[ros-users] ar_pose orientation accuracy

Thomas Ruehr ruehr at in.tum.de
Mon Nov 8 16:37:47 UTC 2010

My collegue Ingo is working on an improved version of ARToolkit Plus for 
the ros community, so if you can wait a few weeks, things might get 
better. (subpixel accuracy, adaptive thresholding etc.)

Generally, ARToolkit is not too good, its not estimating 
subpixel-accurate and only taking the 4 outer corners for the estimation 
of the homography. So you should expect the orientation to jump a lot.
Additionally, the distance will be pretty noisy and have a systematic 
dependency on the shutter, gain and your threshold, as only the outside 
edge from white to black is taken into consideration.

I worked with 12.5x12.5mm ARToolkit markers to detect drawer and cabinet 
handles for a week or two, using PR2 forearm camera (we now have some 
pcl method for detecting handles). This worked, but I wouldn't bother 
looking at the orientation you get. Sometimes the orientation jumped 
between something reasonable and 'the marker pointing directly to the 
camera'. I had to filter this out and then averaged over 20 frames or so 
and would still end up with around +-3 cm distance error and a bad 
orientation. Watch out - averaging quaternions is not necessarily 
trivial. The markers were however very small, maybe 1% of the image area 
or even less.


from TUM

On 11/08/2010 05:10 PM, Eohan George wrote:
> I think its a known problem about artoolkit .. this link Robust Pose 
> estimation from planar target 
> <http://citeseerx.ist.psu.edu/viewdoc/download?doi=> mentions 
> some of those..
> The artoolkitplus has a robust pose estimate api which "reduces" this 
> problem. However, I did see this issue to a smaller extent even with 
> rpp. I am not sure if I was using it the wrong way though.
> Eohan
> On Mon, Nov 8, 2010 at 10:47 AM, Ivan Dryanovski 
> <ivan.dryanovski at gmail.com <mailto:ivan.dryanovski at gmail.com>> wrote:
>     Steven,
>     I'm not sure if this is exactly the problem you are observing, but I
>     have noticed a similar behavior from ARToolkit before. When the camera
>     is nearly perpendicular to the marker, and the marker is in the center
>     of the image, there occurs a singularity. Let's say the angle between
>     the marker normal and the optical axis is 1 degree - ARToolkit will
>     arbitrary flip between +1 and -1 degrees. If you make the angle
>     bigger, the orientation of the marker becomes less ambiguous, and the
>     error diminishes.
>     Ivan Dryanovski
>     On Mon, Nov 8, 2010 at 9:00 AM, Herman Bruyninckx
>     <Herman.Bruyninckx at mech.kuleuven.be
>     <mailto:Herman.Bruyninckx at mech.kuleuven.be>> wrote:
>     > On Mon, 8 Nov 2010, Steven Bellens wrote:
>     >
>     >> 2010/11/8 Steven Bellens <steven.bellens at mech.kuleuven.be
>     <mailto:steven.bellens at mech.kuleuven.be>>:
>     >>> Hi,
>     >>>
>     >>> I'm experimenting a bit with the ar_pose package. I'm using a
>     single
>     >>> fixed camera to track a moving marker. To verify the estimation
>     >>> accuracy, I just leave the marker fixed and I've plotted
>     position and
>     >>> orientation estimates. The position estimates are pretty much
>     >>> constant, but the orientation estimates are oscilating a lot (see
>     >>> appendix), and apparently always between two values. Is this
>     because
>     >>> of the bad capability to estimate that orientation or can this be
>     >>> caused by the environment conditions (light - set-up - distance to
>     >>> marker)?
>     >>
>     >> For clarity, plotted are the 4 components of a unit quaternion.
>     >
>     > So...? What does a jump of "0.1" quaternion units mean? And is
>     this meaning
>     > the same for each of the four components?
>     >
>     > The answer to this last question is probably "no", hence...?
>     >
>     > Herman
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