How to find a local maximum in surf (3D) plot ?

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sung-cheol kwon
sung-cheol kwon 2016-12-19
评论： Scout Patel 2022-8-15
I would like to find a local maximum in 3D surf plot and to draw the identified maximum as a line To further understand what i want to do, i drew a black line by my hand as shown in attached figure,
How could i do this?
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José-Luis
José-Luis 2016-12-19
To me this looks like a catchment delineation problem. You could start with a flow accumulation and follow all the cells that have an accumulation of one and are not on the border.
https://se.mathworks.com/matlabcentral/fileexchange/14504-flow-accumulation--upslope-area-
Scout Patel
Scout Patel 2022-8-15
[TF1,P] = islocalmax(Array,2,'MinProminence',4);%theshold at 4...change accordingly
P(TF1)
V1 = find(TF1, 3, 'first');%this finds the first 3...change accordingly
[y1,x1]=ind2sub(size(Array),V1)
% Fit line to data using polyfit
c = polyfit(x1,y1,1);
% Display evaluated equation y = m*x + b
disp(['Equation is y = ' num2str(c(1)) '*x + ' num2str(c(2))])
% Evaluate fit equation using polyval
y_est = polyval(c,x1);
% Add trend line to plot
hold on
plot(x1,y_est,'r--','LineWidth',0.5)
hold off

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回答（2 个）

Walter Roberson
Walter Roberson 2016-12-19
编辑：Walter Roberson 2016-12-19
Possibly https://www.mathworks.com/help/images/ref/imgradient.html
You can also proceed by taking max() along a dimension of your data. There are a couple of problems you would need to work out with that:
A) The line you are interested in might not cross the entire array, but max() is going to find a maximum for every row or column anyhow
B) The line you are interested in might happen to travel sharply in the direction along which you are taking the max; you would only get one location per row or column, whereas if you had happened to ask along the other dimension you would have gotten a nice answer.
Dean Culver
Dean Culver 2018-3-29
编辑：Dean Culver 2018-3-29
This would be computationally inefficient, but I think it would get the job done:
  • Determine the maximum value of the surface function z on the perimeter. Call it z_(p,max).
  • Determine the row and column indices (i,j) such that z(i,j)=z_(p,max).
  • Record x(i,j) and y(i,j)
  • (begin loop) Sample the points immediately adjacent to the current (i,j).
  • Choose the maximum of these which is not a previously recorded value.
  • Record the new (i,j), x(i,j), y(i,j), and z(i,j).
  • (end loop when you've reached a perimeter and can no longer proceed)
  • plot3 to get the curve

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