MATLAB Answers

2D Convolution - Sobel Filter. What is wrong?

612 views (last 30 days)
Nisreen Sulayman
Nisreen Sulayman on 11 Sep 2019
Commented: Nisreen Sulayman on 20 Sep 2019
img = imread('Davis_Hall.jpg');
% 'Davis_Hall.jpg':color image
img =double(rgb2gray(img));
Gx = double([-1 0 1;-2 0 2;-1 0 1]);
Gx=rot90(Gx,2);
Gy = double([-1 -2 -1; 0 0 0; 1 2 1]);
Gy=rot90(Gy,2);
I= img;
[r,c]=size(I);
Fx = zeros(r,c);
Fy = zeros(r,c);
I = padarray(I,[1 1]);
for i=2:r-1
for j=2:c-1
Fx(i,j)=sum(sum(Gx.*I(i-1:i+1,j-1:j+1)));
Fy(i,j)=sum(sum(Gy.*I(i-1:i+1,j-1:j+1)));
end
end
img=uint8(img);
FMag=sqrt(Fx.^2+Fy.^2);
figure(1)
imshow(img);
title('Original Image');
figure(2)
imshow((abs(Fx))./max(max(Fx)));
title('Gradient in X direction');
figure(3)
imshow(abs(Fy)./max(max(Fy)));
title('Gradient in Y direction');
figure(4)
imshow(FMag./max(max(FMag)));
title('Gradient Magnitude');
% IT IS Not Allowed to use: imfilter, conv2, filter2, conv

  0 Comments

Sign in to comment.

Answers (2)

David Wilson
David Wilson on 12 Sep 2019
Here's my (old) code for a sobel filter:
img = imread('Davis_Hall.jpg');
% 'Davis_Hall.jpg':color image
X =double(rgb2gray(img));
%% Start
Bx = [-1,0,1;-2,0,2;-1,0,1]; % Sobel Gx kernel
By = Bx'; % gradient Gy
Yx = filter2(Bx,X); % convolve in 2d
Yy = filter2(By,X);
G = sqrt(Yy.^2 + Yx.^2); % Find magnitude
Gmin = min(min(G)); dx = max(max(G)) - Gmin; % find range
G = floor((G-Gmin)/dx*255); % normalise from 0 to 255
image(G); axis('image')
colormap gray
Gives the following: DavisH.png

  3 Comments

Nisreen Sulayman
Nisreen Sulayman on 12 Sep 2019
IT IS Not Allowed to use: imfilter, conv2, filter2, conv
Nisreen Sulayman
Nisreen Sulayman on 12 Sep 2019
It is a graded excercise\online course.

Sign in to comment.


Nisreen Sulayman
Nisreen Sulayman on 12 Sep 2019
Here is my results:
untitled.jpg

  10 Comments

Show 7 older comments
Bruno Luong
Bruno Luong on 12 Sep 2019
The second error I spot is the way you deal with padding and boundary
IMO the loop and padding should be (not tested)
[r,c]=size(I);
Fx = zeros(r,c);
Fy = zeros(r,c);
I = padarray(I,[1 1],'replicate'); % lass abrute than 0
for i=1:r
for j=1:c
Fx(i,j)=sum(sum(Gx.*I(i:i+2,j:j+2)));
Fy(i,j)=sum(sum(Gy.*I(i:i+2,j:j+2)));
end
end
Nisreen Sulayman
Nisreen Sulayman on 13 Sep 2019
Didn't work!!
Maybe there is something wrong related to the "greader"
We have a good results ... still didn't accept the answer!! (even with first code I have a good results!!)
OR THERE is that kind of silly bug which I couldn't spot.
Nisreen Sulayman
Nisreen Sulayman on 20 Sep 2019
How to normalize a convolved image?
I have got these messages after running
%read the image
img = imread('Davis_Hall.jpg');
I =double(rgb2gray(img));
%Gx = double([-1 0 1;-2 0 2;-1 0 1]);
Gx=[-1 0 1;-2 0 2;-1 0 1];
Gx=rot90(Gx,2);
%Gy = double([-1 -2 -1; 0 0 0; 1 2 1]);
Gy=[-1 -2 -1; 0 0 0;1 2 1];
Gy=rot90(Gy,2);
[r,c]=size(I);
Fx = zeros(r,c);
Fy = zeros(r,c);
FMag=zeros(r,c);
I = padarray(I,[1 1],0,'both');
for i=2:r-1
for j=2:c-1
Fx(i,j)=sum(sum(Gx.*I(i-1:i+1,j-1:j+1)));
Fy(i,j)=sum(sum(Gy.*I(i-1:i+1,j-1:j+1)));
FMag(i,j)=sqrt(power(Fx(i,j),2)+power(Fy(i,j),2));
end
end
Fx=Fx(2:r-1,2:c-1);
Fy=Fy(2:r-1,2:c-1);
FMag=FMag(2:r-1,2:c-1);
img=uint8(img);
figure(1)
imshow(img);
title('Original Image');
figure(2)
imshow((abs(Fx))./max(max(Fx)));
title('Gradient in X direction');
figure(3)
imshow(abs(Fy)./max(max(Fy)));
title('Gradient in Y direction');
figure(4)
imshow(FMag./max(max(FMag)));
title('Gradient Magnitude');
%Fx may have negative values and values which are greater than 255, hence normalize before visualiz
%Fy may have negative values and values which are greater than 255, hence normalize before visualizin
%FMag may have values which are greater than 255, hence normalize before visualizing

Sign in to comment.

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!