I'm using the ARToolkit as an object tracker in an application, and it
seems like all the transformations that arGetTransMat returns have been
rotated around the X-axis by 180 degrees.  Should the matrix that this
function calculates be the transformation of the object in the camera's
coordinate system ?  

If so, a feducial help upright directly in front of a camera should
return the identity matrix with just a translation in the -Z direction.

My app is returning this matrix, but then multiplied by a 180 degrees
around the X-axis rotation matrix.  But the graphics get drawn correctly
when I load the returned matrix into my (modelview) matrix stack.

what am I misunderstanding?  shouldn't positive z values imply that the
feducial is behind the camera ?

thanks all, 
brendan hannigan

Brendan Hannigan   |   brendan@c ............   |   http://www.hannigan.org
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Hi,

>If so, a feducial help upright directly in front of a camera should
>return the identity matrix with just a translation in the -Z direction.

No.

I put a figure which shows coordinates systems in ARToolKit at:
http://www.sys.im.hiroshima-cu.ac.jp/people/kato/etc/ATK-coordinates.jpg

As we can see in the figure, when the marker is put in front of the
camera, the direction of Y-axis and Z-axis are opposite between
camera coordinates and marker coodinates. This means that
transformation matrix between marker and camera includes 180 degree
rotation in X-axis.
Also translation in Z-axis should be plus.

-- 
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  Hirokazu Kato
  Faculty of Information Sciences
  Hiroshima City University               Phone: 082-830-1705
  Email: kato@s ........................     Fax: 082-830-1435
  URL: http://www.sys.im.hiroshima-cu.ac.jp/people/kato/