Only add to empty elements in an array

Is there a quick way to add an array to another one of equal size, BUT only add if the particular element in the first array is 0?

For example, I start with Original = zeros(3)

I want to add 3 arrays to it. lets say

a = [1 2 3; 0 0 0; 0 0 0]

b = [0 0 0; 4 5 6; 0 0 0]

c = [0 0 0; 1 1 1; 7 8 9]

If I just add a, b, and c to Original I will get [1 2 3; 5 6 7; 7 8 9] but I want the second row of c to NOT add because that row of Original already has values in it.

So it should give Original = [1 2 3; 4 5 6; 7 8 9]

I am looking to use this with 3D arrays, and preferably without for looping through each element.

Answer 1

[Stephen23]

>> d = cat(3,a,b,c);
>> sum(d.*(cumsum(d~=0,3)==1),3)
ans =
     1     2     3
     4     5     6
     7     8     9

[Michael Luo]

I really appreciate the quick answers, I was just wondering if this will work with 3D arrays?

And if not, can I just instead cat on the 4th dimension, and if so what would I need to change?

Thanks so much

[Michael Luo]

Ok so I just tested it and it does technically work if I concatenate in 4th dimension, however not as expected.

So what I'm trying to do is add layers of pixels to an image through a loop, each time adding a bit. To prevent white specks (adding a pixel twice) it must only add when the pixel is fully black (RGB = 0 0 0).

When I run using your code, I think some pixels are not added despite it being black. The resulting image still contains large chunks of black pixels.

Thanks again for the replies

[Michael Luo]

Turns out all I needed to do was tweak your code a tiny bit and it worked. Thanks!!!

Hope you have a good day