Matlab video reader - draw disk on objects - matlab

Question

i try to draw a disk on objects in video (the object are cars are move on road from left to right).
my code:
obj = VideoReader('Cars.avi');
get(obj)
im = read(obj,71);
nframes = get(obj,'NumberOfFrames');
sedisk = strel('disk',10);
im_new = imopen(im,sedisk);
stats = regionprops(im_new);
area_array = [stats.Area];
im2 = read(obj,1);
figure,imagesc(im2);
for i=1:nframes-1
stats(i).Centroid
frame = read(obj,i);
imshow(frame);
end
i see the frames but not the disk on the cars, why it's not working?
maybe something in the logic is wrong?
thank's everyone

Answer 1

You need to put the regionprops code into the for loop, and plot the regions over the image.
Start by following MATLAB regionprops documentation
I built a sample code with traffic.avi as input file.
traffic.avi file comes with my MATLAB installation (in folder toolbox/images/imdata/).
Here is a code sample (please read the comments):
clear
close all
%obj = VideoReader('Cars.avi');
obj = VideoReader('traffic.avi');
nframes = get(obj, 'NumberOfFrames');
%sedisk = strel('disk', 10);
sedisk = strel('disk', 2); %Smaller disk fits traffic.avi (you can keep sedisk = strel('disk', 10))
%Read first image (assume no cars). In your code you can keep: im = read(obj,71);
im1 = read(obj,1);
for i = 2:nframes
im = read(obj, i);
imshow(im); %Show the frame
%Subtract frame form the first frame - assume the cars will pop up in the difference image.
diff_im = uint8(abs(double(im) - double(im1)));
I = rgb2gray(diff_im); %Convert to grayscale image
BW = imbinarize(I); %Convert to binary image.
im_new = imopen(BW, sedisk);
stats = regionprops('table', im_new, 'Centroid', 'MajorAxisLength', 'MinorAxisLength'); %MATLAB documentation code sample
%imshow(im_new);
if (~isempty(stats))
%stats(i).Centroid
centers = stats.Centroid; %Get centers (MATLAB documentation code sample)
%Get radius of the circles (MATLAB documentation code sample).
diameters = mean([stats.MajorAxisLength stats.MinorAxisLength],2);
radii = diameters/2;
%Plot the circles on the displayed video frame.
hold on
viscircles(centers,radii);
hold off
end
pause(0.1); %Pause 0.1 seconds
end
The code is not marking the cars accurately (just demonstrates the stages).
Here is a sample frame: