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fix negative scale factor of coordinate function due to spatial transform axis direction #3992

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Jan 28, 2025
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fix negative scale factor of coordinate function due to spatial transform axis direction #3992

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4 changes: 3 additions & 1 deletion OpenSim/Simulation/SimbodyEngine/SpatialTransform.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -193,7 +193,9 @@ void SpatialTransform::scale(const SimTK::Vec3 scaleFactors)
}
SimTK::Vec3 axis;
transform.getAxis(axis);
double scaleFactor = ~axis * scaleFactors;
// we want weighted aggregate of scale factors but to ignore the sign
// ignoring sign due to issue #3991 resulting -ve scale factor
double scaleFactor = ~axis.abs() * scaleFactors;
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Seems like a reasonable enough change; but it makes me wonder: is there any scenario where we would want to preserve the sign on the scale factor?

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While manual scales can be set to -ve numbers, measurement-based scale factors are ratios between (unsigned) distances so can't be -ve. If we think about it as how much we stretch/shrink segments then this should never be -ve otherwise vectors/directions flip which is unlikely an intended behavior of our 'scaling'. However if you suspect there're other places in the code worth more investigation please let me know @nickbianco

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Sounds good, just making sure I understood this correctly.

// If the function is already a MultiplierFunction, just update its scale factor.
// Otherwise, make a MultiplierFunction from it and make the transform axis use
// the new MultiplierFunction.
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