MATLAB: Canny Edge Detection

Question

The following program can run canny edge detection. But Some edge are missing. How fix this? Please help me if you can.

% program for edge detection 
clc; clear all; close all;
%read an image
[filename, pathname, filterindex] = uigetfile( ...
{  '*.jpg','JPEG (*.jpg)'; ...
   '*.bmp','Windows Bitmap (*.bmp)'; ...
   '*.fig','Figures (*.fig)'; ...
   '*.*',  'All Files (*.*)'}, ...
   'Choose image(s) to be processed', ...
   'MultiSelect', 'off');
if filterindex==0, break;end
filename=cellstr(filename);
y= imread(horzcat(pathname,char(filename)));
% y=imread('car3.jpg');
mm=input('Input your threshold between 0 and 255    ');
% convert color image to gray scale 
x= rgb2gray(y);
imshow(x);
[r,c]=size(x);
max=0;
min=0;
for j=2:r-1
    for i=2:c-1
        p=i-1;
        q=i+1;
        a=j-1;
        b=j+1;
        d1=abs(x(j,i)-x(j,p));
        d2=x(j,i)-x(j,q);
        e1=x(j,i)-x(a,i);
        e2=x(j,i)-x(b,i);
        f1=x(j,i)-x(a,p);
        f2=x(j,i)-x(b,q);
        g1=x(j,i)-x(a,q);
        g2=x(j,i)-x(b,p);
        if (d1 < 0)
            d1=d1*(-1);
        end
        if (d2 < 0)
            d2=d2*(-1);
        end
        if (e1 < 0)
            e1=e1*(-1);
        end
        if (e2 < 0)
            e2=e2*(-1);
        end
          if (f1 < 0)
              f1=f1*(-1);
          end
          if (f2 < 0)
              f2=f2*(-1);
          end
          if (g1 < 0)
              g1=g1*(-1);
          end
          if (g2 < 0)
              g2=g2*(-1);
          end
          if (d1 >= mm) && (d2 >= mm)
              x(j,i)=255;
          elseif (e1 >= mm) && (e2 >= mm)
              x(j,i)=255;
          elseif (f1 >= mm) && (f2 >= mm)
              x(j,i)=255;
          elseif (g1 >= mm) && (g2 >= mm)
              x(j,i)=255;
          else
              x(j,i)=0;  
          end
      end
  end
  figure
  imshow(x);
BW = im2bw(x, 0.5);
figure
imshow(BW)

Answer 1

Yeah, that can happen. Try some edge-linking processes like those listed here:

http://www.csse.uwa.edu.au/~pk/Research/MatlabFns/#edgelink