National Biocomputation Center


Surgical WorkBench (SWB)

Frederic Mazzella, Kevin Montgomery
National Biocomputation Center Stanford University Stanford, CA 94305

Keywords: projection display, virtual reality, virtual surgery, 3D rendering

ABSTRACT: This paper details the construction of a high resolution computer projection display used by the National Biocomputation Center. The projection display was conceived in order to improve the image quality and to reduce the floor space occupied by the previous installations. The project is part of the effort towards the conception of a complete virtual reality environment used for surgical applications. The presentation provides an overview of the scientific context in which this project was accomplished, then addresses the technical challenges encountered and evaluates the practical results obtained. A complete technical description of the material used and how to assemble the different parts is given in the second part.

INTRODUCTION: Conducting research in the virtual reality field, the National Biocomputation Center needed a high quality large display. The virtual reality techniques used by the laboratory are aimed at providing a virtual environment for virtual surgery, a tool that is used to train surgeons. This paper is divided in two main parts. The first part is a presentation of the project, with the scientific context the description of the large display that was built and the results obtained. The second part is a technical description providing all the mathematical equations and figures to explain the practical aspects of building such a system.

Virtual surgery: The National Biocomputation Center is conducting research in the area of virtual surgery.?One of the goal of the laboratory wants to achieve is the construction of a complete virtual reality environment used by surgeons to train for surgeries. Such an environment can be used to train young surgeons, but also to train experimented surgeons for unusual surgeries. Surgery tools linked to computers are the necessary interface between the movements of the surgeon and the modification of the virtual environment. The National Biocomputation Center built its own custom surgery tools from actual tools used in the operating room, such as scissors or needles. Those surgery tools have 3D Birds trackers mounted on and can be linked to force feedback devices to allow a surgeon to see and feel the consequences of his movements in the virtual environment.

This technology is shared by a number of our projects which include the visualization of soft tissues in craniofacial surgery after repositioning bone fragments, the virtual microsurgerytraining tool, and the virtual delivery room.

The virtual environment is usually built from actual patients data, using scanning techniques such as MRI (Magnetic Resonance Imaging) or CAT (Computerized Axial Tomography). The 3D data is used to produce a virtual model on which the surgeon will operate. The virtual tissues generated from those scans are tissues than can be deformed, cut, sewed and the physical model on which our laboratory is conducting many developments is based on a mass-spring mesh implementation.

Virtual reality technique: Many techniques exist to achieve virtual reality sensations. In parallel to the experiments with 3D head mounted displays using little LCD screens, the National Biocomputation Center also experiments the use of large displays (70 inches) monitored by a computer that knows the position of the observer and automatically computes 3D views for each of the eye of the observer. Projection-based VR presents some advantages over traditional head-mounted VR. With projection-based VR, the user doesn't have to carry the display equipment, he/she can see his/her own body in the environment, and it usually brings less fatigue, which is a critical point when it comes to using VR as a tool for simulating surgeries which can be hours long.

To see the virtual environment in stereoscopic 3D, users wear stereographics liquid crystal shutter glasses. Such glasses can let the light go through the glass surfaces or block it, so that each eye sees different images. The opacity of each of the glass of those glasses is remotely controlled by the computer and synchronized with the display. To navigate and interact with the virtual environment, the surgeon uses the custom surgical tools we developed. To update the visual display with respect to the users' position and input, the surgeon is also monitored by a 3D tracking device. Thus, the computer always knows where the observer is standing. The computer produces alternatively two different views of the scene at a high rate. Those two different views correspond to what the two eyes of the observer would see from this point of view in a real situation, looking towards the screen. The impression to see in 3D (virtual reality) is thus achieved.


Motivations of the project: Projection syial.

The rear projection system built at the National Biocomstems still remain the best technique to produce large displays. The quality of the image is excellent when using rear projection optics. However, such displays usually present some drawbacks that are to be considered when it comes to using them as part of a virtual reality environment. They often occupy a considerable floor space, and the high brightness and contrast that can theoretically be achieved is usually considerably altered by ambient lighting. Thus, this type of installation is generally used in a big room and in the dark. Such constraints are not suitable to our virtual reality purposes. We thus decided to build our own computer projection display that would fit our needs, using the excellent projection material we already had (SONY video projector and DRAPER rear projection screen). We wanted to achieve this for a reasonable price, because such systems are really expensive to buy directly from specialized companies ($20,000), and they are not enough customized to integrate our special materputation Center was designed to save valuable floor space by folding the light from the video projector using two optical grade first surface glass mirrors. The installation is using approximately half the floor space which was originally required. We also enclosed the optics behind the screen, thus preventing the ambient light from interfering with the path of the image being projected onto the screen.

Realization: The large display installation we were using before was made of a high quality video projector (SONY VPH-1292Q, see appendix A for detailed characteristics) with a rear projection screen from DRAPER located approximately 3 meters away from the projector. We kept this excellent video projector and the excellent rear projection screen, and built a custom made system integrating those elements.

We had to build a solid and stable frame that could support all the weight of the screen and the two mirrors. Since we had special constraints and wanted a real custom made projection system, we decided to make the frame in wood (redwood), thus facilitating custom cuts and frame assembly. To prevent the light from entering the optical system behind the screen, we hanged removable thin black-painted wood panels on every side of the installation, and on top of it also.

Calculus were made to make an optimal installation in terms of space occupancy and image quality. The length of the optical path for which the system was designed is the optimal length recommended by the video projector's manufacturer and we used high quality first surface mirrors to prevent double images to form. Double images get formed with back surface mirrors, due to double reflections on each side of the glass. This causes the image to blur.


The contrast, brightness and image quality greatly improved the virtual reality sensations. The space occupied by the installation is now a lot smaller. Because ambient lighting is not an issue anymore, it is now possible for the user to see his/her own body in the virtual environment. This was not the case with the previous installation, since it had to be used in the dark. The video projector is able to display 120 images per second, which allows the virtual environment to include two persons at the same time, both getting real 3D sensations: the computer can compute and display 30 images per second in each eye of each person. This feature is useful if a surgery has to be achieved by two surgeons at the same time, and this is one advantage of this installation over virtual reality using head-mounted displays.

This installation was realized in about 100 hours by one person, or two when some big panels or frames had to be displaced. This includes the time used for all the preliminary calculus, and the time to actually build the installation (buying the material, cutting, assembling, painting). The material cost in total $1000.


Description Qty Dimensions: Europe (mm) & US (inches)
Side pannels (wood) 2 (mm)10 x 1550 x 2250 | (inches)1/8 x 61 x 88 & 5/8
Top & bottom panels (wood) 2 (mm)10 x 1570 x 1980 | (inches)1/8 x 61 & 13/16 x 77 & 15/16
Rear panel (wood) 1 (mm)10 x 2000 x 2250 | (inches)1/8 x 78 & 3/4 x 88 & 5/8
Front panel (same as rear panel, but cut to fit the screen dimensions) 1 (mm)10 x 2000 x 2250 | (inches)1/8 x 78 & 3/4 x 88 & 5/8
Vertical axes (wood) 4 (mm)38 x 89 x 2230 | (inches)1 & 1/2 x 3 & 1/2 x 87 & 13/16
Horizontal axes (X) (wood) 4 (mm)38 x 89 x 1474 | (inches)1 & 1/2 x 3 & 1/2 x 58 &
Horizontal axes (Y) (wood) 4 (mm)38 x 89 x 1904 | (inches)1 & 1/2 x 3 & 1/2 x 74 & 15/16
Mirror 1 1 (mm)508 x 1016 | (inches)20 x 40
Mirror 2 1 (mm)1422 x 1803 | (inches)56 x 71
Back Panel (wood ) 1 (mm)508 x 1272 | (inches)20 & x 50 &
Frame (vertical) (wood) 2 (mm)38 x 89 x 700 | (inches)1 & 1/2 x 3 & 1/2 x 27 & 9/16
Frame (horizontal) (wood) 2 (mm)38 x 89 x 1904 | (inches)1 & 1/2 x 3 & 1/2 x 74 & 15/16
Mirror slides (to be cut in 3) 1 (3) (mm)2032 | (inches)80
Screws 20 (mm)10 | (inches)3/8
Back Panel (wood) 1 (mm)1550 x 1854 | (inches)61 x 73
Frame (vertical) (wood) 2 (mm)38 x 89 x 2250 | (inches)1 & 1/2 x 3 & 1/2 x 88 & 5/8
Frame (horizontal) (wood) 2 (mm)38 x 89 x 1904 | (inches)1 & 1/2 x 3 & 1/2 x 74 & 15/16
Mirror slides (to be cut in 3) 1 (3) (mm)4647 | (inches)183
Screws 40 (mm)10 | (inches)3/8
Frame (vertical) (wood) 2 Adjusted to fit the screen size
Frame (horizontal) (wood) 1 (mm)38 x 89 x 1904 | (inches)1 & 1/2 x 3 & 1/2 x 74 & 15/16
Screws 200 (mm)32 | (inches)1 & 1/4
Zinc Corners 32 (mm)150X150 | (inches)5 & 14/16 x 5 & 14/16
Mirror glue 1 tube 250 mL
White primer 1 bucket (mm)3.8 L | (inches)1 Gallon
Black paint 1 bucket (mm)3.8 L | (inches)1 Gallon


This system is designed for a 90 inch creen, dimensions (mm), ratio 4/3:
width: 1829.3
Height: 1372

The side panels are in thin wood and painted in black. The video projector is posed on the bottom of the system. Since the beam goes out of the machine making an angle of 13 degrees with the horizontal if it is posed as is on the floor, we need to set the position of the video projector such that the beam goes out in an horizontal manner. We built a support underneath the video projector to hold it in the correct position, the beam lighting in the correct direction (horizontal).


The main frame was built in redwood and special cuts were made at each angle to make the whole structure stable. More stability is brought with the building of the inner structures such as the two mirror supports and the screen support. There are 4 zinc corners and many screws at each angle also. The structure is very strong and we can hang very heavy panels on it without any fear.


Equations (in mm) :
b1.sin (beta) = 177 = > b1 = 205.03
b2.cos (beta) = 400 - d1 = 149 = > b2 = 294.96
b1 + b2 = 500
500.sin (beta) = 431.63
500.cos (beta) = 252.37

Configuration: The assembly of the different parts of the wood support for this mirror is the same as the one depicted for the second mirror (see the figure in the configuration part of the description of the second mirror).

Equations (in mm) :
a1.sin(beta) = 686
=> a1 = 794.65
a2.cos(beta) = 1500 d3 = 638
=> a2 = 1264.02
a1+a2 = 2058.67
a3 = a2.sin(beta) = 1091.19
(a1 + a2).cos(beta) = 1039.09

Configuration: The panel with the mirror glued on it is hanged on the above structure.


The screen supports were designed to fit the screen we were using (Draper screen). The shape of the screen, and the way to hang it depend highly on the case. The wood was arranged to make a solid and stable structure the hang the screen.


Equations (in mm) :
d1 + d2 + d3 = 2567
d2.cos(pi 2.beta) + x = d3
d2.sin(pi 2.beta) = 1267
d1 + x = 400
d1 = d1 + x
d2 = d2
d3 = d3 - x

d1 + d2 + d3 = 2567
d3 = d2.cos(pi 2.beta)
d2.sin(pi 2.beta) = 1267
d1 = 400

= > 400 + 1267/sin(pi 2.beta) + 1267/tan(pi 2.beta) = 2567
= > beta = 59.686 degrees x = 148.88
d1 = 251.12 d2 = 1453.89 d3 = 861.98

The beam:
Equations (in mm) :
The height of the beam depends on the distance to the projector:
h(d) = (1372 165)/2567 + 165
h(d) = 0.4701986755.d + 165

And so does the width:
w(d) = (1829.3 611)/2567 + 611
w(d) = 0.4746136736.d + 611


Q: Does the beam touch the ground?  A: No
The beam touches the ground for h(d)/2 = 177, that is to say d = 401.96, which is much more than d1.

Q: Does the beam collide with the video projector?  A: No
Because h(d1+b)/2 < c (see figure below)

d1 = 251.12
b = V(110*110 + d1*d1).cos(pi 2.beta Atan(110/d1))
c = V(110*110 + d1*d1 b*b)
b = 136.85
h(d1+b)/2 = 173.55
c = 237.93
h(d1+b)/2 < c

Q: Is Mirror 1 big enough? A: Yes

IN HEIGHT: Because h(d1 + x) = 353.08 < M1.sin(beta) = 431.63

IN WIDTH: Because w(d1 + x) = 800.84 < Width of M1 = 1000

Q: Is Mirror 2 big enough?  A: Yes
IN HEIGHT: Because h(d1 + d2 + y)/2 < M2*cos(pi/2 beta)/2 (see figure below)

y = M2.sin(pi/2 beta)/2 = 353.31
M2.cos(pi/2 beta)/2 = 604.29
h(d1 + d2 + y)/2 = 566.4
h(d1 + d2 + y)/2 < M2.cos(pi/2 beta)/2
Because w(d1 + d2 + y) = 1587.91 < Width of M2 = 1800

Q: Does everything fit in the box, Fred? A: It sure fits!

APPENDIX A: Technical characteristics of the video projector



  • l Multiscan capability : horizontal 15kHz to 135kHz, vertical 38Hz to 150Hz. Incorporates new, high resolution 9-inch electromagnetic focus CRTs.
  • l Incorporates a Universal Optical Coupling. Remarkable light output of 225lm (ANSI lumen), 1000lm (peak white), 300lm (all white. High resolution of 700 TV lines/2000 x 1600 pixels (RGB) in at fH : 94kHz, fV : 60Hz.
  • l Wide RGB bandwidth of 120MHz. Registration adjustment at 21 points on the screen for accurate registration. HACC lens for superior and stable picture performance.
  • l Video decoder circuit for enhanced video picture quality. Responds to both analog/digital RGB signals.
  • l Supplied wired/wireless remote control unit RM-PJ1292 controls all the projector functions such as set-up adjustments and operational functions.
  • l Various optional accessories available for system versatility. Desk top mounted projection, ceiling mounted projection, floor mounted projection and rear projection capabilities.
  • l Can be used with screens from 90-inch to 300-inch in size with simple adjustments.


APPENDIX C: Useful links

Description of the SONY video projector:
Draper, rear projection screen maker:
About caves and VR projection-based environments:
Selecting the Right Rear Projection Screen Surface:
To adjust the display:
Link to the Electronic Visualization Laboratory:

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