How to determine if points fall above an arbitrary line?

Question

James Akula 2020-5-12

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https://ww2.mathworks.cn/matlabcentral/answers/524950-how-to-determine-if-points-fall-above-an-arbitrary-line

评论： James Akula 2020-5-12

采纳的回答： KSSV

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Hello all,

I am trying to find a way to determine if points are above or below an arbitrary line. For example, the code

numXY = randi([1, 19], 1);
lineXs = [0; sort(rand(numXY, 1) * numXY)];
lineYs = rand(size(lineXs));
Xs = rand([100, 1]) * max(lineXs);
Ys = rand(size(Xs));
figure
scatter(Xs, Ys, '.k')
hold on
plot(lineXs, lineYs, 'r')
axis tight

would produce something like

How to determine which of the points in (Xs, Ys) are above or below the red line defined by (lineXs, LineYs)?

Thank you in advance!

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Answer 1

KSSV 2020-5-12

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/524950-how-to-determine-if-points-fall-above-an-arbitrary-line#answer_432070

编辑：KSSV 2020-5-12

在 MATLAB Online 中打开

Read about inpolygon.

numXY = randi([1, 19], 1);
lineXs = [0; sort(rand(numXY, 1) * numXY)];
lineYs = rand(size(lineXs));
Xs = rand([100, 1]) * max(lineXs);
Ys = rand(size(Xs));
idx = inpolygon(Xs,Ys,[0 ; lineXs ; max(lineXs)],[ 0 ; lineYs ; 0]) ;
figure
% scatter(Xs, Ys, '.k')
hold on
plot(lineXs, lineYs, 'r')
plot(Xs(idx),Ys(idx),'*k')
plot(Xs(~idx),Ys(~idx),'*r')

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James Akula 2020-5-12

Thanks. Inpolygon is a nice answer. In the "real world" I might have to select values lower than zero for the lower edge--I will see if -inf works with inpolygon--but at the least, just picking something lower than the lowest Y would do.

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