Object Detection and Tracking Mike Knowles 11 th January

Introduction  Goal – to detect and track objects moving independently to the background  Two situations to be considered:  Static Background  Moving Background

Applications of Motion Tracking  Control Applications  Object Avoidance  Automatic Guidance  Head Tracking for Video Conferencing  Surveillance/Monitoring Applications  Security Cameras  Traffic Monitoring  People Counting

My Work  Started by tracking moving objects in a static scene  Develop a statistical model of the background  Mark all regions that do not conform to the model as moving object

My Work  Now working on object detection and classification from a moving camera  Current focus is motion compensated background filtering  Determine motion of background and apply to the model.

Detecting moving objects in a static scene  Simplest method:  Subtract consecutive frames.  Ideally this will leave only moving objects.  This is not an ideal world….

Using a background model  Lack of texture in objects mean incomplete object masks are produced.  In order to obtain complete object masks we must have a model of the background as a whole.

Adapting to variable backgrounds  In order to cope with varying backgrounds it is necessary to make the model dynamic  A statistical system is used to update the model over time

Background Filtering  My algorithm based on: “Learning Patterns of Activity using Real-Time Tracking” C. Stauffer and W.E.L. Grimson. IEEE Trans. On Pattern Analysis and Machine Intelligence. August 2000  The history of each pixel is modelled by a sequence of Gaussian distributions

Multi-dimensional Gaussian Distributions  Described mathematically as:  More easily visualised as: (2-Dimensional)

Simplifying….  Calculating the full Gaussian for every pixel in frame is very, very slow  Therefore I use a linear approximation

How do we use this to represent a pixel?  Stauffer and Grimson suggest using a static number of Gaussians for each pixel  This was found to be inefficient – so the number of Gaussians used to represent each pixel is variable

Weights  Each Gaussian carries a weight value  This weight is a measure of how well the Gaussian represents the history of the pixel  If a pixel is found to match a Gaussian then the weight is increased and vice-versa  If the weight drops below a threshold then that Gaussian is eliminated

Matching  Each incoming pixel value must be checked against all the Gaussians at that location  If a match is found then the value of that Gaussian is updated  If there is no match then a new Gaussian is created with a low weight

Updating  If a Gaussian matches a pixel, then the value of that Gaussian is updated using the current value  The rate of learning is greater in the early stages when the model is being formed

Static Scene Object Detection and Tracking  Model the background and subtract to obtain object mask  Filter to remove noise  Group adjacent pixels to obtain objects  Track objects between frames to develop trajectories

Moving Camera Sequences  Basic Idea is the same as before  Detect and track objects moving within a scene  BUT – this time the camera is not stationary, so everything is moving

Motion Segmentation  Use a motion estimation algorithm on the whole frame  Iteratively apply the same algorithm to areas that do not conform to this motion to find all motions present  Problem – this is very, very slow

Motion Compensated Background Filtering  Basic Principle  Develop and maintain background model as previously  Determine global motion and use this to update the model between frames

Advantages  Only one motion model has to be found  This is therefore much faster  Estimating motion for small regions can be unreliable  Not as easy as it sounds though…..

Motion Models  Trying to determine the exact optical flow at every point in the frame would be ridiculously slow  Therefore we try to fit a parametric model to the motion

Affine Motion Model  The affine model describes the vector at each point in the image  Need to find values for the parameters that best fit the motion present

Background Motion Estimation  Uses a framework developed by Black and Anandan Black M.J. and Anandan P. The robust estimation of motion models: Parametric and Piecewise-smooth Fields, Computer Vision and Image Understanding, Vol. 63, No. 1, pp , January  For more details see my talk from last year

Examples

Other approaches to Tracking  Many approaches using active contours – a.k.a. snakes  Parameterised curves  Fitted to the image by minimising some cost function – often based on fitting the contour to edges

Constraining shape  To avoid the snake being influenced by point we aren’t interested in, use a model to constrain its shape.

CONDENSATION  No discussion on tracking can omit the CONDENSATION algorithm developed by Isard and Blake.  CONditional DENSity propagATION  Non-gaussian substitute for the Kalman Filter  Uses factored sampling to model non- gaussian probabiltiy densities and estimate propogate them though time.

CONDENSATION  Thus we can take a set of parameters and estimate them from frame to frame, using current information from the frames  These parameters may be positions or shape parameters from a snake.

CONDENSATION - Algorithm  Randomly take samples from the previous distribution.  Apply a random drift and deterministic diffusion based on a model of how the parameters behave to the samples.  Weight each sample on the basis of the current information.  Estimate of actual value can be either a weighted average or a peak value from the distribution

Summary  Static-scene background subtraction methods  Extensions to moving camera systems  Use of model-constrained active contour systems  CONDENSATION