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    <p>This code finds co-linear groups of lines. </p> <p><img src="https://i.stack.imgur.com/v7i19.png" alt="enter image description here"></p> <pre><code>theta = zeros(length(lines),1); rho = zeros(length(lines),1); for k = 1:length(lines) xy = [lines(k).point1; lines(k).point2]; plot(xy(:,1),xy(:,2),'LineWidth',1,'Color','green'); plot(xy(1,1),xy(1,2),'x','LineWidth',1,'Color','yellow'); plot(xy(2,1),xy(2,2),'x','LineWidth',1,'Color','red'); %text(xy(1,1),xy(1,2),[' ' num2str(k)],'fontsize',10,'color',[1 1 1]); theta(k) = lines(k).theta; rho(k) = lines(k).rho; end theta_tolerance = 2; theta2 = abs(bsxfun(@minus, theta, theta')) &lt;= theta_tolerance; theta2(1:size(theta2,2)+1:numel(theta2)) = 0; % zero diagonal rho_tolerance = 1; rho2 = abs(bsxfun(@minus, rho, rho')) &lt;= rho_tolerance; rho2(1:size(rho2,2)+1:numel(rho2)) = 0; % zero diagonal rhotheta2 = sparse(rho2 &amp; theta2); [nc, C] = graphconncomp(rhotheta2); paired = ismember(C,find(hist(C,1:max(C))&gt;1)); % paired lines colors=get(gcf,'DefaultAxesColorOrder'); for line=find(paired) xy = [lines(line).point1; lines(line).point2]; plot(xy(:,1),xy(:,2),':','LineWidth',4,'Color',colors(C(line),:)); text(xy(1,1),xy(1,2),num2str(C(line)),'fontsize',20,'Color',colors(C(line),:)); end </code></pre> <p>It doesn't take care of overlapping. It is not clear how want to treat a case with 3 co-linear segments, where the first segment and the second segment intersect? Do you need to consider both the set containing the first and the third and the set containing the second and the third segment?</p>
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    1. POIdentify all co-linear line segments (in matlab)
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    1. COMany thanks for your very helpful response. It almost answers my question. Only one issue remains -- any idea how to prevent identification of overlapping regions? For example, in your image the segments labelled 6 and 2 are actually the same segment--there appears to be a small deformation that confuses the line segment identification. Is it possible to modify the segment identification that
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      1. USLee Sande
    2. COMy previous comment was incomplete. Many thanks for your very helpful response. Any idea how to prevent identification of overlapping regions? For example, in your image the segments labelled 6 and 2 are actually part of the same segment-- a small inflection confuses the houghline algorithm. Is it possible to modify the segment identification that will tolerate a bit of deformation? I am also not sure that even after identification of intersecting segments, how to decide which line to disregard? However, by visual inspection, it is clear that the there are only 3 pairs of line segments.
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