How make endpoints of bwboundaries consistent in Matlab?

I have an image and if I then plot the boundary of just the left and edges with the code below:

 imshow(I); hold on; [B,L,N] = bwboundaries(I);
    for k=1:length(B),
    	boundary = B{k};
        BL=size(boundary);
        plot(boundary(1,2), boundary(1,1), '*g','MarkerSize',15);
        for j=1:10:BL(1)
            plot(boundary(j,2), boundary(j,1), '.r','MarkerSize',5);
        end
        
    end

I get this image

The starting point (the green star) for the left edge is at the left side of the image, which is what I expected. However, the starting point for the right edge is towards the middle

Apparently this is because `bwboundaries` deals with tracing objects in clockwise direction, whereas the 2nd edge needs to be traced counterclockwise for it to begin and end on the right boundary of the image

I was hoping the start and end points for both edges would be where the blue dots are

I also was hoping the boundary array for the right edge from `bwboundaries` would be sorted from the corrected startpoint to endpoint (like how it is for the left edge), so that for edges that don't touch the border, I can then draw a straight line connecting them (like in red in the last image), and then use `polyshape` and `area` on it, so I can find the area of that closed region, like in the image belo

how can I do this in Matlab?

Answer 1

[Image Analyst]

Blobs are found in column major order, in other words, going down rows in the first column, until it hits something white, and then across to the next column to the right if it didn't find anything. So in your first blob, the upper left endpoint was the leftmost pixel, whereas in your second blob, the middle of the curve was the leftmost white pixel. Once it finds the first starting pixel, it follows the boundary, clockwise (I guess).

Why don't you skeletonize the blobs/snakes with bwmorph(mask, 'skel', inf) and then detect endpoints with bwmorph(mask, 'endpoints')?