i have number of points that have (x,y) .then i define a line and i want to realize which points are under the line?

Then you have to get some x values and get a digital vector of line values. You can use your r locations, or you can use more or less than that. Then see which values are less than the y value of the line. For the same r points

yLine = polyval(coefficients, r);
belowLine = t < yLine; % A logical index vector.

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Mili Kian 2020-4-19

编辑：Image Analyst 2020-4-20

在 MATLAB Online 中打开

Thanks a lot.

My points are :

(x,y)

I can't understand what to do at the end?? Please help me.

Image Analyst 2020-4-20

Those points are just a grid:

I'm not sure what to do either. Can you draw the line on there that you want to detect if some point is below that line, and repost your picture with the line indicated?

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Answer 2

Mili Kian 2020-4-21

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https://ww2.mathworks.cn/matlabcentral/answers/518908-i-have-number-of-points-that-have-x-y-then-i-define-a-line-and-i-want-to-realize-which-points-are#answer_427446

i want to see the effect of topoghraphy

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Image Analyst 2020-4-21

在 MATLAB Online 中打开

Try this:

xy = [...
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x = xy(:, 1);
y = xy(:, 2);
plot(x, y, 'b.', 'MarkerSize', 20);
grid on;
% Make line
xRedLine = unique(x)
redLine = [8, 8, 8, 10, 13.1, 10, 8, 8, 8];
hold on;
plot(xRedLine, redLine, 'r+-', 'LineWidth', 3, 'MarkerSize', 15);
% Find values less than the red line for each x and plot a red square around them
for k = 1 : length(x)
	thisX = round(x(k))
	index = find(thisX == xRedLine);
	if y(k) < redLine(index)
		plot(thisX, y(k), 'rs', 'MarkerSize', 20);
	end
end