How can I track the displacement of a moving object.

I am trying to track the displacement of a white cross from a video file.

I can convert each individual frame to grayscale but can't then calculate the displacement of the white cross (the object I am trying to track) and plot this on a graph vs time. Here is the code I am currently:

v = VideoReader('Test_4.wmv');
video = read(v);
imtool(video(:,:,:,50))

data=size(video)
numframes = data(4)  %Convert to grayscale
for k = numframes-1:-1:1
g(:, :, k) = rgb2gray(video(:, :, :, k));
end

for k = numframes-1:-1:1 %Difference between frames
d(:, :, k) = imabsdiff(g(:, :, k), g(:, :, k+1));
end
imtool(d(:, :, 50), [])

thresh = graythresh(d)%Divides between foreground and background
bw = (d >= thresh * 255); imtool(bw(:, :, 50))

bw2 = bwareaopen(bw, 20, 8); %Removes objects that are too small

s = regionprops(bwlabel(bw2(:,:,1)), 'centroid'); %label each object and calc centre of mass
c = [s.Centroid]

subplot(2, 1, 1)
plot(t, centroids(:,1)), ylabel('x')
subplot(2, 1, 2)
plot(t, centroids(:, 2)), ylabel('y')
xlabel('time (s)')

Help would be much appreciated, thanks.

Answer 1

[Image Analyst]

Well not having the video, or even a frame from it, it's hard to know what to suggest. But assuming you have one cross and you've segmented it so that it's the largest blob in the image, you might use bwareafilt(bw, 1) instead of bwareaopen(bw, 20, 8).

[Matthew Roper]

Sorry, here are the figures produced for one frame with the code I am currently using. As you can see, I haven't been able to segment the image so that it is the largest blob.

Unfortunately I just tried the bwareafilt(bw, 1) and to no avail.

Thanks again.

[Image Analyst]

Can you illuminate the scene better? With the middle frame, there's not even a cross there. The thing in the middle, that is in the location of the cross in the bottom frame, is not a cross shape and can rightfully be considered as some completely different shape. I mean for all we know the shape might alternate between a pattern of six horizontal bar shapes and a single cross shape.

[Matthew Roper]

My issue is that I'm using a high speed camera - the image you see above is with two high powered optical lights illuminating the cross. Do you know of another method to be able to track the displacement, rather than converting to grayscale?

[Image Analyst]

There are things like Kinect and laser range finders. Why doesn't it always look like a cross? Is it because of the lights? Are the lights DC/battery operated, or do they pulse with the power line frequency?

[Matthew Roper]

Those images are all of the same frame at each stage of my code, so I am a bit confused as to why it is not a cross shape throughout. The light is DC operated. Is there a way I can track a single point on the cross instead of calculating the centroid?

[AC]

Dear Sir, I want to use simulink image processing and need to measure white object's displacement graph shown in figure. I attached my video.. How can do this? Anyone can help me ? If i success i will record a real object that moves faster in video..

[Image Analyst]

I don't know how to use Simulink.