mortar block boundary detection

How can i determine this mortar block boundary more precisely??? i have tried canny edge detection and ROI using but i couldn't get a precise result. here is main portion of my code-

clc;`` close all; clear all; a=imread('bef90_c1w5-1_crop.jpg'); a=rgb2gray(a); a=histeq(a); a=imresize(a,[512 512]); figure(45) imshow(a); figure(40) imhist(a); b=im2bw(a); figure(2) imshow(b); k=medfilt2(b,[3 3]); k=imcomplement(k); figure(3) imshow(k); % k=imcomplement(k); se=strel('disk',3); s4=imdilate(k,se); figure(4) imshow(s4); s1=strel('disk',2); d=imerode(s4,s1); d=imfill(d,'holes'); figure(5) imshow(d); % j=imsubtract(s,d); % figure(6) % imshow(j); n = edge(d,'canny'); figure(6) imshow(n);