Superimposing Multiple Images + Adding Colormap - matlab

Question

I'm an undergrad student working in a cell biology lab with a basic background in matlab. I'm working on a project of tracking cell trajectory (time lapse) on a petri dish. Below are two example images that i used the watershed feature to separate from the background. The original pictures had neon green cells, now this is all in black and white/
Let's say i have 20 pictures like this, how might I superimpose one on top of another so they all of equal transparency?
Then, how can i add a colormap that represents time? (The bottom most picture is one end of the colormap and the most recent picture is the opposite end) <- this is extremely challenging as it often things the background is black and not NaN

Answer 1

The Basic Idea
Probably the easiest way to do this, is to take the binary image for each layer, and multiply the image by the time at which it was acquired (or it's index in time). Then you can concatenate all images along the third dimension (using cat). You can compute the maximum value along the third dimension using max. This will make the newer time points appear to be "on top" of the older time points. You can then display the resulting flattened matrix using imagesc and it will automatically map to the colormap for the current figure. Typically we would refer to this as a maximum intensity projection.
Creating Some Data
First since you've only provided two images, I'm going to create some shifted versions of the first image you've provided for the demonstration.
% Create some pseudo-data in a cell array that represents the image over time
im = imread('http://i.imgur.com/xTurvfO.jpg');
im = im(:,:,1);
ims = cell(1, 5);
% Create some shifted versions of im1
shifts = round(linspace(0,1000,5));
for k = 1:numel(shifts)
ims{k} = circshift(im > 100, shifts([k k]));
end
Implementing the Method
Now for the application of the method I discussed
% For each image, multiply the binary mask by the time
for k = 1:numel(ims)
ims{k} = ims{k} * k;
end
% Concatenate all images along the third dimension
IMS = cat(3, ims{:});
% Flatten by taking the maximum value along the third dimension
MIP = max(IMS, [], 3);
% Display the resulting flattened image using imagesc
imagesc(MIP);
% Create a custom colormap with black at the end to create our black background
colormap(cat(1, [0 0 0], parula))
The Result

Answer 2

I have used imfuse to create composite images, which is similar to combining multiple channels on a fluorescent microscope. The Mathworks documentation is http://www.mathworks.com/help/images/ref/imfuse.html.
The tricky part is choosing the vector for color channels. For example, [2,1,2] means choosing B(lue) for image 1, R(ed) and G(reen) for image 2. [2,1,2] is the scheme recommended for colorblind people and gives figure on the left of this image. Using [1,0,2] for red/blue gives the the figure on the right.
fig1 = imread([basepath filesep 'fig.jpg']); %white --> black
fig2 = imread([basepath filesep 'fig2.jpg']);
fig_overlay = imfuse(fig1, fig2,'falsecolor','Scaling','joint', 'ColorChannels', [1,0,2]);
imshow(fig_overlay)