Lesson 1:

Introduction to Ray Tracing

Contents

  1. Overview of main rendering algorithms
  2. Ray tracing pipeline
  3. Recursive ray tracing
  4. What is possible

Rendering

or image synthesis

The process of generating a more or less photorealistic 2D image from a 3D scene as a view from a camera located in that 3D scene by means of a computer program.

  • Efficiency
  • Photorealism
  • Efficiency
  • Photorealism

Rasterization

Ray Tracing

Ray Casting

Rendering Algorithms

main requirements:

Rendering

or image synthesis

The process of generating a more or less photorealistic 2D image from a 3D scene as a view from a camera located in that 3D scene by means of a computer program.

Rasterization

Ray Tracing

Rendering Algorithms

  • Photorealism
  • Efficiency
  • Efficiency
  • Photorealism

Rendering

or image synthesis

The process of generating a more or less photorealistic 2D image from a 3D scene as a view from a camera located in that 3D scene by means of a computer program.

Rasterization

Ray Tracing

Rendering Algorithms

  • Efficiency
  • Photorealism

www.OpenGL.org

www.OpenRT.org

  • Photorealism
  • Efficiency

4 Essential Building Blocks for Rendering

Camera

  • View point
  • Viewing direction
  • Field of view
  • Resolution
  • etc.

Geometry

3D geometry of all objects in a scene

 

Intersection Algorithms

ray - geometry intersection

Light Sources

  • Position
  • Color
  • Power
  • etc.

(Repeatedly reflected light /
indirect illumination)

Shading

  • Color
  • Texture
  • Absorption
  • Reflection
  • Refraction
  • Subsurface scattering
  • etc.

(local property may vary over surface)

4 Essential Building Blocks for Rendering

Camera

Geometry

Light Sources

Shading

Scene

Forward Light Transport

Camera

Geometry

Light Sources

Shading

Scene

  • Shoot photons from the light sources into scene
  • Reflect at geometry's surfaces (according to some reflection model)
  • Wait until they are absorbed or hit the camera sensor (very seldom)
  • Nature: massive parallel processing at the speed of light

Backward Light Transport

Camera

Geometry

Light Sources

Shading

Scene

  • Start at the camera

  • Trace only paths that might transport light towards the camera

    • May try to connect to occluded light sources

  • Ray Tracing

Basic Algorithm Overview

Camera

rt::ICamera

Geometry

rt::IPrim
rt::CSolid

 

Intersection Algorithms

rt::IPrim::intersect(Ray)

Light Sources

rt::ILight

Shading

rt::IShader

Scene

rt::CScene

Ray

rt::Ray

Basic Algorithm Overview

Ray-Generation

Ray-Geometry Intersection

Shading

Pixel Color

rt::ICamera::InitRay()
rt::IShader::shade()
rt::ILight::illuminate()
rt::IPrim::intersect()

Basic Algorithm Overview

Ray-Generation

Ray-Geometry Intersection

Shading

Pixel Color

Ray Generation

  • One ray for every pixel
  • Rays from camera origin along camera directions into the scene
rt::ICamera::InitRay()
rt::Ray

Basic Algorithm Overview

Ray-Generation

Ray-Geometry Intersection

Shading

Pixel Color

Ray Intersection

  • Check if the ray intersects any geometry in the scene
  • Ray-primitive algorithms for most of the geometrical primitives were developed in 1990s
rt::IPrim::intersect()
rt::Ray

Basic Algorithm Overview

Ray-Generation

Ray-Geometry Intersection

Shading

Pixel Color

Shading the Hit-Point

  • Determine pixel color
    • Energy (color) traveling along primary ray
  • Needed:
    • Local material color
    • Object texture
    • Reflection properties
    • Local illumination at the hit-point
rt::IShader::shade()

Basic Algorithm Overview

Ray-Generation

Ray-Geometry Intersection

Shading

Pixel Color

Shading the Hit-Point

  • Determine pixel color
    • Energy (color) traveling along primary ray
  • Needed:
    • Local material color
    • Object texture
    • Reflection properties
    • Local illumination at the hit-point
      • Compute it through recursive tracing of rays to all the light sources in the scene
rt::IShader::shade()

Basic Algorithm Overview

Ray-Generation

Ray-Geometry Intersection

Shading

Pixel Color

Shading the Hit-Point

  • Local illumination at the hit-point
    • Compute it through recursive tracing of rays to all the light sources in the scene
    • Check if the hit-point is occluded
    • Sum-up the light from all un-occluded light sources
    • Multiply it by the primitive's color
rt::IPrim::intersect()
rt::IShader::shade()

Basic Algorithm Overview

Ray-Generation

Ray-Geometry Intersection

Shading

Pixel Color

Shading the Hit-Point

  • Local illumination at the hit-point
    • Compute it through recursive tracing of rays to all the light sources in the scene
    • Check if the hit-point is occluded
    • Sum-up the light from all un-occluded light sources
    • Multiply it by the primitive's color
rt::IPrim::intersect()

Update the resulting pixel color

rt::IShader::shade()

Basic Algorithm Overview

Ray-Generation

Ray-Geometry Intersection

Shading

Pixel Color

Shading the Hit-Point

  • Local illumination at the hit-point
    • Compute it through recursive tracing of rays to all the light sources in the scene
    • Check if the hit-point is occluded
    • Sum-up the light from all un-occluded light sources
    • Multiply it by the primitive's color
rt::ILight::illuminate()
rt::IShader::shade()

Basic Ray Tracing

Features:

  • Direct lightning
  • Hard shadows

primary ray

shadow ray

Recursive Ray Tracing

Features:

  • Direct lightning
  • Hard shadows
  • Reflection / Transmission

primary ray

shadow ray

transmitted ray

reflected ray

Distributed (Stochastic) Ray Tracing

Features:

  • Direct lightning
  • Soft shadows
  • Glossy Reflection / Transmission
  • Anti-aliasing
  • Depth of field
  • Motion blur
  • Spectral rendering

primary ray

shadow ray

transmitted ray

reflected ray

Path Tracing (Global Illumination)

Features:

  • Direct lightning + Indirect lightning
  • Soft shadows
  • Glossy Reflection / Transmission
  • Anti-aliasing
  • Depth of field
  • Motion blur
  • Spectral rendering

primary ray

shadow ray

transmitted ray

reflected ray

Ray Tracing Features

Advantages

 

Automatic, simple and intuitive

  • Easy to understand and implement
  • Delivers “correct“ images by default

 

Ray Tracing Incorporates Into a Single Framework

  • Hidden surface removal
  • Shadow computation
  • Exact simulation of reflection and Refraction (Snell’s law)

 

Limitations

  • Easily gets inefficient for full global illumination computations
  • Many reflections (exponential increase in number of rays)
  • Indirect illumination requires many rays to sample all incoming  directions

What is Possible?

Models Physics of Global Light Transport

Dependable, physically-correct visualization

What is Possible?

VW Visualization Center

What is Possible?

Realistic Visualization: CAD

What is Possible?

Realistic Visualization: VR / AR

What is Possible?

Lighting Simulation

What is Possible?

Huge Models

Logarithmic scaling in scene size

~1 Billion Triangles

12,5 Millions Triangles

What is Possible?

Outdoor Environments

90 x 10^12 (trillion) triangles

What is Possible?

Games

Ray Tracing in CG

In The Past

Only used as an off-line technique

Was computationally far too demanding (minutes to hours per frame)

Believed to not be suitable for a HW implementation

 

More Recently

Distributed Real-time ray tracing on PC 

RPU: First full HW implementation

Commercial tools: Embree / OSPRey (Intel / CPU), OptiX (Nvidia / GPU)

Complete film industry has switched to ray tracing (Monte-Carlo)

 

Ray Tracing outside CG

Volume computation

Sound waves tracing

Collision detection

Rasterization

Primitive operation of all interactive graphics!

Scan convert a single triangle at a time

 

Sequentially processes every triangle individually

Can never access more than one triangle

 

But most effects need access to the world:  shadows, reflection, global illumination, etc.

Ray Tracing

Inspired by Nature!

Follow the path of many photons

Record those hitting the film in a camera

 

 

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