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

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])

Answer 1

[David Goodmanson]

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.

[Emil Amor]

Indeed. Correct up to a point: I still need the fadeout. I didn't make it clear when I asked, sorry. Essenetially, how can I keep the same sort of intensity profile as the plot shown in my original question, while not going through a range of colours? Looking at the central spot in my pic: it has blue, green, orange, yellow. What I was looking for was a single colour but with corresponding levels of opacity. I am very new to Matlab, and every single colormap I try has issues, as well as an insanely bright background.

[David Goodmanson]

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]

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

Answer 2

[KSSV]

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])

[Emil Amor]

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.