how to embed a watermark inside roi in matlab - matlab

Question

the region of interest in the image are calculated. Now how to embed the watermark into the roi..i ve put the code for embedding it in the whole image in lsb. How can it modified for roi alone?
clear all;
file_name='pout.tif';
cover_object=imread(file_name);
file_name='cameraman.tif';
message=imread(file_name);
message=double(message);
message=round(message./256);
message=uint8(message);
Mc=size(cover_object,1);
Nc=size(cover_object,2);
Mm=size(message,1);
Nm=size(message,2);
for ii = 1:Mc
for jj = 1:Nc
watermark(ii,jj)=message(mod(ii,Mm)+1,mod(jj,Nm)+1);
end
end
watermarked_image=cover_object;
for ii = 1:Mc
for jj = 1:Nc
watermarked_image(ii,jj)=bitset(watermarked_image(ii,jj),1,watermark(ii,jj));
end
end
imwrite(watermarked_image,'watermarkedimage','bmp');
figure(1)
imshow(watermarked_image,[])
title('Watermarked Image')

Answer 1

If your roi is rectangular, just loop over the appropriate subsection of the image rather than the whole thing.
If not, and you can define the watermarking as some function:
imgout = watermark(img1, img2);
Then you can use roifilt2 to apply that function just in your roi.
In this simple example,mask is a BW matrix where 1 indicates part of our roi (a mask can be created several different ways including using some of the interactive roi functions, see bottom section). img1 and img2 have already been loaded:
f = #(x) watermark(x,img2); % our very basic function
imgout = roifilt2(img1,mask,f);
Complications may arise if your img2 is smaller than img1 (or if you want to resize it to just cover the roi area). In this case, do everything on a subsection of img1/mask and then assemble the final image:
img1_s = img1(a:b,c:d); % these return the same size as img2
mask_s = mask(a:b,c:d);
imgout_s = roifilt2(img1_s,mask1_s,f);
imgout = img1;
imgout(a:b,c:d) = imgout_s;
A mask can be created several ways, some using the interactive roi functions, some not. A lot depends on how you have your roi in the first place - do you have a centre/radius, or do you want to hand-pick the location interactively, etc. Here are a few examples making use of the Image Processing Tool Box:
Interactive ROI function and createMask.
This will work for any of the interactive roi functions (imellipse, imfreehand, etc). You can interactively adjust the mask between the second and third steps. Do not close the image window before calling createMask.
h = imshow(img); %display image
e = imellipse(gca,[50 50 100 100]); % select roi
mask = createMask(e,h); % return mask
List of x/y points and poly2mask
If you have a list of points defining the outer edge of your roi, a quick non-interactive way of producing a mask is to use poly2mask. The third and fourth values define the total size of the mask returned.
mask = poly2mask(x,y,size(img,1),size(img,2));
Using morphological operators
strel defines a structuring element, and imdilate performs a dilation using that structuring element. Given an image which contains a single point, what that does is replace the point with the structuring element - in this case disk which will produce a circle (or as close as you can get using pixels). Other shapes (e.g. diamond) can be used with strel
mask = zeros(size(img));
mask(a,b) = 1; % a,b is the centre point of the circle
se = strel(disk,r,0); %r is the radius of the circle
mask = imdilate(mask,se);