How to determine if points fall above an arbitrary line?

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James Akula
James Akula 2020-5-12
评论： James Akula 2020-5-12
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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KSSV
KSSV 2020-5-12
编辑：KSSV 2020-5-12
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')
  1 个评论
James Akula
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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