Transactions on Information and Communications Technologies vol 12, 1995 WIT Press, ISSN

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1 Transactions on Information and Communications Technologies vol 2, 995 WIT Press, ISSN The ALEPH project: image synthesis using illusion and physical based rendering F.J. Seron, J.A. Magallon, E. Melendez, P. Latorre Department of Computer Science, Zaragoza University, C/Maria Luna, s/n. E-5005 Zaragoza, Spain Abstract In the present paper, we describe the main features of the rendering general system ALEPHRad v 2.0 based in illusion and physical techniques for the simulation of light transfer as well as the techniques it will include. This kind of system may be useful for applications like: Architectural design, complex lighting systems, lighting industry and special effects.. Introduction Realism is a vague term when applied to computer graphics. It can mean anything from the rigourous simulation of the physical phenomena of light bouncing through an environment to the illusion of realism created by empirical approximations. In the former case, we are attempting to mimic the workings of reality. In the latter, we are representing an impression. Simulating real processes accurately dictates modeling immutable rules of physics and geometry that rigidly govern the technique. This kind of technique has the advantage of producing images that are able to be analysed. Useful data can be extracted (not only presented as a picture) and used to evaluate the performance of a specific scene. Projecting the illusion of realism simply means that elements of realism are generated only up to the degree necessary to convey the idea or impression. The advantage of this technique is that results may be obtained very quickly and easily without fully describing the exact physical characteristics of the whole scene.

2 Transactions on Information and Communications Technologies vol 2, 995 WIT Press, ISSN Visualization and Intelligent Design in Engineering The aims of the ALEPH project, [ ALEPH stands for Advanced Library for Efficient Photorrealism ], were to collect the background information required for a complete understanding of the rendering techniques and implementing a rendering general system based in illusion and physical techniques. Now is a rendering system designed for producing special effects, and it is also being used in the creation of computer generated imagery tailored to the demands of lighting design and architecture. Generally we use physically-based rendering for lighting design and illusion-based rendering for architecture. In figure, appears the temporal evolution of the ALEPH project and a short description of each important item. 2. Illumination models The illumination models used in computer graphics applications divide incoming light into two classes: light arriving directly from light sources and light arriving indirectly from the reflection of the neighbouring bodies. And there are four convenient classes in which to place the light leaving a surface: diffuse reflection, diffuse transmission, specular reflection, and specular transmission. Thus, we have two kinds of incident light and four modes of light transport, giving us a total of eight classes of light-surface interaction to consider. The intensity of light leaving a surface in a given direction is a function of the illuminating light, the properties and geometry of the surface itself, the direction of the viewer and the emissivity of the surface. The computer graphics illumination models used in practice fall into four general classifications: empirical, transitional, analytical and hybrid. The shading techniques that evolved with these models fall into four corresponding classification: incremental, ray tracing, radiosity and hybrid (ray tracing + radiosity) methods. Early incremental models were empirical in nature. They were evaluated after the geometry was transformed into the perspective space (screen space) and used for visible surface computation, [6, 33, 9, 28. Transitional models use prior work in physics and optics to improve the earlier empirical models, [8, 5, 34, 22, 3,, 24]. Transitional models provide a basis for determining the fraction of the incident energy that are reflected and transmitted at a "smooth interface". The ray tracing approach attempts to simulate light rays within a threedimensional scene. Ray tracing is especially appropriated at modeling specular reflection, refraction, and transmission effects. This approach for the simulation of global illumination can be characterised as view-dependent. However, most objects do not have smooth surfaces. With a rough surface the incident and transmitted light is scattered in all

3 Transactions on Information and Communications Technologies vol 2, 995 WIT Press, ISSN Visualization and Intelligent Design in Engineering 3 directions. Analytical approaches make the illumination model the driving force for the application of energy equilibrium techniques to computer imagery, [3, 8,, 27, 26]. The radiosity formulation offers a solution to the restricted problem of global illumination with ideal diffuse reflectors. This approach for the simulation of global illumination can be characterised as view-independent. The hybrid rendering technique integrates the radiosity and ray tracing technique to handle diffuse and specular effects respectively [3]. 3. ALEPHRad v 2.0. Implementation ALEPHRad v 2.0 is a physically-based rendering system tailored to the demands of lighting design and architecture. The simulation uses a hybrid method (simple two-pass approach) [3] and taking advantage of the complementary strengths of radiosity and ray tracing. Practically, it uses radiosity to compute the diffuse interreflection of light between the surfaces, and ray tracing algorithm adds specular reflections/refractions and highlights. Once the radiosity solution is obtained and a viewpoint has been selected, a picture is easily rendered using a classical ray tracer with the following simplification: no shadow rays are needed, since the radiosity solution already takes into account shadowing between surfaces. The software is divided into several different components, which are related and designed to work in concert. The system uses as input data: three dimensional geometric models, specific material properties, specific light sources and textures. The system offers the user to solve the rendering equation with different approximations; it can be included specular and/or diffuse and/or directional-diffuse reflection and transmission in any combination. The algorithms have been taken from the following sources: Radiosity pass: * Progressive refinement (in order to solve the calculation problem) [2, 9, 0, 20]. * Adaptative refinement (in order to solve the computing energy exchanges with groups of patches at once) [, 3, 4, 7] * Ray casting - (in order to determine visibility between two surfaces) [7, 2]. - (in order to calculate the form-factors) [32, 2, 2].

4 Transactions on Information and Communications Technologies vol 2, 995 WIT Press, ISSN Visualization and Intelligent Design in Engineering Ray Tracing pass: The ray tracing technique, [3,, 29, 4, 24, 25, 5], provides a method for modeling diffuse and translucent surfaces, soft shadows, motion blur, and depth of field Rendering [9, 6, 23] The Interface selected between three-dimensional geometric modeling systems and ALEPHRad v 2.0 is RenderMan. One of the great advantages of using RenderMan is the fact that you can describe the appearance characteristic of the objects with as much detail and subtlety as you typically describe the shapes and positions of those objects. The RenderMan Shading Language is a special-purpose programming language for describing appearance characteristic. Shading Language programs, called shaders, can be used to model materials and effects in a physically "realistic" or in an "unrealistic" artistic style [30]. 4. Example As an example of the ALEPH possibilities, we present (Figure 2) a lighting simulation of a synthetic furniture, including books, pottery, glass vases, tv set, video set, pictures within the bookcase, portraits and all the objets inside the display cabinet, in a real room. As lighting simulation of an almost empty room, except for a bookcase, is not very interesting, nor is it very informative, we have composed it with a real scene, taking into account several factors, such as the real lighting parameters measured from the room and used for the calculations of the synthetic furniture. This way, the furniture, shadows and other details provide the visual cues we need to understand the lighting of a space. The resulting image offers the chance of using four different kind of area light sources ( two ceiling fluorescent lamps, a window, a little lamp beside the furniture and four spotlight bulbs within the bookcase), and some textures ( wood, pictures, portraits, cover-books,... as texture maps and glass vases, candelabras,... as procedural textures). The scene geometry was also relatively complex since the virtual camera position and its intrinsic parameters had to perfectly match up to the photograph ones. The materials involved in the synthetic objects calculation were basically plastic (wood and books) and metal (frames,...) to obtain realistic reflections and interactions with the ambient light. Glass and some special procedural materials were also used, paying more attention to the smallest details.

5 Transactions on Information and Communications Technologies vol 2, 995 WIT Press, ISSN Visualization and Intelligent Design in Engineering 5 5. Open problems Simple two-pass techniques are capable of producing highly realistic images. However, they ignore important physical properties of light reflection and offer no guarantee that reflection effects are correctly simulated since they lack of some effects as light-specular-...-specular-diffuse-eye, because when the diffuse surface is found from the eye, there is no information available to compute the illumination of that surface due to specuiar reflectors. The assumption that all surfaces are ideal (diffuse or specular) is rarely met in practice. Deviation from these behaviour is necessary for photorrealistic image synthesis. Unfortunately, non diffuse materials make the global illumination problem significantly more complex, and algortihmic changes are needed to keep this complexity under control. The future work should be to develope a physical-based rendering system to efficiently solve the rendering equation under most conditions. This includes specular, diffuse and directional-diffuse reflection and transmission in any combination to any level in any environment. The simulation blends deterministic and stochastic (Monte Carlo technique) light-backwards ray-tracing method. 6. Conclusions The development of this work has allowed us to understand the natural illumination processes, the developing of illumination models and the experience of implementing a image generation system. This experience has highlighted the problems underlying theory. Now ALEPHRad v 2.0 is a rendering general system based in illusion and physical techniques designed for producing special effects and used in the creation of computer generated imagery and it is also being used in the creation of computer generated imagery tailored to the demands of lighting design and architecture. However the illumination models used in ALEPHRad v 2.0 to model behaviour of light, are still constrained by lack of information. References. Amanatidcs, J., Ray tracing with cones, ACM Computer Graphics (SIGGRAPH 84), vol. 8, nf 3, pp , 984, 2. Amanatides, John, Andrew Woo, Optik (version.2a) ray tracing program, Dynamic Graphics Project, University of Toronto, Arvo, James, Andrew S. Glassner, et al., Graphics Gems II, Academic Press, Boston, Baum, Daniel R., James M. Winget, Stephan Mann, Kevin P. Smith, "Making Radiosity Usable: Automatic Preprocessing and Meshing Techniques for the Generation of Accurate Radiosity Solutions",

6 Transactions on Information and Communications Technologies vol 2, 995 WIT Press, ISSN Visualization and Intelligent Design in Engineering Computer Graphics (SIGGRAPH'9 Proceedings), vol. 25, no. 4, pp. 5-60, August Blinn, J. F. and Newell, M. E, Texture and reflection in computer generated images. Communications of the ACM, vol. 9, n- 0, pp , Bouknight, W. J., A procedure for generation of threedimensional half-toned Computer Graphics presentations. Communications of the ACM, vol. 3, n^ 9, pp , Campbell, III, A.T., Donald S. Fussell, "Adaptive Mesh Generation for Global Diffuse Illumination", Computer Graphics (SIGGRAPH'90 Proceedings), vol. 24, no. 4, pp , August Catmull, E. E., Computer display of curved surfaces. Proceedings IEEE Conference on Computer Graphics, Pattern Recognition and Data Structures, May 975, pp. -7, Chen, Shenchang Eric, "A Progressive Radiosity Method and its Implementation in a Distributed Processing Environment", Master's Thesis, Program of Computer Graphics, Cornell University, January Chen, Shenchang Eric, "Incremental Radiosity: An Extension of Progressive Radiosity to an Interactive Image Synthesis System", Computer Graphics (SIGGRAPH'90 Proceedings), vol. 24, no. 4, pp , August Cohen, Michael, Donald P. Greenberg, Dave S. Immel, Philip J. Brock, "An Efficient Radiosity Approach for Realistic Image Synthesis", IEEE Computer Graphics and Applications, vol. 6, no. 3, pp , March Cohen, Michael, Shenchang Eric Chen, John R. Wallace, Donald P. Greenberg, "A Progressive Refinement Approach for Fast Radiosity Image Generation", Computer Graphics (SIGGRAPH'88 Proceedings), vol. 22, no. 4, pp , August Cook, R. L. y Torrance, K. E., A reflection model for Computer Graphics. ACM Transactions on Graphics, vol., n-, pp. 7-24, Cook, R. L., Stochastic sampling in Computer Graphics. ACM Transactions on Graphics, vol. 5, n-, pp. 5-72, Dippe, M. A. and Wold, E. H., Antialiasing through stochastic sampling. ACM Computer Graphics (SIGGRAPH 85), vol. 9, n* 3, pp , Francois Sillion, Claude Puech, Christophe Vedel, "Improving Interaction with Radiosity-based Lighting Simulation Programs", Computer Graphics (990 Symposium on Interactive 3D Graphics), vol. 24, no. 2, pp. 5-57, March Goldsmith, J., J. Salmon, "Automatic Creation of Object Hierarchies for Ray Tracing", IEEE Computer Graphics and Applications, vol. 7(5), pp. 4-20, May Goral, C. M., Torrance, K. E., Greenberg, D. P. and Battaile, B., Modeling the interaction of light between diffuse surfaces. ACM Computer Graphics (SIGGRAPH 84), vol. 8, n* 3, pp , 984.

7 Transactions on Information and Communications Technologies vol 2, 995 WIT Press, ISSN Visualization and Intelligent Design in Engineering 7 9. Gouraud, H., Continuous shading of curved surfaces. IEEE Transactions on Computers, June 97, pp , Greenberg, Donald P., Michael Cohen, Roy Hall, Holly Rushmeier, Francois Sillion, John Wallace, "Radiosity", SIGGRAPH'9 Course Notes, July, Haines, Eric, John R. Wallace, "Shaft Culling for Efficient Ray- Cast Radiosity", 3D/Eye Inc., New York, January, Hall, R. A. y Greenberg, D. P., A testbed for realistic image synthesis. IEEE Computer Graphics & Applications, vol. 3, n- 8, pp. 0-20, Heckbert, Paul S., "Adaptive Radiosity Textures for Bidirectional Ray Tracing", Computer Graphics (SIGGRAPH'90 Proceedings), vol. 24, no. 4, pp , August Kajiya, J. T., The rendering equation, ACM Computer Graphics (SIGGRAPH 86), vol. 20, n? 4, pp , Lee, M. E, Redncr, R. A. and Uselton, S. P., Statistically optimized sampling for distributed ray tracing. ACM Computer Graphics (SIGGRAPH 85), vol. 9, n? 3, pp. 6-67, Immel, D. S., Cohen, M. F. y Greenbcrg, D. P., A radiosity method for non-diffuse environments. ACM Computer Graphics (SIGGRAPH 86), vol. 20, n? 4, pp , Nishita, T., Okamura, I. y Nakamae, E., Shading models for point and linear sources. ACM Transactions on Graphics, vol. 4, n- 2, pp , Phong, B. T., Illumination for computer generated pictures. Communications of the ACM, vol. 8, n? 8, pp. 3-37, Upstil, S., The RendcrMan Companion: A programmer's Guide to Realistic Computer Graphics. Addison-Wesley, Vcrbeck, C. P. and Grccnbcrg, D. P, A comprehensive lightsource description for Computer Graphics. IEEE Computer Graphics and Applications, vol. 4, n- 7, pp , Wallace, J. R., Cohen, M. F. y Greenbcrg, D. P., A two-pass solution to the rendering equation : A synthesis of ray tracing and radiosity techniques. ACM Computer Graphics (SIGGRAPH '87), vol. 2,n*4, pp , Wallace, John R., Kells A. Elmquist, Eric A. Haines, "A Ray Tracing Algorithm for Progressive Radiosity", Computer Graphics (SIGGRAPH'89 Proceedings), vol. 23, no. 3, pp , July Warnock, J. E., A representation. Ph hidden surface algorithm for half-tone picture D. Dissertation. Department of Computer Science, University of Utah, Salt Lake City, Whitted, T., An improved illumination model for shaded display. Communications of the ACM, vol. 23, n^ 6, pp , 980. Note: This work was supported by the Comision Interministerial de Ciencia y Tecnologia Espanol. Project TIC C02-02

8 8 Visualization and Intelligent Design in Engineering 00 ON ON TH rv. ALEPH \ Radiant Energy \ Isotropic, Anisotropic Opaque Translucent Energy Interchange ;A Physical BDRF-BDTF functions Empirical values O[ Commercial Uses Transactions on Information and Communications Technologies vol 2, 995 WIT Press, ISSN VJ K <N Primitives + Parametric Surfaces 3 S.0.0 (3 "c o "q oo ^ RenderMan «00% - Correct Experimental - Commercial «02-30^ hours : TH i CICYT (TIC92-03-C02) ALEPHRad v 2.0 Diffuse + Specular Energy Isotropic Opaque Translucent Energy Interchange CICYT (TIC92-03-C02) ALEPHRad v.0 Diffuse Energy Isotropic Opaque CONAI (IT-3/90) ALEPHTracer v 4.0 CONAI (IT-3/90) ALEPHTracer vl.o Isotropic Reflectant Translucent Geometrical Optics > < & Isotropic Reflectant Translucent Energy Intechange Geometrical Optics i Empirical & Physical Coefficients Empirical Values Physical Coefficients Empirical Values Empirical Coefficients Empirical Values ) Empirical Coefficients Empirical Values Radiosity W/m^ (lux) ) a TJ (4 PC Dimensionless Dimensionless Primitives + Parametric Surfaces Radiosity + RayTracing (Hybrid - Double-Pass) Polygons Primitives + Parametric Surfaces Primitives + Parametric Surfaces d O G 3 d Tracing HRay to G "C 2 Global Global Ideal Point 8Area Sources.2 _o o crj so U "c G 3 (U - "c 'o 0 2 TJLight Sources RenderMan = 00% - Incorrect Commercial RenderMan Propietary C-Binding :- 00% - Correct Commercial Experimental Propietary C-Binding Experimental ; Energy Balance ^ o Z en w = 0-90 hours» 0 hours = 30 min - 30 min EMPIRICAL Vv\LUES VALIDATICN OF RESULTS Q II Z \ Paradigm Materials Surface Behaviour Optical Characterization of Materials Magnitude Geometrical Modelling I Visual Modelling Illumination J Interface Soft Condition Computer Rendering Time Scientific Problems

9 Transactions on Information and Communications Technologies vol 2, 995 WIT Press, ISSN c CO CD a C/3 OQ' 3' rn CD rjq Figure 2. Synthetic furniture in a real room