重力加速度センサーとデジタルコンパスを用いた自動補正携帯プロジェクタ
Using Tilt Sensor and Digital Compass in Auto-calibration Handheld Projector
Dao Vinh Ninh† 細井一弘† 杉本雅則†
東京大学大学院 新領域創成科学研究科{dao,hosoi,sugi}@itl.t.u-tokyo.ac.jp
Raskar R. et al. [4], introduce a self-contained system, which
1. Introduction
contains a camera, a projector and a tilt sensor. In this system,
In February 2006 a British company has introduced athe camera and the projector form a stereo-pair. By analyzing
matchbox-sized projector for image and video projection,
the dual images, this system can figure out the position andwhich start-ups a bright future of an in-phone projector [1].
inclination of the screen, which then help the projector toThe miniaturization of projector has brought to great
calibrate its projection.
development of projector’s using and researching. The
This system is self-contained, but it requires analyzing the dual
variation of projector using situation also rises up the need ofimages, which is not always possible because the camera needcreating a more flexible projector. When a projector is used
to be able to recognize the projection area, the operation thateverywhere, we can not always use the projector in a sweet
depends very much on background illumination condition. This
position, and usually the projector will create a keystone
method is also error prone because when the projector become
distortion, which distorts the view of original images.
smaller and smaller, the camera comes closer to the projector,
The research of projector calibration, helps to correct keystonewhich results in the fact that, the camera’s view of projection
distortion, has archived much development and is applied toimage becomes similarly to the projector’s image and thepractical use.
analysis of dual-views becomes extremely difficult.
Nowadays, some types of projector are mounted with auto
calibration function, which helps the projectors to auto
3. Proposed Calibration Method
calibrate vertical keystone distortion for the projection on a
vertical screen by using a tilt sensor. A horizontal distortion isTo calibrate a projection, we propose a method of using a tilt
practically manual-calibrated by user.
sensor and a digital compass.
A three-dimension acceleration sensor is used as a tilt sensor toA full auto-calibration method is on researching. Someretrieve a projector’s inclination to vertical axis and its rotation
researchers utilize a camera to correct the distortion. But, thearound horizontal axis. The sensor looses a degree of freedom
approach of using image processing technique is environment
by constraint of gravity acceleration when it is used as a tilt
illumination dependent, time consumed and error prone.
sensor. Thus, a tilt sensor is enough to calibrate a projection ona horizontal plane, which is symmetric to vertical axis.
Here, we propose a full auto-calibration method by combining
the usage of a tilt sensor and a terrestrial magnetic sensor
without any presumed knowledge of display screen’s position
The technique of utilizing a tilt sensor to calibrate a projection
or direction. This method utilized only self-containedon a level surface was applied at CoGAME system [5], an
information. It is robust to environment illumination, simple tointeractive game that players use projectors to lead a turtle-like
configure, speedy and can apply to any projection screen at anyrobot. CoGAME is an application that was exhibited atdirection and inclination.
“International Virtual Reality Competition in 2006”, held at
Tokyo, and received the “Technical Price” with the over all
evaluation ranked 3rd in the competition.
2. Related WorkThe using of a compass gives a complete recognition ofA traditional method of calibrate a projector by using a camera
projector’s direction, which means the projector looses its lastis described by Rahul S. et al. [2]. The system consists of adegree of freedom.
camera and a projector. The system required the camera to be
The using of a projector, mounted with a tilt sensor and aable to recognize the display screen, which has a known
digital compass, to calibrate a projection on an arbitrary plane
dimension ratio. This system also uses image processing
needs a one-step-configuration.
technique for the recognition of projector’s projection area. But
it is not always possible to recognize a screen, because we do
3.1 One Step Configuration
not always use an edged screen for projection.
To calibrate a distorted projection, we need to know theThe using of screen recognition can be replaced by the using ofrelative inclination and rotation of the projector to the display
markers on the display area or on portable device.
screen. We can get the projector’s direction, inclination andInstead of using marker on the display screen, Sugimoto† M. etrotation from sensors’ information; therefore, we can calibrate
al [3] approached the calibration of projector by using an
the projection if we know the display surface’s direction andoutside camera to watch the mobile projector, which isinclination. We need only two parameters to define a planemounted with LED markers for detecting its direction.
because a plane itself is symmetric to its axis.
These approaches require attaching markers to the space
To measure a display screen’s parameters, all what we need tobeforehand, or using device in an information space, thus are
do is choosing a projector’s temporary direction, so that under
not feasible for ubiquitous use in arbitrary environment.
an un-calibrated condition the projector displays an undistorted
image. By pressing a button (which is now represented by a†button on computer) on the projector, we can remember the
東京大学大学院 新領域創成科学研究科
projector’s direction and inclination for that figure. The
Graduate School of Frontier Sciences, The University of
projector’s parameters at undistorted projection representTokyoplane’s parameters.
By comparing projector’s real time parameters with thestandard parameters archived from the configuration, we areable to do a full auto-calibration to the previously unknown
display screen.

4. Homography Transformation and Rendering
After retrieving projector’s relative direction to the display
screen, we can calibrate the distorted projection by several
methods. A general method is to pre-wrap the displayed
images so the distortion will be counterbalance. This method
creates a lost of image quality but it is flexible.
Another method to calibrate a distorted projection is themethod of using a shift-lens projector. By changing thedirection of projector’s lens at the direction inverse to thedirection of the distortion, a projector can display undistortedimages.
In a hand-held projector, the rendering method, which does notcontain a mechanical moving part, is preferred. In this system,
we also adopt the same technique.

in camera view. This, in return, brings the interaction ofcamera-projector system into a new way. Because, theprojector’s images are calibrated, users from different positions
can display images on the same image area.
In a camera-projector system, different projectors canindependently interact with each others without confusing
because a camera view of the projector in the system isunchanged, which help the image recognition without much
image processing effort.

Picture 2: System construction
At this step of implementation, the tilt sensor is speedy andreliable; with the output rate up to 350Hz, the calibration relies
on tilt sensor’s information is almost at real time (CoGAME).
The GPS receiver is mounted with a two-dimension digitalcompass and can only give the output rate at 0.5Hz.
To obtain an accurate direction value, when the projector is nothorizontal, we have to adjust the compass’s data by the tilt
sensor. A compass is also inflected by a strong magneticenvironment. Thus, the system can work well underenvironment without much steel near by.

6. Conclusion

Picture 1: A projector with tilt sensor and a compass uses
image pre-wrapping method for calibration.
To render a counter-distorted image, we calculate theprojection transformation from the projector to the display
screen, and then select a suitable display area inside theprojector’s cover area, called expected display area, so that theprojection will not cause any distortion. By re-converting theexpected display area into the projector’s source image, wearchive the pre-wrapped image needed for the calibration.
The pre-wrap image is created by rendering the original image
using the homography transformation between the original
display area and the counter-distortion area.

5. Implementation and Evaluation
The system was implemented by using a Toshiba’s portable
LED projector (TDP-FF1), a Sunhayato’s acceleration sensor(MM-2860) as a tilt sensor, and a Garmin’s GPS receiver(Geko 301) with digital compass function.
A VAIO type-U is used for information processing and images
rendering.
Utilize this technique as a backbone; a camera is freed fromauto-calibration technique. The camera can come very close to
the projector, projector’s projection area becomes unchanged

We have introduced a method of calibrating a distorted
projection on an arbitrary planar screen by using a tilt sensor
and a digital compass. This calibration method is new, reliableand speedy.
The configuration is quick and easy. This method costs lesstime to analyze the projector’s direction to the screen, so it issuitable for a mobile hand held projector.
By using a high performance HoneyWell’s three- dimension
compass, we are implementing a real time calibration projector
system.
Calibration a projector by sensors make a camera become freeand can come very close to the projector. This brings the visualinteraction into new step because image area recognition is noneeded to be done by a heavy image processing technique.

Reference[1]Edward, B., “Light Blue Optics”, In SID Mobile Displays
Conference, 2006.
[2]Rahul, S., et al., “Smarter Presentations: Exploiting
Homography in Camera-Projector Systems”, In Proc. of ICCV,
2001.
[3]Sugimoto, M., et al. “Intuitive manipulation techniques forprojected displays of mobile devices”. In Interaction 2005, pp.
95–96, 2005.
[4]Raskar, R., et al., “ilamps: Geometrically aware andself-configuring projector”, ACM Transactions on Graphics(TOG), Vol.22, No.3, pp.809-818, 2003.
[5]Interaction Technology Laboratory, CoGAME,
http:://www.itl.u-tokyo.ac.jp/cogame , 2006.