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The Multimedia Room

The multimedia lecture room of the Freie Universität Berlin offers a perfect environment for the use ofE-Chalk. A large, interactive rear projection surface, assembled from four single display screens, offers enough space to the speaker topresent his lecture and provides students with an optimal view. The combined display has a total size of 1,15 x 6,13 m.

The lecturer can use a laser pen as a pointing device to work directly on the video wall. In combination with the E-Chalk software, the videowall acts like a huge, intelligent blackboard. The blackboard is controlled by a program, that detects the light spot of the laserpointer with the help of four fixed video cameras. Once the light spot has been found, the program calculates the desired mouse coordinates.

One of the four displays can be used for displaying a laptop signal. Ater the installation of a small driver program, the laptop mouse canagain be controlled from the projection screen.

The room has a 5.1 6-channel audio exquipment.


An important factor during the planning and development of the system was the use of conventional hardware, that allows for a cheap solution.

The optical system consists of four acrylic glas panes, that are embedded in a wooden socket into one wall of the room. In a room behindthat wall are four video projectors. Each projector beams directly onto a mirror, that is positioned in a way that it's reflection is sharp onthe backside of one of the glas panes. By using mirrors instead of a direct projection, the depth of the room, that is necessary for a sharpprojection can be reduced. There are four firewire-cameras installed, each of the cameras is centered on one of the projection areas andhelps tracking the laser spot over the working area. The current configuration also consists of two computers. A linux system, thatprocesses the camera images and a windows system that runs E-Chalk or any other user application.

None of the used hardware components is special hardware, all individual parts are availble on common hardware shops. Only the cameraracks have been manufactured for this application by the physic's workshop of the Freien Universität.

Operating Mode and Performance

The linux system, that does all the image processing runs permanently, it finds the laser spot of the laserpointer in the camera images and calculates the desired mouse coordinates. Afterwards, the coordinates are transmitted to the windowssystem via network. For the correct calculation of the coordinates, the pen system needs to be recalibrated everytime the position of thecameras, projectors or mirrors is changed.

The video wall has a screen resolution of 4096 x 768 Pixel and allows pixel exact control of the mouse pointer, and therefore normal writing,on the whole board. The processing speed, that means the maximal sampling rate of the light pen is limited to 30 frames per seconds bythe video cameras. A faster movement of the pen cannot be tracked reliably with the current hardware configuration.

Klicken Sie um eine größere Skizze des Stiftes anzeigen zu lassen .

Klick to view this picture

The rear projection display can be operated with a common laser pointing device, nevertheless we developed aspecial stylus for a better usability. It consists of a pen-like, metal enclosing, a light emitting diode and a switch, that activates thelight diode anytime it is set up onto a surface. This design creates a natural and intuitive feeling when writing with the pen.

Mobile Pen-System

The mouse controlling software of the pen system in the multimedia room may also be used in a mobile environment.A speaker, that wants to use a mobile version of the system, only needs a projector, a firewire camera and a light emitting diode, that may befound on keyring pendants. The camera has to be positioned with view on the projection surface, the LED serves as pointing device. After thecamera has been calibrated by the program with a few mouse clicks, the LED may be used to control the desktop.

Mobile Version des Stiftsystems.Eine weitere Einsatzmöglichkeit des mobilen Stiftsystems.

The precision of the mobile version is inferior to the installation in the multimedia room, but exact enough to operate a windows desktop ordraw a simple sketch.


  • Kristian Jantz.
    EinStift-Treiber für eine interaktive Multiprojektionswand. Diplomarbeit, Fachbereich Mathematik und Informatik, FreieUniversität Berlin, Januar 2006
  • Michael Diener.
    Lichtpunkterkennung per Kamera an einer Rückprojektionswand: Ein Lichtgriffel für E-Chalk,Bachelorarbeit, Freie Universität Berlin, 2003

Last modification: Mon, 2010-08-09