houghlines.m

包括进行标准Hough变换

function lines = houghlines(f,theta,rho,rr,cc,fillgap,minlength)
%HOUGHLINES Extract line segments based on the Hough transform. 
%   LINES = HOUGHLINES(F,THETA,RHO,RR,CC,FILLGAP,MINLENGTH) extract line
%   segments in the image F associated with particular bins in a Hough
%   transform.THETA and RHO are vectors returned by function HOUGH. Vectors
%   RR and CC specify the rows columns of the Hough transform bins tu use
%   in searching for line segments. If HOUGHLINES finds two line segments
%   associated with the same Hough transform bin that are separated by less
%   than FILLGAP pixels, HOUGHLINES merges them into a single line
%   segment.FILLGAP defaults to 20 if omitted.Merged line segments less
%   than MINLENGTH pixels long are discarded.MINLENGTH defaults to 40 if
%   omitted.
%
%   LINES is a structure array whose length equals the number of merged
%   line segments found. Each element of the structure array has these
%   fields:
%   
%   point1    End-point of the line segment;two-element vector
%   point2    End-point of the line segment;two-element vector
%   length    Distance between point1 and point2
%   theta     Angle (in degrees) of the Hough transform bin
%   rho       Rho-Axis position of the Hough transform bin
%   采取下面策略：
%   1、将像素位置旋转90－o，以便它们大概位于一条垂直线上。
%   2、按旋转的x值来对这些像素位置分排列。
%   3、使用函数diff找到裂口。忽视掉小裂口；这将合并被小空白分离的相邻线段。
%   4、返回比最小阈值长的线段的信息。

if nargin < 6
    fillgap = 20;
end
if nargin < 7;
    minlength = 40;
end

numlines = 0;lines=struct;
for k = 1:length(rr)
    rbin = rr(k);cbin = cc(k);
    %Get all pixels associated with Hough transform cell.
    [r,c] = houghpixels(f,theta,rho,rbin,cbin);
    if isempty(r)
        continue
    end
    
    %Rotate the pixel locations about (1,1) so that they lie approximately along a vertical line.
    omega = (90-theta(cbin))*pi/180;
    T = [cos(omega) sin(omega);-sin(omega) cos(omega)];
    xy =  [r-1 c-1]*T;
    x=sort(xy(:,1));
    
    % Find the gaps larger than the threshold.
    diff_x = [diff(x);Inf];
    idx = [0; find(diff_x>fillgap)];
    for p = 1:length(idx)-1;
        x1 = x(idx(p)+1);x2=x(idx(p+1));
        linelength = x2-x1;
        if linelength >= minlength
            point1 = [x1 rho(rbin)];point2 = [x2 rho(rbin)];
            %Rotate the end-point locations back to the original angle.
            Tinv = inv(T);
            point1 = point1*Tinv; point2 = point2*Tinv;
            numlines = numlines + 1;
            lines(numlines).point1 = point1+1;
            lines(numlines).point2 = point2+1;
            lines(numlines).length = linelength;
            lines(numlines).theta = theta(cbin);
            lines(numlines).rho = rho(rbin);
        end 
    end
end