CS 655 Distributed Ray Tracing Distributed Ray Tracing
CS 655 Distributed Ray Tracing
Distributed Ray Tracing What is 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. § Also called “distribution ray tracing” or “stochastic ray tracing” § (The images in this presentation are from Andrew G. Zaferakis, Aneesh Naman, Allen Martin, Daqing Xue, Henrik Wann Jensen, and Jinho Lee)
Distributed Ray Tracing • Idea: Introduce noise into the ray tracer to minimize visual artifacts such as aliasing, perfect reflections, etc. • Stocastically distribute rays over: § § § Space – antialiases the image Reflection angle – produces glossy reflections Transmission angle – produces translucency Shadow ray – produces soft shadows (penumbra) Lens area – produces depth of field Time – produces motion blur
Antialiasing • The human eye samples using a Poisson disk distribution: • A finite number of photoreceptors • Cones in the eye are distributed stochastically, but such that no two cones are closer than a certain distance
Antialiasing • We can apply this approach to ray tracing § Send out rays stochastically, but such that they maintain a Poisson disk distribution • This reduces aliasing, but is expensive to compute • However, we can approximate a Poisson disk distribution fairly quickly • Idea: begin with a regular grid • Jitter the ray locations slightly within the grid
Uniform Sampling Prone to aliasing in the image
Jittered Sampling Antialiases the image
Jittered Sampling • Approximates a Poisson disk distribution § Not exact, but much cheaper • Removes high frequencies • Introduces noise in place of high frequencies • Can still have problems § May leave large areas uncovered by samples § May have some areas with more samples than necessary
Improved Jittered Sampling • To improve the image further, send out multiple jittered rays per pixel Similarly for all other pixels • Make the number of sub-pixels easily changeable – start small then increase as you are confident your raytracer works.
Standard Ray Tracing • No antialiasing
Distributed Ray Tracing • With antialiasing
Standard Ray Tracing - detail • No antialiasing
Distributed Ray Tracing - detail • With antialiasing
Glossy Reflections • We can use the concept of ray jittering to produce glossiness (blurred reflections) • Tracing a ray based on perfect reflection angle produces sharp reflections N I R
Glossy Reflections • We can create a blurred reflection by sending out jittered rays about the reflection ray N I R Bound of jittered reflection rays
Glossy Reflections • This has the effect of “blurring” the reflection • The exact reflection vector is computed, then slightly jittered from the original direction • The jittered ray may hit an entirely different object than the one hit by the true reflection ray • This gives a smoothly blurred reflection
Reflections – Standard Ray Tracer • Perfect reflection
Reflections – Distributed Ray Tracer • 10 distributed rays
Reflections – Distributed Ray Tracer • 20 distributed rays
Reflections – Distributed Ray Tracer • 50 distributed rays
Standard Ray Tracing • Perfect reflections
Glossy reflections • 4 rays
Glossy reflections • 16 rays
Glossy reflections • 64 rays
Translucency • We can also apply the concept of ray jittering to produce translucency (blurred transmissions) • Tracing a ray based on perfect transmission angle produces sharp transparencies N I T
Translucency N I • Jittering the rays about the actual transmission angle produces a blurred effect Bound of jittered reflection rays T
Transmissions – Standard Ray Tracer • Perfect transmission
Transmissions – Distributed Ray Tracer • 10 transmission rays
Transmissions – Distributed Ray Tracer • 20 transmission rays
From Jia, Sun, and Xu
Soft Shadows • We have been simulating lights with point light sources • This produces hard shadows Point light source -hard shadow
Soft Shadows • Lights in the real world are not point light sources, thus the shadows are not sharp Area light source umbra Area light source -soft shadow penumbra
Soft Shadows • The penumbra is the portion of the shadow resulting from partially obscured lights Area light source • To simulate penumbra: § § § Send out multiple shadow rays from the intersection point to the area light source Jitter the rays according to the area light source The intensity of the surface point depends on the number of jittered rays that reach the light source Bounds of jittered shadow rays Occluding object Intersection point
Hard Shadows
Soft Shadows
Standard Ray Tracing • Hard shadows
Ray Tracing with an Area Light Source • Visible penumbra and umbra, but too distinct
Distributed Ray Tracing • 10 rays per pixel
Distributed Ray Tracing • 20 rays per pixel
Distributed Ray Tracing • 50 rays per pixel
Depth of Field • Idea: the camera (or eye) should have a fixed focal length § Objects at that distance should be in focus § Objects closer or further away should not be in focus Lens Image Plane Focal Plane
Lens Properties D VD r P Vp Pf C F/n Object in Scene Lens Image Plane PD Focal Plane
Lens Properties F: n: F/n: Pf: P: Focal length aperture number lens diameter the focal point distance from lens to focal point Vp: distance from lens to image plane PD: a point not on the focal plane D: distance from lens to PD r: radius of cone for object at distance D C: circle of confusion D VD Vp C Image Plane Lens r P F/n Pf Focal Plane PD Object in Scene
Simulating a lens • Approach 1: § For an object located at PD, send out a group of jittered rays that lie within a cone of radius r How do we compute r? § This simulates a lens without actually having one, but isn’t very accurate §
Simulating a lens • Approach 2: Find the focal point 1. - Send a ray from the center of the lens (eye point) through the screen and follow it a distance P 2. Choose a jittered point on the lens 3. Trace a ray from that jittered point through the focal point 4. Return intensity information based on what that ray hits
Simulating a lens • Step 1 – determine the focal point by tracing a ray from the lens center through the pixel a distance P
Simulating a lens • Step 2 – Choose a jittered point on the lens New point
Simulating a lens • Step 3 – Trace the ray from the new point through the focal point
Simulating a lens • Step 4 – Return the intensity information
Depth of Field Example
Depth of Field Example From Alan Watt, “ 3 D Computer Graphics”
Motion Blur • Distribute rays over time § Static objects will not change § Moving objects will be blurred, depending on their velocity • How: § § Jitter the rays with respect to time Determine object positions at each of the jittered time values Send each ray with the objects positioned appropriately Combine the rays back into one to get the motion blurred object
Motion Blur Example (from Cook et. al. )
Summary of Distributed Ray Tracing • For each ray do: 1. Jitter the spatial screen location of the ray 2. Select a time for the ray and move the objects to that time Perform depth of field calculation: 3. a. b. 4. 5. 6. 7. Determine the focal point by sending a ray from the eye point (center of the lens) through the pixel, a distance P Determine a lens location by jittering the origin of the ray to a position on the lens Compute the intersection by sending the primary ray from the jittered location through the focal point Trace jittered reflection rays Trace jittered trasmission rays Trace jittered shadow rays
Summary of Distributed Ray Tracing Shadow Ray Reflected Ray Transmitted Ray
Distributed Ray Tracing (Cook et. al. )
- Slides: 58
