Undistortion for Dense reconstruction

[Eva-Maria Steinhaus]

Hello,

can someone explain to me, what exactly happens in the step Undistortion in the beginning of the dense reconstruction process. I thought the pictures must be undistorted earlier in the process, even before the sparse point cloud is calculated.  

Thank you for your help.

Best regards, 

Eva-Maria

[Pierre-Olivier Agliati]

Hi Eva-Maria,

No, images needn't to be undistorted in the SfM step, however, the parameters modeling the distorsion need to be known (or computed) in order to project the 3D key points on image planes and minimize the errors.

Then undistorsion makes it possible to convert images into a pinhole model, necessary for the stereoscopic algorithms of the MVS.

Cheers,

Pierre-Olivier

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[Eva-Maria Steinhaus]

Thank you for your explanation, Pierre-Olivier!

[Johannes Schönberger]

Undistortion does exactly what you describe. It's not necessary to explicitly undistort the entire images before that. Undistortion prior to dense reconstruction is also just a way to enable efficient patch matching across multiple views without undistorting the coordinates on the fly many times during dense reconstruction. Some individual steps require undistortion from image pixels to ray directions, e.g., for triangulation, etc. 

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