Posted by Rallaz on Jan 20, 2012; 4:41pm
URL: https://forum.librecad.org/Where-I-can-find-LFF-specification-arcs-tp5158044p5161018.html

The use of "bulge" or curvature is the most compact form to mix lines and arcs as well as being the "more standard" in a program that uses the dxf format.
I updated the wiki with some comments about bulges.

More comments
From DXF spec:
The bulge is the tangent of one fourth the included angle for an arc segment, made negative if the arc goes clockwise from the start point to the endpoint. A bulge of 0 indicates a straight segment, and a bulge of 1 is a semicircle.

Another possible definition is the relationship between the half length of the chord (segment from start & end points) and the sagitta
sagitta = bulge * length_chord / 2

see:
http://en.wikipedia.org/wiki/Sagitta_(geometry)
http://liutaiomottola.com/formulae/sag.htm