Tangrams do not overlap and so do not have lines that intersect. Tangrams are therefore, simple polygons. Like simple polygons, they can be concave or convex.
A concave polygon has one or more angles pointing inwards and therefore contains angles greater than 180°. Nearly all tangram configurations are concave.
A convex polygon has no angles pointing inwards. More precisely, no internal angle is greater than 180°. Of the thousands of existing tangram figures, there are only 13 convex tangram configurations.
So there you have it, the not so complicated math behind the art!