How to overlay an image over a background in matlab

Question

I have an image in which certain pixels are black. And I also have a background image over which I want to overlay my image. But I want the background to appear only where the pixels are black in the main image. Thanks for any help.

Answer 1

If your images are the same size, this is pretty easy. Suppose you have two images in grayscale, mainImage and backImage. Then you can set the mainImage pixels that are black to backImage values by writing:

mainImage(mainImage == 0) = backImage(mainImage == 0);

and this will set the mainImage black pixels backImage.

If the images are in color then you have to be a bit more careful. The easiest, most understandable way to do this in my mind is to find the zero values in each color plane and then set the mainImage where they are all zero to the backImage. I made a little messy very that converts to linear space and sets the pixels as follows:

[Ny, Nx, Nz] = size(mainImage);
dSize = Ny*Nx;

% Get the location of the black pixels:
bR = mainImage(:,:,1) == 0;
bG = mainImage(:,:,2) == 0;
bB = mainImage(:,:,3) == 0;

blackPixels = find(bR & bG & bB);

mainImage(blackPixels) = backImage(blackPixels);
mainImage(blackPixels + dSize) = backImage(blackPixels+dSize);
mainImage(blackPixels + 2*dSize) = backImage(blackPixels+dSize*2);