BDrate和BDpsnr计算方法

参考VCEG-M33中bitrate和psnr的关系：

SNR = a + b*bit + c*bit2 + d*bit3

bit = a + b*SNR + c*SNR2 + d*SNR3

function avg_diff = bjontegaard(R1,PSNR1,R2,PSNR2,mode)

%BJONTEGAARD    Bjontegaard metric calculation
%   Bjontegaard's metric allows to compute the average gain in PSNR or the
%   average per cent saving in bitrate between two rate-distortion
%   curves [1].
%   Differently from the avsnr software package or VCEG Excel [2] plugin this
%   tool enables Bjontegaard's metric computation also with more than 4 RD
%   points.
%
%   R1,PSNR1 - RD points for curve 1
%   R2,PSNR2 - RD points for curve 2
%   mode - 
%       'dsnr' - average PSNR difference
%       'rate' - percentage of bitrate saving between data set 1 and
%                data set 2
%
%   avg_diff - the calculated Bjontegaard metric ('dsnr' or 'rate')
%   
%   (c) 2010 Gius