Video engraving for virtual environments

Geb Thomasa, Ted Blackmonb, Michael Simsc, and Daryl Rassmussenc

aDepent of Industrial Engineering, University of Iowa, Iowa City, IAbTelerobotics & Neurology Units, U.C. Berkeley, Berkeley, CA

Clntelligent Mechanisms Group, NASA/Ames, Moffett Field, CA

ABSTRACT

Some applications require a user to consider both geometric and image information. Consider, for example, aninterface that presents both a three-dimensional model ofan object, built from a CAD model or laser-range data, and an image•ofthe same object, gathered from a surveillance camera or a carefully calibrated photograph. The easiest way to providethese information sets to a user is in separate, side-by-side displays. A more effective alternative combines both types ofinformation in a single, integrated display by projecting the image onto the model. A perspective transformation that assignsimage coordinates to model vertices can visually engrave the image onto corresponding surfaces ofthe model. Combining theimage and geometric information in this manner provides several advantages. It allows an operator to visually confirm theaccuracy ofthe modeling geometry and also provides realistic textures for the geometric model. We will discuss several ofour procedural methods to implement the integrated displays and discuss the benefits gained from applying these techniquesto projects including robotic hazardous waste remediation, the virtual exploration of Mars, and remote mobile robot control.

Keywords : Telerobotics, remotely operated vehicles, stereoscopic displays, virtual environments, texture mapping, videoengraving.

2. INTRODUCTION

Various researchers associated with the 1MG have pursued or are pursuing the idea of controlling a robot from avirtual environment"2. The principle idea behind this line ofresearch is that ifthe interface can provide a significant sense ofpresence for the robot operator —ifit can provide a convincing illusions so the operator feels as if he or she is physicallypresent at the robot's site — then operation ofthe robot will proceed faster and with fewer errors.

One ofthe challenges in this approach is gathering, transmitting and displaying enough information about the remotescene to provide a convincing illusion for the display. One approach to providing a sense ofpresence is to mount a pan andtilt stereoscopic camera on the robot that tracks the operator's head movement and delivers images ofthe remote scene to ahelmet-mounted display worn by the operator. Unfortunately, image resolution and time delays make this approach lesscompelling than might be imagined. The approach described in this paper attempts to circumvent the need for a continuousstream of images and control loops with long time delays by combining detailed geometric models and images.

A second benefit of Video Engraving is revealed in an application at Sandia National Laboratories. The techniquewas applied to a system developed to direct a robot to remediate nuclear waste3'4. One of the challenges facing the designersof this interface is the fact that the sculpted surface geometry of the sludge and the camera images come from separatesources. The original interface approach was to take these two disparate signals and present them to the operator in separatewindows on the display, or even separate displays. The operator visually compared the two signals to ensure the accuracy ofthe surface contour map and the robot operations.

If the image showed a metal pipe at a particular location, the operator naturally searched the sculpted surface forevidence of the pipe. This process requires the operator to build a mental picture of where the camera was, where the pipemust be in the tank, and where the pipe should appear in the sculpted surface. This task requires a significant mental effortand a significant exposure to human error.

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Figure 1 illustrates an approach that overcomes these difficulties by combining the two sources of information in asingle scene. In a demonstration presented last summer at the 1996 Sandia Forum, the image from the camera was projectedonto the three-dimensional model ofthe tank with a technique called 'Video Engraving'. The effect is as if, in the virtualmodel, a slide projector with the image replaced the camera. The slide projector shined the image onto the sculpted surface,clearly displaying which region ofthe sludge was being viewed, and where the camera was when it took the picture. Thisunified display allowed the operator to avoid building complex internal models for the sake of comparing the imageinformation to the laser-range finder information.

The Video Engraving technique has simultaneously been developed for a number ofother applications including:the visualization of Mars, manufacturing, and remote mobile robot control. This paper presents our techniques, includingmathematical representation and specific computer techniques, Jessons learned in each ofthe applications, and our plans forfuture enhancements.

3. THE VIDEO ENGRAVING PROCESS

The Video Engraving process integrates video information with geometric models in virtual environments. It mapspatches of the video images as textures onto the surface of polygonal model objects. The video textures enhance the model'srealism and bring information from the remote environment to the operator's attention. A major advantage of this approach isthat it allows the operator to view the photo-realistic model from angles other than the camera angle without sacrificing videoinformation, so long as the model itself does not occlude the photographic surface textures.

Video Engraving associates each 3D vertex of the model with a 2D coordinate of the video image through aperspective transformation. The effect is as if, in the modeled world, a slide projector replaces the camera. The projector actslike a light source that casts its image regardless of subsequent viewing angle. The projector shines the video image onto themodel and deposits the pixels directly onto the objects in the model as textures. One might think of the image patches being

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Figure 1: An image of the mock nuclear waste is projected from the direction indicated by the arrow onto thegeometrically modeled surface generated by Sandia's laser-mapping system. The image provides visual informationnot provided by the geometry alone.

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engraved onto the surface ofthe modeled objects so operators can view the modeled objects from many new angles and stillsee the integrated video information. Figure 2 illustrates the basic Video Engraving process.

The elemental function of the Video Engraving process is to associate model vertices, each described by a vector [x,y, z, 1], with texture coordinates [u, v]. A number ofreferences describe the transformation based on tracing rays through thecamera's focal point5. Kim6 provides algorithms to calibrate the transformations between a particular image and the threedimensional objects it views. His procedure gives:

U f 0 0 011 l2 l3 l4

w V = 0 f 0 021 22 23 24 Y

1 0 0 —1 0 C31 C2 C3 C Z

where f is the focal length, w is an arbitrary scale factor, and the c's are determined in the calibration procedure.

Depending on the geometric model and virtual viewing parameters, Video Engraving may introduce noticeabledistortion effects into the rendered image. The success of the procedure depends in a complex way on the accuracy of the 3Dmodel, the size of the facets of the model, the orientation of the facets with respect to the camera line of sight, the number ofimage pixels per facet, the number of facets in the model, and the position of the viewpoint when the operator views the videoengraved model.

The algorithm used in most texture-mapping hardware linearly interpolates a texture image uniformly over eachgeometric facet. This efficient approach leads to distortion in the Video Engraving process if the facets of the model areoblique with respect to the line of sight of the video image, as illustrated in Figure 3.

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Figure 2. Engraving Video On a ModeL The algorithm cuts a camera image of a real block into piecesaccording to the model projection into the image plane. Then it fixes the corners of the image patches to the edges ofthe model. Operators can view the resulting model from any direction.

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The left side ofFigure 3 shows an evenly marked bar tilting away from a camera image plane. The diverging linesindicate rays passing from the focal point, through the virtual image plane, to the bar. The resulting 2D image ofthe barshown in the middle ofthe figure is not an even array ofmarkings, but enlarges the markings close to the focal point andreduces the markings that are farther away. Video Engraving this image onto a single polygonal facet creates a noticeabledistortion, as illustrated on the right side ofthe figure. The model ofthe bar does not have evenly spaced markings, as itshould to match the bar in the real environment.

To date we have minimized such distortion effects by re-tessellating large, oblique polygons into a number of smallerpolygons to adequately cover the projected 2D image plane. However, a tradeoffoccurs between inserting too many polygonsin the model which will slow the rendering rate and potentially limit real-time interaction. As the size ofthe polygonal facetsdecrease and begin to approach the 2D image resolution, image-based rendering techniques7 would be a more suitable meansfor visualization. However, experience through implementation has shown that far fewer polygons than the image resolutionare needed to adequately eliminate human perception ofthe distortion effects due to oblique polygons.

4. APPLICATIONS

4.1 Hazardous waste remediation

To apply Video Engraving to Sandia's hazardous waste remediation task, we modified their Rocinante software(htt://www.sandia.gov/cc_at/Forum96/sancho.html). Rocinante is a passive viewer ofthe operations in the main robot controlinterface, except that Rocinante operators can control their viewpoint independently. The Rocmante viewer provided the bestopportunity to apply the new technique because the main control software provided no opportunities to associate texturecoordinates with individual objects, although this short coming is now being addressed.

For the demonstration, we decided to allow a single, predetermined image to be projected as an engraved textureonto the model. Because ofchallenges facing the calibration ofall the interacting systems, we sought to make this engravinginteractive so the camera location, pointing angle and projection assumptions could be adjusted with different keystrokes.

To gain this level of interaction, we applied the opera-lighting technique available in Silicon Graphics OpenGL andPerformer software packages. This procedure allows the programmer to assign a texture to a spotlight. Originally, thisfeature was probably intended to allow programmers to design their own intensity falloffs from the center of the spotlight sothey create their own light patterns. Because the procedure was so general, one can define an arbitrary image for thespotlight, creating the effect of a slide projector shining on a scene. We defined the texture to be the image captured from thecamera, placed the spotlight in the correct position and wrote routines to adjust the viewing direction, the horizontal field ofview and the aspect ratio.

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Figure 3: A Projected Image Of An Evenly Stripped Bar (Left) Mapped To a Surface Without Correcting forPerspective Distortion (Right).

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The first opportunity to view the real scene and the fmal data set came the night before the demonstration. Wesimply loaded in an image of the scene, interactively adjusted the camera position until we empirically matched the projectionassumptions. We could then interactively fly through the textured, three-dimensional image as if it were any three-dimensional scene.

4.2 Mobile robots

For remotely operated vehicles, a 3D model of the local terrain can be created from a stereo range map calculatedfrom a pair a stereo video images taken by video cameras on-board the robot. Through initial camera calibration andcorrelation of image features, a stereo disparity map is created and the stereo disparities are used to create an image rangemap. This range map is then used to generate a 3D polygonal mesh of the local terrain. The fmal 3D model includes the 3Dterrain mesh with the original image. The 3D panoramic model can be incorporated into a virtual environment and viewedwith a stereoscopic display.

display.

In practice, the 3D mesh which is triangulated from the image range map tends to include so many vertices that real-time interaction is limited by slow refresh rates. To overcome this limitation, the second author has explored selective vertexreduction and non-uniform mesh reconstruction using a Delaunay triangulation8. Another challenge occurs when the image islarge. Due to hardware constraints, texture image files are typically limited to a size of 512 X 512 pixels. Consequently, theoriginal image is split into smaller portions, which must overlap by a single pixel to avoid undesired seams in the rendered

image.

Figure 4 shows a rendered portion of a panoramic terrain model generated during the 'Desert 96' field test ofthe Marsokhodremotely operated vehicle. The 3D rendered scene looks photo-realistic and the user can interactively pan and tilt the virtualviewpoint to gain a greater sense of telepresence. However, when the viewing position is translated significantly away fromthe original camera position, distortion affects begin to dominate the display. Consequently, methods of restricting theviewpoint to a region around the original imaging location is desirable.

4.3 3D modeling of the Viking Lander

Video Engraving was also utilized in creating a 3D computer model of the Viking Lander spacecraft as part of an interactive,virtual reality exhibit to promote unmanned space exploration under development for the National Air & Space Museum. Aprototype of the exhibit was on display at the museum on July 19th & 20th, 1996 to help commemorate the 20th anniversary of

Figure 4. A panoramic terrain model for the Marsokhod remotely operated vehicle. A pair of panoramic,stereo video images from on-board cameras were used to generate a sparse terrain mesh suitable for real-timerendering (left). A single image of the stereo pair was then engraved as a texture onto the resultant mesh to producea photo-realistic 3D model (right) which could be interactively viewed in a virtual environment with a stereoscopic

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Figure 5. Video Engraving process for 3D modeling of the Viking Lander spacecraft. The original trianglemesh composed from 5 separate laser scans (upper-left) was used as a skeleton model to reconstruct a suitable CADmodel (upper-right) with fewer facets and free of sensor noise and occlusion holes. Individual sub-components of theCAD model were then manually aligned with uncalibrated photographs (lower-left) to engrave the portions of thehigh-resolution camera images onto the final model used in the simulation (lower-right).

the Viking Lander 1 touchdown on the surface of Mars9. Without access to CAD files (the Viking spacecraft were fabricatedover 20 years ago), a 3D laser scanner (courtesy of Cyra Technologies) was taken to the California Museum of Science andIndustry to digitize a full-scale model of the Viking Lander.

Laser scans and photographs (uncalibrated with the laser scans) were taken from 5 different angles of the spacecraft.A consensus geometry technique'° combined the 5separate laser scans into a single triangle mesh (Figure 5,upper-right).However, this data set included too many facets for real-time rendering, suffered from occlusion 'holes' in the model, andincluded distortions from averaging noise due to the consensus geometry algorithms. An interactive modeling toolkit wasdeveloped to manually reduce the data set and reconstruct a suitable 3D CAD model of the Viking Lander (Figure 5, upper-

right).

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Once the desired spacecraft sub-components were reconstructed, the operator adjusted the virtual viewpointparameters to align individual sub-components ofthe Viking Lander model with each ofthe five corresponding photographs(Figure 5, lower-left). For each selected model sub-component (or set of sub-components), an algorithm considered thesurface normal ofthe polygon and the viewing direction ofeach ofthe five photographs in order to select the "optimal"photograph for each polygon. The (u, v) texture coordinates for each vertex were then computed using the perspectivetransformation to Video Engrave the digital images onto the 3D model. (Figure 5,lower-right).

4.4 Modeling Mars from outer space

The virtual reality exhibit developed for the National Air & Space Museum also benefited from the Video Engravingprocess in generating a photo-realistic model ofMars utilized for an introductory approach to the Red Planet. For this part ofthe simulation, it was not necessary to incorporate the digital terrain elevation map for the surface of Mars because the virtualviewpoint was a global view of Mars from outer space where surface elevations could not be perceived. However, it wasdesired to have a photo-realistic model which would maintain visual quality while 'flying' the viewpoint towards the planet.

A high-resolution 2D image ofMars showing the Valles Marineris canyon system was downloaded from the WorldWide Web (http://nssdc.gsfc.nasa.gov/photogallery/PhotoGallery-Mars.html). A simple polygonal hemisphere was createdto represent the surface ofMars for this image. Utilizing the known camera position for the 2D image ofMars, this virtualviewpoint was reproduced in the modeling software and the corresponding image was projected onto the portion of thehemisphere subtending its projection onto the 2D image plane. With great delight, it was found that a user could interactively'fly' the virtual viewpoint away from the original camera position and significantly towards the planet surface, while thephoto-realistic quality ofthe resulting computer image was sufficient to give the impression ofa more sophisticated modelingprocess. Figure 6 illustrates the Video Engraving process utilized for the modeling of Mars from outer space.

Figure 6. Video Engraving of a 3D model of Mars from outer space. A high-resolution 2D image of Mars wasdownloaded from the World Wide Web and engraved onto a hemispherical mesh to create a photo-realistic 3D model(left). A user can interactively 'fly' the virtual viewpoint towards the red Planet and the 3D model retains its photo-realistic quality (right).

5. ADVANTAGES OF VIDEO ENGRAVING OVER PRESENT TELEROBOTIC DISPLAYS

Most telerobot displays provide information from the remote environment in one of 3 ways: live video, separatevideo and graphical model displays, or a simulation model blended with the video view. The blended interfaces generallytake the form of the phantom display approach pioneered at MIT" and further developed at the Jet Propulsion Laboratory.'2"3

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A phantom display projects the model into the image plane and superimposes it on the live video view. This technique allowsthe operator to compare the model information with the live information in one scene, but sacrifices the three-dimensionalaspect ofthe model in the projection transformation defined by the physical camera location. The other approaches also haveadvantages and limitations, which Table 1 outlines.

Table 1: Comparison of Different Telerobotic Display Approaches

Technique Advantages Disadvantages

Video-only Clear display of video information. Viewpoint restricted to physical cameralocation;No model information or interaction.

Separate Modeland Video Displays

Allows model interaction to enablesupervisory control;Unlimited viewpoint with model.

Difficult to mentally correlate the model withthe video information.

Model overlay onvideo.

Combines model and video;Allows model interaction to enablesupervisory control;

Viewpoint restricted to physical cameralocation.

Video Engraving Seamless, natural integration ofvideo and model.

Computationally expensive, requiring texturemapping hardware;Potentially introduces distortion of video.

Video-only interfaces limit the operator's viewpoint to the physical camera location. Mechanical mounts can, inturn, limit the camera's viewpoint. Also, video-only displays do not include model information, which aids the operator'sconstruction of a mental map and is the basis for the task simulation and collision avoidance algorithms.

Separate model and video displays provide 3D model information and enable graphical simulations for supervisorycontrol. However, it can be mentally difficult to correlate the 3D model with the 2D video and does not provide an easyvisual mechanism to confirm the accuracy ofthe model. Moreover, the human operator is required to share limited attentionresources with multiple displays.

Phantom robot interfaces, which superimpose the model information onto the video image, offer the advantage ofcombining both the video and model information in a single display. Unfortunately, the phantom paradigm collapses toseparate model and video displays ifthe operator wishes to view the model from a position unattainable by the physicalcamera.

The Video Engraving approach provides several advantages over other techniques:

1. It allows a reduction in model complexity without sacrificing the operator's ability to perceive detail.

2. Discrepancies between modeled geometry and the image information are evident in the display: both in location andmagnitude.

3. The three-dimensional environment provides distinction between depth and scale change.

4. The three-dimensional environment also provides a greater range of viewpoints for interactive exploration.

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6. FUTURE DIRECTIONS

This work is at an early stage of development and faces at least three challenges that offer interesting directions forfuture research. The first two relate to distortion effects that can dominate the scene unless controlled. The third relates tohardware improvements that will significantly improve the power ofthis technology.

6.1 Distortion analysis

As described previously, distortion effects occur in the Video Engraving process when polygonal facets ofthe 3Dmodel are oblique to the line ofsight ofthe video camera (see Figure 3). Currently, this problem is handled by subdividingpolygons where the distortion noticeably occurs so that more image points are explicitly assigned to model coordinates. Weare currently investigating mathematical techniques to quanti!y these distortion effects and overcome this limitation byappropriately warping the 2D video image to correct for the distortion prior to application ofthe texture onto the 3D model.

Distortion effects are also present when the 3D model is not sufficiently accurate. Presently, this is used as visualfeedback for the human operator to subjectively verify the accuracy ofthe 3D model. However, the authors hypothesize thatcomparing the resultant rendered image after Video Engraving with the original 2D video image is an ideal mechanism toautomatically veriiy the accuracy ofthe model. Moreover, it may be feasible to feedback the 'distortion error' throughalgorithmic analysis and quantification in order to continually drive an on-line modeling process utilizing machine vision.

6.2 Re-sampling requirements

A second issue associated with the video engraving technology, and texture maps in virtual environments generally,is defining the limits ofuseflul viewing positions for a texture-mapped scene. The simplest instance ofthe phenomenon is thatwhen the object is approached, individual pixels begin to dominate the display, destroying the photo-realistic effect.However, for the Video Engraving process, other distortions can also spoil the photo-realistic illusion when the virtualviewpoint is moved to areas where the 2D video information is inadequate or does not exist. In our continuing research wewill seek to characterize this process, and quantify its effects in order to define the most effective range of viewpoints for anyvideo-engraved model and determine when an update ofthe video texture is necessary.

6.3 Get video into texture memory

It is desirable to manipulate the virtual environment so that new images are projected onto 3D models as fast aspossible and without re-compilation ofthe computer program. For live video streams, this could possibly require updating thevideo image in texture memory as often at 30 times per second (or even faster for high speed video cameras). When thecamera is not moving relative to the modeled environment, it is not necessary to update the texture image. The rate of updatedepends upon the speed of the physical video camera through the real environment. In order to automate the updating of thetexture image and to maximize the speed of this process, the authors advocate and recommend research efforts towardsconnecting live video streams from frame-grabber hardware directly into texture memory hardware.

7. CONCLUSIONS

The Video Engraving approach provides a practical strategy for integrating three-dimensional and video informationin a single interface environment. The combination of the two information streams provides many synergistic displayopportunities that are not available in any other telerobotic interfaces. The advantages include:

• continuous visual monitoring of model accuracy;

• richer visual displays allowing distinction between viewpoint movement and scale change;

• less complex geometric models; and

• a design that is easily extendible with virtual reality technology.

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The insight into the underlying problems facing modem telerobotics and the creative solutions provided by theVideo Engraving process represent a significant step forward for the telerobotics community.

ACKNOWLEDGEMENTS

Portions ofthis work was performed while the first author held a National Research Council-ARC ResearchAssociateship. Application ofthe Video Engraving process to hazardous waste remediation was supported in part by a grantfrom Sandia National Laboratories. Acknowledgment is given to Phil Hontalas, Eric Zbinden and Lew Hitchner for softwaredevelopment ofthe stereo-video pipeline for the Marsokhod telerobot Desert 96 field test. Development ofthe virtual realityexhibit for the National Air & Space Museum was funded by an educational outreach grant from Steve Brody at NASA HeadQuarters. Thanks is given to Bryan Vandrovec at Cornell University for assistance with software development in modelingthe Viking Lander spacecraft and to Cyra Technologies, Inc. for use ofthe 3D laser scanning hardware and technical support.

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