
The FreeSG
SK class implements a generalpurpose 2D "sketching" engine with dimensioning
and geometrical constraint solving.
Sketches define a set of elements and a set of relations between those
elements.
Elements are organized in a tree (the same structure is used to described group and leaf nodes). Each element in the graph is associated with a transformation matrix. The major sketch elements are points, lines and arcs, but other types can be added through a class registration interface. Relations represent geometric constraints affecting points, lines, circles, segments and arcs. The following constraints are implemented:
The SK_View(3) widget is commonly used to display and edit SK objects. 
AG_Object(3)> SK. 
The SK_New() function allocates, initializes, and attaches a SK object. 
These functions multiply a node's transformation matrix
T with a translation, scaling or rotation matrix.
They are only aliases for
M_Matrix* functions, except that they accept a pointer to a node instead of a matrix.

The SK_Translate*() functions multiply T by a 2D translation matrix. The SK_Scalev() function multiplies T by a 2D scaling matrix. SK_Rotate*() multiply T by a 2D rotation matrix. Angles are given in radians, except for SK_Rotate*d() variants which accept angular arguments in degrees. SK_Rotatev() generates a rotation of theta radians around axis. The SK_Rotate*() variants with the "d" suffix accept angles in degrees instead of radians. The SK_GetNodeTransform() function returns a matrix which is the product of the transformation matrices of the given node and all of its parents. 
M_Matrix(3), M_Vector(3), SG_Intro(3), SK_View(3) 
The SK object first appeared in FreeSG 1.0. 