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Distribution Ray Tracing Academic Press, 1989 Reading Required: Shirley, 13.11, 14.1-14.3 Further reading: A. Glassner. An Introduction to Ray Tracing. Distribution Ray Tracing Academic Press, 1989. [In the lab.] Robert L. Cook, Thomas Porter, Loren Carpenter. “Distributed Ray Tracing.” Computer Graphics Brian Curless (Proceedings of SIGGRAPH 84). 18 (3). pp. 137- CSE 557 145. 1984. Fall 2014 James T. Kajiya. “The Rendering Equation.” Computer Graphics (Proceedings of SIGGRAPH 86). 20 (4). pp. 143-150. 1986. 1 2 Pixel anti-aliasing BRDF The diffuse+specular parts of the Blinn-Phong illumination model are a mapping from light to viewing directions: ns LV I=ILB kds()NL +k N LV ILr f (,LV ) The mapping function fr is often written in terms of No anti-aliasing incoming (light) directions in and outgoing (viewing) directions out: ffrr(,inout) or ( in out ) This function is called the Bi-directional Reflectance Distribution Function (BRDF). Here’s a plot with in held constant: fr (,in out ) in Pixel anti-aliasing All of this assumes that inter-reflection behaves in a mirror-like fashion… BRDF’s can be quite sophisticated… 3 4 Light reflection with BRDFs Simulating gloss and translucency BRDF’s exhibit Helmholtz reciprocity: The mirror-like form of reflection, when used to ff(( )) rrinout out in approximate glossy surfaces, introduces a kind of That means we can take two equivalent views of aliasing, because we are under-sampling reflection (and refraction). reflection. Suppose in = L and out = V: For example: fr (L V) fr (V L) L V We can now think of the BRDF as weighting light Distributing rays over reflection directions gives: coming in from all directions, which can be added up: IIf()VLLLNL ()(r V) d H Or, written more generally: IIf()()( N out inr in out ) ind in H 5 6 Reflection anti-aliasing Pixel and reflection anti-aliasing Reflection anti-aliasing Pixel and reflection anti-aliasing 7 8 Full anti-aliasing Approximating integrals Let’s say we want to compute the integral of a function: Ffxdx () If f(x) is not known analytically (or is not easy to integrate), but can be readily evaluated, then we can approximate the integral by sampling. Our approximate integral value is then something like: Full anti-aliasing…lots of nested integrals! n Ffixx () Computing these integrals is prohibitively expensive, i1 especially after following the rays recursively. where we have sampled n times at spacing x. We’ll look at ways to approximate high-dimensional Evaluating an integral in this manner is called integrals… quadrature. Q: What happens if we have integrals over d dimensions, sampling n times in each dimension? 9 10 A stochastic approach Approximating integrals (cont’d) A good approach to distributing samples unevenly across many dimensions is to do so stochastically. We can also do uneven sampling: Quick review first… For 1D, let’s say the position in x is a random variable X, which is distributed according to p(x), a probability density function (non-negative, integrates to unity). Then: E[X] V[X] E[kX] = Our approximate integral value is then something like: V[kX] = n Ffxx ()ii E[g(X)] = i1 where we have sampled n times at variable spacing xi. V[g(X)] = We can think of the xi as weights on the samples. We can also show that for independent random variables X and Y: Q: When the xi are more closely spaced, do they get larger or smaller weight? E[X + Y] = Q: Which xi should we pick? V[X + Y] = 11 12 Monte Carlo integration Monte Carlo integration (cont’d) Instead, we can estimate the expected value of Consider this function of a random variable: f(X)/p(X) (and thus the integral F) as: fX() n 1()fXi p()X F npXi1 ()i where f(x) is the function we’d like to integrate. What where the X are independent and identically is its expected value? i distributed (i.i.d.) random variables, each with distribution p(x). This procedure is known as Monte Carlo integration. Note that F is a random variable built by summing over functions of random variables f(Xi )/p(Xi ), where each of these functions of a random variable is itself a random variable. What is its expected value of F ? Computing this expected value requires the analytic n integral F, which is not available. 1()fXi E[F ] E npXi1 ()i 13 14 Monte Carlo integration (cont’d) Uniform sampling OK, so now we can say that F is a random variable with expected value equal to F. Suppose that the unknown function we are integrating happens to be a normalized box function What is the variance of this random variable? of width a: 1/axa / 2 n fx() 1()fXi V[F ] V 0otherwise npXi1 ()i Suppose we now try to estimate the integral of f(x) with uniform sampling over an interval of width w (i.e., choosing X from a uniform distribution): 1/wxw / 2 px() 0otherwise where w a. If we can get the variance to 0, then F must be a single value which must be equal to F. Getting to 0 is hard, but getting to low variance is an important goal. What variables and/or functions are under our control here? 15 16 Importance sampling Stratified sampling A better approach, if f(x) is non-negative, would be to An improvement on importance sampling is stratified choose p(x) ~ f(x). In fact, this choice would be sampling. optimal. The idea is that, given your probability function: You can break it up into bins of equal probability area (i.e., equal likelihood). Then choose a sample from each bin. Why don’t we just do that? Alternatively, we can use heuristics to guess where f(x) will be large and choose p(x) based on those heuristics. This approach is called importance sampling. 17 18 Summing over ray paths Summing over ray paths We can think of this problem in terms of enumerated rays: We can incorporate tertiary rays next: 1 The intensity at a pixel is the sum over the primary IIrfrrfrrpixel ()( ijk r ijk ij )( r ij i ) n ijk rays: n 1 Each triple i,j,k corresponds to a ray path: IIrpixel () i n i For a given primary ray, its intensity depends on rrrijk ij i secondary rays: So, we can see that ray tracing is a way to approximate a Ir() Ir ( )( f r r ) iijriji complex, nested light transport integral with a summation j over ray paths (of arbitrary length!). Substituting back in: Problem: too expensive to sum over all paths. 1 IIrfrrpixel ()( ij r ij i ) Solution: choose a small number of “good” paths. n ij 19 20 Glossy reflection revisited Whitted ray tracing Let’s return to the glossy reflection model, and modify Returning to the reflection example, Whitted ray it – for purposes of illustration – as follows: tracing replaces the glossy reflection with mirror reflection: We can visualize the span of rays we want to integrate over, within a pixel: Thus, we render with anti-aliasing as follows: 21 22 Monte Carlo path tracing Importance sampling Let’ return to our original (simplified) glossy reflection The problem is that lots of samples are “wasted.” model: Using again our glossy reflection model: An alternative way to follow rays is by making random decisions along the way – a.k.a., Monte Carlo path tracing. If we distribute rays uniformly over pixels and Let’s now randomly choose rays, but according to a reflection directions, we get: probability that favors more important reflection directions, i.e., use importance sampling: 23 24 Stratified sampling Stratified sampling of a 2D pixel We still have a problem that rays may be clumped Here we see pure uniform vs. stratified sampling over a together. We can improve on this by splitting 2D pixel (here 16 rays/pixel): reflection into zones: Now let’s restrict our randomness to within these Random Stratified zones, i.e. use stratified sampling: The stratified pattern on the right is also sometimes called a jittered sampling pattern. One interesting side effect of these stochastic sampling patterns is that they actually injects noise into the solution (slightly grainier images). This noise tends to be less objectionable than aliasing artifacts. 25 26 Distribution ray tracing DRT pseudocode These ideas can be combined to give a particular TraceImage() looks basically the same, except now method called distribution ray tracing [Cook84]: each pixel records the average color of jittered sub- pixel rays. uses non-uniform (jittered) samples. replaces aliasing artifacts with noise. function traceImage (scene): provides additional effects by distributing rays to for each pixel (i, j) in image do sample: I(i, j) 0 • Reflections and refractions for each sub-pixel id in (i,j) do •Light source area s pixelToWorld(jitter(i, j, id)) • Camera lens area •Time p COP d (s - p).normalize() I(i, j) I(i, j) + traceRay(scene, p, d, id) [This approach was originally called “distributed ray end for tracing,” but we will call it distribution ray tracing (as I(i, j) I(i, j)/numSubPixels in probability distributions) so as not to confuse it with a parallel computing approach.] end for end function A typical choice is numSubPixels = 5*5. 27 28 DRT pseudocode (cont’d) Pre-sampling glossy reflections (Quasi-Monte Carlo) Now consider traceRay(), modified to handle (only) opaque glossy surfaces: function traceRay(scene, p, d, id): (q, N, material) intersect (scene, p, d) I shade(…) R jitteredReflectDirection(N, -d, material, id) I I + material.kr traceRay(scene, q, R, id) return I end function 29 30 Soft shadows The pinhole camera The first camera - “camera obscura” - known to Aristotle.
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