Table-Top Spatially-Augmented Reality:Bringing Physical Models to Life with Projected Imagery

Ramesh Raskar, Greg Welch, Wei-Chao ChenUniversity of North Carolina at Chapel Hill

{raskar, welch, ciao}@cs.unc.eduhttp://www.cs.unc.edu/~raskar/Tabletop/

Abstract

Despite the availability of high-quality graphicssystems, architects and designers still build scaledphysical models of buildings and products. These physicalmodels have many advantages, however they are typicallystatic in structure and surface characteristics. They areinherently lifeless. In contrast, high-quality graphicssystems are tremendously flexible, allowing viewers to seealternative structures, facades, textures, cut-away views,and even dynamic effects such as changing lighting,moving automobiles, people, etc.

We introduce a combination of these approaches thatbuilds on our previously-published projector-basedSpatially-Augmented Reality techniques. The basic idea isto aim multiple ceiling-mounted light projectors inward tographically augment table-top scaled physical models ofbuildings or products. This approach promises to providevery compelling hybrid visualizations that afford thebenefits of both traditional physical models, and moderncomputer graphics, effectively "bringing to life" table-topphysical models.

1. Introduction

In [Raskar98c] we introduced the general notion ofSpatially Augmented Reality (SAR), where physicalobjects are augmented with images that are integrateddirectly in the user’s environment, not simply in theirvisual field. For example, images can be projected ontoreal objects using light projectors, or embedded directly inthe environment with flat panel displays. For the purposeof this paper we concentrate on the former, in particularfor the specific case where multiple ceiling-mountedprojectors are aimed inward so that they illuminate and

can augment table-top scaled physical models of buildingsor other objects.

Figure 1. Two different views of simple physicalmodels augmented with projected imagery. (Theunderlying physical models are shown inFigure 2.)

This setup promises very compelling hybridvisualizations that afford benefits heretofore exclusivelyafforded by either physical or graphics models. Liketraditional physical models, the augmented physical modelcould be viewed in 3D from any position around the table,by multiple people, without head-tracked stereo glasses.However as is typical with modern computer graphics,one could easily depict alternative surface attributes,changing lighting conditions, dynamic objects, and otherhelpful 2D information. If one was willing to wear trackedstereo glasses, one could make virtual modifications to thephysical model, adding or removing components,depicting internal structure, etc. In either case, multipleinward-pointing projectors can be used to afford very highresolution and highly-saturated imagery.

Figure 2. The underlying physical models fromFigure 1. The physical objects are wood, brick, andcardboard.

When more than one inward-pointing projector is usedto create virtual imagery, two central problems need to besolved:• We need to calibrate the display environment and

achieve static registration between the projectors andobjects in the working volume. If 3D virtual imageryis desired, the corresponding head-tracking systemalso needs to be registered.

• We also need to solve the more difficult problem ofgenerating seamless images by achieving geometricregistration between overlapping projections.Building on our previous work [Raskar99a] wepropose to use static video cameras and standardactive computer vision techniques to compute thenecessary 3D representation of the projectorparameters and the surfaces of the real objects. Theproblem of achieving seamless imagery with multiple

projectors has been explored for simpleconfigurations by [Humphreys99] [Panoram][Raskar98d] [Raskar99a] [Trimensions]. However, inthis case, due to the presence of concave objects or acollection of disjoint objects, the regions of overlapbetween the two projectors are not necessarilycontiguous.

1.1 Applications

The hybrid physical/graphics model-based SARapproach described in this paper has certain restrictionswhen compared to pure physical or graphics modelapproaches. However, it offers an interesting new methodto realizing compelling high-fidelity illusions of virtualobjects and surface characteristics coexisting with the realworld. Two example applications are augmentedvisualization of table-top architectural models of one ormore buildings, and augmented visualization of bench-topparts and procedures for assembly line workers or repairtechnicians.

In the first example, an architect could provide clientswith a compelling form of walk-around scaled model ofthe real (proposed) buildings or complex. In a simpledemonstration in [UnderKoffler99], a single projector isused to illuminate blocks on a table-top for urbanplanning. With SAR, at a minimum, assuming the surfacesof the physical model are diffuse white, the approachcould be used to "paint" different colors and textures ontothe surfaces of the physical model. (The textures couldalso convey some notion of 3D surface perturbations byusing bump mapping for example.) In addition she couldshow the clients the building as it would appear undervarying lighting conditions, including night time withbuilding lights on, daylight with the sun in varyingpositions, and both over varying seasons. Finally, shecould show the clients parts of the internal structure of thebuilding, including pipes, electrical wiring, etc.

In the second example, an assembly line worker couldbe guided through the assembly process via spatially-augmented information. Head-mounted display AR hasbeen used for this application at the Boeing Corporation[Curtis98]. Using the techniques in this paper we believeone could achieve the same effects without the need of ahead-mounted display, using inward-pointing projectors torender instructional text or images on a white worksurface.

1.2 Hybrid Model Visualization

In purely virtual environments (VE), one rendersgraphics models of real objects, usually together withcomputer generated virtual objects. In contrast, the basicnotion of Augmented Reality (AR) is to enhance physicalobjects with computer generated virtual objects. In thecase of immersive (HMD-based) VE, the user seesphysical and virtual objects at the same limited spatial andtemporal resolution and fidelity. One advantage ofprojector-based Spatially-Augmented Reality[Raskar98c], like optical-see-through HMD-based AR, isthat the spatial and temporal fidelity of the physical objectis preserved and only the additional data is rendered atlimited resolution. In contrast, with video-see-throughHMD-based AR, images of virtual objects are renderedand superimposed with video images of the physicalobjects, so again the user sees physical and virtual objectsat the same limited resolution and fidelity.

Here we are interested in Spatially-Augmented Realityin the specific case where the physical object beingaugmented by projected imagery is itself a model ofinterest—in fact, a physical model that matches the basicstructure of the graphics model, the representation usedby the computer. In the most basic example, thevisualization of a building (for example) makes use ofboth a physical model and a graphics model of thebuilding. The physical model has the proper structure orshape, but no color or texture. The graphics modelminimally includes the structure (identical to the physicalmodel), the colors, the textures, and any other surfaceattributes. In addition, the graphics model might containsome purely virtual components for which there is nophysical counterpart. In effect, the user is viewing ahybrid physical and graphics model, getting advantagesfrom both.

1.3 Projector Configurations

Previously, multiple overlapping projectors have beenused primarily to create large panoramic displays. Theuser typically stands in front of the displayed images (e.g.InfoMural [Humphreys99], InfinityWall [Czernuszenko97]) or inside the large field-of-view display environment(e.g. Cave [Cruz-Neira93], Office of the Future[Raskar98a] [Raskar99a]). We call this an inside-looking-out projection system. In most cases, one aligns theprojectors so that the neighboring projections overlapside-by-side. The region on the display surfacessimultaneously illuminated by two or more projectors isusually a (well-defined) single contiguous area. Further,the corresponding projector pixel coordinates change

monotonically. This is similar to the monotonic orderingof corresponding pixels in stereo camera pairs.

Here we envision a table surrounded and illuminatedby a collection of ceiling-mounted projectors, where userscan visualize and possibly interact from anywhere aroundthe table. We call this an outside-looking-in projectionsystem. One can imagine using the projectors to renderonto a simple display surface such as a sphere or a cube,creating a crystal-ball type visualization system. Anothersetup would be looking into a concave hemispherical bowlilluminated to render high-resolution 2D or head-tracked3D imagery that you can walk around.

In this paper we are more interested in visualizationsystem where one can change 2D attributes such as coloror texture, and possibly 3D attributes, of known three-dimensional physical models that themselves form thedisplay surface. We have previously demonstrated[Raskar98a] how to render perspectively correct imageson smooth but non-planar display surfaces. In this case,due to the presence of concave objects, or a collection ofdisjoint objects, the regions of overlap between two ormore projectors are not necessarily contiguous, andcorresponding pixels do not maintain monotonic ordering.This is a major difference and creates new challengeswhen a seamless image of the virtual object is to berendered in the overlap region. In this paper, we discussthe motivation for such a system and suggest an approachfor calibration and rendering for such a setup.

2. Usefulness

At one extreme, if a detailed physical model of anobject is available, the model is clearly going to be higherresolution, more responsive, easier on the eyes, essentiallybetter than almost anything Virtual Reality (VR) has tooffer—for a static model. At the other extreme, clearlypure VR has the advantage in that you can show the user"anything," static or dynamic, without the need for aphysical model. We believe that this hybrid Spatially-Augmented Reality approach can offer some of theadvantages of each of the two situations, when a physicalmodel is either readily available or obtainable. We believethat the combination has significant potential. Even simplestatic demonstrations are extremely compelling, bright,clear, and easy to look at. (Please see the video availableat project webpage.)

In general, assuming you want to augment a physicalobject with 2D or 3D graphical information, you have aseveral alternatives [Milgram94]. For example, you coulduse a video or optical see-through head-mounted display.

In fact, one major advantage of Spatially AugmentedReality achieved using light projectors is that the userdoes not need to wear a head-mounted display. (In[Bryson97] and [Raskar98c] the various advantages ofspatially immersive displays over head-mounted displaysfor VR and AR have been noted.) In video see-throughAR, or pure VR for that matter, the physical and virtualobjects are both rendered at a limited pixel resolution andframe rate i.e. limited spatial and temporal resolution. Inthe hybrid SAR approach however, the spatial resolutiondepends only on the display parameters of the projectorsuch as its frame buffer resolution, field of view anddistance from the illuminated object. The spatial andtemporal resolution of static scene is independent of theviewer location or movement. Thus, using a fixed set ofprojectors much higher resolution imagery, text or finedetail can be realized.

If only surface attributes of real objects are to bechanged, then the calibration, authoring and rendering aremuch easier. In this case, the rendering is viewerindependent, no stereo display (projection) is necessaryand multiple people around the real object cansimultaneously see the augmentation. Even if the virtualobjects are not strictly surface attributes, but are near thereal surfaces on which they are displayed, the eye-accommodation is easier. Most of these advantages areshared by all spatially-augmented reality setups.

To be fair, such a hybrid approach has somedisadvantages. The approach cannot in general be said tobe better than pure physical or graphics models, but betterthan each in certain respects under certain circumstances,and worse in others. For example, you must have or beable to obtain (using our methods for example) a graphicsmodel of the physical model. Also, one of the advantagesof video see-through AR is that virtual imagery can easilyocclude the images of real objects. In projector-basedSAR, if the surfaces of the physical model are not purewhite, one might not be able to completely occludeportions of the physical model, should that be necessary.

3. Methods

We have developed a simple interactive approach tomodifying the surface characteristics of multiple table-topphysical models. The approach essentially involvesmanually adjusting projected image texture coordinates tovisually align with the physical models. While notsophisticated, we have shown the results to many people,and the overwhelming consensus is that these simpleresults are extremely compelling. (Please see the videoavailable at project webpage.)

More significantly, building on our previous work wehave developed a comprehensive automatic approach formodifying the surface characteristics of the physicalmodel, and adding 3D virtual objects. While we are stillworking on demonstrating this full approach, we havedemonstrated individual portions, and hope to have a fulldemonstration soon.

The full approach for augmenting physical modelsinvolves first determining the relationships betweenvarious components in the environment and theirparameters. These components include video cameras,light projectors, physical model and the head-trackingsystem. We refer to this as the calibration phase. Next,the user might need to interactively associate parts of thegraphics model with the corresponding parts of thephysical model, or they might want to alter parts of thegraphics model. We refer to this as authoring. Finally,during run time we use advanced rendering techniques toaugment the physical model with perspectively correctvirtual objects for the head-tracked user.

3.1 Calibration

We propose to use multiple ceiling mounted inward-looking static video cameras to capture geometricinformation about the physical model. The video camerascan themselves be calibrated by observing a commoncalibration pattern such as a cube with carefully pastedcheckerboards on each of its visible side[Tsai86][Faugeras93]. After the intrinsic and extrinsicparameters of the cameras are computed, the calibrationpattern can be removed. By projecting active structuredlight with projectors, calibrated stereo camera pairs can beused to compute the depth in the scene. The primitives inthe structured light could be a dense set of binary encodeddots projected by each projector. By stitching togetherthe depth values computed by each stereo camera pair,one can create a 3D surface representation of the entirephysical model. Since multiple projectors will be used, itis necessary to create a unique and continuous geometricrepresentation of the physical model so that we candisplay overlapping images without visible seams. Theextracted physical model can be stored as a polygonalmodel, the graphics model. During depth extraction, wecan also determine the correspondences between 2D pixelcoordinates of a given projector and the 3D locationsilluminated by those pixels. If corresponding pixels for sixof more 3D surface points are known, one can calibratethe projector and find the projection parameters of thatlight projector. Finally, if a head-tracking system is used,the transformation between the tracker’s coordinatesystem and the working volume’s coordinate system can

be computed by taking readings of the tracker sensor atmultiple positions and corresponding positions of thesensor computed by triangulation with calibrated stereocamera pairs.

When more than one projector illuminates a part of thephysical model, we need to ensure that the projectedimages are geometrically aligned. This is analogous tocreating photo mosaics by stitching images together as in[Szeliski96][Sawheney97]. We need to computecorrespondences between multiple projector pixels. Eachcamera observes which pixel of different projectorsilluminated the same surface point on physical model. Theset of projector pixels in correspondence can be indirectlycalculated from these observations [Raskar98d][Raskar99a].

3.2 Authoring

One of the important tasks in achieving compellingaugmented reality is to create association between thephysical objects and the graphics primitives that willenhance those objects when projected. Examples ofgraphics primitive are lines, text, texture mappedpolygons or even complete 3D (virtual) objects. Forexample: which texture image should be used for the faceof a building model? What color distribution will lookbetter for a physical model? A user interface is critical increating the graphics primitives with different shape, colorand texture. A similar user interface is required forpositioning and aligning the graphics primitive so that it iscorrectly projected on the desired part of the physicalmodel.

3.3 Rendering

If one wants to change only the surface attributes ofthe physical model such as color or texture, then it maynot be necessary to completely compute the 3D graphicsmodels of the physical model or the projection parametersof the light projectors. For example, if the user wants tochange color of one face of a building on a tabletoparchitectural model then s/he only needs to find the set ofpixels from one or more projectors that illuminates thatface of the building. Those pixels can be determinedinteractively without explicit 3D representation. Thepixels can be colored or applied pre-warped textures tochange the appearance of the face of the building. On theother hand, if the 3D graphics model of the building andprojector parameters are known, then one can easily pre-compute the set of projector pixels that illuminate the faceof the building. When only surface attributes (for diffuse

surfaces) are changed, the rendering can be assumed to beview-independent and no head tracking is necessary.

When virtual objects are to be rendered in head-tracked 3D, one can use the two-pass rendering methoddescribed in [Raskar98a]. With this method virtual objectscan be made to appear perspectively correct even whenthe underlying surfaces of the physical model is notplanar. In the first pass, the desired image of the virtualobject for the user is computed and stored as a texturemap. In the second pass, the texture is effectivelyprojected from the user’s viewpoint onto a polygonalgraphics model of the physical model. The polygonalgraphics model, with the desired image texture mappedonto it, is then rendered from the projector’s viewpoint.This is achieved in real-time using projective textures[Segal92]. As described in [Raskar99a], usually a thirdpass of rendering is necessary to ensure that theoverlapping images projected from multiple projector aregeometrically aligned.

When multiple projectors overlap, the luminance inthe overlap region may be much greater than that inregions illuminated by only one projector. Thus inaddition to geometric alignment between projectedimages, it is also necessary to achieve intensitynormalization. The problem of generating seamlessimages using multiple projectors has been explored forlarge wide-field-of-view displays [Panoram][Trimensions] [Raskar99a] [Raskar99b], as well as two-dimensional arrays of flat projections [Humphreys99][Czernuszenko97]. In such cases, the overlap region istypically a (well-defined) contiguous region on displaysurface as well as in each projectors frame buffer. Theintensity of projector pixels is weighted using feathering(also known as intensity roll-off or soft-edge) techniquesso that the overlapping images blend to create a singleseamless image. In case of multiple projectors lookinginwards, if we have a single convex physical objectilluminated by a rectangular projected image, the overlapregion for any two projector is also contiguous. However,typically the physical model is made up of non-convexobjects or a collection of disjoint objects resulting inoverlap regions that are fragmented in each projector’sframe buffer. In [Raskar99a] we described anddemonstrated an image blending technique to achievegeometric alignment and intensity normalization to createseamless images from multiple projectors. The imageblending technique can be used even if the singlecontiguous overlap region is not rectangular or theilluminated surface is not flat. When the overlap region isnot contiguous, however, one first needs to identify thepixels in each projector’s frame buffer that illuminate

surface(s) also illuminated by at least one other projector.Using a simple region-growing algorithm in eachprojector’s frame buffer it should be possible to identifythe different islands of overlapping regions. The imageblending technique described in [Raskar99a] can then beused for each of these islands.

4. Registration Issues

In augmented reality, preserving the illusion thatvirtual and real objects coexist requires proper alignmentand registration of virtual objects to real objects[Azuma94][State96][Neumann96]. Traditional ARmethods use body-centric coordinate system to rendersynthetic objects, and SAR methods use a fixed worldcoordinate system to render them. However, in both cases,the static and dynamic registration errors are caused by anumber of factors such as system delay, optical distortionand tracker measurement error, and are difficult to addresswith existing technology. The tracking requirements forregistration in SAR are similar to spatially-immersivedisplay (SID-VR) systems because real and virtual objectslie in the same fixed world-coordinate system. Thus, staticcalibration errors can play an important role inregistration. They include correct estimate oftransformations between display devices, tracker andworld coordinate system. In the case of HMD-based AR,such errors result in the apparent translation of virtualobjects with respect to the real objects. As noted in [Cruz-Neira93] and [Raskar98c], in SAR such errors lead tofundamentally different types of artifacts. For example,when the additional imagery is simply modifying thesurface attributes, the rendered imagery is viewerindependent and remains registered with static realobjects. If 3D virtual objects are displayed on part of thephysical model with which they are expected to beregistered, then as described in [Cruz-Neira93][Raskar98c], the dynamic errors results in shearof virtual objects instead of translation. Finally, if floating3D objects are to be displayed, the mis-registration issimilar to HMD-based AR. This will also be the case ifinteraction with virtual objects involves movement withrespect to the real objects.

5. Conclusion

In this paper we have presented the idea of augmentingphysical models by surrounding them with light projectorsand displaying seamless images on the surfaces of thoseobjects. This method appears to be especially effectivewhen the surface attributes of the real objects need to be

augmented, for example surface color or texture. Multipleusers can stand around and view the modified surfaceattributes without stereoscopic projection, glasses orHMD. We have described how the setup can be used toaugment physical models by displaying perspectively-correct 3D virtual objects.

The hybrid visualization method can augment physicalmodels with white diffuse surfaces by blending imagesfrom multiple projectors. However, currently thistechnique appears to be somewhat limited to visualizationand not suited for complicated interaction with the virtualobjects. One also needs to address the issue of aliasing ifphysical models with sharp edges are illuminated bylimited resolution images. Shadows can also create aproblem.

We look forward to refining our ideas and the relatedalgorithms, developing better authoring programs, and topursuing some of the many applications we have in mind.In our current setup, we have simply painted differentcolors and textures on top of the physical model.However, we plan to construct a complete setup withhead-tracking to display 3D virtual objects in the next fewmonths. In the end we believe the approach promises verycompelling hybrid visualizations that afford the benefits ofboth traditional physical models, and modern computergraphics, effectively "bringing to life" table-top physicalmodels.

6. Experiment

We have demonstrated a simple table-top physicalmodel illuminated by two video projectors and augmentedby painting different textures and colors. The video of thedemonstration is available at the project website. Asshown in Figure 2, the scene is made up of white-coloredwooden objects, cardboard boxes and bricks. The texturesand colors are interactively painted using AdobePhotoshop. For facades, it is sufficient to specify fourpoints to achieve the necessary pre-warping of textures.

Figure 3. The images in projector framebuffers forthe scene in Figure 1.

Figure 1 shows the augmented scene with two projectors.The video also shows how colors can be interactivelychanged (in this case spray-painted). Then we showcontribution of each projector. When we turn on the roomlights, one can see the simplicity of the physical model(also shown in Figure 2). The images in each of the twoprojector framebuffers are shown in Figure 3. Althoughthis experiment uses only two projectors and a simplephysical model, complex architectural models whenilluminated with multiple projectors and viewed with headtracking will be more pleasing to look at than with thepure VR or HMD-based AR displays. Please see theproject web site http://www.cs.unc.edu/~raskar/Tabletopfor more media.

Acknowledgements

This research is supported by the National ScienceFoundation agreement ASC-8920219: “Science andTechnology Center for Computer Graphics and ScientificVisualization” , Intel Corporation, and the “National Tele-Immersion Initiative” sponsored by Advanced Networks& Services, Inc. We would like to thank members of theOffice of the Future group at UNC Chapel Hill and inparticular Prof. Henry Fuchs and Herman Towles foruseful discussions and support.

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