Determine point position regarding a line

Hi Matlab experts,

I was wondering if anyone would have an idea on how to determine the position (right or left) of a point (in 2D) regarding a line (a segment of a line if we want to be precise) that I have extracted using 'improfile'.

Here's what I've tried:

[cx,cy,c] = improfile(label,xi,yi,j-y_cc); % extracting my line segment
m = [cx(1) cy(1)];
n = [cx(end) cy(end)];
coeff = [[1; 1]  m(:)]\n(:); %find the slope and the intercept
                for ii = 1:256
                    for jj = 256:-1:1
                        if label(ii,jj,k) == 11 || label(ii,jj,k) == 12
                            pos = sign((n(1) - m(1)) * (jj - m(2)) -...
                            (n(2) - m(1)) * (ii - m(1)));
                            %test = jj-ii*coeff(1)-coeff(2); % I have tried looking at the sign of this too
                            if pos <= 0
                                labelvst_l(ii,jj,k) = 1;
                            end
                    end
                end
            end

I would be very grateful if anyone could help with this! Thank you

Ismail

Answer 1

[KSSV]

If (x1,y1) and (x2,y2) are two points P1,P2. You can decide which point comes first by calculating difference between x coordinates i.e (x2-x1). If (x2-x1) is +ve P1 comes first and then P2. If (x2-x1) is negative P2 comes first and then P1.

[Ismail Koubiyr]

Thank you for your answer. The thing is I'm trying to know whether a point P is above or under a line (segment) that is defined by two points Q1 and Q2 that I'm getting from the function 'improfile'

[KSSV]

It is a easy task. Refer the below link.

http://math.stackexchange.com/questions/324589/detecting-whether-a-point-is-above-or-below-a-slope

Answer 2

[Morteza Adl]

You can use cross product.