MATT OZTALAY: All righty. Hi, everybody. Welcome to the Unreal Engine
learning theater here at GDC 2023. My name is Matt Oztalay. I am a senior developer relations
technical artist here at Epic Games. And today, I get to talk to you
about the future of materials in the Unreal Engine. That future is called Substrate. If you've been snooping
around our source code or you've been watching
this on Twitter, you may have heard of
something called Strata. That's what we're talking about
today, same feature, new name. And if you hear me say
Strata, I'm sorry. I mean Substrate all the
time, totally. Substrate is experimental. Substrate is paradigm shifting. And it is going to open up
a lot of new possibilities for surface representation
in real-time rendering. Now, Substrate, it is meant to
be a more modular, expressive, and unifying framework for
describing material appearances. And what I've got
for you today isn't so much the SIGGRAPH-level talk. It's not going to be a
super comprehensive guide. There's a lot of depth
to this system already. And I'm going to have some
links available at the end of the show for you to check
out even more about Substrate because I think it's really cool. Now, like I said,
it is experimental. And as Arran mentioned
earlier, if you've ever spent any time with
the Unreal Engine, you know that experimental means
something very specific to us. It means that it is
not going to support all the features in the Engine. It means that it is probably going
to have some bugs and crashes. And you should
absolutely, positively not use it in a
shipping product. And with Substrate, one of
the big reasons for that is it is mutually exclusive of
the legacy material system. In fact, in order to get
working with Substrate, you have to explicitly enable
it in your project settings. And it's going to remind
you that it's experimental. And there's two reasons for that. One, this is a really big change
to the whole render pipeline. And they just can't coexist. It's not like, oh, we could have
Cascade particles and Niagara running at the same time. No. No. No. No. This is more like PhysX and Chaos. You got to pick one. The other reason is that when you
convert your project to Substrate, it's going to add in this
Substrate Legacy Conversion node. And that's fine. Actually, this is a really
good thing because it means you don't have to
convert all of your materials to Substrate right out of the gate. The bad news is, if you save
a change to this material and then you disable
Substrate in your project, it's going to break a
bunch of connections. And you're going to
lose a bunch of work. I don't want you to
lose a bunch of work. We don't want you to
lose a bunch of work. You don't want you to
lose a bunch of work. So our general
recommendation is start looking at it in a separate project. So what are we looking at here? Because you're going to
see, you've seen already, the Material Editor
isn't going away. That's staying. What's changing is
basically this purple node. And if we all remember in
the legacy material system, because it wasn't that long
ago, we had this list of static shading models. And all these shading models were
kind of doing the same thing. And then on the right there, we had
this big, monolithic material input node that was mapping
to a bunch of G buffers. And with Substrate, those are gone. Don't need them anymore. Because we're replacing it
with the Substrate Slab BSDF. So you're going to look at this. You're going to ask
me two questions. One, what is a BSDF? A BSDF just stands for Bidirectional
Scattering Distribution Function. Effectively, that's a shading model. A shading model is a BSDF. And the next thing you're going
to ask me is, what is a slab? And a slab, very basically, is meant
to represent both the interface, the point at which light
interacts with matter, and how light scatters
through the medium, the meat of a slab of matter. And then we're going to
look at that node again. Because you're going
to notice that there are some missing
inputs to this node. And the reason there's a bunch
of stuff missing and changed is because one of the things we're
doing with Substrate-- the big thing we're doing with Substrate is we
are removing a bunch of abstractions from the legacy material
system and replacing them with the actual physical
properties of matter, so that we can more
accurately model how light is going to
interact with those different properties of matter. And as we work our way down,
the first two you're going to notice are missing is
specular and metallic. Those were abstractions that we
used in the legacy material system in the default lit
shading model in order to generate dielectric
and metallic surfaces. Now we're going to use the actual
physical properties of matter to do that, which is the
reflective index of matter. And we have abstracted that a little
bit out to these F0 and F90 inputs. So F0, very simply, is the
amount of light reflected back at the viewer when your view is
perpendicular to the surface. And F90 is the amount
of light reflected back at the viewer when the view
is tangent to the surface. In effect, this just means specular
color and edge specular color. So if I wanted to represent
a metallic surface, I'm going to do it with its
actual physical properties. So a metallic surface has no
albedo component, scientifically. If I want to make it shiny,
I'm still going to use roughness. But instead of passing in some
sort of special base color and calling it metallic, I'm going
to use the actual reflective index of something like gold. And I've got a resource available
in the links at the end of the show, if you want to go look up a bunch
of these reflective indices. And so I plug that in. And now I have gold using the
actual physical property of matter that makes something look like gold,
which is that reflective index. Now, like I said,
this is still experimental. The UX of this is going to change. We want to keep working on
it, making it easier for everybody to use. And we want to make sure
that this is a system that's going to work for everybody because
this is where we're going, right? And if you want to
check it out right now, we've got the content examples
available for 5.1 actually has a Strata hallway. So what you can do is pull
down the 5.1 content examples, duplicate a version of that
project to enable Substrate in it. And you're good to go. You're off to the races. Because like I said,
we want you to check this out. We want you to ask questions. We want you to give
us feedback on it because this is
where we're going. And this is paradigm shifting. And I'm trying not to
overhype this too much. But that's really, really difficult
because I'm standing and walking around on a stage at GDC with
a microphone attached to me. So I get to be really
excited about stuff. And like I said,
try not to overhype it. But I feel like this is
the next thing after PBR. It's another
generational leap in how you're able to
describe materials and how materials are going to--
how we're going to render materials. Like we had diff norm spec. And that was really cool. And that a lot of stuff
changed when we introduced physically based rendering. This is the next thing. So let's take a look at some of
the stuff you can do with it. Because like I said,
very some really cool stuff here. So the first thing is that,
with these different BSDFs, you can layer them together. Or I'm sorry. You can blend them
together horizontally, which opens up a lot of really
interesting possibilities. So if I came in here and wanted
to make something like copper, I can come in here. I have the reflective
index of copper. And now I get to talk about
some really interesting stuff. So if I wanted to do something like
copper oxide in the legacy material system, we really struggled there. The legacy material
system was really good when metallic equals one,
so a metallic surface, or when metallic equals zero,
giving us a dielectric surface. So copper oxide is
a really tricky one because that's a
semimetallic surface. And that midrange of metallic
equals 0.5 was really-- it was a real struggle
for us to physically-- to properly represent that. So I can come in here and
grab my copper oxide textures. And I've got a base color, a
normal, and a roughness map for it. So I'll get those two
things plugged in first. And then I get to talk
about the helper nodes. So you're going to hear me today
talk a lot about physical accuracy, properties of matter, realistic,
blah, blah, blah, blah, blah. That's all well and good. But sometimes those abstractions
were really helpful for us. And the good news is
we understand that. And we've created a
bunch of helper nodes. So if you ever want to find anything
related to Substrate in the Material Editor, just click that right. Do that right click. And then everything
in the context menu is going to have all
of those nodes for you. And we're going to use this
metalness to diffuse color F0. So that lets me plug
in that base color. And then I can set my metallic
and specular values to 0.5. And this node is going to distribute
base color between diffuse albedo and F0, allowing me to create a
more accurate semimetallic surface. And then-- got ahead of myself. And then I can use the
Substrate Horizontal Blend node to blend these two together. I'll just grab a
little blend texture. And then I'll do a
height lerp on it. Wrong height lerp. Height blend. And plug that in. And plug this in. I've got this rusty,
streaky copper look here that is a separate
BSDF being rendered in a physically accurate manner
separate from the copper BSDF that's next to it. But that's not all. One of the other fun things
that we get to do with Substrate is that we get to layer
BSDFs on top of each other. So in the legacy material system,
if you wanted to have a clear coat, that was a separate shading model
in order to get that to work right. But now with Substrate,
we don't need separate shading. We don't need separate
shading models that are built into the Engine because we're exposing that
to you with this slab BSDF. So if I want to vertically
layer a coating on top of this, I just make another slab,
set it's diffuse and roughness. And now I get to talk
about the other abstraction that we've done away
with, which is down here. So you'll notice we've got
these three nodes with prefixed with SSS, Subsurface Scattering. And then the next one is MFP. And this is a fun one. This is another physical
measurable property of matter. And that stands for Mean Free Path. It's the mean distance that
different wavelengths of light will traverse through a media
before it runs into something. This is a measurable phenomenon. And if you want to get into
it, everything in the world has a little bit of subsurface
scattering to it because there's space between matter. And it might be on the
order of nanometers. But it's still there. And we can still
kind of perceive it. And this MFP is the
physical property that we previously abstracted with things like subsurface color. But subsurface color was a thing
that was really useful to me because I understood
it as an artist. So the interesting thing and the
sort of counterintuitive thing is that, if I wanted to
make this a blue coating, I would, oh,
I'll put a blue color into MFP. But what this says is the mean free
path of red light through the media is one millimeter, as with green. And then blue light is going
to be able to go further through that media, which means
I end up with a yellow coating. But sometimes that's
not how my brain works. And the good thing is
we have another helper node for that,
which is the Substrate Transmittance-To-MeanFreePath
node, which just lets me translate this color to a mean free
path value in centimeters that will work for our vertical layering. So I grab that. I plug that into the top. I plug our bottom into the bottom. And then I can right click and
do Connect To Front Material. And now I have a colored coating
on top of my rusty copper in a way that I can control. And I can do some really
fun stuff with this. I can come in here and use
a Substrate Coverage Weight node, which is just going to
say, how much should this be BSDF be contributing to the final pixel? And I can come down here
and grab our height lerp, whoop, swap this out
for a scratch mask, plug that in,
plug that into the vertical layer. And now I can scratch
that coating off of everything else underneath it. And I'm not paying to
light that top level BSDF. Let's see what we can do with this. So I've got these painted metal
milk crates here, milk cans here that I got from Quixel Megascans. And this is the same system. Basically, I've got a painted
metal with a coating on it. And then I've got a horizontal
blend between rust and bare metal. And then we're vertically
layering all of that together. Nothing up my sleeve. But these milk cans, with their
clearcoat and their really nice paint, don't look quite
right in this environment. So what I can do is start
to chip that clearcoat away. And as I chip that clear coat away
to reveal the paint underneath, that paint is going to start to
get scratched up a little bit. And then we start to
reveal the bare metal. And the bare metal is going
to start turning to rust as it's exposed to everything
in the environment. And this is a really
fun thing that we can do with Substrate because
of all of this blending that's available to you. Now, one of the fun
things about blending-- I had this idea. I was up in Tahoe this past weekend. And there was so much snow up there. I've never seen that much
snow in my entire life. And it got me thinking
about all the cool things that we can do with Substrate. And I was like, man,
snow would be a really cool thing to do with Substrate. But it was the Saturday before GDC. And there was
absolutely no way that I was going to sneak
snow into this demo even though two of the other demos
we're doing today already have snow. Like, that's a bummer. I just think this kind of
stuff is really, really cool. Now, what may not be
apparent from this is that,
because these are separate BSDFs, each BSDF gets its
own specular lobe. So if I'm layering a coating
on top of another coating, the top coating is going
to have its own specular response separate and distinct from
the specular response underneath it. And the physical properties
of the top coating are going to affect the
specular response of everything underneath it. So if I jump back into
our slab-- and you'll bear with me for a second
while I rip most of this out. Let's go back to our bare
metal, our copper here. I'm going to grab into that. And then maybe I want to have a
really just a super clear coating. And again, you're going to hear
me talk about physical properties of matter and realistic lighting. Just because the system
that is doing the lighting is going to do so in a
physically accurate way does not mean that
you have to create physically plausible surfaces. You can do whatever you
want with these numbers. And we have figured out how to use
these numbers to create physically plausible lighting results. So I can have something. I can have direct control
over this MFP value to create something like a-- we're
going to do like a satin coating on top of the copper. And so the satin coating,
because roughness is an abstraction
of a concept called microfacets and microfacets are going to diffuse light,
those microfacets diffuse light outward and through. And so it's going to create
a more diffused specular response on the surface underneath. Now, again, this might not be
super visible at this point because it's flat on top of flat. So it can come in here and
grab a hammered copper normal, plug that into my normal map. And now, we're going to see a kind
of very solid white hot specular in the middle that is the
specular response of the coating. And then underneath, ,
hopefully you can start to see it. We're going to see
a different lighting response on the copper itself. But again, this might be
a little tricky to see. So I'm going to do a fun
thing with mean free path. And we're going to talk about this
top thickness value over here. So I can set the mean free path. Again, directly to
whatever value I want. So I could say at most the mean free
path of light through this media is going to be half a centimeter. And then because I
have direct control over these physical
attributes, I'm going to say this coating also
has an albedo value. And then if I grab the height
map of map of my copper-- so the way I've got this setup is
that a value of one, in this map is the values of
the hammered copper. And then if I multiply this by
two, what do we think is going to happen to the light that
is propagating through the media? Because again,
we're trying to deal with energy conservation and light propagation. If the mean free path of
light through the media is only a half a centimeter. And I say that the media is
2 and 1/2 centimeters thick that's going to absorb a bunch
of the light moving through it. And then we get to fill in all of
the valleys of this hammered copper with this weird kind
of milky clearcoat. And then hopefully this makes
it a little easier to see, that separate specular response
of the surface underneath. Just look at the hills
of that hammered copper. And you start to see that
separate specular response. There's a lot of really fun
things that we can do with this. And again, we're going to
talk a lot more about stuff. And we're going to talk about this. This is my very crude representation
of thin-film interference. So light has wavelength. We're not going to get super
deep into the wave/particle nature of light. But light is a wave. And light has a
wavelength, visible light between 380 and 740 nanometers. Now, if you have a coating
that is within that range, that basically sub
1,000 nanometer range. Light gets into that coating. And because of the index
of refraction of the media, it's going to start bouncing around. And when it starts bouncing around. It's going to create these
constructive and destructive interference patterns, which makes
some really interesting results. You're all looking at me like I
have horns growing out of my head. But I promise you've all
seen this effect before. And I'm going to say this too now-- just sneak this in-- I'm talking a lot
about physical stuff. I'm talking a lot about
optics and physics. I went to art school. If I can figure this
out, you can too. Don't worry. You've all seen this before. You're all familiar with
soap bubbles, right? We've all seen a soap bubble. You know those really weird, funky
color patterns on a soap bubble? That's thin-film interference. And then that lets me talk about
our next helper node, which is the Substrate Thin-Film node. So all I have to do is set
the index refraction of that. And then I set the thickness of
that using good ole T_Noise01. So the input thickness value
here is in 10 micrometers, which is 10,000 nanometers,
which is a little too high for me. So I'm going to scale that down
to the 1,000 nanometer range. And then it gives me
these specular color and edge specular color values out. And now I get a physically
plausible thin-film interference on the surface of this bubble. And now this is the thing that is
totally achievable in the legacy material system. In fact, Ben Cloward did
a great video about this. He released it on YouTube last week. Definitely go check it out. But what's interesting
about this effect is that, as I move the
camera around the bubble, that pattern is going to
change because we are viewing the film through different
thicknesses, which means different amounts of light
are going to bounce out and give us an interesting response. There's a really cool
application of this. I don't know about
the rest of y'all. I spent a lot of time
on blacksmithing YouTube in the early days of the pandemic. And I learned about tempering steel. And when you heat steel up to a
certain nonmelting temperature, you change its
crystalline structure, which makes it
harder or stretchier. And these properties
are really valuable. But how do you know how
hot a piece of steel is without touching it
with a temperature probe? And the cool thing about steel
is that, as you heat it up, it creates a layer of
iron oxide on the surface. And as it gets hotter
and hotter and hotter, it creates a thicker and thicker
layer of iron oxide in the zero to 1,000-nanometer range, which
gives us thin-film interference. So as you increase the temperature
of the steel, that coating, thickness is going to increase,
which gives us different colors. And that's how blacksmiths
and metal workers are able to figure out how
hot a piece of steel got. And there's a really fun-- so basically,
any time a piece of steel gets hot, it's going to create
that iron oxide coating. And one of the things that
gets really, really hot is rocket engines, right? So what I can do is start to get
this rocket engine a little hotter. And you can start to see around
the end of the engine bell here, that's going to start to
develop a little color to it. And then you're going
to start to see, as the engine runs a little longer,
these precombustion chambers are going to start to
develop that thin coating. And it's going to change a little
bit into that pale straw color. And we start getting into
the deeper blues and purples as that engine has been
running for longer. You can see right here,
in that upper left area, that coating thickness is
going to change that specular response as we view the surface
from different glancing angles. I just think this is really cool. So Substrate is opening up
a lot of new possibilities for surface representation. These were things that
were either previously very difficult or
expensive or things that were just downright impossible
to achieve in the legacy material system. So I just started wearing these
glasses about a month ago. I'm not super used to them. These are totally
fogged over and blurry. Can't see any of
your faces right now. And that lets me talk about rough
refractions and frosted glass and weird, thick coatings. So in the legacy material
system, if you wanted to do something like
that, you would need to use the
SpiralBlur-SceneTexture node or scene texture average. And that will give
you kind of OK results or really good results that
would be really expensive. We don't want that. So I'm going to show you how
easy this is in Substrate. And I'm going to do it from scratch. So all I have to
do is set the Blend Mode to
TranslucentColoredTransmittance. And then I set the Refraction
Method to Index Of Refraction. And then I'm going
to set the roughness, so that I don't spoil the surprise. Set the index of
refraction of the material. And now what I've got is a totally
opaque, translucent surface. But that's not quite what we want. And you're going to notice that
there's no opacity input pin here. And that's because we've also-- opacity was itself an
abstraction of mean free path. So if I wanted something
that is super, super clear, that means the mean free path of
light moving through that media is going to be really, really high. So if I wanted to make
this super translucent or super, super transparent,
I just set the mean free path to a really high number. And now I've got a properly
translucent glass sphere. And if I wanted to make
this a frosted glass sphere, all I have to do is frost the glass. And now I've got frosted glass. And all I needed to do
was these 3-4 nodes. And I have direct access to this
because the roughness of the media, again, those microfacets, are
going to bounce and diffuse light both going into and coming
out of the surface, which is why we get that beautiful
rough refraction going there. The other fun thing
that we get to do is I get to expose
myself as a liar. I had snow in the whole
scene the entire time because I really wanted to do snow. And the reason that I got to do
this without really affecting too much of the performance is
because Substrate's coverage weight means that, if you have two
slabs on top of each other but one of those slabs
isn't visible, you're not paying to light that slab. So I got to put snow on top
of everything in my scene without having to pay any additional
cost because the snow wasn't visible. So one of the things
with Substrate is that, as you increase
the number of layers, the number of visible BSDFs
in a single pixel, that's going to increase the pixel cost at
a slightly higher than linear rate. But because of that Substrate
Coverage Weight node, I didn't have to worry about
that if it wasn't-- if the snow wasn't visible. So all of these little
green areas here are where we're blending between
the base material and snow. But once we get into the snow,
that's still just one BSDF. And if I get rid of all the
snow, you can see that the
whole scene is just one BSDF. I wasn't worried about having
to pay that cost too much. Again, I think this is some
of the really cool stuff that you can start
doing with Substrate. And on that note, that's my time. Thank you all so much
for sitting and watching. Really appreciate your
time and attention. Don't forget to thank your
conference associates. Feel free to scan that QR code. That's all the links and more that
I talked about during the show. And get home safe. And if you want to come out, see
me after class, I'll be over here. And I'll be answering
questions about Substrate. Thanks again.
