Background
Subtraction
Shashank Dhariwal
Manmeet Singh Kapoor
Adham Beyki
1
Agenda


Introduction



Challenges



Various Techniques &Comparison



Mixture of Gaussians



Advantages & Disadva...
Introduction
A

computational vision process

 Identifies

frame

A

moving objects from the video

class of techniques...
The Problem


Aim: Given a frame sequence from a fixed
camera, detecting all the foreground objects.



Approach: detect...
Challenges
Illumination
Changes
Motion
Changes
Changes in
Background
Geometry

• Gradual
• Sudden
• Camera Oscillations
• ...
Various Techniques
 Running

Gaussian Average
 Temporal Median Filter
 Mixture of Gaussians(MoG)
 Kernel Density Estim...
Comparison
Method

Speed

Memory

Accuracy

I

I

Low-Medium

Temporal Median Filter

ns

ns

Low-Medium

Mixture of Gauss...
Mixture of Gaussian

Deals with

• Repeated motion
• Background clutter
• Long term scene change

8
Mixture of Gaussian


Problems with other techniques [3]
1. Averaging images over time
-Not robust to scenes with slow mo...
Mixture of Gaussian
 Hypothesis

:

If we model each pixel as a mixture of Gaussians to
determine whether or not a pixel ...
Mixture of Gaussian

Fig 1 . [ 3 ]
11
Background Subtraction -MoG


Pixel process : It’s the history of the pixel ‘X’’s value
from frame 1 to ‘t’. [4]



Part...
Background Subtraction -MoG


Decision making : For each Gaussian in the mixture
of pixel X


if pixel X <= 2.5 (probabi...
Background Subtraction -MoG


Part 2 : Updating of parameters and using a suitable
heuristic for distributions that repre...
Background Subtraction -MoG


Advantages :





Robust against movement that are part of
background, e.g moving branch...
Implementation
Outdoor Scene
Moving cars
Stopped cars
Pedestrians
Luminosity changes
Shadows
Leaves and branches of trees
...
Implementation
Frame Difference

as the easiest method
o Objects with uniformly
distributed intensity
o Objects must be mo...
Implementation
MoG

a complex method
o Parametric model
o Mixture of Gaussian
components
o Comparing pixel value with
trac...
Implementation

19
References

1.
2.

3.

4.

5.

6.

7.

Tarun Baloch, MSc Thesis ‘Background Subtraction in Highly Illuminated Indoor
Envir...
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Background subtraction

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Background subtraction

  1. 1. Background Subtraction Shashank Dhariwal Manmeet Singh Kapoor Adham Beyki 1
  2. 2. Agenda  Introduction  Challenges  Various Techniques &Comparison  Mixture of Gaussians  Advantages & Disadvantages  Implementation & Results  References 2
  3. 3. Introduction A computational vision process  Identifies frame A moving objects from the video class of techniques for segmentation[1] 3
  4. 4. The Problem  Aim: Given a frame sequence from a fixed camera, detecting all the foreground objects.  Approach: detecting foreground as difference between the current frame & an image of the scene’s static background. | framei – backgroundi| > Th 4
  5. 5. Challenges Illumination Changes Motion Changes Changes in Background Geometry • Gradual • Sudden • Camera Oscillations • High Frequency Objects • Parked cars… 5
  6. 6. Various Techniques  Running Gaussian Average  Temporal Median Filter  Mixture of Gaussians(MoG)  Kernel Density Estimation(KDE)  Sequential Kernel Density Approximation(SKDA)  Co-occurrence of Image Variance  Eigenbackgrounds 6
  7. 7. Comparison Method Speed Memory Accuracy I I Low-Medium Temporal Median Filter ns ns Low-Medium Mixture of Gaussians m m High Kernel Density Estimation n n High Sequential KD Approximation m+1 m Medium-High Co-occurrence of Image Variance 8n/N2 nK/N2 Medium M n Medium Running Gaussian Average Eigenbackgrounds Table 1 – Comparison [2] 7
  8. 8. Mixture of Gaussian Deals with • Repeated motion • Background clutter • Long term scene change 8
  9. 9. Mixture of Gaussian  Problems with other techniques [3] 1. Averaging images over time -Not robust to scenes with slow moving objects -Single Threshold for entire scene 2. Modelling each pixel using Kalman filter -Not robust to backgrounds with repetitive change -Takes significant time to re-establish the background 3. Modelling each pixel using single Gaussian -Good indoor performance - Not good enough out-door scenes for repetitive change 9
  10. 10. Mixture of Gaussian  Hypothesis : If we model each pixel as a mixture of Gaussians to determine whether or not a pixel is part of the background, then we will arrive at an effective approach to separate the background and foreground, which can be used for real-time tracking.[ 3]  Used in tracking of moving objects: 1. Separating Foreground from Background (our agenda) 2. Tracking Objects in Foreground (not in the scope) 10
  11. 11. Mixture of Gaussian Fig 1 . [ 3 ] 11
  12. 12. Background Subtraction -MoG  Pixel process : It’s the history of the pixel ‘X’’s value from frame 1 to ‘t’. [4]  Part 1 : For above pixel process of ‘X’ the probability of observing a ‘X’ at frame ‘t’ is as follows, K = no. of Gaussian in the mixture (generally 3-5). [4] weight(i,t) = estimate of the weight of Gaussian in the mixture means (i,t) = mean value of the Gaussian at time ‘t’ Covariance(i,t) = Covariance matrix of Gaussian in the matrix = Variance * Identity matrix 12
  13. 13. Background Subtraction -MoG  Decision making : For each Gaussian in the mixture of pixel X  if pixel X <= 2.5 (probability of .996) standard deviation form the mean then the Gaussian is said to be ‘matched’ - Increase the weight - Adjust the mean closer to X(t) - Decrease the Variance  Else the Gaussian is ‘unmatched’ - Decrease the weight  If all the Gaussians in the mixture for pixel X(t) are unmatched - Mark X(t) as foreground pixel - Find the least probable Gaussian in the mixture and replace it with a new Gaussian with the following parameters: - Mean = X(t) i.e present value of X - Variance as a high value - Weight as a low value 13
  14. 14. Background Subtraction -MoG  Part 2 : Updating of parameters and using a suitable heuristic for distributions that represent background pixels Based on the decision made , change the following parameters using the equations given below: Where - α (alpha) is the learning parameter. - M (i,t) value is set to 1 for model that matched and 0 for rest - µ(mean) and σ (Std Deviation) for unmatched remain same and changes for the matched distributions - ρ is the updating parameter 14
  15. 15. Background Subtraction -MoG  Advantages :    Robust against movement that are part of background, e.g moving branches of a tree Robust against rain , snow, etc…. Disadvantages:     Not a good subtraction when shadows are there Difficulty with objects overlapping Fast lighting changes were also an issue. Gives false positives 15
  16. 16. Implementation Outdoor Scene Moving cars Stopped cars Pedestrians Luminosity changes Shadows Leaves and branches of trees 16
  17. 17. Implementation Frame Difference as the easiest method o Objects with uniformly distributed intensity o Objects must be moving all the time!  Computationally cheap  Highly adaptive background model  Tuning threshold value (=25 for our example) 17
  18. 18. Implementation MoG a complex method o Parametric model o Mixture of Gaussian components o Comparing pixel value with tracking Gaussian components  Very good at separating objects  Suppressing background noise  Parameter optimisation  Not quickly enough adaptive background model 18
  19. 19. Implementation 19
  20. 20. References 1. 2. 3. 4. 5. 6. 7. Tarun Baloch, MSc Thesis ‘Background Subtraction in Highly Illuminated Indoor Environment’ Indian Institute of Technology, 2010 M. Piccardi. Background Subtraction techniques : A Review . In IEEE International Conference on Systems, Man and Cybernetics, 2004, Volume 4, pages 3099– 3104, 2005. URL http://dx.doi.org/10.1109/ICSMC.2004.1400815. Eric Thul , ECSE-626 Final Project: An evaluation of Chris Stauffer and W. E. L. Grimson’s method for background subtraction, 2008, www.cs.mcgill.ca/~ethul/pub/course/ecse626/project-report.pdf C. Stauffer and W. E. L. Grimson. Adaptive background mixture models for realtime tracking. IEEE Computer Society Conference on Computer Vision and Pattern Recognition, 1999, 2:252, 1999. URL http://doi.ieeecomputersociety.org/10.1109/CVPR.1999.784637. A A Mazeed, Mark Nixon and Steve Gunn, Classifiers Combination for Improved Motion Segmentation, 2004,eprints.ecs.soton.ac.uk › ECS › Research › Publications Omar Javed, Khurram Shafique and Mubarak Shah, A hierarchical approach to robust background subtraction using colour and gradient information,2002, visionnas2.cs.ucf.edu/papers/javed_wmvc_2002.pdf Sen-Ching S.Chung and Chandrika Kamath, Robust techniques for background subtraction in urban traffic video, 2004, www.llnl.gov/casc/sapphire/pubs/UCRL-CONF-200706.pdf 20

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