http://www.mathworks.com/access/helpdesk/help/techdoc/ref/convhull.html

xx = -1:.05:1; yy = abs(sqrt(xx));
[x,y] = pol2cart(xx,yy);
k = convhull(x,y);
plot(x(k),y(k),'r-',x,y,'b+')

This should get you where you are going.

plot(X, Y, '-');

so that I get a continuous line plot of the object perimeter. This requires getting the x coordinates representing the top half of the object in increasing order, but the x coordinates for the bottom half of the object in decreasing order.

I have tried to get the coordinates sorted using dummy columns in a matrix using code such as below and using sortrows:

coords = [X Y X>mean(X) Y<mean(Y)];
coords = sortrows(coords, [4 1 3 2]);
plot(coords(:,1), coords(:,2), '-r');

This doesn’t quite work and I get a horizontal line across the middle of my object. I feel I am close using this approach, but I just can’t get it. Any suggestions?
Thanks.
Fred

http://www.mathworks.com/access/helpdesk/help/techdoc/ref/sort.html

Use sort with the second output argument. That gives the absolute index, but you can use IND2SUB to get back to row and column.

-Doug

I need to sort the pixel intensity values of an image in descending order, but then also be able to keep track of each intensities original coordinate in the image. I assume I will need to create an Nx3 matrix where N is the total number of pixels and the 3 column values are: 1. the pixel intensity, 2. the original col, and 3. the row coordinate.

Could you help me to get started doing this?

Thanks,
Mark

What you need to do is use the transpose operator (‘) to transpose the rows to columns. Then you can use sort rows and transpose right back.

Doug

Thx for your time!

You need to show all the relevant code to get help.

Doug

??? Index exceeds matrix dimensions.

How can I “sort” this problem ;-)

Please watch the video again. You will see Sortrows at the end. This is the function you are looking for.

Doug