Face Detection using the Spectral Histogram representation

1. Face Detection using the Spectral Histogram representation By: Christopher Waring, Xiuwen Liu Department of Computer Science Florida State University Presented by: Tal Blum blum+@cs.cmu.edu

2. Sources • The presentation is based on a few resources by the authors: • Exploration of the Spectral Histogram for Face Detection – M.Sc thesis by Christopher Waring (2002) • Spectral Histogram Based Face Detection – IEEE (2003) • Rotation Invariant Face Detection Using Spectral Histograms & SVM – CVPR submission • Independent Spectral Representation of images for Recognition – Optical Society of America (2003)

3. Overview • Spectral Histogram • Overview of Gibbs Sampling + Simulated annealing • Method for Lighting Normalization • Data used • 3 Algorithms • SH + Neural Networks • SH + SVM • Rotation Invariant SH +SVM • Experimental Results • Conclusions & Discussions

4. Two Approaches to Object Detection • Curse of dimensionality • Features should be: (Vasconcelos) • Independent • have low Bayes Error • 2 main Approaches in Object Detection: • Complicated Features with many interactions • Require many data points • Use syntactic variations that mimic the real variations • Estimation Error might be high • Assuming Model or Parameter structure • Small set of features or small number of values • This is the case for Spectral Histograms • The Bayes Error might be high (Vasconcelos) • Estimation Error is low

5. Why Spectral Histograms? • Translation Invariant • Therefore insensitive to incorrect alignment. • (surprisingly) seem to be able to separate Objects from Non-Objects well. • Good performance with a very small feature set. • Good performance with a large rotation invariance. • Don’t rely at all on any global spatial information • Combining of variant and invariant features • Will play a more Important role

6. What is Spectral Histogram

7. Types of Filters • 3 types of filters: • Gradient Filters • Gabor Filters • Laplasian of Gaussians Filters The exact composition of the filters is different for each algorithm.

8. Gibbs Sampling+ Simulated Annealing • We want to sample from • We can use the induced Gibbs Distribution • Algorithm: • Repeat • Randomly pick a location • Change the pixel value according to q • Until for every filter

9. Face Synthesis usingGibbs Sampling + Simulated Annealing • A measure of the quality of the Representation

10. Comparison - PCA vs. Spectral Histogram

11. Reconstruction vs. Sampling Reconstruction sampling

12. Spectral Histograms of several images

13. Lighting correction • They use a 21x21 sized images • Minimal brightness plane of 3x3 is computed from each 7x7 block • A 21x21 correction plane is computed by bi-linear interpolation • Histogram Normalization is applied

14. Lighting correction

15. Detection & Post Processing • Detection is don on 3 scaled Gaussian pyramid, each scale down sampled by1.1 • detections within 3 pixels are merged • A detection is marked as final if it is found at at least two concurrent levels • A detection counts as correct if at least half of the face lies within the detection window

16. Adaptive Threshold

17. Algorithm Iusing a Neural Network • Neural Network was used as a classifier • Training with back propagation • Data Processing • 1500 Face images & 8000 Non-Face images • Bootstrapping was used to limit the # non faces (Sung Poggio) leaving 800 Non-Faces • Use 8 filters with 80 bins in each

18. Alg. I - Filter Selection • 7 LoG filters with • 4 Difference of gradient: Dx Dy Dxx Dyy • 70 Gabor filters with: • T = 2,4,6,8,10,12,14 • = 0,40,80,120,160,200,280,320 • Selected Filters (8 out of 81) • 4 LoG filters with: • 3 Difference of Gradiant: Dx Dxx & Dyy • 1 Gabor filter with T=2 and

19. Spectral Histograms of several images

20. Algorithm I – Resultson CMU test set I

21. Algorithm I – Resultson CMU test set II

22. Algorithm IIusing a SVM • SVM instead of a Neural Network • They use more filters • 34 filters (instead of 7) • 359 bins (instead of 80) • 4500 randomly rotated Face images & 8000 Non-Face images from before

23. Algorithm II (SVM)Filters • The filters were hand picked • Filters: • The Intensity filter • 4 Difference of Gradient filters Dx,Dy,Dxx &Dyy • 5 LoG filgers • 24 gabor filters with • Local & Global Constraints • Using Histograms as features

24. Spectral Histograms of several images

25. Algorithm II (SVM) Results

26. Old Results

33. Algorithm IIIusing SVM +rotation invariant features • Same features as in Alg. II • The Features enable 180 degrees of rotation invariance • Rotate the image 180 degrees and switchHistograms achieving 360 degrees invariance

34. Rotating 180 degrees

35. Combining the two classifiers

36. ResultsUpright test sets

37. ResultsRotated test sets

40. Rotation Invariation Results

41. More pictures

44. Conclusions • A system which is rotation & translation invariant • Achieves very high accuracy for frontal faces and rotated frontal faces • The system is not real time, but is possible to implement convolution in hardware • Uses limited amount of data • Accuracy as a function of efficiency

45. Conclusions (2) • Faces are identifiable through local spatial dependencies where the global ones can be globally modeled as histograms • The problem with spatial methods is the estimation of the parameters • The SH representation is independent of classifier choice • SVM outperforms Neural Networks • The Problems and the Errors of this system are considerably different than of other systems

46. Conclusions (3) • Localization in Space and Scale is not as good as other methods • Translation Invariant features can enable a coarser sampling the image • Use adaptive thresholding • Use several scales to improve performance • SH can be used for sampling of objects