Interpolating contour plot using user input

Hello,

I have created a 2D contour map using a 25x19 matrix and was wondering how to interpolate the value at certain user-input x-y coordinates? Essentially, I want the user to enter coordinates that are either integer or decimal, and for the code to output the value at that corresponding location. Any help would be appreciated :)

data = readmatrix('pixeltxt.txt');

contourf(data);

grid minor

Answer 1

[Bram Schroeders]

I think taking a look at this documentation may help you https://www.mathworks.com/help/matlab/ref/scatteredinterpolant.html

[Radhika Kulkarni]

I am a bit confused on the parameters - would v be my 25 x 19 matrix? and if so, do the x and y have to be the same dimensions? Or would I use x and y as my user-inputted coordinates? If you could clarify further, that would be great.

[Radhika Kulkarni]

Is it possible to convert my 25x19 matrix using Matlab code (picture attached) to fit a function in terms of z = f(x,y)? That way, I can enter x and y coordinates and the output will be the cooresponding value at that location.

[Bram Schroeders]

This is exactly what the scatteredinterpolant function does for you. x and y would in this case be the indices of the values in the matrix. To create a vector to input in the function you can use a for loop like this:

A = rand(25,19) %In your case this would be your matrix and not 'rand(25,19)'
x = [];
y = [];
v = [];

for i = 1:size(A,1)
    for j = 1:size(A,2)
        x = [x; i];
        y = [y; j];
        v = [v; A(i,j)]
    end
end

Now you can create the interpolant function like this:

f = scatteredInterpolant(x,y,v)

and you can read the values like this:

xi = % The x value you want to interpolate at
yi = % The y-value you want to interpolate at

I = f(xi,yi) % The interpolated value

[Radhika Kulkarni]

I got an error saying: "

Error using scatteredInterpolant. The input points must be specified in column-vector

format.

Error in pixelexport (line 13)

f = scatteredInterpolant(x,y,v) "

[Bram Schroeders]

Oh, i see i made a typo. The for loop should be like this:

for i = 1:size(A,1)
    for j = 1:size(A,2)
        x = [x; i];
        y = [y; j];
        v = [v; A(i,j)]
    end
end

I also corrected my previous comment

[Radhika Kulkarni]

That works, thank you so much!!

[Radhika Kulkarni]

I do have one final question: Is there a way for me to keep my plot as it is w/ the values, but change the axis? I want the x axis to be -21.4 to -19.6 going up by 0.2. And I want the y axis to be -17.5 to -15 going up by 0.5

I tried set gca but it did not work - it gave me a blank axis overall

Thanks in advance

[Bram Schroeders]

use:

grid on;
xlim([-21.4 -19.6]);
ylim([-17.5 -15]);

[Radhika Kulkarni]

It returned this:

[Bram Schroeders]

This is exactly what i would expect that it returns. If this is not what you want, i don't understand your question.

[Radhika Kulkarni]

I have another scatter plot of nodes on a mesh with the axes I stated previosuly. So, I wanted to modify this contour plot to match that axes so that I can match the position of the nodes to the value in my contour plot. For example, a node positioned at -21, -17 would coorespond to ___ value in the contour plot. I am essentially looking to overlay the contour plot over the scatter plot. I hope this makes more sense

[Bram Schroeders]

You can use the rescale function:

So for instance you have a vector called A and you want to set the x-axis to [-21.4 -19.6] and y-axis to [-17.5 -15] you can do this:

A(:,1) = rescale(A(:,1),-21.4,-19.6);
A(:,2) = rescale(A(:,2),-17.5,-15.0);

I think this is what you mean

[piston_pim_offset]

Thanks for your answers Bram Schroeders