How do I superimpose images in MATLAB?

% Create the background
%   This example uses a blend of colors from left to right, converted to a TrueColor image
%   Use repmat to replicate the pattern in the matrix
%   Use the "jet" colormap to specify the color space
bg = ind2rgb(repmat(1:64,64,1),jet(64));

% Create an image and its corresponding transparency data
%   This example uses a random set of pixels to create a TrueColor image
im = rand(100,100,3);
% Make the image fade in from left to right by designing its alphadata
%   Use repmat to replicate the pattern in the transparency fading
imAlphaData = repmat(0:1/size(im,2):1-1/size(im,2),size(im,1),1);

% Display the images created in subplots
hf = figure('units','normalized','position',[.2 .2 .6 .6]);
ax1 = subplot(2,3,1);
ibg = image(bg);
axis off
title('Background')
ax2 = subplot(2,3,4);
iim = image(im);
axis off
title('Image without transparency yet')

%   Now set up axes that overlay the background with the image
%   Notice how the image is resized from specifying the spatial 
%   coordinates to locate it in the axes.
ax3 = subplot(2,3,[2:3, 5:6]);
ibg2 = image(bg);
axis off
hold on
%   Overlay the image, and set the transparency previously calculated
iim2 = image(im,'XData',[30 50],'YData',[10 30]);
set(iim2,'AlphaData',imAlphaData);
title(sprintf('Using transparency while overlaying images:\nresult is multiple image objects.'))

If you want to extract a circle from the image, and overlay just that (NOTE: This part of the example requires the Image Processing Toolbox function ROIPOLY to create the circle mask.):

%   Add another image to the background, but use the 'AlphaData' property
%   to just overlay a circle instead.  
%
%   First define the polygon region of interest
t = 0:.1:2*pi;
[imheight,imwidth] = size(im(:,:,1));
x = round(20*cos(t)+round(imwidth/2));
y = round(20*sin(t)+round(imheight/2));
%   Extract the polygon mask
imCircleAlphaData = roipoly(im,x,y);
axes(ax3);
%   Overlay the image, and set the transparency previously calculated
iim3 = image(im,'XData',[0 30],'YData',[30 60]);
set(iim3,'AlphaData',double(imCircleAlphaData));

2. Using indexing to superimpose images: Use this method if you have an image that you want to place in a scene, but prefer to have only a single image as a resultant. The following example achieves this by replacing the scene pixels with the object image pixels.

% Create the scene
%   This example uses a random set of pixels to create a TrueColor image
scene = rand(100,100,3);

% Create the object image
%   This example uses a blend of colors from left to right, converted to a TrueColor image
%   Use repmat to replicate the pattern in the matrix
%   Use the "jet" colormap to specify the color space
obj = ind2rgb(repmat(1:64,64,1),jet(64));

% Display the images created in subplots
hf2 = figure('units','normalized','position',[.2 .2 .6 .6]);
axi1 = subplot(2,3,1);
iscene = image(scene);
axis off
title('Scene')
axi2 = subplot(2,3,4);
iobj = image(obj);
axis off
title('Object image')

% Now replace pixels in the scene with the object image
result = scene;
%   Define where you want to place the object image in the scene
rowshift = 20;
colshift = 0;
%   Perform the actual indexing to replace the scene's pixels with the object
result((1:size(obj,1))+rowshift, (1:size(obj,2))+colshift, :) = obj;
%   Display the results
ax3 = subplot(2,3,[2:3, 5:6]);
iresult = image(result);
axis off
hold on
title(sprintf('Using indexing to overlay images:\nresult is one image object'))

If you want to extract a circle from the object, and overlay just that: (AGAIN, NOTE: This part of the example requires the Image Processing Toolbox function ROIPOLY to create the circle mask.):

%   Let's add another image to the scene.  
%   First define the polygon region of interest
t = 0:.1:2*pi;
[objheight,objwidth] = size(obj(:,:,1));
x = round(20*cos(t)+round(objwidth/2));
y = round(20*sin(t)+round(objheight/2));
%   Since the mask is 2D, repeat it through the 3D RGB matrix
%   while extracting the polygon mask
objCircleMask = repmat(roipoly(obj,x,y),[1,1,3]);
%   Define where we want to place the circle object in the scene
rowshift = 0;
colshift = 30;
result2 = result;
%   Perform the actual indexing.  First calculate the replacement pixels
%   by multiplying the scene by the negative of the mask, and then adding 
%   the object multiplied by the mask.  Multiplying the mask chooses all 
%   pixels corresponding to a 1 in the mask, and removes color from the 
%   pixels corresponding to a 0.
replacementPixels = ( result((1:size(obj,1))+rowshift, (1:size(obj,2))+colshift, :) ...
     .* double(~objCircleMask) ) + ( obj .* double(objCircleMask) );
%   Now replace the pixels with the ones we just calculated
result2((1:size(obj,1))+rowshift, (1:size(obj,2))+colshift, :) = replacementPixels;
%   Display the results.  Notice that we placed the circle object in the scene
%   that resulted from the previous object placement.
axes(ax3);
delete(iresult)
iresult2 = image(result2);