Fresnel Reflections

What we refer to in CG by the term “Fresnel Effect” or “Fresnel Reflections”, is the way Specular Reflection intensity changes according to light \ surface incident angle, and it is a basic optical property surfaces.

Specular reflection intensity changes according to light incident angle, and behaves almost like a perfect mirror at grazing view angle.
The reason we call this natural reflection behavior “Fresnel Effect” or “Fresnel Reflection” is that the equations describing the how reflection intensity changes according to incident angle were invented by the French Physicist Augustin-Jean Fresnel, and in early CG days, not all systems knew how to calculate natural reflections or reflections at all for that matter. So in CG we ended up treating this as something special, when in fact it’s not special in nature, it was just special in the early days of ray-tracing.

When rendering Fresnel Reflections, the reflection intensity isn’t determined by a linear blending percent like mixing a layer.
It’s determined by a factor called “Refractive Index” or “Index Of Refraction” i.e. IOR.
The IOR value is derived from the physical material’s density, which is the key factor determining both reflection intensity and refraction amount.

Examples of some physical IOR values*:
Air (vacuum): 1.0
Water: 1.33
Glass: 1.52
Diamond: 2.417
* Physical values differ between different measurements and samples of materials so you might see differences between different data sources.

FResnel_Off

This ball is rendered without “Fresnel Reflections”.
Its Specular reflection is blended consistently at 50% over the diffuse color (reflection), not affected by the light/view incident angle.
The result looks wrong for a natural material. It may look like a dielectric material (non metal) that’s coated with a silvery coating, but it can’t look correctly like glossy plastic or glass.

FResnel_On

This ball is rendered with “Fresnel Reflections”.
The reflections look natural for a dielectric material (non metal), because they are dim at perpendicular incident angle and intense at grazing view angle, hence seen mostly at the sides of the ball accentuating its contour.

Theoretically Specular Reflection for all types of materials should be calculated using what we refer to in CG by the term “Complex Fresnel”, that is reflection equations that take into account both the Refractive Index (IOR) and Extinction Coefficient for 3 primary colors (spectrum wave lengths).
*Complex fresnel component values for different materials can be found on https://refractiveindex.info/.
In practice, for Dielectric materials (non metals), most common production rendering systems use what we refer to in CG by the term “Simple Fresnel” or “Simple IOR”, that is calculating the reflection for all 3 primary colors using a single Refractive Index value, which is the Refractive index of the Green primary color.
This method has proven itself to be very efficient for rendering non-metallic surfaces (dielectric materials).
Rendering metallic reflection using complex IOR produces the most realistic color and reflection* for metals.
*In metallic surfaces the color is the reflection color itself and not a separate Diffuse component.
Some rendering systems like Arnold 5 for example have implemented a general form* of Complex IOR into their physical surface shader, Complex IOR reflection can also be rendered via OSL shaders that can be found on the web (or written..).
*I’m using the term ‘general form’ because these implementations don’t include input for Complex IOR values but just a general metallic reflection curve, that interpolates manual color selection.
Popular useful cheats for mimicking metallic reflection without complex IOR are to set a very high (non physical) simple IOR value, like 15 to 30 which forces the Fresnel reflection to become more metal-like, or turn Fresnel reflection completely off, turning the specular reflection into a perfect mirror reflection, or create a custom reflection/angle curve/ramp that produces the effect of the metallic reflection color and intensity changing by incident angle, see example here.

In many popular production renderers, the physical surface shader uses a single IOR parameter. Some rendering systems allow using 2 different IOR parameters, one for calculating reflections and the other for calculating Refraction.
* physically correct dielectric materials should be defined with the same IOR value for both reflections and refraction. using different IOR values for reflection and refraction allows useful cheats like creating transparent a material that is modeled without any thickness or defining a transparent glass that has silver reflective coating like sunglasses sometimes have.

Notes:

  1. IOR lists on the web, that display only simple IOR values like this list, are not valid for metals, and produce wrong results.
    *Using simple IOR values for dielectric materials however is very efficient.
  2. There are parts in the CG industry where in daily slang language, the term “Fresnel” is used to refer to any shading effect that is view-angle dependent,
    Usually referring to the shading properties appearing at the “sides” or contours of the model.
  3. There are some CG systems that use the term Fresnel to refer to a simple linear or non-linear incident angle blending effect, that should actually be called “Facing ratio” or “Perpendicular-Parallel” blending (falloff).
    This is wrong because IOR based Fresnel reflection intensity produces a specific physical Reflection intensity/view angle function curve, and not just a linear or simple power function.
    See example in UE4’s Fresnel node.

Related:

  1. V-Ray Next’s new metallic material option.
  2. Creating a rich metallic shader in UE4.
  3. Complex Fresnel Texture for Cycles.
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Arnold for Maya – Transmission Scattering (Ray Traced SSS)

Software:
Maya 2018 | Arnold 5

Scatter

The Arnold Standard Surface Shader’s Transmission Scattering options can be used for simulating highly realistic volumetrically ray-traced sub-surface-scattering suitable for materials like wax, soap, milk etc.

While the Transmission Depth attribute controls volumetric light absorption within the object (fog), the Scatter attribute controls what percentage of the light will be scattered instead of absorbed, effectively creating the murky effect of semi-transparent materials.

Note that for the scattering effect to work Scatter must have a dominant percentage value, and the Depth attribute must generally be much lower (shallower) than what would create coloring without scattering, otherwise the object will continue to look transparent and lacking the internal substance that we want to simulate.

Also note that the Opaque attribute must be unchecked in the Arnold attributes of the object’s shape node for the light to be able to pass into the mesh and illuminate the volume.
*This is actually a “cheat”, because physical semi-transparency has to be simulated by indirect light calculation or caustics, but for dense volumes like wax it’s very effective and the loss of realism is insignificant.

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You can simulate the effect more accurately by rendering caustics,
In that case the Opaque attribute in the Arnold attributes of the object’s shape node must be checked and more steps must be taken allow refractive caustics to be ray-traced.

Note that simulating the effect using caustics will be very demanding in Transmission samples and Ray Depth.

Scatter_Caustics

Related:
Translucency

Using Arnold’s Ambient Occlusion node to create an eroding paint shader effect

Software:
Maya 2018 | Arnold 5

Arnold’s Ambient Occlusion (dirt) node can be used as a procedural mask to create interesting material effects like in this example of paint that is eroded at the model’s bulging areas to reveal metal beneath it.

In this shader’s case the Ambient Occlusion node is connected to the Mix property of an Arnold Mix shader, that blends between two different Arnold Standard Surface shaders, one simulating the underlying tin metal, and the other simulating the red paint that covers it.

Note that in the Ambient Occlusion node the Invert Normals property is checked, so that the effect will create a mask for the bulges and not for the creases,
And also that the Self Only property is checked so that the node will behave as a fixed object mask disregarding the proximity of other objects.
In this example the output of the Ambient Occlusion node is also process using a Remap Value node to increase it’s contrast so it will define borders between the areas.

Paint-Metal.jpeg

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VRayMtl’s new metallic goodness

Software:
3ds max 2019 | V-Ray Next | V-Ray next GPU

In V-Ray Next (4) for 3ds max, a new Metalness parameter has been added to the Reflect parameters of the VRayMtl material.
This allows for easy creation of realistic metal materials and also effectively enables a PBR workflow with V-Ray.

When the Metalness parameter is set to 1.0, the material becomes completely reflective, the Diffuse color controls the general reflection color, and the Reflect color (which should generally be set to white) controls the reflection color at grazing incidence angle.

The IOR parameter still controls how fast will the main reflection color (set by the Diffuse color) blend into the Reflection color, and I don’t know whether some form of complex Fresnel has been implemented or if it’s just a dielectric simple Fresnel blended over a reflective surface.

An excellent article on the new Feature from the Chaos Group blog:
https://www.chaosgroup.com/blog/understanding-metalness

VRay_Next_Metalness

Related:

  1. Fresnel Reflections
  2. Complex Fresnel for Blender & Cycles
  3. Metal material in UE4

 

Basic Toon Shading in Blender & Cycles

Software:
Blender 2.79

  1. Set a Toon BSDF as your model’s material.
  2. Activate the Freestyle option in the Render Settings, and set Line Thickness,
    * More line style settings are found in the Render Layers settings.
    * Freestyle lines are only rendered in the final render (F12) and are not rendered in the Cycles viewport interactive render mode.

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A more controllable setup:
2 Toon BSDF’s, one for color shading and one for highlights added together, and a Background shader override to control shaded area darkness:
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Randomize textures in V-Ray for Maya

Software:
Maya 2018 | V-Ray 3.6

Connect the different texture options as inputs to a VRayMultiSubTexNode and connect it to the wanted material input.
In theĀ VRayMultiSubTexNode attributes, set Get ID From to Random by Render ID.
* Press add new item in theĀ VRayMultiSubTexNode attributes to add one or more inputs to the list.

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Arnold for Maya Standard Surface Shader Translucency / Paper Shader

Software:
Maya 2018 | Arnold 5

Translucent
In this example, the lamp shade has a Translucent material

The Subsurface component of the Arnold Standard Surface shader (aiStandardSurface) controls Sub Surface Scattering (SSS).
When the ‘Thin Walled’ option is checked in the Geometry attributes of the shader, the Subsurface isn’t rendered as a full volume of material like soap or skin/flesh (the effect that is traditionally called Subsurface Scattering – SSS) but as a thin paper-like translucent surface like paper, thin cloth thin leaves, lamp shades etc. (the effect traditionally called Translucency or ‘Paper Shader’)

* Note that this option is suitable mainly for polygon surfaces without thickness (just one side)

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To create a Translucent shader with Arnold:

Subsurface Weight must be higher than 0.0 for the effect to be computed.

In Geometry, check Thin Walled for the SSS to be rendered as Translucency (Paper Shader).

Extra options:

Use samplerInfo Node Facing Ratio output in Subsurface weight to add realism by changing the weight by angle.

Multiply weave texture with Facing Ratio to simulate fabric translucency.

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