MATLAB: 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

Question

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?

i write this :

….

r=[0 10 15 20 30];

t=[10 10 15 10 10];

f= line(r,t)

axis([0,30,0,20]);

d=(eldof(:,10))& (eldof(:,11)) <= f

then all of points are zero

why??

Answer 1

You need to call polyfit:

coefficients = polyfit(r, t, 1);

or you can call fitPolynomialRansac if you want the line to go exactly through the points on the bottom of your plot.

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.