Advanced Ray Tracing

1
Advanced Ray Tracing

• Mani Thomas
• CISC 440/640
• Computer Graphics

2
Review

• Ray tracing
• Compute 3D ray into the scene for each 2D image
  pixel
• Compute 3D intersection point of ray with nearest
  object in scene
• Test each primitive in the scene for intersection
• Find nearest intersection
• Recursively spawn rays from the point of
  intersection
• Shadow Rays
• Reflected rays
• Transmitted rays
• Accumulate the color from each of the spawned
  rays at the point of intersection

3
Review

• Ray object intersection
• Intersection with a plane
• Implicit form
• Intersection
• Intersection with a sphere
• Implicit form
• Intersection

4
Review

• Shadow feelers
• Spawn a ray from P to the light sources
• If there is an intersection of the shadow ray
  with any object then P is in shadow
• Reflection
• Angle of incidence angle of Reflection

5
Review

• Refraction
• Ray passing through media of different refractive
  indices bend towards/away from the normal
• Snells Law
• ni and nr are the refractive indices of the two
  media
• Transmitted ray

6
Road map

• Super sampling
• Acceleration techniques
• Monte Carlo methods
• Distributed ray tracing
• Bidirectional ray tracing
• Caustics
• POV-ray

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Anti aliasing

• Ray tracing gives a color for every possible
  point in the image
• But a square pixel contains an infinite number of
  points
• These points may not all have the same color
• Sampling choose the color of one point (center
  of pixel)
• This leads to aliasing
• jaggies
• moire patterns
• Aliasing means one frequency (high) masquerading
  as another (low)
• e.g. wagon wheel effect

8
Super-sampling

• Ray tracing is a point-sampling process
• Take discrete looks at the scene along individual
  rays passing through each pixel
• Reduce aliasing due to this discrete signal
  sampling

Courtesy F.S. Hill, Computer Graphics using
OpenGL
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Adaptive Super-sampling

• Instead of shooting one ray per pixel, shoot four
  rays through the corners of a pixel
• Color at the pixel is the average of the colors
  at each corners
• Adaptive super-sampling (Whitteds approach)
• Compute the intensity variation between the four
  corners with the average
• Shoot more rays through corners with higher
  intensity variation
• Compute final color as a weighted average rather
  than the regular average

Courtesy of C. Rasmussen, CISC 640
10
Stochastic Super sampling

• Visible aliasing is possible even with adaptive
  super-sampling
• sampling grid interacts with regular structures
• objects aligned with sampling grid
• Stochastic sampling
• instead of a regular grid, subsample randomly
• keep taking samples until the color estimates
  converge
• jittering perturb a regular grid

Courtesy of C. Rasmussen, CISC 640
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Using Extents

• Ray tracing is slow, performing the same
  functions.
• Most of the time is spent in computing
  intersections
• Each ray should be intersected with every object
  in the scene
• Each ray, spawns out reflected/transmitted rays
  which have to be interested with the objects in
  the scene

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Using Extents

• Extent of an object is a shape that encloses the
  object
• Compute complicated intersections if and only if
  the ray hits the extent
• Two shapes most commonly used as extents
• Sphere specified by a center and radius (C , r)
• Box specified by sides aligned to the
  coordinate axis

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Distributed Ray Tracing

• Distributed ray tracing is NOT ray tracing on a
  distributed system.
• Distributed ray tracing is a ray tracing method
  based on randomly distributed oversampling to
  reduce aliasing artifacts in rendered images
  (Allen Martin, http//www.cs.wpi.edu/matt/courses
  /cs563/talks/dist_ray/dist.html)

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Distributed Ray Tracing

• Developed by Cook, et. al. (Distributed Ray
  Tracing, Computer Graphics, vol. 18, no. 3, pp
  137-145, 1984)
• Stochastic Oversampling (http//www.cs.virginia.ed
  u/cs551dl/lecture11/sld016.htm)
• Pixel for antialiasing
• Light source for soft shadows
• Reflection function for soft (glossy) reflections
  
• Time for motion blur
• Lens for depth of field

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Distributed Ray tracing

• Gloss (fuzzy reflections)
• Partially reflecting surfaces
• Traditional ray tracing
• reflections look identical to the scene they are
  reflecting
• reflections are always sharp
• Randomly distributing the rays reflected by the
  surface
• Send out a packet of rays around the reflecting
  direction.
• The actual value of reflectance is the
  statistical mean of the values returned by each
  of these rays

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Distributed Ray tracing

• Distributing a set of reflection rays by randomly
  perturbing the ideal specular reflection ray.
• The spread of the distribution determines the
  glossiness where a wider distribution spread
  models a rougher surface.  

taken from http//www.cs.wpi.edu/emmanuel/courses
/cs563/write_ups/zackw/realistic_raytracing.html
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Distributed Ray tracing

• first image is from the traditional ray tracer
• second one uses 16 rays in place of the single
  reflected ray
• third image uses 64 rays

taken from http//www.uwm.edu/People/dtstrock/grap
hics/mcrt.html
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Distributed Ray tracing
taken from http//www.cs.wpi.edu/emmanuel/courses
/cs563/write_ups/zackw/realistic_raytracing.html
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Distributed Ray tracing

• Fuzzy translucency
• Same as glossy reflections, but you jitter the
  refracted ray
• Analytical function similar to the shading
• A transmission function is used instead of a
  reflectance function
• Light is gathered from the other side of the
  surface.
• Cast randomly distributed rays in the general
  direction of the transmitted ray from traditional
  ray tracing.
• The average value computed from each of these
  rays the true translucent component.

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Distributed Ray tracing

• first image is obtained from a traditional ray
  tracer
• Second image uses 10 rays for the transmitted ray
• third image uses 20 rays

taken from http//www.cs.wpi.edu/matt/courses/cs5
63/talks/dist_ray/dist_trans_fuzzy_20.html
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Distributed Ray tracing

• first image is from the traditional ray tracer
• second one uses 16 rays in place of the single
  reflected ray
• third images uses 64 rays

taken from http//www.uwm.edu/People/dtstrock/grap
hics/mcrt.html
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Distributed Ray tracing

• Penumbras (soft shadows)
• Traditional ray tracing shadows are discrete
• Shadow feelers used to check if a point is in
  shadow with respect to a point light source
• Incorrect for large light sources and/or light
  sources that are close to the object
• The transition from fully shadowed to partially
  shadowed is gradual.
• Due to the finite area of real light sources, and
  scattering of light of other surfaces

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Distributed Ray tracing

• Penumbras (soft shadows)
• A set of rays are cast about the projected area
  of the light source.
• The projected area helps tackle the large area
  light source
• The amount of light transmitted by the ratio of
  the number of rays that hit the source to the
  number of rays cast

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Distributed Ray tracing

• In case of a point source, the occluder would
  create a sharp shadow boundary
• In an area light source or if the light source is
  closer to the object
• Creation of a penumbra region
• Sending out shadow feelers to capture the
  penumbra region

taken from http//www.cs.wpi.edu/emmanuel/courses
/cs563/write_ups/zackw/realistic_raytracing.html
25
taken from http//www.cs.wpi.edu/emmanuel/courses
/cs563/write_ups/zackw/realistic_raytracing.html
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Distributed Ray tracing
taken from http//www.cs.unc.edu/andrewz/comp238/
hw2/
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Distributed Ray tracing

• Depth of field - the distance that objects appear
  in focus
• Objects that are too far away or two close will
  appear unfocused and blurry
• pinhole camera model does not truly mimic the
  real world situation
• Pinhole assumed to be infinitely small
• Changing focal length change field of view but
  does not change focus

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Distributed Ray tracing

• Distributed ray tracing creates depth of field by
  placing an artificial lens in front of the view
  plane.
• Randomly distributed rays are used once again to
  simulate the blurring of depth of field.
• The first ray cast is not modified by the lens.
• focal point of the lens is at a fixed distance
  along this ray
• Rest of the rays sent out for the same pixel will
  be scattered about the surface of the lens
• Points in the scene that are close to the focal
  point of the lens will be in sharp focus.
• Points closer or further away will be blurred

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taken from http//www-courses.cs.uiuc.edu/cs419/m
p2/gallery-sp04/
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Distributed Ray tracing
taken from http//www-courses.cs.uiuc.edu/cs419/m
p2/mp2-gallery-sp05/
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Distributed Ray tracing

• Motion blur
• Temporal sampling rather than spatial sampling
• A Frame represents an average of the scene during
  the time that the camera shutter is open
• Before each ray is cast, objects are translated
  or rotated to their correct position for that
  frame.
• The rays are averaged to give the actual value.
• Objects with the most motion will have the most
  blurring in the rendered image.

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Distributed Ray tracing
taken from http//www-courses.cs.uiuc.edu/cs419/m
p2/mp2-gallery-sp05/
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Distributed Ray tracing
taken from http//www-courses.cs.uiuc.edu/cs419/m
p2/mp2-gallery-sp05/
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Bidirectional Ray tracing
caustic
Created by H. Wann Jensen
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Bidirectional Ray tracing

• Caustic - (Concentrated) specular
  reflection/refraction onto a diffuse surface
• Standard ray tracing cannot handle caustics

caustic
Created by H. Wann Jensen
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Light Paths

• Interactions of the light ray can be expressed
  using regular expressions
• L is the light source
• E is the eye/camera
• D is a diffuse surface
• S is a specular surface

from Sillion Puech
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Light Paths

• Direct visualization of the light LE
• Local illumination LDE, LSE
• Ray tracing LSE, LDSE
• Caustics LSDE

from Sillion Puech
Taken from cisc 440/640 Fall 2005
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Diffuse Surfaces

• Uncertainty in the direction that a photon will
  take for diffuse surfaces
• For specular surfaces, the BRDF (probability
  that incoming photon will leave in a particular
  direction) has a thin profile
• We can predict the direction of the outgoing
  photon
• For an ideal diffuse surfaces, the BRDF would be
  spherical
• The photon can travel along any of the direction
  with equal probability

from Sillion Puech
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Bidirectional Ray tracing
from P. Heckbert

• Idea Trace forward light rays into scene as well
  as backward eye rays
• At diffuse surfaces, light rays additively
  deposit photons in radiosity textures, or
  rexes, where they are picked up by eye rays
• The rays of the forward and backward pass "meet
  in the middle" to exchange information.

Paul S. Heckbert, Adaptive radiosity textures
for bidirectional ray tracing , SIGGRAPH 1990
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Radiosity

• Handling cases such as LDE
• Color Bleeding

courtesy of Cornell
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Softwares

• Two beautiful rendering/modeling softwares
• POV-ray (http//www.povray.org/)
• Persistence of Vision - ray tracer
• A free rendering tool (not a modeling tool)
• Uses a text based scene description language
  (SDL)
• Available on Windows/linux/MAC OS
• Blender (http//www.blender3d.org)
• Modeling, Animation, rendering tool
• Especially useful in 3D game creation
• Available for Windows, Linux, Irix, Sun Solaris,
  FreeBSD or Mac OS X under GPL

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POV-ray
created using POV-ray, http//www.povray.org/
43
POV-ray
created using POV-ray, http//www.povray.org/
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POV-ray
created using POV-ray, http//www.povray.org/
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POV-ray
created using POV-ray, http//www.povray.org/
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Conclusion

• Traditional Ray tracing
• Shadow feelers
• Reflection
• Refraction
• Distributed Ray tracing
• Jittered sampling
• Bidirectional Ray tracing
• Caustics

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Thank you