code for qint function

function [p,y,a] = qint(ym1,y0,yp1)

%QINT - quadratic interpolation of three adjacent samples

%

% [p,y,a] = qint(ym1,y0,yp1)

%

% returns the extremum location p, height y, and half-curvature a

% of a parabolic fit through three points.

% Parabola is given by y(x) = a*(x-p)^2+b,

% where y(-1)=ym1, y(0)=y0, y(1)=yp1.

p = (yp1 - ym1)/(2*(2*y0 - yp1 - ym1));

y = y0 - 0.25*(ym1-yp1)*p;

a = 0.5*(ym1 - 2*y0 + yp1);