Changing color of pixels around certain pixels

So I have an image named "0001.tif". I managed to import the image and do basic manupulations and save it.

What I am strugeling with is that when a pixel (that satisfies a certain criteria) is detected, the pixels in a 5 pixel radius should become white. In other words, these surrounding pixels' RGB values should change to 255.

Note: There can be more than one set of white pixels in the original image.

    img = imread(append('0001.tif'));
 
    R = img(:,:,1);
    G = img(:,:,2);
    B = img(:,:,3); 
    
    RGB = sqrt(im2double(R).^2+im2double(G).^2+im2double(B).^2);
    
    idx = RGB<0.3;

    R(idx) = 255;
    G(idx) = 255;
    B(idx) = 255;
    
    imgNew(:,:,1) = R;
    imgNew(:,:,2) = G;
    imgNew(:,:,3) = B;
    
    imwrite(imgNew,append('0001Processed.tif'));

Is there any way to acomplish this while using vectorization and without having to run through a nested for loop?

Answer 1

[Steve Eddins]

Suppose mask is a logical matrix that indicates which pixels satisfy your condition. (That seems to be the purpose of idx in your code.) Then dilate mask using a circular structuring element with radius 5. Use the dilated mask to set the red, green, and blue component values to 255. It might look something like this:

se = strel('disk',5,0);
dilated_mask = imdilate(mask,se);
R(dilated_mask) = 255;
G(dilated_mask) = 255;
B(dilated_mask) = 255;
imgNew = cat(3,R,G,B);

Note that strel and imdilate are in Image Processing Toolbox.

[Image Analyst]

@Edrich Engelbrecht Note that instead of

R = img(:,:,1);
G = img(:,:,2);
B = img(:,:,3); 

you can do

[R, G, B] = imsplit(img);

and instead of

imgNew(:,:,1) = R;
imgNew(:,:,2) = G;
imgNew(:,:,3) = B;

you can do

imgNew = cat(3, R, G, B):