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Thomas Seeholzer
01-21-2004, 05:33 PM
Dear Peter

I do not know the specifics of your system.
We use a 2D DLT algorithm to calibrate non-perpendicular planes. This method
requires 4 points with known real world coordinates. SIMI-Motion has also an
extended version (called grid calibration) which allows as many points as
you want. This method is also using 2D DLT to interpolate between the given
points and should be used in cases of heavy lens distortion.
I think that linear interpolation (between the grid points) needs much more
points to be as accurate as the mentioned DLT algorithm.

A non-perpendicular calibration has the same accuracy as the perpendicular
type as long as the angle between your camera and the measurement plane is
not too small (60 to 90) Obviously you will not be able to measure
anything when the camera's view is parallel to the plane.
But its sensitivity to "out-of-plane" points will get worse with each degree
less than 90.

Regards,
Thomas.

Thomas Seeholzer
SIMI Reality Motion Systems GmbH
Tel: +49 89 321459-0
Fax: +49 89 321459-16
Mail: seeholzer@simi.com
Web: http://www.simi.com


> -----Original Message-----
> From: Biomechanics and Movement Science listserver
> [mailto:BIOMCH-L@NIC.SURFNET.NL] On Behalf Of Peter Sinclair
> Sent: Thursday, January 22, 2004 6:31 AM
> To: BIOMCH-L@NIC.SURFNET.NL
> Subject: [BIOMCH-L] 2D digitising with non-perpendicular camera
>
>
> Dear BIOMCH-L members,
>
> Does anyone have any experience with calibrating 2D images with
> non-perpendicular camera views? We are using the APAS
> digitising system
> which seems to do be doing a good job. With 18 co-planar calibration
> control points spread through the field of view, the APAS calibration
> routine accommodates the changing apparent image size to produce a
> reasonable calibration. The system will measure a constant
> distance between
> markers on a stick moved across the screen, but we haven't done more
> rigorous tests at this stage.
>
> We are only in the preliminary stage of testing the accuracy;
> needing to
> measure (amongst other things) the accuracy's sensitivity to
> out of plane
> movements. Are there theoretical reason's why the calibration of a
> non-perpendicular camera position would be less reliable than
> the usual
> perpendicular camera position?
>
> Thanks very much for your assistance,
>
>
>
> Peter Sinclair
> Lecturer in Biomechanics
> School of Exercise and Sport Science
> The University of Sydney
> p.sinclair@fhs.usyd.edu.au
>
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