Original subtitles

You don't understand this node. Yes, the principal BSDF. The very first node that appears when you open the shader editor.

What really is anisotropy? What is the new subsurface scattering? What the hell is sheen?

And don't get me started on the shit this index of refraction is up to.

Today I answer all these questions in an order that gets increasingly harder to grasp and stick around till the end

because understanding the last concept can literally make or break your renders. So let's get right into it.

BSDF stands for Bidirectional Scattering Distribution Function. It's a complex concept but I can break it down for you.

Essentially, essentially BSDF is a mathematical tool used to...

Whoa, hey, hey. I think I lost you there for a second. Hang on.

The principal BSDF. Yeah, this one node contains not one or two but sometimes three or even four layers.

The topmost layer is called sheen which is followed by coat and then what I like to call the base layer.

The base layer can take up to four modes, metal, diffuse, transmission and emission.

Okay, let's start from the easiest layer to understand and move to the most complex layer. Let's break it down.

Starting from the topmost layer, sheen. Sheen basically simulates very small fibers on the surface it is applied to.

For cloth, this adds a soft velvet like reflection near the edges.

It can also be used to simulate dust on arbitrary materials.

Weight basically controls the intensity of the sheen, zero being off and one being on.

Roughness controls the amount of color that is reflected back to the camera.

Higher values reflect more color and can give a dusty appearance while lower values look fuzzy and darker.

Moving on, we have coat. Coat is added on top of materials to simulate a clear coat or a lacquer or a car paint.

Weight does weighty things, roughness does roughness things.

Next up, we have coat IOR. IOR controls the amount light bends when it enters the coat layer and this is how it affects the surface.

It also controls the falloff of the coat tinting.

Higher the IOR, greater will be the falloff.

Next input is a normal input. After doing blender for four years, I found out if you hook this up to a noise layer and crank up the noise, turn the distortion on a little bit, you can make it look exactly like your ball sack.

Next up, we have base layer. As I said before, base layer can take up to four forms, metal, diffuse, transmission and emission, starting off with the metallic form.

The metallic input basically controls the metalness of your material.

While blender gives you the option to vary between full metal and diffuse, if you want a totally photorealistic result, set the value of metalness either one or zero because there's no half metal in real life.

Next up, we have the transmission layer.

Turning on this layer makes your surface look like glass. Again, set this value between zero and one because there's no half glass or half transmission in real life.

Next up, we have the emission layer. You know this layer. It's pretty self-evident. I don't need to explain this.

Next up, we have diffuse layer. This layer can be further divided into plane diffuse or subsurface and specular.

Starting off with subsurface.

Subsurface is used to simulate that semi-translucent kind of surface of organic matter such as skin and milk.

Light basically scatters below the surface to create a soft, glowing kind of effect.

Blender presents us with three methods to render subsurface, that is Christensen-Burley, Random Walk and Random Walk Skin.

Christensen-Burley method is an approximation to actual volumetric scattering and is a trade-off between render speed and noise for accurate photorealism.

Random Walk, on the other hand, uses true volumetric scattering inside the mesh to get a more photorealistic result.

Overlapping faces and holes in the mesh can cause problems, so be wary of that.

Random Walk Skin is optimized for skin rendering. It makes it so that your human models actually look like humans and not a shitty excuse for a sex doll.

Under subsurf, we have weight, which is there to remind you that you're a land whale.

The scale of subsurface scattering defines the scattering depth.

The larger the scale, the softer the final material will appear to be.

The radius of scattering basically gives the average distance light travels below the surface.

It takes in three inputs, x, y, z, and this corresponds to the RGB channels.

Increasing the value of x will increase the depth by which the red colored light rays will travel into the surface.

The overall effect basically tints the subsurface scattering.

To simplify this, you could just control the entire thing with an RGB node.

Anisotropy basically defines the directionality of the subsurface scattering.

Zero scatters light rays uniformly in all directions.

With increasing values, light rays are more biased to scatter forward in one direction.

The overall effect of this input basically increases that glow effect that occurs when light directly strikes a subsurface material.

To achieve photorealism, it is important to know the anisotropic value of subsurface materials.

For example, skin has an anisotropy of 0.8.

Moving on to the specular layer.

Specularity is basically a means to control reflections on a surface.

Again, Blender presents us with two methods to render specularity.

The first method is ggx.

This method is faster, but it is less physically accurate.

The second method is multi-scatter ggx.

This is more physically accurate, but takes longer to render.

Before we go ahead, I think it is important for us to understand what's the difference between this index of refraction, or what I like to call the global index of refraction, and the other IORs.

Each layer in the principal BSDF is set up in such a way that light interacts with the topmost layer first and then the bottom layer.

That means light will first go through sheen, then coat, then to the base layer.

The existence of these four layers implies the simulated light ray will have to travel between multiple mediums, and hence there exists an index of refraction for each of the layers, except for sheen.

Don't question, I don't know why.

Traditionally, index of refraction is understood as the bending of light when it travels through different mediums.

But in actuality, it has one more important function.

It also controls the falloff of the reflections that happen on the surface between the grazing angles and the head-on angles.

Notice this, as I increase the IOR, more the reflections around the head-on angles intensify.

It is something similar to the Fernel node.

Okay, back to specular.

Specular has an input called IOR level.

It basically controls the intensity of reflections that occur on the head-on angles.

0.5 means it presents the reflections as they are.

0 removes all the reflections, and 1 doubles the intensity of them.

Next up, we have tints.

Specular tint is of two kinds, non-metallic and metallic.

Non-metallic tints are activated when the metallic is turned all the way down.

They provide an artistic control over the color of the specular reflections at head-on angles,

while grazing angle reflections remain white.

Metallic tints only color the grazing angles or the edges.

Next up, we have anisotropy.

Anisotropy for reflections basically decides the shape of the reflections.

Higher values give elongated highlights.

This is meant to replicate microstructures called strands that occur on a surface,

which bias the reflection in a particular direction.

And yeah, that covers all the inputs in the Principle BSDF.

I hope you learned as much as I did making this video.

I made this video because I recently upgraded to the new version of Blender,

and the new Principle BSDF stumped me quite a bit,

and I realized I didn't know how shit went down behind the scenes.

That about wraps the video.

Check me out on Instagram.

Thanks for watching. Bye!