How to implement convolution instead of the built-in imfilter

m=13; n=13;

 sigma=15;

 [h1 h2]=meshgrid(-(m-1)/2:(m-1)/2, -(n-1)/2:(n-1)/2);

 hg= exp(-(h1.^2+h2.^2)/(2*sigma^2));            %Gaussian function 
 h=hg ./sum(hg(:));

Now I want to use these Gaussian kernels to implement linear filtering with on image

    OriginalRGB = imread('LeerNaam.png');                     % read image

filter=h; s = size(OriginalRGB);

r = zeros(s);

for i = 2:s(1)-1

    for j = 2:s(2)-1

        temp = OriginalRGB(i-1:i+1,j-1:j+1)) .* filter;

        r(i,j) = sum(temp(:));
    end
end

[Valeska Pearson]

I need to know how to fix the temp = .... the .* doesn't work

[Chad Gilbert]

Are you happy to use conv2? Or are you avoiding it along with imfilter?

[Image Analyst]

Don't do that - it's not linear filtering, that's masking. See my demo below.

[Ferdie]

Thanks... The reason for not implementing imfilter is because I need to know what imfilter does. It is for a project and I can't just use built-in functions.

Answer 1

[Image Analyst]

Try this demo:

clc;    % Clear the command window.
clearvars;  % Erase all existing variables.
workspace;  % Make sure the workspace panel is showing.
format longg;
format compact;
fontSize = 20;
close all;

% Check that user has the Image Processing Toolbox installed.
hasIPT = license('test', 'image_toolbox');
if ~hasIPT
  % User does not have the toolbox installed.
  message = sprintf('Sorry, but you do not seem to have the Image Processing Toolbox.\nDo you want to try to continue anyway?');
  reply = questdlg(message, 'Toolbox missing', 'Yes', 'No', 'Yes');
  if strcmpi(reply, 'No')
    % User said No, so exit.
    return;
  end
end

% Read in a standard MATLAB color demo image.
folder = fullfile(matlabroot, '\toolbox\images\imdemos');
baseFileName = 'peppers.png';
% Get the full filename, with path prepended.
fullFileName = fullfile(folder, baseFileName);
if ~exist(fullFileName, 'file')
  % Didn't find it there.  Check the search path for it.
  fullFileName = baseFileName; % No path this time.
  if ~exist(fullFileName, 'file')
    % Still didn't find it.  Alert user.
    errorMessage = sprintf('Error: %s does not exist.', fullFileName);
    uiwait(warndlg(errorMessage));
    return;
  end
end
rgbImage = imread(fullFileName);
% Get the dimensions of the image.  numberOfColorBands should be = 3.
[rows, columns, numberOfColorBands] = size(rgbImage);
% Display the original color image.
subplot(2, 1, 1);
imshow(rgbImage);
title('Original Color Image', 'FontSize', fontSize);
% Enlarge figure to full screen.
set(gcf, 'units','normalized','outerposition',[0 0 1 1]);

% Extract the individual red, green, and blue color channels.
redChannel = rgbImage(:, :, 1);
greenChannel = rgbImage(:, :, 2);
blueChannel = rgbImage(:, :, 3);

% Construct Gaussian Kernel
m=13; 
n=13;
sigma=15;
[h1 h2]=meshgrid(-(m-1)/2:(m-1)/2, -(n-1)/2:(n-1)/2);
hg= exp(-(h1.^2+h2.^2)/(2*sigma^2));            %Gaussian function
h=hg ./sum(hg(:))
% Could be done easier with fspecial though!

% Convolve the three separate color channels.
redBlurred = conv2(redChannel, h);
greenBlurred = conv2(greenChannel, h);
blueBlurred = conv2(blueChannel, h);

% Recombine separate color channels into a single, true color RGB image.
rgbImage2 = cat(3, uint8(redBlurred), uint8(greenBlurred), uint8(blueBlurred));
% Display the blurred color image.
subplot(2, 1, 2);
imshow(rgbImage2);
title('Blurred Color Image', 'FontSize', fontSize);

[Ferdie]

Thank you very much! But I also need to do the convolution without using the conv2 built in function. I tried for loops, but I have problem when doing .* multiplication. Matlab gives errors. How can I code conv2?