在绘制云图等时，需要用颜色来表征一维数值的大小，如果用灰度表示，那映射非常简单，可以是线性映射。如果用彩色来表示，则需要一个从一维数值到三维RGB颜色空间的映射，希望该映射能够覆盖大部分颜色空间，并且在不同区间都能比较明显地反映数值变化。

Matlab中绘制云图时，默认采用的是jet映射算法，随着数值的增大，颜色从蓝到红，即由冷色到暖色。它是让三原色RGB分别由小到大，保持一段之后再由大表小，并且将三原色的这种变化错开，否则就成了灰度了。

在Matlab中该算法描述实现：

function J = jet(m)
%JET    Variant of HSV.
%   JET(M), a variant of HSV(M), is an M-by-3 matrix containing
%   the default colormap used by CONTOUR, SURF and PCOLOR.
%   The colors begin with dark blue, range through shades of
%   blue, cyan, green, yellow and red, and end with dark red.
%   JET, with no arguments, is the same length as the current colormap.
%   Use COLORMAP(JET).
%
%   See also HSV, HOT, PINK, FLAG, COLORMAP, RGBPLOT.

%   Copyright 1984-2002 The MathWorks, Inc. 
%   $Revision: 5.7 $  $Date: 2002/04/01 21:01:50 $

if nargin < 1
   m = size(get(gcf,'colormap'),1);
end
n = ceil(m/4);
u = [(1:1:n)/n ones(1,n-1) (n:-1:1)/n]';
g = ceil(n/2) - (mod(m,4)==1) + (1:length(u))';
r = g + n;
b = g - n;
g(g>m) = [];
r(r>m) = [];
b(b<1) = [];
J = zeros(m,3);
J(r,1) = u(1:length(r));
J(g,2) = u(1:length(g));
J(b,3) = u(end-length(b)+1:end)

对应的C++实现为： double** jet(int m) { int i=0; //n = ceil(m/4); int n = ceil(m/4.0); //u = [(1:1:n)/n ones(1,n-1) (n:-1:1)/n]'; int u_len = n*3-1; double* u = new double[u_len]; for(i=0;i<n;i++){ u[u_len-i-1] = u[i] = double(i+1)/n; u[n+i] = 1; } //J = zeros(m,3); double** J = new double*[m]; for(i=0;i<m;i++){ J[i] = new double[3]; for(int j=0;j<3;j++){ J[i][j] = 0; } } //g = ceil(n/2) - (mod(m,4)==1) + (1:length(u))'; //g(g>m) = []; //J(r,1) = u(1:length(r)); int t = ceil(n/2.0) - (m % 4 == 1); int g_len = m-t; for(i=0;i<g_len;i++){ J[t+i][1] = u[i]; } //r = g + n; //r(r>m) = []; //J(g,2) = u(1:length(g)); int r_len = m-t-n; for(i=0;i<r_len;i++){ J[t+i+n][0] = u[i]; } //b = g - n; //b(b<1) = []; //J(b,3) = u(end-length(b)+1:end); int b_start = n-t; for(i=b_start;i<u_len;i++){ J[t+i-n][2] = u[i]; } delete[] u; return J; }

该代码可以在我的代码仓库中下载。

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