# Contour question in matlab

• MATLAB

## Main Question or Discussion Point

Hello everyone
If I have a plot of a function of two variables and I want the whole curve over the whole x and y intervals but in addition I want to highlight with a black colour a specific region when the function is between two values
F between -8 and 0 or what ever
How can I do that !???????
Thank you

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kreil
Gold Member
This question is poorly presented, so I'll just take a guess as to what you mean.

If I have a plot of a function of two variables and I want the whole curve over the whole x and y intervals but in addition I want to highlight with a black colour a specific region when the function is between two values
This sounds like it has nothing to do with contours, but that you just want to plot a certain interval of the function in a different color. In that case you just split the domains up and make separate plots overlaid.

Code:
[x1,y1] = meshgrid(-8:0.25:-5,-8:0.25:8);
[x2,y2] = meshgrid(-5:0.25:5,-8:0.25:8);
[x3,y3] = meshgrid(5:0.25:8,-8:0.25:8);
f = @(x,y) x.^2 - 3*y.^3;
figure
hold on
plot3(x1,y1,f(x1,y1),'b','LineWidth',2)
plot3(x2,y2,f(x2,y2),'k','LineWidth',2)
plot3(x3,y3,f(x3,y3),'b','LineWidth',2)
xlabel('x')
ylabel('y')
zlabel('z')
view(-142,14)

#### Attachments

• 5.3 KB Views: 411
kreil
Gold Member
I reread your question and realized you might have meant you want to only change the color of the function when the function value is in some interval (instead of the function being evaluated in an interval as in my last post). In this case, you can just use logical indexing to pick out the relevant function values.

Notice the use of surfc here, which plots the surface and also adds a contour plot underneath.

Code:
[x,y] = meshgrid(-8:0.25:8);
f = @(x,y) x.^2 - 3*y.^3;
z = f(x,y);
q = find(z>0 & z<2);
figure
hold on
surfc(x,y,z)
plot3(x(q),y(q),z(q),'*r')
xlabel('x')
ylabel('y')
zlabel('z')
view(-143,42)

#### Attachments

• 37.5 KB Views: 392
Thank you very much for your time
The second part is what I want where I want to highlight a specific region.
I would love to ask some questions :
1-Can I add the color of the region to the color bar or not?
2- Can I change the steps of The numbers of the color bar instead of 10 20 30 I want them to take steps of 5 or some variable ?

Thank you for your time and I hope the questions make a bit more sense than last time

kreil
Gold Member