SpHEAR/calibration/src/sphear_qint.m

%% -*- mode: octave -*-
%%
%% https://ccrma.stanford.edu/~jos/sasp/Matlab_listing_qint_m.html
%%
%% QINT   Quadratic interpolation of 3 uniformly spaced samples
%%
%%               [p,y,a] = qint(ym1,y0,yp1) 
%%
%%       returns extremum-location p, height y, and half-curvature a
%%       of a parabolic fit through three points. 
%%       The parabola is given by y(x) = a*(x-p)^2+b, 
%%       where y(-1)=ym1, y(0)=y0, y(1)=yp1. 

function [p,y,a] = sphear_qint(ym1,y0,yp1) 
  p = (yp1 - ym1)/(2*(2*y0 - yp1 - ym1)); 
  if nargout>1
    y = y0 - 0.25*(ym1-yp1)*p;
  end;
  if nargout>2
    a = 0.5*(ym1 - 2*y0 + yp1);
  end;
end