cairo:matrix-transform-distance (matrix dx dy)

matrix -- a cairo:matrix-t instance
dx -- a number coerced to a double float for the x component of a distance vector
dy -- a number coerced to a double float for the y component of a distance vector
tdx -- a double float for the transformed x component of a distance vector
tdy -- a double float for the transformed y component of a distance vector

Transforms the distance vector (dx,dy) by matrix. This is similar to the cairo:matrix-transform-point function except that the translation components of the transformation are ignored. The calculation of the returned vector is as follows:

tdx = dx * a + dy * c
tdy = dx * b + dy * d  

Affine transformations are position invariant, so the same vector always transforms to the same vector. If (x1,y1) transforms to (x2,y2) then (x1+dx1,y1+dy1) will transform to (x1+dx2,y1+dy2) for all values of x1 and x2.