How to plot a line with angle of 'Orientation' and length of 'MajorAxisLength' through 'Centroid'?

Dear All,

I would like to plot a line with angle of 'Orientation' and length of 'MajorAxisLength' through 'Centroid'.

Similar as shown in image bellow:

http://uk.mathworks.com/help/images/ref/referencemtoz11.gif

The code I'm using right now, is piloting the line with the right angle, but it starts from the center

   st = regionprops(Image,'Centroid','Orientation','MajorAxisLength');
   x = st.Centroid(1) + st.MajorAxisLength * cosd(st.Orientation);
   y = st.Centroid(2) - st.MajorAxisLength * sind(st.Orientation);
   line([c(1) x],[c(2) y]);
How can do it right, maybe I should use BoundingBox as well?

Thanks in advance for any help,

I

[Nici Me]

     st = regionprops(Image,'Centroid','Orientation','MajorAxisLength', 'Extrema');
     line([st.Extrema(7),st.Extrema(4)],[st.Extrema(15),st.Extrema(12)], 'Color', 'red', 'Linestyle', '--');
     plot(st.Extrema(4),st.Extrema(12), 'r*')
     plot(st.Extrema(7),st.Extrema(15), 'g*')

Answer 1

[David Young]

Your original code is almost right. You just need to think through the geometry a little more. This works:

st = regionprops(Image,'Centroid','Orientation','MajorAxisLength');
hlen = st.MajorAxisLength/2;
xCentre = st.Centroid(1);
yCentre = st.Centroid(2);
cosOrient = cosd(st.Orientation);
sinOrient = sind(st.Orientation);
xcoords = xCentre + hlen * [cosOrient -cosOrient];
ycoords = yCentre + hlen * [-sinOrient sinOrient];
line(xcoords, ycoords);

[Ivan Shorokhov]

Hello David,

Thanks a lot for your help!

[Maria Pardo Garcia]

I'm having some problems implementing this code, because it tells me:

Error using /

Too many input arguments.

How can I fix it? I want to do this because I will like to understand the values that ' Orientation' gives

Thank you in advanced

María

[Image Analyst]

Maria: The code was meant for the case where you had only one blob. You have multiple blobs so you'll have to index st and do it for every blob that st has information on. If you have no idea how to do it, post your image and your code to a new question (not here).

Answer 2

[Sid]

Hi Ivan,

Would this example from Steve be helpful?

Essentially, I grabbed what Steve had done before, and tinkered it a little bit with the direction you had taken. I am using 'rice.png' as my source image.

I simply divided the Major/Minor Axis Lengths by 2, and then also changed the angle for Minor Axis orientation.

Hope this helps!

Cheers,

Sid

_____________________________________

clc; clear all; close all;

I = imread('rice.png');

bw = im2bw(I,graythresh(I));
bw = bwareaopen(bw,10);

s = regionprops(bw, 'Orientation', 'MajorAxisLength', ...
    'MinorAxisLength', 'Eccentricity', 'Centroid');

imshow(bw)
hold on

phi = linspace(0,2*pi,50);
cosphi = cos(phi);
sinphi = sin(phi);

for k = 1:length(s)

%     k = 10;
    xbar = s(k).Centroid(1);
    ybar = s(k).Centroid(2);

      a = s(k).MajorAxisLength/2;
      b = s(k).MinorAxisLength/2;

      theta = pi*s(k).Orientation/180;
      R = [ cos(theta)   sin(theta)
           -sin(theta)   cos(theta)];

      xy = [a*cosphi; b*sinphi];
      xy = R*xy;

      x = xy(1,:) + xbar;
      y = xy(2,:) + ybar;

      plot(x,y,'r','LineWidth',2);

      hold on

      % Major Axis Values

      xMajor1 = xbar + (((s(k).MajorAxisLength)./2) * cosd(s(k).Orientation));
      yMajor1 = ybar - (((s(k).MajorAxisLength)./2) * sind(s(k).Orientation));

      xMajor2 = xbar - (((s(k).MajorAxisLength)./2) * cosd(s(k).Orientation));
      yMajor2 = ybar + (((s(k).MajorAxisLength)./2) * sind(s(k).Orientation));
      line([xMajor1 xMajor2],[yMajor1 yMajor2],'color','g')

      % Minor Axis Values
      xMinor1 = xbar + (((s(k).MinorAxisLength)./2) * sind((180-s(k).Orientation)));
      yMinor1 = ybar - (((s(k).MinorAxisLength)./2) * cosd((180-s(k).Orientation)));

      xMinor2 = xbar - (((s(k).MinorAxisLength)./2) * sind((180-s(k).Orientation)));
      yMinor2 = ybar + (((s(k).MinorAxisLength)./2) * cosd((180-s(k).Orientation)));
      line([xMinor1 xMinor2],[yMinor1 yMinor2],'color','c')

end
hold off

[Ivan Shorokhov]

Hi Sid,

Thank you for your interest and willing to help. I already saw this example and tried to use it,but unfortunately it didn't help a lot...

[David Young]

Looks a bit more complex than is needed, Sid. It's most help if you give simple solutions with the minimum of code.