Hi all,
I like to add to this discussion on re-topology "Instant Field Aligned Meshes".....
"....The software converts geometry to uniform quad or triangle meshes, automatically following the curvature of the geometry and snapping edges to sharp features on the model."...etc
http://www.cgchannel.com/2015/11/download-free-auto-retopology-tool-instant-meshes/
http://igl.ethz.ch/projects/instant-meshes/
https://www.foundry.com/trends/design-visualisation/mitsuba-renderer-instant-meshes

regards
O.