How to create personalized colormap that starts from a specified value

Hello everyone

I've plotted a surface which it's value when z>=0 is what I want to report. When I use zlim([0 n]) command, the colormap won't update and the peaks are not distinguishable as you see in pictures attached. Is there a way that I could create a personalized colormap which it starts from z=0 and vary with z value? (For example in the grey gradient in pictures attached, the color of z=0 becomes black and the color of highest value of z becomes grey)

Thanks and regards

Answer 1

[Florian Bidaud]

Hi

If Z is the map you plot, you can replace all the negative values by 0 like that :

Z = [0 1 8; -1 5 10;-6 -3 14]

Z = 3×3

0 1 8 -1 5 10 -6 -3 14

Z_negative = Z;
Z_negative(Z<0) = 0

Z_negative = 3×3

0 1 8 0 5 10 0 0 14

And then plot this new map

If you don't want these values at all, you can replace them by NaN :

Z = [0 1 8; -1 5 10;-6 -3 14];
Z_negative = Z;
Z_negative(Z<0) = NaN

Z_negative = 3×3

0 1 8 NaN 5 10 NaN NaN 14

[Reza Lashani]

replacing all negative values by zero is not desirable because it creates a z=0 plate which is all black.

I have tested also replacing negative values by NaN but in the boundaries (close to z=0) the surface takes a cogged form and no matter how much I refine the mesh. I persue a solution that creates a smooth result like when using zlim([0 n]).

[Florian Bidaud]

I'm not sure what you really want. zlim removes the values from the plot as follows:

Maybe what you want is changing the color scale ? In this case, you need to use caxis([0 n]).

x = [0 1 2 3 4];
y = [0 1 2 3 4];
z = [1 5 -2 4 6;-1 5 -6 7 8; 1 2 5 9 10; -2 -3 -4 -5 -7;5 -1 6 -2 3];

surf(x,y,z)
zlim([0 max(max(z))])

caxis([0 max(max(z))])

[Reza Lashani]

thank you very much Florian. It is exactly what I wanted.