Suggestions on how to count on only one side of a drawn line

I have to count the number of green dots in a set of pictures. I have already written the code to count the dots. The image is divided by the drawn pink line and I only need to count the dots on the left side of the line. I am thinking of using a condition to count the cell only if it is on the left side, but I am not sure how to or how to find the coordinates of the line.

[Rik]

Maybe inpolygon will help? You'll have to use the line to determine a polygon.

Answer 1

[Arun]

Hey Marianne,

I understand that you want to determine the count of the points that are on the left of a given line. I will assume that you possess the coordinates of the points and the equation of the line.

One approach to solve this issue is to form a polygon on the left with the line as one of its edges and then count the point in that polygon. Here is a code that you can use for solving the issue:

% random data for sample that represent points data 
%generate N random numbers in the interval (a,b) with the formula r = a + (b-a).*rand(N,1). 
points= [(-2000 + (2000+2000)*rand(100,1)),(-2000 + (2000+2000)*rand(100,1))]; 
leftMost=  min(points(:,1));%for left edge of the polygon 
upperMost= max(points(:,2));%for top edge of the polygon 
lowerMost= min(points(:,2));%for bottom edge of the polygon 

% line  asumming the equation to be y=3x + 5 
lowerIntersection=(lowerMost-5.0)/3.0; 
upperIntersection=(upperMost-5.0)/3.0; 

%you can use the the below code if you have point coordinates for  the line  
    % lowerIntersection=find(points(:,2)==lowerMost); 
    %  upperIntersection=find(points(:,2)==upperMost); 

%edge of polygon are 
polyx=[leftMost,lowerIntersection,upperIntersection,leftMost]; %x-coordinates for the polygon edges
polyy=[lowerMost,lowerMost,upperMost,upperMost]; %y-coordi
nates for the polygon edges

%inpolygon function
[in,on]=inpolygon(points(:,1),points(:,2),polyx,polyy); 

pointsLeft=numel(points(in,1)); %points on left or on the line 
pointsOnLine=numel(points(on,1));  %points on the line 
pointsRight=numel(~points(in,1)); %points on the right of the line 

pointsLeft 

pointsLeft = 52

pointsOnLine 

pointsOnLine = 3

pointsRight 

pointsRight = 52

In case you are unfamiliar with the process of obtaining the line equation, you can refer the following link for guidance: https://www.mathworks.com/matlabcentral/answers/295992-finding-the-equation-of-a-line-passing-2-points .

Please refer the shared link for more information regarding “inpolygon” function: https://in.mathworks.com/help/matlab/ref/inpolygon.html

Hope this helps.