# slicing an M-by-N-by-3 matrix using an M-by-N matrix of logical values

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Luca Amerio on 11 Apr 2018
Commented: Walter Roberson on 12 Apr 2018
Hi everybody
I have an RGB image as a MxNx3 matrix.
I would like to slice it using an MxN matrix of logic values (one for each pixel) and add a 1x1x3 value to those pixels. I'm trying to a clean way to do it.
In other words, I'm trying to do something like this
A = rand(200,200,3);
i = rand(200,200) > 0.5;
C = cat(3,0.3,0.4,0.2);
A(i) = 0.5*A(i,:) + 0.5*C; %doesn't work!
The only ways I found is either "unpacking" each MxN submatrix to a new variable like this
R = A(:,:,1);
G = A(:,:,2);
B = A(:,:,3);
R(i) = 0.5*R(i) + 0.5*C(1);
G(i) = 0.5*G(i) + 0.5*C(2);
B(i) = 0.5*B(i) + 0.5*C(3);
Anyone got a better idea?

Walter Roberson on 12 Apr 2018
A = A + repmat(i, [1, 1, 3]) .* repmat(C, [size(A,1), size(A,2), 1]);
Since R2016b this can be expressed as
A = A + i .* C;

Luca Amerio on 12 Apr 2018
The first one is nice, but the second one is great!
Nice one, thank you!
Walter Roberson on 12 Apr 2018
dpb's Answer takes into account the selective 1/2 where mine does not. The fix would be
A = A + (C - A)/2 *.i;

dpb on 12 Apr 2018
A=0.5*(A.*i+C.*i);

#### 1 Comment

Walter Roberson on 12 Apr 2018
Note that this requires R2016b or later due to differences in the sizes of the objects.