MATLAB Answers

Amanda
1

Convert CurrentPoint to X and Y Value.

Asked by Amanda
on 7 Feb 2013
Latest activity Edited by Jan
on 4 Nov 2019
This is where we are getting stuck. We would appreciate any insight.
We have a basic figure (linear plot using: and we are using ‘CurrentPoint’ in the code to extract a value from the graph.
Here is the input:
function FcnName(src,evnt,a)
cp = get(gcf, 'CurrentPoint')
disp('click down!!!!')
disp(a)
end
Here is the output:
cp = 341 257
but what we need is the "real" x and y value. Is there a way to accomplish this?
Amanda

  1 Comment

Jan
on 8 Feb 2013
What are "real x and y values"? Realtive to the AXES, in screen coordinates or in the real world?

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4 Answers

Answer by Azzi Abdelmalek
on 7 Feb 2013
 Accepted Answer

x=get(gco,'Xdata');
y=get(gco,'Ydata')

  6 Comments

get(gca,'Xdata')
Error: The name 'Xdata' is not an accessible property for an instance of class 'axes'.
to get all axes properties, Type
set(gca)
To get just the current point coordinate, there is a function ginput
If you select the axes, gco do the same thing then gca, if you select a figure, gco is the same then gcf
I guess I'm still not seeing it. Let's say a user plotted, say, 1:10, and then the user clicked in the middle of the plot. The user wanted to retrieve the coordinate (5,5) but instead got some kind of pixel coordinates like 341,257 (like the poster said). I'm just not seeing how your code turns the 341,257 into 5,5. But whatever - I don't have the code like Amanda does and she said it works for her so that's what counts.

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Answer by Jan
on 8 Feb 2013
Edited by Jan
on 4 Nov 2019

I guess that you do not want the coordinates relative to the figure, but relative to the data in an AXES object. Then:
CP = get(gca, 'CurrentPoint');
x = CP(1);
y = CP(2);
The CurrentPoint property of the axes replies a 2x3 array, which defines the viewing line through a 3D scene. But in standard 2D view, the CP(1, 1:2) contain the current 2D position already. This property is the "location of last button click, in axes data units", see Doc: axes_props.

  1 Comment

Thank you so much, Jan. You saved my life...orz

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Answer by Image Analyst
on 8 Feb 2013
Edited by Image Analyst
on 8 Feb 2013

Amanda, give this a try. It will plot some data points, then ask you to click near one, and it will tell you which point of your data that you clicked closest to.
% Plot data - a line from (1,1) to (10,10).
h=plot(1:10, 'bs-')
grid on;
axis equal;
xlim([0 11]);
ylim([0 11]);
datacursormode on;
% Enlarge figure to full screen.
screenSize = get(0,'ScreenSize')
set(gcf, 'units','pixels','outerposition', screenSize);
% Ask user to click on a point.
uiwait(msgbox('Click near any data point'));
% Print the x,y coordinates - will be in plot coordinates
[x,y] = ginput(1) % Will be close to 5,5 but not exactly.
% Mark where they clicked with a cross.
hold on;
plot(x,y, 'r+', 'MarkerSize', 20, 'LineWidth', 3);
% Print the coordinate, but this time in figure space.
% Coordinates will be way different, like 267, 196 instead of 5,5.
cpFigure = get(gcf, 'CurrentPoint')
cpAxis = get(gca, 'CurrentPoint')
% Print coordinates on the plot.
label = sprintf('(%.1f, %.1f) = (%.1f, %.1f) in figure space', x, y, cpFigure(1), cpFigure(2));
text(x+.2, y, label);
% Tell use what ginput, cpFigure, and cpAxis are.
message = sprintf('ginput = (%.3f, %.3f)\nCP Axis = [%.3f, %.3f\n %.3f, %.3f]\nCP Figure = (%.3f, %.3f)\n',...
x, y, cpAxis(1,1), cpAxis(1,2), cpAxis(2,1), cpAxis(2,2), cpFigure(1), cpFigure(2));
uiwait(msgbox(message));
% Retrieve the x and y data from the plot
xdata = get(h, 'xdata')
ydata = get(h, 'ydata')
% Scan the actual ploted points, figuring out which one comes closest to 5,5
distances = sqrt((x-xdata).^2+(y-ydata).^2)
[minValue minIndex] = min(distances)
% Print the distances next to each data point
for k = 1 : length(xdata)
label = sprintf('D = %.2f', distances(k));
text(xdata(k)+.2, ydata(k), label, 'FontSize', 14);
end
% Draw a line from her point to the closest point.
plot([x xdata(minIndex)], [y, ydata(minIndex)], 'r-');
% Tell her what data point she clicked closest to
message = sprintf('You clicked closest to point (%d, %d)',...
xdata(minIndex), ydata(minIndex));
helpdlg(message);

  7 Comments

Make sure you have a bunch of points that are whatever in 'x' but not in y
x = rand(1,500)*10;
y = rand(1,500)*10000;
Now click near a point that has a second point at about the same y but much different x.
I'll see if I can find my code.
OK I see what you mean. My code does calculate the closest point numerically but it may not APPEAR to be the closest point on the screen because it's so compressed vertically. So if you click on a point in the middle of the screen, the closest point may be only 0.5 away but that appears way on the other side of the plot, while one that appears right next to the point, but may actually be 50 units away, and that is probably the one the user wanted to specify. It's closer on the screen but farther away numerically.
Baha
on 13 Nov 2014
Dear Image Analyst,
This was actually a great tutorial for what I was thinking if it was possible to do in matlab. Is it possible to extend this code for clicking in 3D and finding the closest data point? I appreciate your guidance...
Baha

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Answer by Simão Faria on 11 Oct 2016

I think you just have to change gcf to gca.
The coordinates you are getting are relative to the whole figure object and not the current axes.
If you use:
cp = get(gca, 'CurrentPoint')
you should get the coordinates according to the X and Y axis assigned to the plot area

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