# Implicit Surface Rendering on GPU - IIIT Hyderabad Real Time Ray-Tracing Implicit Surfaces on the GPU Jag Mohan Singh IIIT Hyderabad IIIT, Hyderabad Implicit Surfaces Implicit Surface which can be described by an equation S(x,y,z) = 0. This can be of different kinds Algebraic Non- Algebraic eg. Transcedental, Irrational, Rational etc. Implicit Surfaces are used for fluid simulation, modeling IIIT Hyderabad of fire, waves and natural phenomena described by equations Thesis Contributions IIIT Hyderabad

Analytical (Exact) Root Finding at frame-rates of 1100 5821 for surfaces up to fourth order Mitchells Interval method (first time on GPU) at frame-rates of 60 965 for surfaces up to fifth order Marching Points at frame rates of 38 825 for arbitrary implicits Adaptive Marching Points (a new method) for arbitrary implicits at frame rates of 60 920 Traditional Methods of Rendering Rasterization IIIT Hyderabad Ray Tracing Rendering Implicit Surfaces Polygonization using Marching Cubes Marching Cubes gives a 3d mesh for the input implicit surface Rasterization of this 3d mesh gives the rendering Ray Tracing

IIIT Hyderabad Shoot rays towards the implicit surface and intersect them with these Ray-Tracing Implicit Surfaces S(x,y,z) = 0 Ray: P = O + t D t0 Can express as: f(t) = 0 Desired: smallest +ve real root t0 IIIT Hyderabad Normal at t0 = (Sx, Sy, Sz) at (O + D t0) Root Finding Methods IIIT Hyderabad Analytical (Exact) exists for polynomials up to fourth order Iterative Methods exists for arbitrary

implicits but have problems related to initialization and convergence. Searching based methods which search for the root along the ray using surface properties Related Work (Exact) Loop and Blinn [ Siggraph 06] Piecewise algebraic surfaces up to order four. The roots are computed by converting the polynomial to Bezier form. Coefficients are interpolated in vertex shader. If root is inside the Bezier tetrahedron then surface normal and perpixel lighting done. Problems in quartic root finding due to extreme self intersections Quadric root finding on GPU IIIT Hyderabad Sigg , PBG 06 Toledo, INRIA Tech Report 06 Ranta , ICVGIP 06 Iterative Methods Newton Raphson Method xn+1 = xn-f(xn)/ f (xn)

Laguerres method ( Similar to Newtons) Newton Bisection Method IIIT Hyderabad Given interval [t1,t2] Choose one of the intervals [t1,tm] or [tm,t2] where tm is the midpoint Interval based Iterative Methods Newtons Interval Method xn+1 = xn- f(xn)/ F(xn) IIIT Hyderabad Krawczyk Method xn+1 = xn-f(xn)/f(xn) + (I- J( xn) / f( xn)) (Xn xn ) Recent Related Work (Iterative) IIIT Hyderabad Knolls Affine Arithmetic [ CGF 08] Compute affine extension of function as F If 0 F then the interval contains the root Compute maximum depth (dmax) of bisection

based on user defined threshold If depth is dmax then we hit the surface Else increment depth and reduce the stepsize by half Back recursion helps in visiting other unvisited nodes in the tree. In the worst case it can lead to visiting all the nodes of the tree. Related Work (Searching) IIIT Hyderabad LG Implicit Surfaces [ Kalra and Barr, Siggraph 89] Lipschitz constants (L,G) for ray tracing implicits. L is equal to maximum rate of change of f(x) over R. G is equal to maximum rate of change of g(t). Compute Bounding Box (B) divide it into sub-bounding boxes (b) Compute L for b If |f(x0)| > Ld reject b else continue recursive subdivision. For each ray compute bounding box extents t1,t2 and midpoint tm If |g(tm)| > Gd If F(t1) and F(t2) are of opposite signs then find the root using Newtons method. Else there is no intersection in t1,t2

Else if |g(tm)| < Gd Call the function recursively on intervals [t1,tm] and [tm,t2] Related Work (Searching) IIIT Hyderabad Sphere Tracing [Hart, Visual Computer 96] Compute while t < D d = f(r(t)) ( Geometric Distance) If d < epsilon then return t t=t+d where Geometric Distance = Signed Distance/ Lipschitz Constant (L) Analytical (closed-form) Roots ( Our Work) Roots are computed in power basis Limitations: Not available for polynomials of order > 4! Difficult for non-algebraic equations IIIT Hyderabad Must use iterative methods

for others Analytical Root Finding Cubic Roots Equation (Homogenous Form) : Ax3+3Bx2w+3Cxw2+Dw3 = 0 Compute: 1= AC-B2 , 2 = AD-BC, 3=BD-C2 , (discriminant) = 4 1 3- 22 IIIT Hyderabad The sign of the discriminant and the values of is determine if it has one triple root, one double and a single real root, three distinct real roots or one real root and one complex conjugate pair as roots. Analytical Root Finding IIIT Hyderabad Quartic Roots The equation is first depressed by removing the cubic term t4+pt2+qt+r = 0 If r is zero then the roots are the roots of cubic

equation and zero. If r is non zero then rewrite as (t2+p)2+qt+r = pt2+p2 This is followed by a substitution y s.t. RHS becomes a perfect square (t2+p+y)2 = (p+2y)t2-qt+(y2+2yp+p2-r) Now, for RHS to be a perfect square its discriminant must be zero which yields a cubic equation in y. Now resubstitute to get two quadratic equations. Interval Arithmetic IIIT Hyderabad Two Intervals a = [x, y] and b = [z, w] Addition a + b = [x + z, y + w] Subtraction a b = [x w, y z] Multiplication a * b = [min(xz,xw,yz,yw), max(xz,xw,yz,yw)] Division a/b = a * (1/b) = a* [ 1/w,1/z] Mitchells Interval-Based Method Initialize interval to [ta, tb] = [tnear, tfar] Compute interval extension of function f ([ta, tb]) and its derivative ft ([ta, tb]) If f ([ta, tb] contains 0, root exists in it. If ft ([ta, tb]) contains zero, multiple roots.

Divide into [ta, tm] and [tm, tb] around the midpoint Recurse into right half only if left has no root. IIIT Hyderabad Else, single root. Proceed to root finding Continue till tb - ta < Mitchell, Graphics Interface 90 Interval Extensions Natural: Uses end-points only. f ([ta, tb]) = [min(f(ta), f(tb)), max(f(ta), f(tb))] Centered: f ([ta, tb]) = f (tm) + ft ([ta,tb]) * [ta - tm, tb - tm] Exact: Use critical points ta < t1 < t2 < < tb of f() IIIT Hyderabad f ([ta, tb]) = [min(f(ta), f(t1), f(t2) , f (tb)), max (f(ta), f(t1), f(t2) , f(tb))] Mitchells Method: Discussion

Advantages: Robust, based on interval arithmetic Fast as the order is logarithmic due to bisections Disadvantages: Good interval extension needed Not obvious for general functions Not easy even for polynomials Difficult on SIMD/GPU IIIT Hyderabad Calculations in f(t), derivatives needed Interval extension used is exact Two-Step Root Finding Bracketing the root Find a (small) bracket/interval that contains the first positive root. Between tnear and tfar Find the root in the interval Newton bisections IIIT Hyderabad

Always converges, no special situations Best for GPU/SIMD as uniform calculations Marching Points (Sign Test) IIIT Hyderabad Divide the parameter domain into equal width intervals from tnear till tfar Compute the function value at endpoints of these intervals. Return the interval with the first sign change. Marching Points (Taylor Test) IIIT Hyderabad Divide the parameter domain into equal width intervals from tnear till tfar Compute the values p, q, r and s for an interval and the interval checked for sign change is [min(p,q,r,s), max(p,q,r,s)]

S(x, y, z) versus f(t) S(x, y, z) is the given form. Relatively simple with dozen or so terms For a given t, evaluate (x, y, z) and S(.). Good for GPU; compose shader on the fly f(t) is different for each ray/pixel. IIIT Hyderabad Evaluates to a large number of terms About 1500 terms for a 10th order polynomial Not suitable for GPU/SIMD Results: Implicit Surfaces IIIT Hyderabad (Marching Points) Marching Points: Discussions Advantages: Easy Implementation Suited for SIMD, fast on current GPUs No need for derivative or coefficient computation

Disadvantages: IIIT Hyderabad Linear in number of intervals as all may be evaluated Sign Test Not robust. Multiple and close roots are problems No structured way to decide interval size. Adaptive Marching Points IIIT Hyderabad Algebraic distance is used as a measure for searching the root Step-size depends on algebraic distance (S(p(t)) and silhouettes (F(t)) Adaptive Marching Points IIIT Hyderabad Silhouette Adaptation Self Shadowing

IIIT Hyderabad Shoot a secondary (shadow) ray towards the light source from intersection point. If this ray intersects the surface in between then the point is in shadow. Only need to bracket the root; no need to find the root. IIIT Hyderabad Shadowing of Surfaces Dynamic Implicit Surfaces IIIT Hyderabad Implicit Surfaces whose equation varies with time. Blobby Molecules and Twisted Superquadric IIIT Hyderabad

Analytic Roots Surface Name FPS for 512x512 Sphere Quadric 5821 Cylinder Quadric 4358 Cayley Cubic 3750 Ding Dong Cubic 3400 Steiner Surface 1400 Tooth Surface

1100 Torus Surface 1200 Surface Name FPS for 512x512 ( Loop and Blinn) Cylinder Quadric 1200 Steiner Surface 500 IIIT Hyderabad Mitchells Interval Method Surface [Order] Iterations Dervish  86

Kiss  65 Peninsula  60 Cushion  53 CrossCap 52 Miter  52 Tooth 50 Cayley  27 Ding dong  18 FPS 60 77 85 170 195 186 195 580

965 Marching Points: Results IIIT Hyderabad Surface [Order] Iterations FPS (Sign Test) FPS (Taylor Test) Chmutov  400 85 38 Chmutov 400

55 48 Sarti 300 60 53 Barth  300 92 105 Endreass  300 140

179 Chmutov  250 185 195 Kleine 400 285 290 Hunt  400 230 225

Barth 125 300 310 Heart  120 265 260 Dervish  300 285 275 Peninsula 

85 370 447 Torus  50 410 430 Blobby Surface 250 160 305 Scherks Surface 250

200 315 Diamond Surface 250 260 306 Superquadric 150 105 125 Adaptive Marching Points: Results IIIT Hyderabad Surface [Order]

Iterations FPS (Sign Test) FPS (Taylor Test) Chmutov  100 98 60 Chmutov 100 125 95 Sarti 100

86 75 Barth  100 150 115 Endreass  96 190 208 Chmutov  64 215

216 Kleine 48 435 385 Hunt  84 240 325 Barth 60 325 310

Heart 48 420 320 Dervish  45 285 280 Peninsula  35 512 435 Torus 

24 555 525 Blobby Surface 50 329 300 Scherks Surface 100 358 322 Diamond Surface 100

With Shadows Without Shadows With Shadows Chmutov  98 70 60 45 Chmutov  125 95 95 75

Sarti  86 78 75 49 Barth  150 110 115 79 Endreass 190 140

208 140 Labs 232 165 310 155 Chmutov 418 280 325 235 Hunt

240 182 310 155 Dervish 285 250 280 175 Kiss 428 325 435

265 Tooth 617 425 542 287 Blobby 329 265 300 195 Diamond 360

208 330 199 Superquadric 185 145 155 105 IIIT Hyderabad Comparison with Knolls Affine Arithmetic Surface FPS (Knolls ANE) FPS (AMP Sign)

Steiner 38 212 Teardrop 121 178 Tangle 71 196 Barth Sextic 88 120 Kleine

101 170 Mitchell 60 176 Barth Decic 16 94 IIIT Hyderabad Results: Robustness Top row: Steiner Surface Bottom row: Cross Cap Surface (Sign Change, Taylor and Interval) Limitations

IIIT Hyderabad Chmutov 20 and 30 (Exterior, Interior) Numerical precision is a issue large number of roots are present in the exterior of Chmutov Surface [0.99,1.0] Taylor test produces false roots for extreme self intersections (Cushion and Piriform) What do we need on the GPU? Number format: Exact implementation of IEEE 754 (Limited) Double precision support Beam-Tracing: Transfer roots from one pixel to neighbour Recursive ray-tracing IIIT Hyderabad Fixed stack on GPU IIIT Hyderabad

Video Conclusions IIIT Hyderabad MP and AMP methods are widely applicable in terms of Implicit Surfaces and are also SIMD amenable as cost per root finding is low Analytical Method has limited applicability However it is SIMD amenable Mitchells method has limited applicability and is not SIMD amenable. Thesis Publications IIIT Hyderabad Related Publications GPU Objects Sunil Mohan Ranta , Jag Mohan Singh and P.J. Narayanan Proc. Fifth Indian Conference on Computer Vision, Graphics and Image Processing (ICVGIP), LNCS Volume 4338, Pages 352-363, 2006, Madurai, India Real time Ray tracing of Implicit Surfaces on the GPU Jag Mohan Singh and P. J. Narayanan

IEEE Transactions on Visualization and Computer Graphics, 2008 (Under Revision) Other Publications Progressive Decomposition of Point Clouds without Local Planes Jag Mohan Singh and P. J. Narayanan LNCS Volume 4338, Pages 364-375, Proc. of Indian Conference on Computer Vision, Graphics and Image Processing (ICVGIP), 2006 Point Based Representations for Hierarchical Environments Kedarnath Thangudu , Lakshmi Gade,Jag Mohan Singh, and P J Narayanan. Pages 574-578, IEEE Computer Society Press, Proc. of International Conference on Computing: Theory and Applications(ICCTA),2007 IIIT Hyderabad Thank you! CPU and GPU versions Frame Rates for 512x512 Position of Sphere Sphere (Quadratic) z=0 z=1

z=4 z=5 1000 922 790 720 GPU Mitchell 1665.0 1662.0 1662.0 1662.0 CPU Point Sampling 0.8124

0.7939 0.7310 0.7196 2.113 2.113 2.113 2.113 z=0 z=1 z=4 z=5 GPU Point Sampling 825.3

724.7 447.5 367.5 GPU Mitchell 955.4 953.4 952.2 952.2 0.7626 0.7258 0.664 0.656 1.109

1.108 1.108 1.108 z=0 z=1 z=4 z=5 GPU Point Sampling 410.96 383.77 273.50 230.06 GPU Mitchell

381.06 380.16 379.23 379.23 CPU Point Sampling 0.2202 0.2074 0.1791 0.1759 0.186 0.185 0.185 0.185

GPU Point Sampling CPU Mitchell Position of Cubic Ding Dong (Cubic) CPU Point Sampling CPU Mitchell Position of Torus Torus IIIT Hyderabad (Quartic) CPU Mitchell Discussion CPU vs GPU SIMD amenable AMP method GPU is able to achieve higher speedups than for Mitchells method. IIIT Hyderabad

Interval method is faster for lower order surfaces than AMP. This advantage is nullified for higher order surfaces. IIIT Hyderabad Results: Some More IIIT Hyderabad Results: More Alg Surfaces