How do i draw a line between a known point already on the loop and an unkown one, also on the loop?

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Andreea Stanescu 2019-12-14

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评论： Andreea Stanescu 2019-12-14

Hello! I have a problem where i have to draw a closed loop that can contain 4 squares. I made a loop defined by 15 points. How do i draw a line between a known point already on the loop and an unkown one, also on the loop? The thing is that those 15 point define The squares but i have to prove that. Can you, please, give pointers for this as well?

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Image Analyst 2019-12-14

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You forgot to include your loop. I don't know if the loop you're talking about is

a for loop
a loop defined by some curve you're plotting, or
both.

Also, this sounds like it's probably homework. Is it?

And have you tried using randperm() to get a random index on your loop (I use loop here to mean a list of x,y coordinates that define some curve).

To create a "closed" loop, you can simply tack on the first row of (x,y) coordinates to end of the the 2-D N-by-2 array:

xy = [xy; x(1), y(1)]; % Close the loop.

Andreea Stanescu 2019-12-14

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Here is what i used to define the loop. It looks like this because i wanted to have those 15 points with their respective coordinates and to not cross itself.

This is for a project.

As for the randperm(), no, i didn't try to use it. I haven't heard of it till now. Can you tell me more about it?

x = [5.5, 6, 4.5, 3.5, 5, 5, 3, 2, 2.5, 3, 2, 2.5, 4, 5, 4.5]  
y = [3, 2, 2.5, 1.5, 1.5, 1, 0.5, 1, 2, 3, 4, 4.5, 5, 4, 3.5]
% Append first point to last to close the curve
x = [x, x(1)];
y = [y, y(1)];
plot(x, y, 'r*');
grid on;
knots = [x; y];
areaOfPolygon = polyarea(x,y);
numberOfPoints = length(x);
% Interpolate with a spline curve and finer spacing.
originalSpacing = 1 : numberOfPoints;
% Make 9 points in between our original points that the user clicked on.
finerSpacing = 1 : 0.1 : numberOfPoints;
% Do the spline interpolation.
splineXY = spline(originalSpacing, knots, finerSpacing);
% Plot the interpolated curve.
hold off;
plot(knots(1, :), knots(2, :), 'ro', 'LineWidth', 2, 'MarkerSize', 16);
hold on;
plot(splineXY(1, :), splineXY(2, :), 'b+-', 'LineWidth', 2, 'MarkerSize', 16);
title('Blue Spline Between Red Knots', 'FontSize', fontSize);
legend('Knots', 'Spline');
xlabel('X', 'FontSize', fontSize);
ylabel('Y', 'FontSize', fontSize);
grid on;
hold on;

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Answer 1

Image Analyst 2019-12-14

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https://ww2.mathworks.cn/matlabcentral/answers/496611-how-do-i-draw-a-line-between-a-known-point-already-on-the-loop-and-an-unkown-one-also-on-the-loop#answer_406358

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Try this:

clc;    % Clear the command window.
close all;  % Close all figures (except those of imtool.)
clear;  % Erase all existing variables. Or clearvars if you want.
workspace;  % Make sure the workspace panel is showing.
format long g;
format compact;
fontSize = 15;
x = [5.5, 6, 4.5, 3.5, 5, 5, 3, 2, 2.5, 3, 2, 2.5, 4, 5, 4.5]  
y = [3, 2, 2.5, 1.5, 1.5, 1, 0.5, 1, 2, 3, 4, 4.5, 5, 4, 3.5]
% Append first point to last to close the curve
x = [x, x(1)];
y = [y, y(1)];
plot(x, y, 'r*');
grid on;
knots = [x; y];
areaOfPolygon = polyarea(x,y);
numberOfPoints = length(x);
% Interpolate with a spline curve and finer spacing.
originalSpacing = 1 : numberOfPoints;
% Make 9 points in between our original points that the user clicked on.
finerSpacing = 1 : 0.1 : numberOfPoints;
% Do the spline interpolation.
splineXY = spline(originalSpacing, knots, finerSpacing);
% Plot the interpolated curve.
hold off;
plot(knots(1, :), knots(2, :), 'ro', 'LineWidth', 2, 'MarkerSize', 16);
hold on;
plot(splineXY(1, :), splineXY(2, :), 'b+-', 'LineWidth', 2, 'MarkerSize', 8);
title('Blue Spline Between Red Knots', 'FontSize', fontSize);
legend('Knots', 'Spline');
xlabel('X', 'FontSize', fontSize);
ylabel('Y', 'FontSize', fontSize);
grid on;
hold on;
% Get a known index.  "known" because it's one of the training points.
knownIndex = randperm(length(x), 1)
% Get a unknown index.  "unknown" because it's one of the interpolated points.
unknownIndex = randperm(length(finerSpacing), 1)
% Get the x,y coordinates for these indexes.
xKnown = knots(1, knownIndex)
yKnown = knots(2, knownIndex)
xUnknown = splineXY(1, unknownIndex)
yUnknown = splineXY(2, unknownIndex)
% Now draw a line between them in dark green.
darkGreen = [0, 0.5, 0];
plot([xKnown, xUnknown], [yKnown, yUnknown], 'o-', ...
	'Color', darkGreen, 'MarkerSize', 24, 'LineWidth', 3);
legend('Knots', 'Spline', 'Line between random knot and random point');