Local thresholding in MATLAB - matlab

Question

I am trying to implement local thresholding in MATLAB 7.7. This is what my original image looks like:
As seen the the word Test is covered in black. This image is a PNG image having dimensions 919x551. I want to apply local thresholding to this image so that I can get the word Test to be visible clearly.
I have implemented the following code that works by dividing the entire image into sub images of 60*60 blocks.
However, when I am doing so, I am not getting the desired output.
My code:
clc;
clear all;
close all;
im = imread('C:\samples\test100.png');
subplot(3,3,1);
imshow(im);
title('original image');
im = rgb2gray(im);
im = double(im);
subplot(3,3,2);
imshow(im);
title('gray scale image');
[row col] = size(im);
max_im = max(max(im));
h = zeros(1,max_im+1);
!1st block
for n = 1:1:60
for m = 1:1:60
a(n,m) = im(n,m);
end
end
a = a+1;
for n = 1:1:60
for m = 1:1:60
t = a(n,m);
h(t) = h(t)+1;
end
end
subplot(3,3,3);
bar(h)
[X,Y] = ginput(1);
for n = 1:1:60
for m = 1:1:60
if a(n,m)<X
a(n,m) = 0;
else
a(n,m) = 255;
end
end
end
subplot(3,3,4);
imshow(uint8(a))
title('1st block image');
!2nd block
for n = 1:1:60
for m = 61:1:60
b(n,m-60) = im(n,m)
end
end
b = b+1;
for n = 1:1:60
for m = 1:1:60
t = b(n,m);
h(t) = h(t)+1;
end
end
figure(2)
bar(h)
[X,Y] = ginput(1);
for n = 1:1:60
if b(n,m)<X
b(n,m) = 0;
else
b(n,m) = 255;
end
end
imshow(uint8(b))
!3rd block
for n = 61:1:120
for m = 1:1:60
c(n-60,m) = im(n,m);
end
end
c = c+1;
for n = 1:1:60
for m = 1:1:60
t = c(n,m);
h(t) = h(t)+1;
end
end
figure(3)
bar(h)
[X,Y] = ginput(1);
for n = 1:1:60
for m = 1:1:60
if c(n,m)< X
c(n,m) = 0;
else
c(n,m) = 255;
end
end
end
imshow(uint8(c))
!final block
for n = 1:1:row
for m = 61:1:col
d(n-60,m-60) = im(n,m);
end
end
d = d+1;
for n = 1:1:60
for m = 1:1:60
t = d(n,m);
h(t) = h(t)+1;
end
end
figure(4);
bar(h);
[X,Y] = ginput(1);
for n = 1:1:60
for m = 1:1:60
if d(n,m)<X
d(n,m) = 0;
else
d(n,m) = 255;
end
end
end
imshow(uint8(d))
s = [a b;c d];
figure(5);
imshow(uint(s))
When I try to run the entire code I get an error as:
??? Undefined function or method 'local' for input arguments of type 'char'
However, when I run only the code for the 1st block I get the following output.
How will I get the word Test visible by creating sub-images and then merging them together?

Answer 1

You can scan the greyscale image horizontally and then find the position of non-zero (or above the threshold) values and set that interval to be filled with white (256 or 1 if using im2double).
for j=1:551
row = im(:,j)
test = 0;
im2=zeros(size(im))
i=0;
%Left black area
while (test == 0 && i<919)
im2(i,j)=0;
if row(i)>threshold
test=1;
end;
i=i+1;
end;
%White inner area
while (test == 1 && i<919)
im2(i,j)=1
if row(i)>threshold
test=0;
end;
i=i+1;
end;
%Left black area
while (i<919)
im2(i,j)=0;
i=i+1;
end;
This doesn't work with letters that have an empty area (such as 'p'), but you can modify the code a little to do that.