How to find the maximum value of the 3-D plot in matlab?

How to find the maximum value of the 3-D plot in matlab? At present ,I am clicking with the help of Cursor after the generation of the 3-D plot . But is there any specific command for that? I have tried max(abs(figure)), but that generates an array and not a particular single value, I am in need to find the maximum single amplitude value of the plot. Kindly help

Answer 1

[Star Strider]

If your plot is for instance:

figure(1)
surf(X, Y, Z)

where all are matrices, convert ‘Z’ to a vector and then take the max:

Zmax = max(Z(:));

If you want to know where the maximum occurs, this will give you the indices:

[Zmax,Idx] = max(Z(:));
[ZmaxRow,ZmaxCol] = ind2sub(size(Z), Idx);

[Abhay]

Respected Sir, Thanks very much for enabling me to find the maximum value. The way you suggested me to find the indices value is giving me only a single result and indices value is displaying as 1, although it should show me the value in coordinates form when i am using the command [ZmaxRow,ZmaxCol] = ind2sub(size(Z), Idx). Kindly clarify

[Star Strider]

I would need your function or data to clarify.

This works for me:

[X,Y] = meshgrid(-5:0.1:5);
Z = -((X+1.2).^2 + (Y-1.5).^2);

[Zmax,Idx] = max(Z(:));
[ZmaxRow,ZmaxCol] = ind2sub(size(Z), Idx);

figure(1)
mesh(X, Y, Z)
hold on
stem3(X(ZmaxRow,ZmaxCol), Y(ZmaxRow,ZmaxCol), Z(ZmaxRow,ZmaxCol)+5, 'bp', 'MarkerSize',15)
hold off
grid on
legend('z(x,y) = -(x^2 + y^2)', 'Maximum', 'Location','N')

[Abhay]

Respected sir, Thanks very much for the clarification. I am attaching my code for your kind reference,the code is for finding out the max frequency response. As per the way suggested by your good-self,the Zmax occurs at (1,1), and the max value is 0.869.Kindly see that whether it is correct or not.

[Star Strider]

I would have to see the figure your code produces. If the maximum is 0.869 and it occurs at (1,1), then I will not argue with it. (I would use the meshgrid function to produce the grid matrix arguments to your function, but if your code produces the results you want, it works for you and I cannot criticise it.)

I cannot tell if it is correct or not, because I have no idea what you want to do.

[Abhay]

Dear Sir, Thanks for your expert advice ,I am attaching the figure generated from the code. Actually from the matlab figure,we can find the max using the cursor also but without the idea of command, it is difficult to know whether this maximum occurs at (1,1)or not, by seeing from the mat-lab figure only. With Regards, Abhay

[Star Strider]

It looks as though it might have several equivalent maxima. In that event, use the find function:

Idx = find((Z(:) == max(Z(:))));
[ZmaxRow,ZmaxCol] = ind2sub(size(Z), Idx);

That will return all the values that equal the maximum, and their coordinates. Normally ‘maximum’ implies only one, but if you have several that could equal the maximum, this change will return all of them.

[Abhay]

Respected Sir, You are absolutely correct as this plot has more than one maxima.You have suggested a very good command for finding out the same.Thanks for a very good insight. When I use this find command, the following values are returned by the mat-lab, Idx=[1;80;6321;6400], ZmaxRow=[1;80;1;80]and ZmaxCol=[1;1;80;80]. Can you please tell me what all these values specify? or what the variable Idx is giving? There is one more query regarding plot. Is there any way of defining/restricting the dimension of these figures. As I have to generate 5 plots using Mat-lab and paste all these generated figures in journals. These journals have ceretain conditions and the allow a particular dimension for these figures? Can you please help in this case? With Sincere Regards, Abhay

[Star Strider]

The ‘idx’ is simply the linear index into the ‘Z’ matrix. The individual row elements of ‘ZmaxRow’ and ‘ZmaxCol’ are the index coordinates into the ‘Z’ matrix that are the various maxima. So for example, one maximum would be at (ZmaxRow(1),ZmaxCol(1)) the second at (ZmaxRow(2),ZmaxCol(2)), and so for the rest.

As you posted them, the coordinates would be:

           Row         Col
             1           1
            80          80
          6321           1
          6400          80

So the ‘first’ maximum would be at (1,1), the ‘last’ maximum would be at (6400,80), and so for the ones between them. You would use these to address the ‘Z’ matrix as indices into it. You would have to use them as indices into the respective x and y coordinate values as well to get them in the units of the coordinate axes, if they were different than the matrix indices.

[Abhay]

Respected Sir, Thanks very much for the clarification. With Sincere Thanks, Abhay

[Star Strider]

My pleasure.

If my Answer solved your problem, please Accept it.

[Abhay]

Respected Sir, I have a Mat-lab generated figure which I saved in jpg , its dpi value is 96, I have to change its dpi value to 300. Can you please help me? Regards, Abhay

[Star Strider]

See the documentation for print and saveas for details on how to do that.