Replace multiple values in matrix / image

Hey there,

I have two matrices. One is an image, the other one is as big as the image with only one dimension. The second matrix indicates where I want to replace pixels in the image by a certain colour. For now I have replaced the pixels using loops. I wanted to get rid of those loops but still have no idea on how to do so.

My code so far looks like this:

(the mask contains values to indicate which pixels to replace. Contains different values for multiple replacements; toReplace contains the values in the mask, that should be replaced in the image)

for c=1:length(toReplace)
    [Y, X] = find(mask == toReplace(c));
    for p=1:size(Y,1)
        img(Y(p),X(p),:) = color;    % color is a 1x3 vertor
    end
end

My closest try so far (At least 1 loop less):

o = ones(size(mask));
r = img(:,:,1);
g = img(:,:,2);
b = img(:,:,3);

for c=1:length(indicator)
    r(mask==toReplace(c)) = o(mask==toReplace(c)).*color(1);
    g(mask==toReplace(c)) = o(mask==toReplace(c)).*color(2);
    b(mask==toReplace(c)) = o(mask==toReplace(c)).*color(3);
end

img(:,:,1) = r;
img(:,:,2) = g;
img(:,:,3) = b;

No idea if it will bring any performance improvements in the end :-)

Thanks in advance,

Patrick

[Neuropragmatist]

I think I need a little more info on your code.

From what I understand you have an MxNx3 RBG image (img) and you have an MxNx1 logical matrix that defines pixels you want to replace (mask).

But you also have another matrix that has colours you want to replace in img called toReplace? What size is that matrix?

In the first piece of code you have posted where does the variable 'color' come from? And why are you testing where mask == toReplace(c)?

[Patrick Rausenberger]

The toReplace matrix contains the IDs that should be replaced. These IDs are given to each pixel by the mask matrix. mask is not like a 0/1 mask but more like a multi times mask containg multiple IDs for different mask outs. In my case it is a labeling of the image.

Color is just a variable given to the overall function and is a 1x3 matrix as written in the code. It only defines the colour that should be given to the pixels.

By mask == toReplace(c) I get the indeces of pixels I want to replace.

The solution by Rik looks good, thanks for your effort anyways.

Answer 1

[Rik]

You can use ismember to remove a loop. I put in a loop over the colors, but since that is only 3 iterations, that will not cause any issues.

im=rand(10,12,3);
mask=randi(10,size(im,1),size(im,2));
toReplace=[1 3];
color=[0 1 0];

im_out=im;
L=ismember(mask,toReplace);
[x,y]=find(L);n=numel(x);
for color_channel=1:3
    ind=sub2ind(size(im),x,y,color_channel*ones(n,1));
    im_out(ind)=color(color_channel);
end

figure(1),clf(1)
subplot(1,2,1)
imshow(im),title('original')
subplot(1,2,2)
imshow(im_out),title('processed')

[Patrick Rausenberger]

Thanks, ismember seems to do the trick ;-)