Very cool!  But its not a true sine wave, its a parabolic wave if that's a
real term.  One segment of a triangle wave is a line. The algorithm is an
integrator, and the integral of a line like y = x is y = (x^2)/2+c, a
parabola (let c be your DC offset).  Maybe someone else can tell us what
the error magnitude amounts to comparing the "parabolic wave" to a sine?

Jim Hartmann