a simplified artist interface physically based shading niklas hansson head teacher game programming...

A simplified artist interface

Physically Based Shading

Niklas HanssonHead 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 thematerials 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

Image from “Real-Time Rendering 3rd Edition”

•Microfacets•A surface consists of many small perfectly flat surfaces.•How much they differ in normal determines how light interact

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

ReferencesSébastien 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 Hoff mann, “Crafting Physically Moti vated Shading Models for Game Devel op ment”, SIGGRAPH 2010

Dim i tar Lazarov, “Physically- based lighting in Call of Duty: Black Ops” SIGGRAPH 2011

Chris ti an Schüler, “An Efficient and Physically Plausible Real- Time Shad ing Model.” ,” The Blinn- Phong Normalization Zoo”

Akienne- Möller, Haines and Hoff mann, “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

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