How can I track and measure the velocity of a moving object in a video?
The video itself is quite simple. It's a high contrast capture of a droplet moving across the screen. How can I calculate the velocity of the droplet given that I already have the scale (pixels to mm)? Any input/direction would be appreciated.
Kshitij Singh answered .
2025-11-20
You can easily calculate the velocity multiplying the distance of centroids between previous frame and current frame, the frame rate of the video and the scale of the unit is meter/pixel.
scale = 1/320; % meter/pixel frameRate = 30; % frame/second velocity = velociy_pix * frameRate * scale; % pixel/frame * frame/second * meter/pixel
Here is an example code that calculates the velocity of a moving ball (requires Computer Vision System Toolbox):
%%Create a video for detection
writerObj = VideoWriter('movingBall.avi');
open(writerObj);
bgI = zeros(240,320,'uint8');
pos_org = [160 120];
r = 10;
for k = 1: 300
f = 0.2*(1-exp(-k/200));
pos = pos_org + 50*[cos(2*pi*f/30*k) sin(2*pi*f/30*k)];
I = insertShape(bgI, 'FilledCircle', [pos r], 'Color', 'white');
I = I + uint8(randn(size(I)))*10;
writeVideo(writerObj,im2frame(I));
end
close(writerObj);
%%Calculate velocity of the ball
videoFileReader = vision.VideoFileReader('movingBall.avi');
S = info(videoFileReader);
frameRate = S.VideoFrameRate; % frame/second
scale = 1/320; % m/pixel
videoPlayer = vision.VideoPlayer('Position',[100 100 600 400]);
oldPoints = [];
while ~isDone(videoFileReader)
videoFrame = step(videoFileReader);
G = rgb2gray(videoFrame);
BW = G > 0.5;
BW2 = bwareaopen(BW, 30);
BW3 = imfill(BW2, 'holes');
stats = regionprops('table',BW3,'Centroid');
points = table2array(stats);
if ~isempty(oldPoints)
% Calculate velocity (pixels/frame)
vel_pix = sqrt(sum((points-oldPoints).^2,2));
vel = vel_pix * frameRate * scale; % pixels/frame * frame/seconds * meter/pixels
else
vel_pix = 0;
vel = 0;
end
% Visualize the velocity
videoFrameOut = insertObjectAnnotation(videoFrame, 'circle', ...
[points 10*ones(size(points,1),1)], ...
cellstr(num2str(vel,'%2.2f')));
step(videoPlayer, videoFrameOut);
oldPoints = points;
end
release(videoFileReader);
release(videoPlayer);
