Surf(x, y, z) with a colormap that is only two colours

Question

Emil Amor 2020-9-27

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https://ww2.mathworks.cn/matlabcentral/answers/600568-surf-x-y-z-with-a-colormap-that-is-only-two-colours

评论： Emil Amor 2020-9-28

How can I use surf with a colormap that assigns a single colour (say red) to all negative values of z and another colour (say blue) to all the positive values of z. Looking at the simple code below, once it runs, you can grab the rotation tool and see the plot from above (see attached picture): how can I get only two colours in this top view, one red (for -ve values) and one blue for the +ve values? You can see I didn't even specify a colormap, so that Matlab goes to the default cmap. Any help? Thanks.

clear all
close all
x = linspace(-20,20,2400);
fz = exp(-x.^2./0.2).*cos(5.*x);
[X, Y] = meshgrid(x, x);
z = exp(-X.^2-Y.^2);
f = z.*meshgrid(fz,fz);
figure
surf(X, Y, f, 'LineStyle', 'None')
xlim([-5 5])
ylim([-5 5])

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Answer 1

David Goodmanson 2020-9-27

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https://ww2.mathworks.cn/matlabcentral/answers/600568-surf-x-y-z-with-a-colormap-that-is-only-two-colours#answer_501079

编辑：David Goodmanson 2020-9-27

Hi Emil,

x = linspace(-20,20,2400);
fz = exp(-x.^2./0.2).*cos(5*x);
[X, Y] = meshgrid(x, x);
z = exp(-X.^2-Y.^2);
f = z.*meshgrid(fz,fz);
  
switch 2   % 1 or 2
  case 1 % original question
  figure(1)
  surf(X, Y, sign(f), 'LineStyle', 'None')
  colormap([1 0 0; 0 0 1])          % make a colormap with two choices 
  colorbar
  case 2 % new question, not documented, see comments 
  figure(1)
  surf(X, Y, f, 'LineStyle', 'None')
  % pick a couple of rgb colors for the ends of the colorbar
  c2 = [0 .8 .8]  % top
  c1 = [0 .3 .3]   % bottom
  % standard colormap has 64 gradations
  map = [linspace(c1(1),c2(1),64)' ...
         linspace(c1(2),c2(2),64)' ...
         linspace(c1(3),c2(3),64)']; 
colormap(map)
colorbar
end
xlim([-5 5])
ylim([-5 5])
xlabel('x')
ylabel('y')
zlabel('z')
view([0 0 1])

The result for swiich case 1 looks funny but I believe it is correct.

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David Goodmanson 2020-9-27

编辑：David Goodmanson 2020-9-27

Hi emil,

see modified answer switch case 2 for a do-it-youself colorbar. Quite frankly for this particular example, since the gaussian function envelope drops off so quickly you are never going to see more than one oscillation. Also, in this example the function is near 0 most of the time. Therefore f~~0 dominates and gives the 'background' color. The background has to have 'bright colorness' to some extent because f~~0 is somewhere in the middle of the colorbar and there has to be room for f<0.

Emil Amor 2020-9-28

Thanks. The modulation can always be changed to see more cycles. One is enough for the current purpose.

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Answer 2

KSSV 2020-9-27

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https://ww2.mathworks.cn/matlabcentral/answers/600568-surf-x-y-z-with-a-colormap-that-is-only-two-colours#answer_501082

You can try something like this

x = linspace(-20,20,2400);
fz = exp(-x.^2./0.2).*cos(5.*x);
[X, Y] = meshgrid(x, x);
z = exp(-X.^2-Y.^2);
z(z>=0) = +1 ;
z(z<0) = -1 ; 
f = z.*meshgrid(fz,fz);
%
figure
surf(X, Y, f, 'LineStyle', 'None')
xlim([-5 5])
ylim([-5 5])

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Emil Amor 2020-9-27

编辑：Emil Amor 2020-9-27

Thanks for the reply. I got the attached pic, which still has blue, green, orange , yellow colours in the 'positive' z values section. I am trying to have all that as one signle colour with perhaps just the intensity (or the opacity) dropping. Thanks.

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