# How can blur an image

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Jorge Ortiz on 16 Mar 2019
Edited: DGM on 28 Feb 2022
Write a function called blur that blurs the input image. The function is to be called like this:
output = blur(img,w);
where img, the input image is a two-dimensional matrix of grayscale pixel values between 0 and 255. Blurring is to be carried out by averaging the pixel values in the vicinity of every pixel. Specifically, the output pixel value is the mean of the pixels in a square submatrix of size 2w+1 where the given pixel sits in the center. For example, if w is 1, then we use a 3x3 matrix, that is, we average all the neighboring pixels of the given pixel and itself. Only use valid pixels when portions of the blurring matrix fall outside the image. For example, the blurred value corresponding to w = 1 at index (1,1) would be the mean of of elements (1,1), (1, 2), (2,1) and (2, 2). Both input img and output output are of type uint8.
Sana Hafeez on 5 Jun 2021
Thank you so much

Dipesh Poudel on 9 Apr 2020
function [output] = blur(A,w)
[row col] = size(A);
A=uint8(A);
B=nan(size(A) + (2*w));
B(w+1:end-w,w+1:end-w)=A;
output = 0*A;
for i=w+1:row+w
for j=w+1:col+w
tmp=B(i-w:i+w,j-w:j+w);
output(i-w,j-w)=mean(tmp(~isnan(tmp)));
end
end
output=uint8(output);
Image Analyst on 11 Feb 2021
Your MyBlur() function, which takes an RGB image needs to call imsplit() and call your blur(), which deals with monochrome images:
function blurredRGBImage = MyBlur(rgbImage, windowWidth)
% Split into separate color channels.
[redChannel, greenChannel, blueChannel] = imsplit(rgbImage);
% Blur each color channel independently.
blurredR = blur(redChannel, windowWidth);
blurredG = blur(greenChannel, windowWidth);
blurredB = blur(blueChannel, windowWidth);
% Recombine into a single RGB image.
blurredRGBImage = cat(3, blurredR, blurredG, blurredB);
Then call it:
blurredRGBImage = uint8(MyBlur(rgbImage, 9));
imshow(blurredRGBImage)

Yaksha SJ on 10 May 2020
function out = blur(img,w)
% convert to double for doing calculations
imgD = double(img);
[row, col] = size(img);
out = zeros(row, col);
for ii = 1:row
for jj = 1:col
% Get the indices for a submatrix
r1 = ii-w;
r2 = ii+w;
c1 = jj-w;
c2 = jj+w;
% Test that indices are valid
% If not, set to min/max that is valid
if r1 < 1
r1 = 1;
end
if r2 > row
r2 = row;
end
if c1 < 1
c1 = 1;
end
if c2 > col
c2 = col;
end
% Get the submatrix and assign the mean to the output pixel
m = imgD(r1:r2, c1:c2);
out(ii,jj) = mean(m(:));
end
end
% convert back to uint8
out = uint8(out);
end
##### 2 CommentsShowHide 1 older comment
Kartik agarwal on 14 May 2020
Edited: Kartik agarwal on 14 May 2020
Hey, if I produce this change in the last segment of your code, I am getting errors :
Original code :
% Get the submatrix and assign the mean to the output pixel
m = imgD(r1:r2, c1:c2);
out(ii,jj) = mean(m(:));
end
end
% convert back to uint8
out = uint8(out);
end
Changed code :
% Get the submatrix and assign the mean to the output pixel
m = imgD(r1:r2, c1:c2);
out(ii,jj) = uint8(mean(m(:))); % uint8 conversion done here itself%
end
end
end
The error says that Variable output must be of data type uint8. It is currently of type double.
I am new to Matlab. I can't understand the error in doing conversion the way I did in changed code. Please help

KSSV on 16 Mar 2019
Edited: KSSV on 16 Mar 2019
for i = 1:3
I(:,:,i) = uint8(conv2(I(:,:,i),ones(20)/20^2,'same'));
end
imshow(I) ;
Image Analyst on 23 Jun 2020
What code? Jorge's or KSSV's? Or one of the other multitude of places where code appears on this page?

Image Analyst on 16 Mar 2019
Can your function call the built-in functions conv2() or imfilter()?
If not, see, and adapt, my attached manual convolution program.
##### 2 CommentsShowHide 1 older comment
Image Analyst on 22 Mar 2019
Edited: Image Analyst on 4 May 2020
Well the solution you chose is not very robust, but it will work in some cases, not all cases.

Jaimin Motavar on 4 Jul 2019
can someone help me to figure out what is wrong in this code?
function output = blur(img,w)
B=double(img);
[m,n] = size(B);
k=2*w+1;
for i = 1:m
for j = 1:n
p=i-fix(k/2);
q=i+fix(k/2);
r=j-fix(k/2);
s=j+fix(k/2);
if p<1
p=1;
end
if q>m
q=m;
end
if r<1
r=1;
end
if s>n
s=n;
end
A=B([p:q],[r:s]);
B(i,j)=mean(A(:));
end
end
output=uint8(B);
end
Walter Roberson on 23 Jul 2019
You are overwriting your input data in B as you do the calculation, so your calculation is being based upon the already-smoothed pixels above and to the left instead of on the original pixels above and to the left.

Brandon McLaughlin on 22 Jul 2019
Help! I'm almost done with this course but for some reason I can't get this function to work. I have my code all set, but it keeps saying I'm doing it wrong. What am I doing wrong?
function output=blur(img,w)
img=double(img);
d=w*2+1
s=size(img)
output=[]
for r=1:d:s(1)
row=[];
for c=1:d:s(2)
if r+w<=s(1) && c+w<=s(2) && r-w>=1 && c-w>=1
tot=sum(sum(img((r-w):(r+w),(c-w):(c+w))));
val=tot/(d*d);
nxt=ones(d,d);
nxt(1:end,1:end)=val;
row=[row nxt];
elseif c+w<=s(2) && r-w>=1 && c-w>=1
tot=sum(sum(img((r-w):end,(c-w):(c+w))));
val=tot/(d*(s(1)-r+w));
nxt=ones((s(1)-r+w),d);
nxt(1:end,1:end)=val;
row=[row nxt];
elseif r+w<=s(1) && r-w>=1 && c-w>=1
tot=sum(sum(img((r-w):(r+w),(c-w):end)));
val=tot/(d*(s(2)-c+w));
nxt=ones(d,(s(2)-c+w));
nxt(1:end,1:end)=val;
row=[row nxt];
elseif r+w<=s(1) && c+w<=s(2) && r-w>=1
tot=sum(sum(img((r-w):(r+w),1:(c+w))));
val=tot/(d*(c+w));
nxt=ones(d,(c+w));
nxt(1:end,1:end)=val;
row=[row nxt];
elseif r+w<=s(1) && c+w<=s(2) && c-w>=1
tot=sum(sum(img(1:(r+w),(c-w):(c+w))));
val=tot/(d*(r+w));
nxt=ones((r+w),d);
nxt(1:end,1:end)=val;
row=[row nxt];
elseif r+w<=s(1) && c+w<=s(2)
tot=sum(sum(img(1:(r+w),1:(c+w))));
val=tot/((r+w)*(c+w));
nxt=ones((c+w),(r+w));
nxt(1:end,1:end)=val;
row=[row nxt];
elseif r-w>=1 && c-w>=1
tot=sum(sum(img((r-w):end,(c-w):end)));
val=tot/((s(1)-r+w)*(s(2)-c+w));
nxt=ones((s(1)-r+w),(s(2)-c+w));
nxt(1:end,1:end)=val;
row=[row nxt];
elseif r+w<=s(1) && c-w>=1
tot=sum(sum(img(1:(r+w),(c-w):end)));
val=tot/((r+w)*(s(2)-c+w));
nxt=ones((r+w),(s(2)-c+w));
nxt(1:end,1:end)=val;
row=[row nxt];
elseif c+w<=s(2) && r-w>=1
tot=sum(sum(img((r-w):end,1:(c+w))));
val=tot/((s(1)-r+w)*(c+w));
nxt=ones((s(1)-r+w),(c+w));
nxt(1:end,1:end)=val;
row=[row nxt];
end
end
output=[output;row];
end
row=[]
s2=size(output)
if s2(1)<s(1)
for c=1:d:s(2)
if c+w<=s(2) && c-w>=1
tot=sum(sum(img((s2(1)+1):s(1),(c-w):(c+w))));
val=tot/((s(1)-s2(1))*d);
nxt=ones((s(1)-s2(1)),d);
nxt(1:end,1:end)=val;
row=[row nxt];
elseif c+w<=s(2)
tot=sum(sum(img((s2(1)+1):s(1),1:(c+w))));
val=tot/((s(1)-s2(1))*(c+w));
nxt=ones((s(1)-s2(1)),(c+w));
nxt(1:end,1:end)=val;
row=[row nxt];
elseif c-w>=1
tot=sum(sum(img((s2(1)+1):s(1),(c-w):end)));
val=tot/((s(1)-s2(1))*(s2(2)-c+w));
nxt=ones((s(1)-s2(1)),(s2(2)-c+w));
nxt(1:end,1:end)=val;
row=[row nxt];
end
end
output=[output;row];
end
row=[]
col=[]
s3=size(output)
if s3(2)<s(2)
for r=1:d:s(1)
if r+w<=s(1) && r-w>=1
tot=sum(sum(img((r-w):(r+w),(s3(2)+1):s(2))));
val=tot/(d*(s(2)-s3(2)));
nxt=ones(d,(s(2)-s3(2)));
nxt(1:end,1:end)=val;
col=[col;nxt];
elseif r+w<=s(1)
tot=sum(sum(img(1:(r+w),(s3(2)+1):s(2))));
val=tot/((r+w)*(s(2)-s3(2)));
nxt=ones((r+w),(s(2)-s3(2)));
nxt(1:end,1:end)=val;
col=[col;nxt];
elseif r-w>=1
tot=sum(sum(img((r-w):(r+w),(s3(2)+1):s(2))));
val=tot/((s3(1)-r+w)*(s(2)-s3(2)));
nxt=ones((s3(1)-r+w),(s(2)-s3(2)));
nxt(1:end,1:end)=val;
col=[col;nxt];
end
end
s4=size(col)
if s4(1)<s3(1) || s4(1)<s(1)
tot=sum(sum(img((s2(1)+1):s(1),(s3(2)+1):s(2))));
val=tot/((s(1)-s2(1))*(s(2)-s3(2)));
nxt=ones((s(1)-s2(1)),(s(2)-s3(2)));
nxt(1:end,1:end)=val;
col=[col;nxt];
end
output=[output col];
end
output=uint8(output);
Image Analyst on 1 Aug 2019
Glad you got it figured out.

Image Analyst on 23 Jul 2019
See my manual convolution routine, attached.

shreyansh pitroda on 29 Apr 2020
%% can some one find what is error in this code
function [output] = blur(img,w)
img = uint8(img);
[a,b] = size(img);
output = zeros(a,b);
h = 2*w+1;
for i = 1:a-h
for j = 1:b-h
value = img(i:(h+i-1),j:(h+j-1));
output(i,j) = mean(value(:));
end
end
output = uint8(output);
end
Sana Hafeez on 5 Jun 2021
Thanks a lot!!

David Gonzalez on 25 May 2020
function output = blur(img,w)
mirror = -1*ones(size(img) + 2*w);
mirror(w+1:end-w,w+1:end-w) = img;
output = zeros(size(img));
for ii = w+1:size(mirror,1)-w
for jj = w+1:size(mirror,2)-w
submatrix = mirror(ii-w:ii+w,jj-w:jj+w);
sum_values = sum(submatrix(submatrix>=0));
valid = sum(submatrix(:)>=0);
output(ii-w,jj-w) = sum_values/valid;
end
end
output = uint8(output);

Divya Ratna on 25 May 2020
i figured it out by myself...
i know its too lengthy..
function out = blur (img, w)
temp = img;
s = size(img);
if s(1) > w && s(2) > w
%1
for i = 1 : w
for j = 1 : w
temp (i,j) = sum(sum(img(1:i+w , 1:j+w))') / ((i+w)*(j+w));
end
end
%2
for i = 1 : w
for j = w+1 : s(2)-w
temp (i,j) = sum(sum(img(1:i+w , j-w:j+w))') / ((i+w)*(w+w+1));
end
end
%3
for i = 1 : w
for j = s(2)-w+1 : s(2)
temp (i,j) = sum(sum(img(1:i+w , j-w:s(2)))') / ((i+w)*(s(2)-j+1+w));
end
end
%4
for i = w+1 : s(1)-w
for j = 1 : w
temp (i,j) = sum(sum(img(i-w:i+w , 1:j+w))') / ((w+w+1)*(j+w));
end
end
%5
for i = w+1 : s(1)-w
for j = w+1 : s(2)-w
temp (i,j) = sum(sum(img(i-w:i+w , j-w:j+w))') / ((w+w+1)*(w+w+1));
end
end
%6
for i = w+1 : s(1)-w
for j = s(2)-w+1 : s(2)
temp (i,j) = sum(sum(img(i-w:i+w , j-w:s(2)))') / ((w+w+1)*(s(2)-j+1+w));
end
end
%7
for i = s(1)-w+1 : s(1)
for j = 1 : w
temp (i,j) = sum(sum(img(i-w:s(1) , 1:j+w))') / ((s(1)-i+1+w)*(j+w));
end
end
%8
for i = s(1)-w+1 : s(1)
for j = w+1 : s(2)-w
temp (i,j) = sum(sum(img(i-w:s(1) , j-w:j+w))') / ((s(1)-i+1+w)*(w+w+1));
end
end
%9
for i = s(1)-w+1 : s(1)
for j = s(2)-w : s(2)
temp (i,j) = sum(sum(img(i-w:s(1) , j-w:s(2)))') / ((s(1)-i+1+w)*(s(2)-j+1+w));
end
end
end
out = temp;
end
para preethi on 22 Jan 2021
This is not working

Chandrashekhar Dhangar on 16 Jul 2020
%can someone pls help with this?
%Error=Variable output has an incorrect value.
%Tested with a 5x5 uint8 matrix with all 255-s in the first and last rows and columns and zeros everywhere else and w = 1
function output=blur(img,w)
[m, n]=size(img);
im=double(img);
S=zeros(2*w+1,2*w+1);
out=zeros(m,n);
for i=1:m
for j=1:n
for u=1:2*w+1
for v=1:2*w+1
if i-w>0 && j-w>0 && i-w+u-1<=m && j-w+v-1<=n
S(u,v)=im(i-w+u-1,j-w+v-1);
else
S(u,v)=0;
end
end
end
out(i,j)=sum(S(:))/(2*w+1)^2;
end
end
output=uint8(out);
end
Sana Hafeez on 2 Jun 2021

aniket GIRI on 18 Jul 2020
Edited: DGM on 28 Feb 2022
function out = blur(img,w)
% convert to double for doing calculations
imgD = double(img);
[row, col] = size(img);
out = zeros(row, col);
for ii = 1:row
for jj = 1:col
% Get the indices for a submatrix
r1 = ii-w;
r2 = ii+w;
c1 = jj-w;
c2 = jj+w;
% Test that indices are valid % If not, set to min/max that is valid
if 1 < 1
r1 = 1;
end
if r2 > row
r2 = row;
end
if c1 < 1
c1 = 1;
end
if c2 > col
c2 = col;
end
% Get the submatrix and assign the mean to the output pixel
m = imgD(r1:r2, c1:c2);
out(ii,jj) = mean(m(:));
end
end
% convert back to uint8
out = uint8(out);
end
Image Analyst on 23 Jul 2020
If row is 1, then row-w is zero or negative. You can't start at row 1 unless you check for the case where the window would be off the image, in which case you'd have to truncate the window. See my attached manual convolution.

Josh Yaksich on 13 Jun 2021
Edited: Josh Yaksich on 13 Jun 2021
This is what I did and it worked. Please don't copy it directly, as you will only be hurting yourself. Hopefully it helps when deciding what approach to take:
function output = blur(img,w)
%save dimensions of img
y = size(img,1);
x = size(img,2);
%make a duplicate img for modification
imgdup = img;
%create a larger matrix of all -1's that's the same size as image plus a border
bsquare = ones(y + 2 * w,x + 2 * w) * -1;
%copy img inside of larger matrix
bsquare((w + 1):end - w, w + 1:end - w) = img;
for i = 1:y
for j = 1:x
%make a small matrix for an individual pixel plus its surrounding pixels
pixelraw = bsquare([i:2 * w + i],[j:2 * w + j]);
%calculate the average value in the small matrix while ignoring -1
pixelavg = mean(pixelraw(pixelraw ~= -1));
%copy the average value to an individual pixel in the duplicate image
imgdup(i,j) = pixelavg;
end
end
%convert duplicate image to unint8 and assign to output
output = uint8(imgdup);

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