How do I select a region of interest across many frames?

I want to select a same region interest across many frames, I also want this region to be fixed for all the frames because I want to detect the movement of a particle across these frames. I would really appreciate any assistance. Thank you

Answer 1

[Image Analyst]

You can specify a region of interest in many ways.

You can interactively draw it using tools like roipoly(), roipolyold(), rbbox(), imrect(), or imfreehand(). You could specify a box by hard coding in the rows and columns. You could do segmentation, for example by intensity thresholding or color segmentation.

However you do it, you have a binary image that is the mask. You can get a frame of your video and then mask it with the ROI mask like this:

% Mask the image.
maskedRgbImage = bsxfun(@times, rgbImage, cast(mask,class(rgbImage)));

Here's a snippet you might find useful:

% Display image
subplot(2,2,1);
imshow(rgbImage);
% Have user freehand draw a mask in the image.
uiwait(msgbox('Draw a region with left button down.  Double click inside to finish it'));
h = imfreehand();
vertices = wait(h);
% Create a binary mask
mask = createMask(h);
subplot(2,2,2);
imshow(mask, []);
% Mask the image.
maskedRgbImage = bsxfun(@times, rgbImage, cast(mask,class(rgbImage)));
subplot(2,2,3);
imshow(maskedRgbImage);

Answer 2

[Matt J]

If your frames are held in a 3D array, A, and M is the logical mask for your ROI, you can do

B=reshape(A,[],size(A,3));

ROIs = B(M,:);

Now, ROIs(:,i) contains the ROI data the i-th frame.