How to find image boundary is above or below the given line,

grayImage = imread('w.png');

I = im2bw(grayImage);

[y,x] = find(~I) ;

idx = boundary(x,y) ;

 % GEt boundary of the human 
% Boundary points 
x = x(idx) ; y = y(idx) ;
% 
imshow(I) ;
hold on
plot(x,y,'b') ;
% [xx,yy] = getpts() ;  % this forms straight line desired 
uiwait(msgbox('Locate the top of the ear?'));
[x2,y2] = ginput(1);
loacationEar=plot(x2,y2,'r+', 'MarkerSize', 10); 
uiwait(msgbox('Locate the point in starting point of your spine?'));
[x1,y1] = ginput(1);
loacationSpine=plot(x1,y1,'r+', 'MarkerSize', 10); 
xx = [x2 y2] ;
yy = [x1 y1] ;
plot([x2 x1], [y2 y1],'r')
end

Using this code i able to do get below result

[jonas]

What exactly is the question?

[cha bha]

How can i know whether red line is below the blue line or above the blue line ?

Answer 1

[Ameer Hamza]

You can use inpolygon() to find you if the points on the red line are inside the blue polygon. Since the blue polygon is non-convex, you need to take several points on the red line to check whether all the points lie inside the blue polygon. If the condition is true, and the red line is completely inside the polygon then you can conclude that it is below the blue line. Add the following lines at the end of your code

X1 = [x1 y1];
X2 = [x2 y2];
t = linspace(0,1,100);  % how many points to take on the red straight line
pts = (X1.*(1-t)' + X2.*t')
isInside = all(inpolygon(pts(:,1), pts(:,2), x, y))

If I draw the points like this

the code will return

isInside =
logical
 1

Similarly, if you draw a line outside the blue polygon, isInside will be false.

[cha bha]

Is there any way to get the points which lies on the boundary?

if true
  spine_points = [spine_X spine_Y];
neck_points = [neck_X neck_Y];
create points to take on the spine ->neck straight line
t = linspace(0,1,50);  
pts = (spine_points.*(1-t)' + neck_points.*t');
find points are inside the polygon or outside the polygon
[in,on]=inpolygon(pts(:,1), pts(:,2), boundary_x, boundary_y);
xq=pts(:,1);
yq=pts(:,2);
plot(xq(in),yq(in),'g+') % points inside
plot(xq(on),yq(on),'r+')
plot(xq(~in),yq(~in),'bo') % points o utside
end

i use " plot(xq(on),yq(on),'r+') " to get points which lies on the boundary,but it didn't show any points?

[Ameer Hamza]

Have you checked whether the on vector have any 1s in it?

[cha bha]

now its working,thank you.