plotting evenly spaced lines across object

Dear all,

After finding an object boundary, I would like to plot evenly spaced lines (lets say every 5 degrees angle moving clockwise) that will cross the object center and stop at the object periphery at both sides. Something like cutting a cake into pieces.

For example,for an elliptical object,after finding long axis of it (let's call it 0-180 degrees axis), I would like to plot lines every given angle starting from 0-180 axis for entire 180 degrees. So if I would move every 5 degrees, I would get 36 lines.

Does anyone know how to do it and how to get the length of these lines? much appreciate jakub

Answer 1

[Sven]

Hi Jakub,

I think this does exactly what you're looking for. I've commented the code so it's easy to follow. Note that I've used the intersections entry on the file exchange.

% Make a blob
BW = false(20);
BW(4:16,6:12) = true;
% Get its boundary and a center location
bb = bwboundaries(BW);
bbXY = bb{1}(:,[2 1]);
centXY = mean(bbXY,1);
figure, imagesc(BW), hold on, plot(bbXY(:,1),bbXY(:,2),'g',centXY(1),centXY(2),'yo')

% Make a line that is sure to extend past the object
cutLineXY = [-1 0; 1 0] * sum(size(BW).^2);

% Define how many times we will rotate it
thetas = 0:15:360;
sth = sind(thetas);
cth = cosd(thetas);

for i = 1:length(thetas)
    % Rotate the line
    R = [cth(i) sth(i); -sth(i) cth(i)];
    rotLineXY = cutLineXY * R;
    % Shift it to the center
    rotLineXY = rotLineXY + [centXY;centXY];
    plot(rotLineXY(:,1),rotLineXY(:,2),'k')
    % Check where it intersects our boundary
    [X,Y] = intersections(rotLineXY(:,1),rotLineXY(:,2),bbXY(:,1),bbXY(:,2));
    % Every 2nd intersection will be "inside" the blob
    for cutNo = 1:2:length(X)
        plot(X(cutNo:cutNo+1), Y(cutNo:cutNo+1),'-w.')
    end
end

Does that answer your question?

[Jakub]

Hey Sven,

Sorry for the late reply. Awesome, this is exactly what I needed. And would you know a fast way to put X and Y coordinates of the same line from all lines into array so that I could calculate the lengths.

The idea behind all of this is that I try to quantify direction of the shape change by compering lengths of the lines in two consecutive frames.

Much appreciate your help, cheers, jakub

[Sven]

Instead of plotting, you could just store the XY coordinates of each line.

Before the loop you could set:

% Gotta be a cell in case you could get more than 1 interection.
allLines = cell(size(thetas));

Then inside the loop:

allLines{i} = [X(:) Y(:)];

If you do that you'll collect all of your lines in a cell for future calculations.

[Jakub]

Cheers Sven!!! Thanks again for your help.

jakub

[Jakub]

Hi Sven,

Sorry for bothering you again. The code works very well, it plots all the lines correctly, however, when I want to see the XY coordinates of intersections, there are only 2 numbers instead of 25 if one use thetas spacing of 0:15:360. Would you know where could be teh problem?

Much appreciate, jakub

[Jakub]

I am sorry again, please ignore previous message; all clear, it is just not my day,

cheers, j