@ @ Computer Graphics, Volume 25, Number 4, July 1991

Design and Simulation ofOpera Lighting and Projection Effects

Julie O’B. Dorse.y

Fran~ois X. Sillion

Donald P. Greenberg

Program of Computer Graphics

Cornell University

Ithaca, New York i 4853

Abstract

A major problem challenging opera designers is the inability to co-ordinate lighting, projection systems, and set designs in the prelim-inary planning phase. New computer graphics techniques, whichprovide the set and lighting designer [be opportunity to evaluate,test, and control opera designs prior to the construction of full scalesystems are presented, These techniques—light source input. simu-lation of directional lighting, modeling of scenic projection systems,and full three-dimensional simulation—show the potential for theuse of computer graphics in theater design,

The light source input component consists of a program for as-signing light source attributes with a set of theater lighting icons.This module allows a designer to specify light source characteristicsin a way familiar to the discipline and to make preliminary evalua-tions of the lighting conditions.

An extended progressive radiosity method is introduced to sim-ulate the directional lighting characteristics which are specified bythe input program.

A new projection approach is presented to simulate the opticaleffects of scenic projectors. In addition, a solution to the distortionproblem produced by angular projections is described.

The above components are integrated to produce full three-dimensional simulations of the global illumination effects in anopera scene.

CR Categories and Subject Descriptors: 1.3.0 [ComputerGraphics]: General; 1.3.7 [Computer Graphics]: Three Dimen-sional Graphics and Realism; J.2 [Computer Applications]: Per-forming Arts.General Terms: AlgorithmsAdditional Keywords and Phrases: opera and stage design, angu-lar projection. simulation, radiosity, directional light sources, tex-ture mapping.

1 Introduction

Opera stage design is an extremely difficult task as, in addition totbe standard architectural and aesthetic considerations, a number of

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additional issues are present, such as dynamic and intricate lightingand sets, projected background scenery, changing focus of attention,manipulation of implied perspective, multiple viewing points, mo-tion of performers, and synchronization with music. Stage and light-ing designers, as well as conductors, rarely have [he opportunity toevaluate these effects together, Consequently, stage set and lightingdesigns are currently developed separately—being combined onlyin the Iast step of the process.

Presently. the only feasible method available for combining alimited stage and lighting design is the construction of small scalemodels. While this process dms give some insight into the visualimpact of the final production, it is a laborious. costly, incomplete,and time-consuming endeavor. Furthermore, because of their smallscale, these models are so inadequate for the evaluation of complexlighting effects that they are not commonly used, Thus, in pmctice,the stage and lighting designers will often work in isolation fromeach other. The bulk of the lighting designer’s task. then. occursat the last minute—after the sets are assembled and in place on thestage.

The primary objective of this paper is to provide the stage andlighting designer the opportunity to design and e\’a/uat~ /he Ii<qhringand projected scenery prim- to (he actual implementutimr. In partic -ular, techniques for light source descriptions and specification, thesimulation of directional lighting, and the modeling of scenic pro-jection systems have been developed. In addition, a solu(ion for thedistortion problem in angular projections is introduced. The proce-dures have been combined to provide full three-dimensional simula-tions so that the proposed design strategy can be evaluated from anyviewer position, The variables are the positions of the stage sets, thelocations, orientation, spatial emittance, and color of the lights, thenumber of lights which are illuminated, the background projectionsystems and scenery, all within the given theater geometry.

Three famous opera houses have been selected to demonstratethe system: the Metropolitan Opera at Lincoln Center in New YorkCity, La Scala in Milan, Italy, and the Staatsoper in Vienna, Austria.Due to space limitations. only the Metropolitan Opera is illustrated.

2 Input for Lights

In general, light sources have well-defined tinite geometries thatgreatly affect the distribution of the light emitted from the source.There are three types of abstract emissive geometries: point sources(zero dimensional), linear sources (one dimensional), and areasources (two dimensional) [23 ]. The light sources used in operaproduction can be treated as point sources, since the lights are verysmall and are located at a significant distance from the stage.

Perhaps the most important characteristic of a Iuminaire that mustbe included in a complete model of a light source is the luminous

( 199! AcM-()-89791-436-H/91 /(M)7/(MMl $00.7s 41

SIGGRAPH ’91 Las Veaas. 28 JuIv-2 Auaust 1991

intensity distribution. In contrast to the assumption typically usedin computer graphics, most of the lights used in opera productiondo not emit light of constant intensity in all directions.

A non-uniform intensity distribution must be specified, whichdescribes the variations of the emitted intensity with direction.llte lighting industry uses goniometric diagrams to represent thesevector-valued functions for easy interpretation [3]. These diagramsrepresent a planar slice through the vector field and thus plot therelative intensity as a function of angular direction (Figure 1). For

I12. 12.5

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(a)

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angle

(b)

Figure 1: A sample emission distribution. (a) Polar goniometricdiagram. (b) Corresponding cartesian diagram.

htrninaires with concentrated beams, such as spotlights, cartesiancoordinates are preferred because of the need for more precisionthan a polar curve allows.

2.1 Instruments and Lamps

In lighting design for opera, many different types of Iuminaires areused. Although there appear to be a large number of different in-strument styles used, each style is a variation on five particukw in-struments: the ellipsoidal reflector spotligh~ the Frcsnel spotlight;the stripligh~ the ellipsoidal reflector floodlight; and the beam pro-jector. The optical characteristics of each of these instruments (andvariations thereof) have been modeled.

2.2 Assigning Light Source Attributes

An interactive graphical program has been developed to allow oneto design a lighting scenario. While a final lighting layout is primar-ily a tool for communicating the designer’s concept and intentionsto the electricians, lighting crews, and board operators, who must“hang” the design and execute it in a performance, this programprovides the means to develop ideas. experiment, move and changeinstruments and their attributes, and iteratively refine a design.

The input program allows for the complete specification of at-tributes for stage lighting. The user can specify the position of eachinstrument, its intensity pattern, color, projection pattern, and thearea which it illuminates in addhion to indicating its height abovethe floor and the angles of the beams of light. As the parameters as-sociated with a light are adjusted, the lamp is instantly updated withthe resulting beam and field angles as well as throw distance. Thisfeature makes it possible to combine light sources and evahrate thedesign implications (e.g. if their beams overlap). Thus, while stillin the modeling phase, one has a good idea of the overall lightingscheme. Once a preliminary design is specified, the user can simu-late the illumination effects and, with a separate program, view theresults to further refine the lighting design.

An attempt has beerr made to carefully design the graphical in-terface so the process of assigning attributes is similar to the way inwhich it is physically performed. A menu is available which con-tains a two dimensional graphical representation of the five major

42

categories of lights used in opera production. Once a category hasbeerr selected, it is possible to choose from a variety of instrumenttypes and manufacturers within that category.

Most of the Iuminaircs used in theater production have a spotand flood focus intensity distribution associated with them. Whena lamp has been selected, a two dimensional icon is drawn in onewindow, and the components of the lamp which move during thefocusing of that particular instrument can be varied interactively,making it possible to focus the instrument to the desired setting(Figure 2). To specify the intensity distribution, one window showseither a cartesian or polar goniometric diagram of the current lu-minous intensity dh-ibution or candle power distribution curve forthe light source. This diagram is updated continuously as the instru-ment is focused. To scale the dkribution, the maximum intensitywhich the lamp emits at the center of its beam is specified in unitsof cartdelas.

Each light source can have a unique pattern or slide. A library ofpatterns and slides to be used in projection has been compiled. Thedesigner can select a pattern from the library and associate it with agiven light source.

Color can be controlled by placing a transparent color filter be-tween the light source and the receiving surface(s). Using the filtersection of the input program, an interactive color tool allows theuser to vary the characteristics of the filter used to color the lightemitted by a lamp.

It is possible to position a lamp at arty location relative to the stageenvironment. Most light sources are positioned on the light bridges,but occasionally they are placed on the front edge of the stage asfootlights or on temporary ladder-like structures along the sides. Toaid the user in positioning the hunps, one viewport displays the lightsource with three dimensional transparent cones attached to it (op-tionally displayed) in the model of the stage. These cones representthe beam and field angles as well as the throw distance of the in-strument (see Appendix A). As the lights are positioned relative tothe stage area, the cones allow the user to visualize the direction inwhich the light will be emitted as well as how much illumination aparticular area will receive.

3 Simulation of Theater Lighting Conditions

Radiosity methods, derived from the field of radiative heat transfer,have been successfully applied to the area of realistic image synthe-sis [4, 5, 6, 9, 16, 24]. The radiosity method has the attractive char-acteristic of providing a view-independent solution. Hence, oncethe solution has been performed, a hwdware renderer can be usedto display the scene from changing viewpoints at interactive rates.

3.1 Modeling Directional Light Sources with Progres-sive Radiosity

The progressive radiosity method [4] can be extended to account fordirectional variations in a light source with non-uniform emissiondistributions (Figure 1). In this implementation, the form-factorsare computed using the ray-traced form-factor approach proposedby Wallace et al. [24]. To account for the variation in light source di-rectionality, the form factor from the light source to a vertex is com-puted as usual and then weighted by a directionality scaling factor,s. Each directional light source has a distribution associated with itwhich describes its normalized light source intensity versus angle.The value ofs is obtained from this distribution for each elementvertex based on the direction 82 (the angle between the directionvector of the light source and the direction of an element vertex).

In this way, the amount of light which is transfemed from the lightsource to the environment is weighted accordhg to the directional

Computer Graphics, Volume 25, Number 4, July 1991

(4 (b)

Figure 2: Light Source Input and Attribute Assignment. (a) The Fresnel spotlight - spot and flood foci. (b) The beam projector - spot andflood foci.

Figure 3: Modeling directional light sources with progressive ra-diosity.

distribution (Figure 3). The illumination received at vertex 1 froma spot light 2 can be represented by a weighted form-factor basedon that light source’s emission distribution:

For directional light sources, a single ray is traced from each ver-tex to the light for shadow testing. A single ray is sufficient becausepoint sources are used. The amount of energy per unit area receivedat each vertex is then weighted as shown above. Figure 4 shows aradiosity rendering of several directional light sources.

In this method, the directional light sources initially shoot outtheir directional energy once, and calculations for any subsequentradiant energy exchange due to secondary reflections can be treatedin the standard manner.

4 Projected Scenery

4.1 Overview

An effective method of creating a scenic background is to project aslide onto a neutral surface. Architectural features, general views,natural objects, cloud formations, and similar objects can be pro-jected on to a backdrop. Projected scenery is fundamentally dif-

43

5 SIGGRAPH ‘91 Las Vegas, 28 July-2 August 1991

Figure 4: Radiosity rendering of directional light sources.

ferent from built or painted scenery in that it achieves its effectsthrough the use of light. Because color in light is more brilliant thanin paint, and has an unlimited value scale by comparison, its use inprojection is far more dramatic and eye-catching. Additionally, itcan give a greater illusion of size and depth in a setting [27]. In asense, projection techniques create an imaginary space which ex-tends the bounds of the real space defined by the set geometry. Fur-thermore, they permit not only rapid changes, but also gradual tran-sitions from one setting to the next, such as from daylight, throughsunset to night-all with the same background [ 1 I].

Typically, the picture to be projected is painted by hand or photo-graphically produced. Slides for scenic projectors range in size from5 to IO inches square. Reduced to its simplest elements, the projec-tion process consists of a light source, the object or slide, the pro-jected image, and the projection surface [18]. There are two typesof projection: shadow projection and lens projecrion. The term lensprojection is used to define all projections obtained using one ormore lenses; shadow projection is used to describe projections ob-tained without the use of lenses [26].

4.2 Simulation of Projected Scenery

The simulation of background scenery using slide projection tech-niques is common in opera production, but is new to the field ofcomputer graphics. It is important to derive methods for projec-tion simulation which will maintain the resolution and quality ofthe original slide, provide soft-focusing according to the optics ofthe projection system, allow arbitrary geometries for the receivingsurfaces, mimic the correct dispersion and attenuation of light, andbe computable in tractable amounts of time.

4.3 A Radiosity Projection Technique

This section describes an extension to the progressive radiosity al-gorithm which allows for the projection of scenery. The energy re-ceived at the surface of the backdrop from the projection is a func-tion of the emission distribution of the light source, the transmissiv-ity values of the slide, and the orientation and distance of elementvertices on the backdrop relative to the projection system. The tech-nique (Figure 5) can be expressed as follows:

I. A two-dimensional array, or “texture map,” of values is ob-tained by scanning photographs or artistic renderings of actualimages to be projected.

2. The backdrop/receiving surface is discretized into a series ofelement vertices, the locations of which are determined based

Figure 5: Radiosity Scenic Projection Technique. The backdropis discretized according to the projected resolution of the slide. Theinitial radiosity at each element vertex is then based on the emissionof the light source, the transmissivity of a point in the slide, and thecolor of the backdrop.

on the intersection of casting a single ray from the projectorthrough each point of the slide to the backdrop. The resolutionof the slide varies according to the desired fuzziness or clarityof the projected image on the backdrop (typically a slide witha resolution between 50x50 and IOOxlOO = 2,500 to 10,000element vertices is used).

3. The radiosity of each of these vertices is then based on thecolor/transmissivity of the relevant point in the slide, the back-drop color, and the emission of the light source in the directionof the element vertex.

The initial radiosity at a vertex on the screen due to the projection(source 2) only is expressed by this modified radiosity equation:

It should be noted that the effect of a directional light source,~(02) (projector) is also included in the above equation.

It is also possible, using computer graphics, to model the depthof focus of the projection system. One can “blur” the slide using afilter, sized according to the distance of the projector to the back-drop, so that the entire projection has the same out-of-focus effect.Figure 6 shows a sample projection which was generated using thismethod.

The radiosity projection technique closely simulates the real pro-jection system in that it yields a view-independent solution with in-terpolation at the backdrop based on projection characteristics at themaximum stored resolution of the slide. In addition, the backdropis subdivided according to the projection itself, rather than accord-ing to a separate and unrelated element meshing. Finally, the lightattenuation of the projector is modeled precisely.

4.4 A General Solution for the Distortion Problem inAngular Projection

It is usual to place the projector(s) behind the proscenium, hiddenfrom the view of the audience, and to project downward at an an-gle to the backdrop. Furthermore, it is common for the backdrop to

@Q Computer Graphics, Volume 25, Number 4, July 1991

Figure 6: Radiosity Scenic Projection

Figure 7: Uncorrected Angular Projection - A square slide in theprojector will produce a distorted backdrop image for a viewer.

be non-planar and shaped in some way (e.g. a curved cyclorama).A slide in the projector will therefore produce a distorted backdropimage, which increases in size with the distance from the projectorIO the backdrop as shown in Figure 7. The problem then is how topredistort a slide such that, when projected and viewed from the po-sition of an “ideal viewer,” the backdrop image appears undistorted.

Currently, when a scene is to be projected, the lighting designerwaits until the sets are built and in place. Then, a slide (known as arusr~r) of a regular grid of guide marks is projected from the posi-tion where the projector(s) will be located. Next, the resulting pro-jected image (now a distorted grid) is photographed and examinedfrom various viewer positions. Based on the distortion apparent inthe projected grid at an ideal viewer location, a predistorted slide isproduced using the distorted grid lines as guides. When the slide isprojected. the viewer is presented with an undistorted scene. Gen-erally, several iterations of this process are necessary to achieve thedesired slide. This trial and error process is time-consuming, laborintensive. and restricted to a given set/projection geometry.

This process can be computer simulated using a digital imagewarping technique-providing a designer the opportunity to pre-

view a projection and produce the distorted slide long before sets arebuilt. The algorithm involves modeling the distortion which wouldresult based on the geometry of a hypothetical projection schemaand generating a pre-distorted slide to counteract the distortion.

Image warping is a growing branch of image processing thatdeals with the geometric transformation of digital imagesl28). A ge-ometric transformation is an operation that redefines the spatial rela-tionship between points in an image. The basis of geometric trans-formations is the mapping of one coordinate system onto another.This mapping is defined by means of a spatial transformation-amapping function that establishes a spatial correspondence betweenall points in the input and output images.

Geometric transformations were originally introduced to correctthe distortions introduced by remote sensing methods [2, IOj. Thisprocess involved estimating the distortion model, usually by meansof a set of reference points. The algorithm presented below differsfrom these previous techniques in three important respects. First.in this method. one begins with the undistorted image and modelsthe distortion which is introduced. Second, the method is very gen-eral in that it does not rely on a particular sensor: it can be used tomodel any projection system and can account for a projection fromany location. Third, this technique involves a two-step approach.because the new value at each point is not merely taken from themapped coordinates (as is usually the case) but is instead derivedfrom a second mapping (ideal/desired transformation).

4.4.1 Definitions

In discussing the problem of an angular projection, it is useful tointroduce detinitions of the three pertinent coordinate systems. Apoint on the slide in the projector is referred to as ~,,y,,:,,. a pointin the projected image on the backdrop as s~Y~z~,. and a point in theviewer’s/spectator’s space as .r,.y, z,.. The physical slide defines thecontinuous function of (J’ q,, pz,,) specifying a color at each position.

The ideal situation would be one in which the projection systemwould associate points in the projector (slide) and viewer space suchthat the viewer simply sees a window into the original picture. Thisoperation would define an affine transformation T such that,

(I,..Y,.--, ) = (s,,yp=,,)T (3)

The color (intensity) at some point s,.!,,.z, would then simply de-pend on the color of the slide at point Is,,y,,z,,) = (s,.!/, z,.)T-‘.

where the function F describes the effect of light attenuation andillumination (Its exact form does not need to be specified here).

However, in reality, a point on the slide .r,,y,,+ will be firstmapped to the backdrop by the projection. according to some pro-jection transformation, P. The resulting point on the backdrop willin turn be transformed to the viewer’s space by the usual perspectiveviewing transformation. I’. Thus,

and,

(sf,yt,If,) = (-(.,‘Y*,-,‘)~’ (5)

(~-tty,,--c,) = (.o,yf>=f,)~’ = (s,>y,>-,>JI’l’ = (.r,y,,-,)D (6)

The transformation, D = f I.. represents the combined mappingfrom the projection and viewing systems or the distortion, which isultimately seen by the viewer. There is a parallel between the realdistortion, D and the ideal transformation, T.

The problem is to create a pre-distorted slide such that, after ap-plying the physical transformation D to this slide. the color at each

45

SIGGRAPH ’91 Las Vegas, 28 July-2 August 1991

point in the viewer space is the same as the color obtained by ideallytransforming the original picture using the transformation T:

V(zvgv2u) F(tiiginal_slide( (zvyoz.)T - ‘)) =

F(d2storted_slide((zv gvzv)D-’ )) (7)

A simple way of guaranteeing this is to define the distorted slide as:

V(2PyPzP) distorted_slide(xpyPzP) =

original .sJ2de((zPyPzP)DT– 1) (8)

4.4.2 Producing a Pre-distorted Slide

An algorithm to produce a pre-distorted slide which simulates theactual process used by lighting engineers is presented below. By us-ing Equation (8), the color at every point on the slide is computed.In order to evaluate this equation, a way to compute the distortionD is needed. In general, D is a complex non-linear transformation,and thus it is not practical to evaluate it analytically. The algorithmproposed computes D exactly for a small number of points and usesa linear interpolation to approximate D at all other points. A regularorthogonal grid (the resolution of which can be varied) cart be usedas a means to quantify the distortion function, D. Rays are sent outfrom the projector at regular intervals (based on the resolution of thegrid and the beam angle of the projector) to the projection surface.By transforming the set of grid points from the projector coordi-

m“’”‘(Xpypzp)Pv ‘v ‘

‘r ,d

ideal viewer

Figure 8: Modeling the distortion in a projection. A regular gridis projected from the projector to the backdrop and mapped to theviewer. The projection system P maps a point from the projectorto the screen. The observer’s eye-system V maps a point from thescreen to the viewer.

nates, XPVPZP to the coordinates on the backdrop/projection surfaceXb!/b.zbthe function p is simulated. Next, the projected points aretransformed into the viewer’s coordkates, Zing”z., based on a cam-era specification which represents the view of an ideal spectator.This transformation represents the function V.

The relationship between the original undistorted grid (the setof points ZPVP,ZP)and the resulting points as seen by the viewer(zvyvzu) represents the distortion, D, which was produced by theprojection (Figure 8).

To compute the final slide, the following steps we taken:First, the virtual, undistorted grid is overlayed onto the slide to begenerated as shown in Figure 9. Next, for each point of the slide,(Zpyp.zp):

46

Original picture(xp,y;,z~)

) (

\(x,

1IIII

I

I

III

,yv,2

Predistorted slide(+.Yp.zp)

Viewer’s

[deal transformation image Physical transformationspace

Figure 9: Process for finding the color of a point in the pre-distortedslide.

1.

2.

3.

4.

5.

Find which grid cell of the undistorted grid the point is in,

Find the coordinates (u, v) of the point within the undistortedgrid cell (the local coordinates within the cell),

Compute the coordinates of the transformed point (xVyVz.) inthe distorted grid. This computation is accomplished by bilin-ear interpolation (or a higher order interpolation if necessary)using the four transformed comers of the cell. (At this pointthe effect of D has been evaluated),

Find the point (z~y~z~) in the original image associated to

z.%zV by the ideal transformation T– 1. This is a standardwindowing operation involving a simple affine transforma-tion,

The color of the distorted slide at point (zPyP ZP) is the colorof the original slide at point (z~y~z~).

The result of this procedure is the predistorted slide.Figure 10 shows a projected image of a New York City sky-

line onto a curved backdrop along with an illustration of thecorrected/pre-distorted slide which was generated using the abovemethod.

5 Simulation Results

The geometric information in the opera hall is su~tvided into twoparts, the geometry of the permanent structure and the geometry andposition of the stage sets.

Fixed Geometry. Definition of the Opera Hall

The geometry of the structure, such as the shell or roof over thestage, as well as the auditorium in general, is fixed. Since thefocus of the simulation is only on the stage, a detailed model ofthe stage portion of the hall has been constructed along with anabstract overall building model. This detailed model includes theproscenium and stage with operable stage lifts as well as the lightbridges/gantries where the lights are positioned. The model was

@Q Computer Graphics, Volume 25, Number 4, July 1991

(4 @I (cl

Figure IO: Radiosity scenic projection of a New York City skyline using a predistorted slide. The projection is from a height of 5 1.5 feetdown at an angle to the curved backdrop. (a) Abstract plan of the environment showing the location of the projector, an ideal viewer, and theshape of the backdrop. (b) Final projected image on the backdrop. (c) Predistorted slide.

made to scale using precise dimensions from drawings and pho-tographs received from the Metropolitan Opera House in New YorkCity.

Variable Geometry - Definition of the Stage Sets

Detailed models of the stage sets constitute the second part of thegeometric definition. Most of the sets have been defined as extrudedcontours. Once a set has been modeled, it is possible to position itat any location on the stage and to view it from any position in thehall.

Examples of Lit Opera Scenes

To demonstrate the usefulness of the techniques, three full simu-lations of real productions from the Metropolitan Opera have beengenerated and are shown in Figures II, 12 and 13. Each of these ex-amples shows the specialized lighting effects-projected scenery,angular projection solutions using pre-distorted slides, and stagelighting-+ombined with high quality view-independent simula-tions for practical use in opera design.

Figure 1 I shows two views for Giinther Schneider-Siemssen’sdesign for the Palazzo on the Grand Canal in Venice from Les C’on-fes d’H@inann. The left and right sides of the set are modeled andtexture-mapped. The facades in the center background were pro-jected from stage left at a height of 46.5 feet onto a curved back-drop using a pre-distorted slide. Striplights are used to provide anoverall wash of light on the set. A series of floodlights were usedto simulate moonlight on the facades to the right. A high intensityspotlight produces the effect of the lantern in the doorway.

Figures 12 and 13 show two distinct lighting schemes for Franc0Zeftirelli’s design of a Parisian garret from La BohPme. A promi-nent feature in these simulations is the sky, which was projectedusing two angular projections. The projectors are positioned on alighting bridge at a height of 53 feet on the left and right sides ofthe stage. The slides were predistorted to account for the angularprojections. A variety of stage lights are positioned on the bridgesand used to illuminate the set.

6 Conclusions

A set of computer graphics techniques for the design and simulationof opera lighting effects has been presented. The light input, pro-jection, and simulation components give the stage and lighting de-

signer a unique opportunity to design, preview, and assess an operadesign prior to the construction of full-scale systems. The results ofthis research clearly demonstrate that the use of computer graphicsin theater design holds great promise, particularly since these tech-niques afford the opportunity for aesthetic evaluations to be madeearly in the design process and consequently allow many design pro-fessionals to work in unison in the preliminary design phase.

Future directions include the use of a higher order interpolationscheme for the projection distortion algorithm, anti-aliasing in thegeneration of a pre-distorted slide, and the control of the lighting asa function of time.

7 Acknowledgements

The authors are extremely grateful to the Hewlett Packard Corpora-tion and the Digital Equipment Corporation for donating the equip-ment which was used for the simulations and displays. The authorsalso thank the National Science Foundation for long term supportof the research at the Program of Computer Graphics. The RobertJames Eidlitz Traveling Fellowship provided funding for a site visitto the Staatsoper, La Scala, and the Metropolitan Opera. Specialthanks to Gil Wechsler and Wayne Chouinard at the MetropolitanOpera for providing documentation of real set and lighting designsand for their helpful discussions and input into this research. MarkShepard modeled the La B&me environment used in the simula-tions. Thanks to Kurk Dorsey, Hurf Sheldon, Ben Trumbore, KevinNovins, Harold Zatz, Ellen French, and Fran Brown for their gen-eral assistance. All images were photographed by Emil Ghinger.

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[II

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: SIGGRAPH ‘91 Las Vegas, 28 July-2 August 1991

Figure 11: Two views of the Palazzo on the Grand Canal in Venice from Les Conks d’Hoffmznn. The left and right sides of the set aremodeled. The canal scenery in the center background was projected at an angle onto a curved backdrop using a predistorted slide. A series offloodlights are used to simulate a dim moonlight on the facades to the right. A high intensity spot is used to light the entrance to the left.

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630 Comwter GraDhics. Volume 25. Number 4. Julv 1991

Figure 12 (top and left): Two views of a garret above the rooftops ofParis from La BohPme (evening). The scene contains two angular pro-jections (using predistorted slides) for the sky and a variety of spots andfloods. The focus of the lighting is on the entries and the interior of thegarret. Figure 13 (right): Alternative lighting scheme for the garret fromLa BohPme (daytime). The projected slides have been re-colored to ob-tain the day effect, and additional colored spot lighting emphasizes thecenter of the set and the right entry.

SIGGRAPH ’91 Las Veaas. 28 JuIv-2 Auaust 1991

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Appendix A — Beam and Field Angles

The beam angle is the central cone of light emitted from an instru-ment (Figure 14). The limit of the beam angle is usually defined as

field

/

an Ie(18%)

. P’-i

“$ L9JFlgurc 14: Relationship between the beam and field angles of alighting instrument.

that point where the light diminishes to 50 percent of its intensitywhen compared with the center of the beam: The field angle is des-cribed as that point where the light diminishes to 10 percent of theoutput of the center of the beam.