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## PowerPoint Slideshow about 'Advanced Ray Tracing' - shiela

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Presentation Transcript

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

Review

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

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

Review

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

Road map

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

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

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”

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 (Whitted’s 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

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

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

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

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)

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.edu/~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

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

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

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/graphics/mcrt.html

Distributed Ray tracing

taken from http://www.cs.wpi.edu/~emmanuel/courses/cs563/write_ups/zackw/realistic_raytracing.html

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.

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/cs563/talks/dist_ray/dist_trans_fuzzy_20.html

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/graphics/mcrt.html

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

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

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

taken from http://www.cs.wpi.edu/~emmanuel/courses/cs563/write_ups/zackw/realistic_raytracing.html

Distributed Ray tracing

taken from http://www.cs.unc.edu/~andrewz/comp238/hw2/

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

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

taken from http://www-courses.cs.uiuc.edu/~cs419/mp2/gallery-sp04/

Distributed Ray tracing

taken from http://www-courses.cs.uiuc.edu/~cs419/mp2/mp2-gallery-sp05/

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.

Distributed Ray tracing

taken from http://www-courses.cs.uiuc.edu/~cs419/mp2/mp2-gallery-sp05/

Distributed Ray tracing

taken from http://www-courses.cs.uiuc.edu/~cs419/mp2/mp2-gallery-sp05/

Bidirectional Ray tracing

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

caustic

Created by H. Wann Jensen

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

Light Paths

- Direct visualization of the light: LE
- Local illumination: LDE, LSE
- Ray tracing: LS*E, LDS*E
- Caustics: LS+DE

from Sillion & Puech

Taken from cisc 440/640 – Fall 2005

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

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

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

POV-ray

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

POV-ray

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

POV-ray

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

POV-ray

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

Conclusion

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

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