Technical description of method used by Pix4D for 3D-reconstruction

Dear Community

I am looking for a paper or technical description of the method applied by Pix4Dmapper to reconstruct 3D-models out of the orientated imagery (standard cameras).

Reason is to get a better understanding of the imagery used and possible improvements while acquiring images in difficult environments (caves).
A better understanding could help to improve the images taken in the caves to have better results of the modelling process later (e.g. contrast, …).

Looking forward to any hints.

Cheers,
Yvo