Physically Based Shading
A simplified artist interface
Niklas Hansson
Head Teacher Game programming
The Game Assembly.
Physically Based Shading
• What is it ?
• Why should I use it ?
• How do I integrate it into a project ?
Basic BRDF Theory
• Boundary : When light goes from one material to
another
• Light splits up into two directions : reflection and
refraction.
• The amount of light
reflected depends on the
materials reflective index.
Image from “Real-Time Rendering 3rd Edition”
Basic BRDF Theory
• Basic Material
- Part is Reflected
- Part enters the material
• Metal
-Part is Reflected
-The part entering the
Material is absorbed
Image from “Real-Time Rendering 3rd Edition”
Basic BRDF Theory
•Non metals
- Light that enters the material scatters internally
- Is absorbed
- and often exits the surface again.
Image from “Real-Time Rendering 3rd Edition”
Basic BRDF Theory
• Sub surface scattering
– Distance between enter and exit positions are
determined by the material.
Image from “Real-Time Rendering 3rd Edition”
Basic BRDF Theory
• Point model
– By ignoring distance between exit and enter points
we can model lighting with an BRDF.
Image from “Real-Time Rendering 3rd Edition”
Basic Microfacet BRDF Theory
•Microfacets
•A surface consists of many small
perfectly flat surfaces.
•How much they differ in normal
determines how light interact
Image from “Real-Time Rendering 3rd Edition”
Basic Microfacet BRDF Theory
Images from “Real-Time Rendering 3rd Edition”
Shadowing & Masking
• Not all micro facets facing half vectors direction
contribute to lightning
•Shadowing
•Masking
Images from “Real-Time Rendering 3rd Edition”
So why is specular so important ?
Images from “Everything is Shiny” by John Hable
So why is specular so important ?
Images from “Everything is Shiny” by John Hable
The Microfacet BRDF
Fresnel Reflectance
• Returns a Value from 0-1 in RGB
– Fraction of light reflected from optically flat surface
given Light directon l and Half vector h
Fresnel Reflectance
• Depends on refraction index (in our terms the
substance of the object) and the light angle.
Image from “Real-Time Rendering 3rd Edition”
Fresnel
Images from “Everything has Fresnel” by John Hable
Schlick’s Approximation
• Decently Accurate, uses F(0) the refractive index
as input.
– Which is our substance term.
Geometric term
•Gives a scalar value between 0 and 1
• Contains the fraction of non obscured microfacets
• Various functions exists most using roughness as input
Distribution function
• Calculates factor of microfacets reflecting light towards
the eye
• The function determines size and shape of the specular
highlight
Blinn Phong Distribution Functions
• Blinn Phong Distribution Function
• Modified Blinn Phong
Beckman Distribution function
Conversion :
Blinn Phong Energy Conversation
• If the specular highlight is larger the specular
intensity must be lower.
• N = roughness
Blinn Phong Energy Conversation
Blinn Phong Energy Conversation
Blinn Phong Energy Conversation
Blinn Phong Energy Conversation
Blinn Phong Energy Conversation
• 0.397436n+0.0856832
Blinn Phong Energy Conversation
• 0.397436n+0.03183
Blinn Phong Energy Conversation
Energy Conversation
• Our Normalized equation for specular light
• Lambert
Cdiff*PI <=1
• Full BRDF
Cdiff+Cspec <=1
Ambient Light
• Ambient Light is just Light
– Has a specular component too
• reflections
• roughness controls reflections blurriness
substance controls reflection strength.
• Pre blurred cubemaps
Ambient Light
• ATI cubemap gen has nice functions for
preblurring mipmaps.
– For correctnes the blurring needs to match your
specular light
Blurred cubemap
• Use the angular extent and always sample from top mip.
• Use the glossiness for that mipmap level and evaluate max size of
highlight for size of angular extent.
• Use the phong function to evaluate a contribution for all the pixels
inside the extent.
Why physically based ?
• It’s more realistic thanks to being physically based?
• Materials does not require as much retweaking under
changing lighting conditions.
• You can create more easily varied materials with lesser
number of values
• It saves artist time and simplifies workflow
Simplified Artist interface
• Reduces number of maps from 4-5 to 2.
• Simplifies Junior artist training
– No longer take albedo and desaturate for intensity
– And then lower color for gloss
– We have all seen it
Simplified Artist interface
• PBL simplifies this by creating a material interface
that makes sense.
– Substance
• Is the index of refraction for the material
• No need to hand paint.
• Mathematically matches old intensity
Color picker
Simplified Artist interface
• Roughness
– The old gloss
– But thanks to Normalization a lot more useful
– Where hand painting happens
– Controls reflection blurrinees and spec
– Surface Roughness that normal maps can’t show
– Touch
– Observe
Example Material rusty Paint
Flexibillity
Sample Oskar stålberg
Why physically based ?
• Lower numbers of values to tweak
• Smaller tighter gbuffer
– Low bandwitdh
– Small ALU increase
– Ideal for deferred rendering
• While still allowing very
varied materials
How to use it
• What did we do ?
• Microfacet BRDF with
– Normalized Blinn Phong
– Fresnel reflectance
– Void Geometry Function
• G(h) = 4(n dot l)(n dot v)
– Prefiltered glossy reflections from cubemap probes
– Ambient from ambient probes
What we did
• Cref is the value from the reflection probe
• Camb is the value from the ambient probe
• Fschlick(v,h) is the fresnel term using the view direction instead of
the light direction
Ambient vs reflection Energy Conversation
Lessons learned
• Substance and Roughness vs Intensity and Gloss
– Spend time training and communicating to your
artists
– Help them use the new interface
• Tools is essential
– Beware of bugs
Maya plugin
What we did
• Flexible
• Kind of projects
–Space shooter
–Realtime Strategy
–First Person shooter
Space shooter
Fps project
• Goals
– Resolve all issues found during the last project
– Get an easy to light solution that took little rebuild
time and would work with just a few spot lights.
• Techniques
– All enviroment and ambient probes generated by the
ingame engine to achieve consistency.
– Box projected cubemaps was used for reflections and
a variation was used for ambient
– Simple Multi bounce solution for lightning
Occlusion
• Specular and reflection doing more work
– Will look weird in areas normally don’t reach by much
light
• Use Ambient occlusion
– Multiply to spec
– Multiply to reflection
• If needed add special Specular occlusion
References
Sébastien Lagarde ”Adopting a physically based shading model “,”Feeding a physically
based shading model”
Roy Driscol
“Energy Conversation In Games”
Fabien Giesen “Blinn-Phong normalization factor”
Nathaniel Hoffmann, “Crafting Physically Motivated Shading Models for Game Development”, SIGGRAPH 2010
Dimitar Lazarov, “Physically-​based lighting in Call of Duty: Black Ops” SIGGRAPH 2011
Christian Schüler, “An Efficient and Physically Plausible Real-​Time Shading Model.” ,”
The Blinn-​Phong Normalization Zoo”
Akienne-​Möller, Haines and Hoffmann, “Real-​Time Rendering” book
Yoshiharu Gotanda, “Practical Implementation of Physically-​Based Shading Models at
tri-​Ace”, SIGGRAPH 2010
Yoshiharu Gotanda, “Practical Physically ​Based Rendering in Real-time”, SIGGRAPH
2012
Going forward
• Testing on real projects
• Better Geometry term
• Using the proper mipmapping for cubemaps
• Some kind of Diffuse- specular energy
conversation
Physically based for non realistic rendering
• Can we do it ?
– Yes just a material interface
– Rules can be bent and broken.
• Specular not that important for cartoonish look
– Diffuse light ,albedo,normal,AO more important
RTS game
More non photo