As one branch of subpixel technology, curve interpolation method is the most popular for its higher precision and less
complexity. But the method has longer running time and bigger direction error, since it makes use of convolution
between direction templates and pixel gray values to get Gradient direction which is interpolating direction. So a new
method is proposed in which unisometric cubic spline interpolation is operated along the normal direction of the
edge-point. It illustrates not only how to choose sampling points for interpolation, but also how to figure node values of
those points, then establishes the suitable interpolating boundary conditions. The method reduces not only the uncertainty
but also the runtime to 1/2, and improves measuring precision and efficiency.