the base color parameter. However, the reflection color for metals (exclusively) at grazing angles can be edited through the use of Edge Color


(Some controls are only enabled depending if you're using non-metal or dielectric materials) 


Examples: The following example showcases the differences metalness can make on any given material simply by changing metalness mode to metal or non-metal(dielectric). On the left is a metallic body with a glossy coat against a glossy plastic material (Non_Metal) on the right image.


(Full-size comparison link: https://corona-renderer.com/comparer/wm6OnQ)


You can easily match the "look" of metals based on real-life references by adjusting their Base and Edge Color, which serves as an artistic interpretation of the end result; this is ideal for the majority of cases. However, in case a more realistic result is preferred, the use of Complex IOR is suggested, as we will see further down in the article. In the following, the default edge colors were procured through the use of Complex IOR (left) against a custom pure green using Edge Color (right).


(Full-size comparison link: https://corona-renderer.com/comparer/phHbaC)

(Click on the image to enlarge)


Note: You can also map the metalness of a Physical Material using a texture to define the base layer material type. In such a case, the values of 0% (black color in the texture) correspond to non-metal areas, while the values of 100% (white color in the texture) correspond to metal areas. In-between values can create a mixture of metal and non-metal areas. While a single material cannot be both metal and non-metal at the same time, this is allowed in some cases - for example, if there is a transparent layer of non-metal on top of a metal (e.g., a spray-painted metal) or if the two kinds of surfaces meet and the transition between them is not sharp (which will always happen because of anti-aliasing and texture filtering). ,


IOR (Non-Metal only) for Reflection & Refraction.

. For


(Champagne glass, generic IOR 1.52)


Contrary to the old CoronaMtl, now labeled as Legacy Material, Corona Physical's IOR is bound to a physically plausible range of 1.0 and up to 3.0, and its reflection/refraction IOR values are interlinked in a physically plausible manner.


(IOR can only be controlled through the Base layer, and it affects both reflection and refraction.)


Examples: The first example showcases how IOR can affect refractive distortion and reflection strength on applied materials (for the sake of realism, the impure glass left image had its absorption slightly darkened). From left to right there are, generic-glass (impure) 1.52 IOR, flint-glass pure 1.62 IOR, lead-glass (crystal) 1.8 IOR.


(Full-size comparison link, only for  generic and flint glass: https://corona-renderer.com/comparer/bWLX7s)


Note: With the new Corona Physical Material, you can now have anisotropic refraction to go along with anisotropic reflection, something that was previously impossible.


Roughness

The Roughness parameter controls the smoothness of the base layer's surface. A value of 0% (color black if using a map) gives a completely smooth surface which leads to sharp reflections from the base layer. On the opposite way, a value of 100% (color white if using a map) gives fully rough surfaces leading to blurred reflections. A smooth surface also influences diffuse reflections, yet a rough one leads to a more flat-like appearance.


(Low roughness against higher value roughness: https://corona-renderer.com/comparer/zjUHXx)


Examples: The following examples showcase how roughness values can affect the rendered outcome of a material, how roughness can affect refractive materials but also opaque ones. As a first example, a metallic pole with a roughness value of 10% (left side) against a value of 50% (right side).


 (Full-size comparison link: https://corona-renderer.com/comparer/dFxcad)


Next up, a frosted lamp-bulb coated with glossy finish against a clear glass one, same IOR values different roughness. The frosted lamp has a roughness of 90%, while the clear glass one a 2%: 


(Full-size comparison link: https://corona-renderer.com/comparer/fjMoOJ)


Roughness values affect both reflection and refraction equally. Rough refractive materials like etched glass (frosted, sandblasted, etc.) won't return any glossy reflections if their roughness value is high, something that was possible to do with the Corona Legacy Material. In a proper manner, a coated rough surface can introduce both underlying rough surface but also glossy coating through the use of Clearcoat, as we will see below.,


(Full-size comparison link: https://corona-renderer.com/comparer/szNYfS)


Note: Roughness mode can be modified to Glossiness in the "Advanced" section within the material (per material change); additionally, in the Corona Renderer Preferences, menu Corona > Preferences > Physical material defaults, it can be changed as a roughness/glossiness global default. The terms Glossiness and Roughness are interchangeable; they are simply the inverts of each other. In the case of inverting glossiness maps into roughness within Cinema 4D, do note to avoid using linear invert function like those from CoronaColor Correct. Instead, resort to inverting the native C4D bitmap's "Black Point"/"White Point" values (from zero to one and vice-versa), or you can use a Filter Shader and invert the "Low Clip" and "High Clip" values as well.. Instead


Below you will find an example rendered with glossiness (left) and roughness maps (inverted) (right); the rendered result remains unchanged between the two modes.


(Full-size comparison link: https://corona-renderer.com/comparer/NH6AhF)


Clearcoat

A Clearcoat layer can be defined as a transparent layer of varnish/finish that can be used to cover a surface. In real-world applications, the clearcoat is one of several layers of paint that can cover a coat of paint, for example.  In the case of metallic panels, It usually begins with a base coat which acts as a primer, and eventually, the base-colored coat will be covered by the clear coat. Generally and in the case of the Corona Physical Material, the base color will mostly consist of matte surfaces that are being coated by a clear coat for one of the following reasons:


  • Cover a surface with a finish/Varnish.
  • To change the reflective index or the type of glossiness of a surface.
  • Introduce coloration or enhance the thickness of a base color through clearcoat absorption
  • Introduce additional bump details on a surface.


(Rough surface plastic base with highly glossy coating through the use of the clearcoat layer)


Clearcoat can be controlled by the amount parameter on how strong the effect of the layer will be; values from 0-100% can be used. Its roughness (similar to base color roughness with values 0-100%), Index of refraction (1.0-3.0), separate bump map, and absorption color that influences all the layers below it. 


Examples: As we will see in the following examples, clearcoat can offer great visual variability to applied assets as well as add realism to the rendered material. Separate bump for the base and coat The base layer and clear coat can have different bump maps. On the left image on the wooden mannequin, we have a subtle bump wood map; a clear coat is still applied but with no bump of its own. Right image clearcoat bump is being introduced in the form of a strong grunge mask that adheres to the weathering of the varnish; both bumps are being applied.


(Full-size comparison: https://corona-renderer.com/comparer/UBqpXV)


Clearcoat absorption can introduce a significant difference in the diffuse base of materials. In the case of an instrument like a violin, the raw unedited wood has a rough surface (Roughness amount ~ 70%) and a low IOR of 1.35, as well as a consistent bump map following its wooden texture.


(Clearcoat example settings setup)


Through the use of a clear coat, we can emulate a varnish/finish look on the material; with an increased clearcoat IOR of 1.4 and significantly lower roughness levels, the material now becomes more glossy. The addition of clearcoat absorption color is introduced as a form of varnish-thickness. In reality, violin coating consists of numerous coats that add up to coat thickness and darkened coloration of the underlying base.


(Full-size comparison link: https://corona-renderer.com/comparer/n5b6pe)


(Full-size comparison link: https://corona-renderer.com/comparer/3nvVoy)


Car paint is also a great example of how the clearcoat can help to achieve great results (rather than using layeredMtl). 

Similar to the above, for the base layer, a rough colored surface can be used as a primer, coated thereon by a glossy clear coat. The addition of clearcoat absorption color can help achieve further coloration.


(Full-size comparison link: https://corona-renderer.com/comparer/d2YphT)

(Clearcoat absorption color can be changed to affect the base layer)


Note: In cases where the clearcoat layer has a weathered coating, this can be emulated by mapping its amount parameter. This will help introduce patchy-looking paintwork or a surface look of "skin shedding," scratches, and other forms of damage, as seen in the previous examples."


Sheen
Sheen can be used to approximate the effect of subsurface scattering in microfibers for cloth-like surfaces such as velvet, satin (etc.). Layer strength can be controlled through the amount parameter, while roughness can offer further control of specular highlights or overall sheen reflectance. Sheen Color can be edited to a specific color for a more preferred visual outcome (although in reality, the sheen is exclusively white color). All of Sheen's parameters can be mapped to offer a further variation, irregularity on the applied effect.


(Example of sheen applied on fabric)


Note: If the Roughness mode in the material's advanced section is set to Glossiness, the parameter's value is treated as glossiness, which works in a reversed manner as seen for the  Base Roughness parameter.


Complex IOR for Metals

Dielectric materials (non-metals) can have their Fresnel effect rendered based on their refractive index alone; for metals, however, their reflectance curve also depends on other complex variables. In order to achieve a precise Fresnel effect for a given metal (e.g., gold, copper, etc.), you can use Complex IOR instead of base and edge color. For a detailed explanation and practical examples, please visit the following guide: How to use Complex IOR for Corona Physical Material?


(Metals created through the use of CoronaPhysical - Complex IOR)


Note: Base and edge colors should be used primarily since they offer more flexible control of the material. Using Complex IOR settings without reference values is not recommended.


Volumetric and Subsurface scattering (SSS)
Volumetric and Subsurface scattering properties can also be used within the Corona Physical Material. Volumetric scattering can only be enabled when the material has refractive properties, while Subsurface scattering (SSS) can only be used if the Refraction channel is not being used. Do note that volumetric and Subsurface scattering parameters are only enabled for Non-Metal materials.


(Example of volumetric and subsurface scattering)


Examples: A material like marble can benefit from using Volumetric or Subsurface scattering, with the latter being much faster to set up and to render. Below, you will find an example of a statue with Subsurface scattering and without.


(Full-size comparison link: https://corona-renderer.com/comparer/srkpp5 )


Thin Shell (no inside)

The previous Thin (no refraction) function is preserved from the Corona Material (Legacy Material) to the Corona Physical Material but renamed to Thin Shell (no inside). The current parameter, when enabled, simulates a  thin shell with no internal volume (hollow). 

Such material offers no actual refraction nor any volume or subsurface scattering. Its refraction is replaced by opacity, and its subsurface scattering is replaced by diffuse and translucency. This parameter is best enabled when recreating "fast to render" windows/glass or leaf materials that are assigned to a single-faced/plane model.,


(Example of a thin-shell leaf, assigned on a plane mesh with opacity)


Presets

The Corona Physical Material comes with 34 presets you can choose from. These don’t include any maps, just preselected settings in the material to give you a great starting point for many common types of materials that you’ll be using in your scenes.


(Preset list along with some preset CoronaPhysical materials)


Most of the metallic presets are split into three categories. A brushed preset that has a strong roughness value along with surface anisotropy that simulates a "brushed" effect on the material.

A foil preset, in order to represent a flattened mostly smooth metallic surface (very thin sheet or leaf-like material, example of a copper foil, or aluminum foil). 

Generally followed by low roughness values, overall more glossy surface, and smaller amounts of surface anisotropy. And lastly, rough as an in-between of foil and brushed types, with average rough values and low anisotropy. 


(Some of the available CoronaPhysical Mtl Presets rendered) 


For the dielectric presets (with the exception of the Iron preset), there is not a particular categorization other than specific material properties per case. Some examples:


  • The Diamond preset, high IOR, enabled and correctly set dispersion.
  • The Glass Architectural preset differs from the regular Glass preset by having a distinct absorption color added to it.
  • The Velvet preset tries to emulate a silk-type surface by utilizing anisotropy and taking into advantage the new implementation of the Sheen layer.
  • With the Plastic PVC opaque, having a generic plastic example coated with a plastic clear coat layer of small thickness (clearcoat amount of 0.5).

Miscellaneous Information


Specular to IOR mapping

IOR mode can now be set to Specular; in such a case, the value of IOR will be treated as a specular value, which is then internally converted to IOR using an established formula. This parameter can be found in the Advanced section within the Corona Physical Material. It can also be changed into a global parameter within the Corona Renderer Preferences, menu Corona > Preferences > Physical material defaults: default IOR mode.


In the following image comparison, the material on the left utilizes a specular map for its base specular, which can be set as Disney Specular from the IOR mode parameter. On the right side, you will see the same material with an unmapped IOR and a default value of 1.5; its IOR mode is set as the default IOR.,


(Full-size comparison link: https://corona-renderer.com/comparer/H6WJaV)


Converting Corona Legacy Materials to Corona Physical Materials

With the release of Corona version 7.0, Corona Converter was also updated; it is now possible to convert all of the Corona Legacy Materials in the scene into Corona Physical Material.