Finding intersection points with refline

Im trying to find the intersection points of a plot and a refline#

The reflines are the straight horizontal lines in the figure

I have attached the figures

Cant seem to figure out the code for it

Thanks in advance

Answer 1

[Star Strider]

It would be easier if you defined both of your functions in each plot with the same X-data. Then you could use the much more efficient code in How to change axis of graph and interpolate data.

This works for today’s plots (I tested it on all of them):

% GET INFORMATON FROM FIGURE: 
openfig('Figure(7).fig');
h1c = get(gca, 'Children');
Xdc = get(h1c, 'XData');
Ydc = get(h1c, 'YData');
maxlen = max(cell2mat(cellfun(@max, cellfun(@size, Xdc, 'Uni',0),'Uni',0)))
Xd2 = cell2mat(Xdc(2));
Yd2 = cell2mat(Ydc(2));
Xd = Xd2;
Yd1 = cell2mat(Ydc(1));
Yd = [Yd1(1)*ones(size(Xd2)); Yd2];

% CALCULATIONS: 
Ydn = diff(Yd, [], 1);              % Subtract line from curve to create zero-crossings
Zx = circshift(Ydn, [0 1]) .* Ydn;  % Use circshift to detect them
Zxi = find(Zx < 0);                 % Their indices

for k1 = 1:length(Zxi)              % Use interp1(Y,X,0) to get line intercepts as Xzx
    Xzx(k1) = interp1([Ydn(Zxi(k1)-1) Ydn(Zxi(k1))], [Xd(1,Zxi(k1)-1) Xd(1,Zxi(k1))], 0);
end

% PLOT ZERO-CROSSINGS ON FIGURE TO CHECK: 
hold on
plot(Xzx, repmat(Yd(1,1),1,length(Xzx)), '*r')
hold off

[Vaultec]

just a quick question what does the maxlen = max(cell2mat(cellfun(@max, cellfun(@size, Xdc, 'Uni',0),'Uni',0))) that line do?

[Star Strider]

That line finds the maximum of the lengths of the 'XData' vectors. I initially used it to determine the argument to the ones function that creates a vector of y values for the constant line to use in the call to interp1. The values are in cell arrays, explaining the nested calls to cellfun.

Thanks!

[Vaultec]

How would i go about if I wanted to have the intersection points be placed as a matrix/ or find the values of the intersection points?

[Star Strider]

The x-coordinates of the intersections are in the Xzx vector. (That is the same as for the code in your earlier question as well. I used essentially the same code for both.) The y-values of the intersections are the value of the constant line. If you want a matrix of the x and y coordinates of the intersections, create it as:

XYintx = [Xzx; repmat(Yd(1,1),1,length(Xzx))];

The x values are in the first row, the y values in the second. Call it whatever you like. (The rows of the matrix are the arguments of the plot function in this and the code I provided for your earlier Question.)

[Vaultec]

Ok thank you very much

[Star Strider]

My pleasure!

[Edwin]

Hi,

This is a very great thread! Thank you for your elucidation on finding intersections on the current gca. I've been trying to understand the code better but I've seem to run into a bit of an issue. IDK if this is the correct analysis but this code seems to not work if there are an odd number of intersection points. Whenever there are even number of intersection points, it does seem to work. I thought a remedy to this would be to try to add an arbitrary line to increase the number of intersections to an even number but the added line seems to create another error.

In essence, to the original problem; whenever there are an odd number of intersections, I get the following error:

Attempted to access Ydn(0); index must be a positive integer or logical.

This seems to arise because Zxi has a value of '1' in its array. Again, I'm still trying to figure out exactly how the code works to find the intersection. But Any ideas or suggestions would be greatly appreciated. Thank you for your help!

Cheers

[Star Strider]

The problem is not the code, but that you are attempting to reference ‘Ydn(0)’. Zero is not a positive integer. Arrays in MATLAB are only allowed to be positive integers: (1, 2, ...).