How to find the angle between the two lines from an region of interest of an image

With reference to the mathwork example "Measuring Angle of Intersection", how to find the initial point on each boundary and trace the boundaries. And can you give me examples of each command, as i couldn't able to understand its working.

This program actually starts from "finding the initial point on each boundary of the cropped image". Since, i studied cropping and changing to grey-scaling a image, i haven't included it.

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im = size(BW);

% horizontal beam

col1 = 4;

row1 = min(find(BW(:,col1)));

% angled beam

row2 = 12;

col2 = min(find(BW(row2,:)));

boundary1 = bwtraceboundary(BW, [row1, col1], 'N', 8, 70);

% set the search direction to counterclockwise, in order to trace downward.

boundary2 = bwtraceboundary(BW, [row2, col2], 'E', 8, 90,'counter');

imshow(RGB); hold on;

% apply offsets in order to draw in the original image

plot(offsetX+boundary1(:,2),offsetY+boundary1(:,1),'g','LineWidth',2);

plot(offsetX+boundary2(:,2),offsetY+boundary2(:,1),'g','LineWidth',2);

ab1 = polyfit(boundary1(:,2), boundary1(:,1), 1);

ab2 = polyfit(boundary2(:,2), boundary2(:,1), 1);

vect1 = [1 ab1(1)]; % create a vector based on the line equation

vect2 = [1 ab2(1)];

dp = dot(vect1, vect2);

% compute vector lengths

length1 = sqrt(sum(vect1.^2));

length2 = sqrt(sum(vect2.^2));

% obtain the larger angle of intersection in degrees

angle = 180-acos(dp/(length1*length2))*180/pi

intersection = [1 ,-ab1(1); 1, -ab2(1)] \ [ab1(2); ab2(2)];

% apply offsets in order to compute the location in the original,

% i.e. not cropped, image.

intersection = intersection + [offsetY; offsetX]

inter_x = intersection(2);

inter_y = intersection(1);

% draw an "X" at the point of intersection

plot(inter_x,inter_y,'yx','LineWidth',2);

text(inter_x-60, inter_y-30, [sprintf('%1.3f',angle),'{\circ}'],...
     'Color','y','FontSize',14,'FontWeight','bold');

interString = sprintf('(%2.1f,%2.1f)', inter_x, inter_y);
text(inter_x-10, inter_y+20, interString,...
     'Color','y','FontSize',14,'FontWeight','bold');

[Walter Roberson]

I am not sure why you have tagged this with Simulink ?

Answer 1

[Image Analyst]

I can't do much with this since you didn't supply BW. But anyway, I don't know what it does but I never use bwtraceboundary(). I always get boundaries with bwboundaries() because it is much easier. Would that work for you?

[Sai]

sir actually i didn't use this program. It was an example from mathworks "Measuring Angle of Intersection". I just wanted to understand that program. Can you explain me those commands and its working sir ? Its not about the program, but the commands used. I want to understand those commands as i couldn't able to get it with any other picture other than the one specified in that example.

This was the first half of the program:

RGB = imread('gantrycrane.png');

imshow(RGB);

text(size(RGB,2),size(RGB,1)+15,'Image courtesy of Jeff Mather',...
     'FontSize',7,'HorizontalAlignment','right');

line([300 328],[85 103],'color',[1 1 0]);

line([268 255],[85 140],'color',[1 1 0]);

text(150,72,'Measure the angle between these beams','Color','y',...
     'FontWeight', 'bold');

% you can obtain the coordinates of the rectangular region using

% pixel information displayed by imtool

start_row = 34;

start_col = 208;

cropRGB = RGB(start_row:163, start_col:400, :);

imshow(cropRGB)

% Store (X,Y) offsets for later use; subtract 1 so that each offset will

% correspond to the last pixel before the region of interest

offsetX = start_col-1;

offsetY = start_row-1;

I = rgb2gray(cropRGB);

threshold = graythresh(I);

BW = im2bw(I,threshold);

BW = ~BW;  % complement the image (objects of interest must be white)

imshow(BW)

[Image Analyst]

It looks explained in the comments. Which comment or function is not understandable? You know that functions are described in the help, right?