[IGSTK-Developers] Bug in igstkTracker with calibration transform

[Julien Jomier]

Hi David,

Thanks for the explanation. I think I understand the way it works and I 
agree with the design but I think the implementation is wrong.
Let me try to explain.

 From the documentation:

   T ' = P * R^-1 * T * C

   where:
   " T " is the original tool transform reported by the device,
   " R^-1 " is the inverse of the transform for the reference tool,
   " P " is the Patient transform (it specifies the position of
         the reference with respect to patient coordinates), and
   " T ' " is the transformation that is reported to the spatial objects
   " C " is the tool calibration transform.

So we are interested by this part of the equation here:

T*C = RotationT * (RotationC+TranslationC)+ TranslationT
T*C = RotationT*RotationC + RotationT*TranslationC + TranslationT (1)

Looking at the code:

// start with ToolCalibrationTransform
rotation = toolCalibrationTransform.GetRotation();
translation = toolCalibrationTransform.GetTranslation();

// transform by the tracker's tool transform
rotation *= transform.GetRotation();
translation = transform.GetRotation().Transform(translation);
translation += transform.GetTranslation();

Which can be written as:

T*C = RotationC*RotationT + RotationT*TranslationC + TranslationT (2)

Therefore, comparing (1) and (2) the rotation is computed in the wrong 
order or maybe I'm missing something really obvisous...

And the m_PatientTransform is following the same implementation so might 
be wrong as well.

Let me know what you think,

Julien


David Gobbi wrote:
> Hi Julien,
> 
> I think that the tool calibration transform code is correct the way it 
> is, but note that the
> way the code is written, the tool calibration transform goes from the 
> "calibrated tool"
> coordinate system to the "raw tool" coordinate system.  This might be 
> the opposite of
> what you expect.
> 
> There is a very good reason for having the calibration transform go in 
> this direction.
> You know how an igstkTransform defines a rotation followed by a 
> translation? Since the tool tip is at (0,0,0) in the "calibrated" coord 
> system, our "calibration transform"
> applies a rotation about the tool tip, which is at (0,0,0) in the 
> calibrated coord system,
> followed by a translation to the "raw" tool tip location.
> 
> This means that even if the rotation portion of the calibration 
> transformation is incorrect,
> the "tool tip" portion of the calibration will still give the correct 
> tool tip location, since the
> rotation part of the calibration occurs around the tool tip position.  
> If the calibration
> transform was the other way around, then we would not get this nice 
> independence of
> translation from rotation.
> 
> If you already had your calibration transformation going in this 
> direction, email me and I
> will take a close look at the code to make sure that there is no bug.
> 
> - David
> 
> 
> Julien Jomier wrote:
> 
>> Hello,
>>
>> There is an important bug in the igstkTracker class preventing any 
>> calibration matrix to be applied correctly.
>>
>> The problem is in the UpdateStatusSuccessProcessing() function.
>>
>> The calibration transform should be applied first (rotation variable) 
>> and then the transform from the tracker (transform variable).
>> However the code is as follow:
>>
>> rotation *= transform.GetRotation();
>>
>> whis is in the wrong order and should be:
>>
>> rotation = transform.GetRotation()*rotation;
>>
>> Let me know if I should fix it, or if someone else is currently on the 
>> igstkTracker class right now or if I'm missing something...
>>
>> Julien
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>>
> 
> 
>