Journal ArticleAlthough ray tracing has been successfully applied to interactively render large datasets, supersampling pixels will not be practical in interactive applications for some time. Because large datasets tend to have subpixel detail, one-sample-per-pixel ray tracing can produce visually distracting popping and scintillaction. We present an algorithm that directs primary rays toward locations rendered in previous frames, thereby increasing temporal coherence. Our method tracks intersection points over time, and these tracked points are used as an oracle to aim rays for the next frame. This is in contrast to traditional image-based rendering techniques which advocate color reuse. We so acquire coherence between frames which reduces temporal artifacts without introducing significant processing overhead or causing unnecessary blur

Martin, William

Shirley, Peter S.

English

The University of Utah: J. Willard Marriott Digital Library

Temporally Coherent Interactive Ray Tracing W. Martill S. Parker E. Reillhard P. Shirley W. Thompsoll UUCS-01-005 School of Computing University of Utah Salt Lake City, UT 841 12 USA May 15, 200 1 Abstract Although ray tracing has been successfully applied to interactively render large datasets, su-persampling pixels will not be practical in interactive applications for some time. Because large datasets tend to have subpixel detail, one-sample-per-pixel ray tracing can produce visual ly distracting popping and scintil lation . We present an algorithm that directs primary rays toward locations rendered in previous frames, thereby increasing temporal coherence. OUf method tracks intersection points over time, and these tracked points are used as an or-acle to aim Tays for the next frame . This is in contrast to traditional image-based rendering techniques which advocate color reuse. We so acquire coherence between frames which reduces temporal artifacts without introducing significant processing overhead or causing unnecessary blur. Temporally Coherent Interactive Ray Tracing W. Martin S. Parker E. Reinhard P. Shi rley W. Thompson University of Utah www.cs.utah.cdu Abst ract. Although ray tracing has been successfully applied to interactively render large dalasels, supersampling pixels wi ll not be practical in interactive ap-plications for some time. Because large dataset') lend to have subpixel detail, one-sample-per-pixcl ray tracing can produce visually dislrill:ting popping and scintillation. We present an algorithm thaI direcTS primary rays toward locat ions rendered in previous frames, thereby increasing temporal coherence. OUT method tracks intersect ion points I.Iver lime, and these tracked points are used as an oracle 11.1 aim rays for the nexi frnmc. T his is in contrast tl.l trad itional image-based ren-dering tcchniques which advocate color reusc. We so acquire coherence between frames which reduces temporal artifacts without introducing significam process-ing overhead Of" causing Wlneccssary blur. 1 Introduction Improvements in general-purpose computer systems have recently allowed ray tracing to be used in interactive applications [10, 13, IS}. On some applications such as isosur-face rcndcring [ 14], ray tracing is superior to evcn highly optimizcd z-buffcr methods. In this paper we present an algorithm to reduce dynamic artifacts in an interactive ray tracing system. This algori thm is relatively simple, and attacks what we believe is the dominant visual artifact in current interactive ray tracing systems. Because ray tracing has a computational cost roughly linear in the number of sam-ples (primary rays), interactive implementations have used one primary ray per pixel and relatively small image sizes, with 512 by 512 images being common. As raw pro-cessor performance increases, these systems can improve by creating larger images, increasing frame rate, or by taking multiple samples per pixel. Once the fra merate is at interactive levels (approximately 15 frames per second or higher), our experience is that increasing the number of pixels is usually a better option than using multiple pri-mary rays per pixel. Current systems that ray trace large dalasets are approximately one hundred limes too slow to render to the highesI resolution screens at thi rty frames per second l. This suggests Ihal even if Moore's Law continues to hold fo r another ten years, we will still be taking only one primary ray per pixel unti l at kast lhat time. Unfortunately, one sample per pixel ray tracing introduces aliasing issues which need to be addressed. Most anti-aliasing approaches were developed in an era where the available processi ng power allowed only relatively simple scenes to be rendered and displayed. As a consequence, the ratio of polygons 10 pixels was low, usually resulting in long straight edges. Aliasing issues are particularly evident in such images, revealing Ihcmselves in Ihe form of jaggies and Moire patterns (1,2]. Animations of such scenes cause the ruMing anls effect along edges and visually distracting scintillations when texture mapping is appl ied. I Recent advances in display technology hayc produ«d LCD displays with Ippro~iIDItely 9 million pixc!s, which is. fictor of 3 10 8 higher than currently common. Fig. 1. Point tracking algorithm. Two solutions are common: spatially filtering Ihe scenc before sampling, or spa-tially filtering the image aftcr sampling [5, 9, 11,12). Pre-filtering the scenc, e.g. usi ng LOD management or MIP-mapping, may not always be possible, as is the case for the 35 mill ion sphere crack propagation dataset used in Parker et a!. [ 14J. Super-sampling the image is considered too costly for interactive systems. as argued above. Another trade-off exists in the fonn of stochastic sampling to tum aliasing into visually less of-fensive noise [4,6]. A by-product of spatial filtering is that temporal art ifacts are also reduced, although temporal anti-aliasing methods have received some attention [8, 16). Since the development of these anti-aliasing schemes, two things have been chang-ing. Scene complexity now commonly far exceeds pixel resolutions and display devices are about to become available at much higher resolutions. The combination of these two leads to the presence of an approximate continuum of high frequencies in the image, which causes spatial aliasing to be visually masked [3, 7] . Whenever this occurs, spatial filtcring produces images that contain more blur than necessary. Highcrscene complex-ity and device resolution do not tend to mask temporal al iasing artifacts as well. A good solution to these issues would therefore preserve spatial crispness while at the same time minimizing temporal arti facts . Our approach is to revert to point sampling where sample localions in the current frame are guided by results from previous frames. Such guidance introduces temporal coherence that will reduce scintillation and popping. II is also computationally inexpen-sive so that most of the processing power can be devoted to producing point samples of highly complex scenes. Scene complexity in tum masks spatial aliasing. The method is demonstrated using an interactive ray tracing engine for complex terrain models. 2 Point tracking algorithm For high polygon to pixel ratios, the scene is sparsely sampled. In interactively ray traced animations, a brute-force approach is likely to possess little temporal coherence. With high probability, each pixel will have di fferent scene features projected onto them from frame to frame. This causes popping/scintillation which is visually distracting. This effect can be minimized by using an algorithm which projects the same feature within a region of a scene onto the screen over the course of a number of frames. We therefore seek a method which keeps track of which points in the scene were sampled during previous frames. This can be easily accomplished within a ray tracing frame-work by storing the intersection points of the primary rays with their corresponding pixels. Prior to rendering the next frame. these intersection points are associated with new pixels using the perspective projection detennined by the modified camera param-eters. Instead of painting the reprojected points directly on the screen [19]. which may lead to inaccuracies, this reprojection serves the sole purpose of directing new rays towards objects that have been previously hil. After the reproject ion of the last fra me's intersection poims. for each pixel one of three situations may arise: • One imersection point was projected onto a pixel. This is the idcal case where scintillation is avoided. • Two or morc intersection points were projected onto a single pixel. Here, an oracle is required to choose which of the points is going to be used. We use a s imple heuristic which chooses the point with thc smallest distance to the camera origin. Points funher away are more likely to be occluded by intervening edges and are therefore less suitable candidates. • Zero points were projected onto a pixel. For such pixels no guidance can be provided and a regular or jiUered ray is chosen instead. For each pixel a new ray is traced. Instead of using image space heuristics to choose ray directions, an object space criterion is used: we aim for the intersection points that were projected onto each pixel. A new regu lar or j ittered ray direction is chosen only in the absence ofa reprojccted point for a given pixcl. Once stored inter.)ection points have been used for the current frame, they are dis-carded. The ray tracing process produces shaded pixel information for the current frame as well as a list of intersection points to be used for the next frame. 3 Results To demonstrate the effectiveness of this point tracking method, we use the interactive ray tracer of Parker et. al . [13) to render two differen t terrain models, which are called Hogum and Range-400. Images of these data sets are given in Figure 2 and their geo-metric complexity is given in Table I . The intersection routine is similar to that used for isosurface rendering [ 14]. The height fie ld is stored in a multiply nested 2D regular grid with the minimum and maximum heights recorded for each grid cell. When a leaf cell is an intersection candidate, the exact intersection is computed with the bilinear patch defined by the four points. The measurcmcnts presented in Ihis paper are all collected using a 32 processor SGT Origin 3800. Each processor is an RI2k running at 400 MHz. We used 30 processors for each run plus an additional processor runni ng the display thread. The computational cost of the point tracking system is assessed in Figure 3. In this graph, we compare the interactive ray tracer with and without poi nt tTacking for different image resolutions. For both data sets it appears that satisfactory frame-rates can be achieved for image sizes up to 5122 pixels. The performance penalty of using the point tracking algorithm is roughly 20%, which is small enough to warrant its use. Although we compare with point sampled images, to obtain similar (but more blurred) results, one would have to compare its perfonnance with super-sampled images, whose cost is linear in the number of samples taken per pixel. Both single sample images with and without point tracking are vastly less expensive than super-sampled images. The visual appearance of the animations with and without point tracking are pre-sented in the accompanying video2• Assuming a worst-case scenario, the point tracking algorithm is most effective if a single intersection point projects onto a given pixel. This l "Thc vilko is in SVHS format and is captur!:d OIl ing the di= t SVltS output or the Origin ~ySkm. This docs l:aWlC sorm bturril\l and al$osorm extra popping. HOI'lew:r, the n: la tiv( IMgnlludc orthe visua l ar1ifa!:ts IS approlllmate1y what wc ob!;cn·c on the actual disp lay. ~ E ~ ~ 0 ! 1 ~ Ii. • ~ Fig. 2. Hogum and Range-400 terrain dmaselS. Terrain Hogum Range-400 Resolution 175 x 199 3275 x 2163 Texture resolution 1740 x 1980 3275 x 2163 Table I . Terrain complexity Time per frame vs. image size: O.'~:=7:==~:=;==-==~--=------~ -&-- HOIUm (brule fon;f:) 0.35 0.3 0.25 0.3 0.15 0.1 0.05 0 -0.050 -A- Range- 400 (brule fon.:c) -e- Hogum (point tracking) ..."... Ran e--400 ( inc cracki" ) S.O fps ~ 9.4 fps " 44- 57 fp$ 20.8 fp$ 2 .68 Image sile (in pi~e ls) 10 2.8 fp, 3.7 fps S.4 fps 12 X 10' Fig. 3. Hogum alld Ran/e-400 data sets rendered with and wilhour point lrackillg al image resolll/ions 0[2562. 384 . 5122. 6402. 7682. 8962. and 10242 pixe&. Shown is the overage [rame.rate achieved[or each animation. IntcrsectlOn points per pixel Terrain 0 I 2 3 4 Hogum 21.93% 58.22% 18.1 8% 1.63% 0.04% Range-400 19.68% 64.73% 14.2 1% 1.33% 0.05% Table 2, Percentage oj pixels that have 0 to 4 ifllf!Tsection points reproject to them. The ideal case is whert / intersection poifll projects /0 a pixel. is when perfect frame-to-frame coherence is achieved. When zero points project to a pixel, a Tay cast for that pixel is likely to produce a contrast change, causing scintil-la tion. When more than one point reprojects to a single pixel, some information will locally disappear for the next fra me. This may also lead to scintillation. For the above experiments, roughly 60% of the pixels in each frame are covered by exactly one intersection point (Table 2). In the remaining cases, either zero or more than one intersection point reprojects to the same pixel. Hence, onc could argue that our algorithm presents a 6()01o visual improvement over standard point sampled animated imagery. Note that these numbers are dependent on both the model and camera speed. 4 Conclusions Our paper has assumed that multisampled spatial filtering will not soon be practical fo r interactive ray tracing. Anticipating a trend towards higher resolution display devices. it is also likely that spatial filteri ng to achieve anti-aliasing will become less nccessary, because the effect of spatial al iasing will largely be masked. However, the same is not true for tempora l aliasing. Combining the performance advantage of point sampl ing over super-sampling with a mechanism to reduce temporal aliasing, we have achieved visually superior animations at a cost that is only slightly higher than point sampling alone. Moreover, its computational expense is much less than the cost of conventional anti-aliasing schemes such as super-sampling. Using this point tracki ng algorithm, the incre3se in computational power which is to be expected according to Moorc's Law can be spent on rcndering larger images, rather than on expensive filtering operations. We speculate that, in the future, the point tracking technique may be applied inde-pendent of the reconstruction used for display. If we altow tracked points to persist even when they co-occupy a pixel , then more sophisticated reconstruction methods could be applied to increase point reuse. Thi s would be in the spirit of Simmons et al. [17] and Popescu et al. [1 5]. We have not pursued this in the present work, noting that the fonnc r use sparscr sample sets than we do, and the lattcr technique requires special pur-pose hardware. Nonetheless, we believe progress in computational perfonnance will continue to outpace improvements in screen resolution, and conclude that such sophis-ticated reconstruction using tracked points wi11 eventually be possible. Acknowledgments The authors would like to thank Brian Smits for insightful discussions. This work: was supported in part by the NSF Science and Technology Center for Computer Graph-ics and Scientific Visualization (ASC-89-20219), and NSF grants 97-31859, 99-78099, and 97-20192. All opinions, fi ndings, conclusions or recommendations expressed in this document arc those of the authors and do not necessarily reflect the views of the sponsoring agencies. References I. BLINN, J . F. Return of the jaggy. IEEE Compllfcr Graphics and Applica/ions 9, 2 (March 19R9), 82- 89. 2. BLINN, J. F. What we need around here is more alia.~ing. IEEE Computer Graphics and Applications 9, I (January 1989).75-79. 3. BOLIN, M. R., AND MEYER, G. W. A perceptually based adaptive sampling algorithm. Proceedings of SIGGRAPH 98 (July 1998),299- 310. ISBN 0-8979 1-999-8. Held in Or-lando, Florida. 4. COOK, R. L. Stochastic sampling in computer graphics. ACM Transac/ions on Graphics 5, I (January 1986),51 - 72. 5. CROW, F. C. A comparison of antialias ing techniques. {EEE Compr.ller Graphics and Applica/ions /, I (January 1981),40--48. 6. D1PP~. M. A. Z., AND WOLD. E. H . Antial iasingthrough stochastic sampling. In Computer Graphics (S{GGRAPH '85 Proceedings) (July 1985), B. A. Barsky, [d., vol. 19, pp. 69- 78. 7. F/i.RWJ;R DA, J. A., PATTANAIK, S. N., SIlIRLEY, P., AND GREeNIJERG, D. P. A model of vi~ual masking for computer graphics. Proceedings of SIGGRAPII 97 (August 1997), 143- 152. ISBN 0-89791-896-7. Held in Los Angeles, California. 8. GRANT, C. W. Integrated analytic spatial and temporal anti-a liasing for polyhedra in 4-Space. In Computer Graphics (SIGGRAPfI '85 Proceedings) (July 1985), B. A. Barsky, Ed.,voI.19,pp.79- 84. 9. MITCHELL, D. P. Generating antialiased images at low sampling densities. In Complller Graphics (SIGGRAPH '87 Proceedings) (July 1987), M. C. Stone, Ed .. vol. 21. pp. 65- 72. held in Anaheim, California; 27 - 31 July 1987. 10. Muuss, M. J. /owards real-time ray-tracing of combina/orial solid goome/ric models. In Proceedings o f BRL-CAD Symposium (June 1995). II. NORTON, A., ROCKWOOD, A. P .. AND SKOLMOSKI , P. T. Clamping: A methodofan-lialiasing textured surfaces by band ..... idth limiting in object space. In Computer Graphics (SIGGRAPH '82 Proceedings) (July 1982), vol. /6. pp. 1-8. /2. PAINTER, J. , AND SLOAN , K. Antialiased ray tl"Ocil/g by adaplive progressive rejillemelll. In Computer Graphics (SIGGRAPH '89 Proceedings) (July 1989). 1. LAne. Ed .. vol. 13. pp. 181-288. held in Boston. Massachusetts: 31 July - 4 Augllst 1989. Il. PARKER, S., MARTIN , W., SLOAN, P.-P. J., SIII RL J;Y, P., SMITS, B., AND HANSEN, C. Imeraclive ray tracing. 1999 ACM Symposium on Interactive 3D Graphics (April 1999), 119--{16. 14. PARKER, S., PARKER, M., LlVNAT, Y., SLOAN, P.-P., HANSEN, C. , AN D SHIRLEY, P. {meroctive ray tracing for volume visualizmion. {n IEEE Transactions on Visualization and Computer Graphics (July-September 1999). 15. POI'ESCU. V., [YLES, 1.. LASTRA, A., STEtNlIURST, J., ENGLAND. N., AND NYLAND, L. The WarpEngine: An architecture for the pas/-polygonal age. Proceedings of SIG-GRAPH 2000 (July 1000). 433--441. {SBN /-58//3-108-5. 16. SHINYA, M. Spatial allli-aliasing for allimatioll sequel/ccs with spatio-temporalfiltering. {n Computer Graphics (SIGGRAPH '93 Proc~edings) (1993). J. T. Kajiya. Ed., vol. 27. pp. 289--296. 17. SIMMONS, M., AND SEQUIN, C. H. Tapestry: A dynamic mesh-based display represema-tionfor imeraCI;ve rendering. Rendering Tc<:hniqucs 2000; 11th EurographiC5 Workshop on Rendering (June 10(0). 319- 340. {SBN 3-11 1-83535-0. /8. WALU, I. , SL\J~A U. F.K, P. , BENTHIN , C., AN ]) WAGN/i.R. M. {merae/h'e rendering Wilh coherent ray tracing. In Eurographies 200 1 (1001). 19. WALTER, B., DRIOTTAKIS, G., AND PARKER, S. {merac/h'e rendering using lhe render cache. In Rendcring Techniqucs '99 (/999). D. Lischinski and G. W. Larson. Eds .. Euro-graphics, Springer- Verlag Wien New York,pp. 19- 30. 

Temporally coherent interactive ray tracing

https://collections.lib.utah.edu/dl_files/7a/50/7a508431ee25ebb4a17a370b820d4b0da3ea84b5.pdf

Temporally coherent interactive ray tracing

Abstract

Similar works

Full text

Available Versions

The University of Utah: J. Willard Marriott Digital Library