Velvia documentation inconsistent
The tooltip for mid-tone bias (https://github.com/darktable-org/darktable/blob/master/src/iop/velvia.c) says :
"how much to spare highlights and shadows"
The user manual (http://www.darktable.org/usermanual/ch03s04s03.html.php) says:
"Velvia can reduce its effect for mid-tones to avoid unnatural skin tones. The mid-tones bias slider controls this selectivity; reducing its value reduces mid-tone protection and makes the overall velvia effect stronger."
The two behaviours are exact opposites ('spare highlights and shadows' vs. 'reduce its effect for mid-tones').