If the 3D image is a color image, you can take individual color channels like this:
% Extract the individual red, green, and blue color channels.
redChannel = rgbImage(:, :, 1);
greenChannel = rgbImage(:, :, 2);
blueChannel = rgbImage(:, :, 3);
Or you could use a weighted average of the red, green, and blue channels like this:
grayImage = rgb2gray(rgbImage);
Or if you have a volumetric image (like a CT or MRI image or other image distributed in 3 spatial dimensions) you can use the mean (or max or min) like this
xyImage = mean(volumeImage, 3);
xzImage = mean(volumeImage, 2);
yzImage = mean(volumeImage, 1);
They will get you the average of the 3D volumetric image projected along one of the dimensions.
