Vive Tracking Setup done wrong?

Studio Operators Composer Camera Tracking

 

Hello Aximmetry-Team :) In our Facebook group we are discussing the right way to set up the values for camera tracking with Vive (or Antilatency). This is the way I thought would be correct:

In our group someone pointed out that it would be better to measure the disctance not to the sensor, but to the entrance pupil of the lens. Could you please point us in the right direction? Thanks :)

   gaffer-tape

 
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Zoltan@Aximmetry
3/27/2021 10:24:11 AM
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Hi,

Well, the short answer is yes.

The apparent vertex of the perspective projection is at the entrance pupil, and we should place the virtual camera there. We call it "Eye Point" in Camera Calibrator.

This point varies with the current zoom and focus. But considering that
- the basic lens calibration method we published so far does not propose any method for determining the Eye Point
- in practice, if you use a wider zoom, you won't experience any serious degradation of the tracking quality if the Eye Point is a few centimeters away from the real one (especially since you do not either have the lens distortion parameters)
you can use sensor center as an approximation safely.

But this does not mean you cannot add this correction in Aximmetry if you have a method for measuring the Eye Point offset. E.g. check this out:
https://www.vrphotography.com/data/pages/techtutorials/technotes/nodalptalign-tn.html

a) If you use Manual Lens mode in your scene, simply add this offset to the Delta Head Transf in ORIGIN.

b) If you use Camera Calibrator then

- In the Camera Properties define the Delta Head Transform for the sensor center. Note that you have to use centimeters here.

- Then for each calibration point you can define an individual offset, which is the distance of the Eye Point from the sensor plane along the optical axis. Positive value means a forward offset.

The default is Auto which means the system uses the current Focal Length as the offset - but it is only an approximation of course, the actual Eye Point usually differs from it significantly.
But you can choose to measure the offset for each calibration point, in this case turn off Auto and enter the measured value in mm.

 
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ericmarodon
4/1/2021 9:01:48 AM
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Just to be sure, In Aximmetry composer, Delta head transform is expressed in METERS, the coordinate system is left-handed (Y up, X is pointing to the left, and Z is depth, pointing away), is that correct?

Also, the reference point (0,0,0) of the delta is the tracker, not the camera, so that if the tracker is pointing forward and camera is below, right and behind the tracker, the delta head should be :

X: Negative distance from the tracker to the sensor in meters.

Y: Negative distance from the tracker to the sensor in meters

Z: positive distance from the tracker to the sensor in meters.


Also, which part of the tracker is considered the origin (0/0/0) ? The 1/4" inch screw at the bottom ?

 
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Zoltan@Aximmetry
4/1/2021 5:36:18 PM
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Hi Eric,

- Yes, in the Composer on the ORIGIN panel it is in meters.

- Almost. The X points right, so to summarize
X: right
Y: up
Z: forward
and yes, this is a left-handed system.

- Yes, the reference point (and orientation!) is the tracker.
X would be positive, since the camera is to the right
Y would be negative, since the camera is below
Z would be negative, since the camera is behind

I'm not sure about the tracker, please check the tech info on Vive,  but yes, it's probably at the screw.



 
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gaffer-tape
4/2/2021 9:44:01 AM
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Thank you Zoltan for clarifying :D