Learning to Detect Objects in Images via a Sparse, Part-Based Representation

1. Learning to Detect Objects in Images via a Sparse, Part-Based Representation S. Agarwal, A. Awan and D. Roth IEEE Transactions on Pattern Analysis and Machine Intelligence Antón Escobedo cse252c

2. Outline • Introduction • Problem Specification • Related Work • Overview of the Approach • Evaluation • Experimental Results and Analysis • Conclusion and Future Scope

3. Introduction • Automatic detection of objects in images • Different objects belonging to the same category can vary • Successful object detection system • Proposed solution – Sparse-Part based representation • Part-based representation is computationally efficient and has its roots in biological vision

4. Problem Specification • Input: An image • Output: A list of locations at which instances of the object class are detected in the image • The experiments are performed on images of side views of cars but can be applied to any object that consists of distinguishable parts arranged in a relatively fixed spatial configuration • The present problem is a “detection” problem rather than a simple “classification” problem

5. Previous Related Work • Raw Pixel Intensities • Global Image • Local features • Part Based Representations using hand labeled features

6. Algorithm Overview Four Stages: • Vocabulary Construction: Building a vocabulary of parts that will represent objects • Image Representation: Input images are represented in terms of binary feature vectors • Learning a Classifier: Two target classes +feature vector (object) and –feature vector (nonobject) • Detection Hypothesis Using the Learned Classifier: • Classifier activation map for the single-scale case • Classifier activation pyramid for multiscale cases

7. Vocabulary Construction • Extraction of interest points using Forstner interest operator • Experiments carried out on 50 representative images of size 100 x 40 pixels. A total of 400 patches, each of size 13 x 13 pixels were extracted • To facilitate learning, a bottom-up clustering procedure was adopted where similarity was measured by normalized correlation • Similarity between two clusters C1 and C2 is finally measured by the average similarity between their respective patches:

8. Vocabulary Construction Forstner applied to sample image Sample patches Clusters from sample patches

9. Image Representation • For each patch q in an image, a similarity-based indexing is performed into the part vocabulary P using: • For each highlighted patch q, the most similar vocabulary part P*(q) is given by:

10. Image Representation: Feature Vector • Spatial relations among the parts detected in an image are defined in terms of distance (5 bins) and directions (8 ranges of 45 degrees each) giving 20 possible relations between 2 parts. • 2-6 parts per Positive Window • Each 100x40 training image is represented as a feature vector with 290 elements. • Pn(i): ith occurrence of a part of type n in the image (1≤n≤270; n is a particular part-cluster) • Rm(j)(Pn1, Pn2): jth occurrence of relation Rm between a part of type n1 and a part of type n2 (1≤m≤20; m is a distance-direction combination)

11. Learning a Classifier • Train classifier using 1000 labeled images, each 100 x 40 pixels in size • No synthetic training images • +ve examples: Various cars with varied backgrounds • -ve examples: Natural scenes like buildings, roads • High dimensionality of feature vector: 270 types, 20 relations, repeats. • Use of Sparse Network of Winnows (SNoW) learning architecture. • Winnow: to reduce in number until only the best are left

12. SNoW: Sparse network of linear units over a Boolean or real valued feature space (activation) Target Nodes Edges are allocated dynamically Input Layer= Feature Layer set of examples e (represented as a list of active features)

13. SNoW: Predicted target t* for example e Activation calculated by the summation for target node t θ - Ω Learning Algorithm Specific Sigmoid function whose transition from an output close to 0 to an output close to 1, centers around θ  .

14. SNoW: Basic Learning Rules • Several weight update rules can be used: update rules are variations of Winnow and Perceptron • Winnow update rule: The number of examples required to learn a linear function grows linearly with the number of relevant features and only logarithmically with the total number of features.

15. 2, 1, 1, 2 2, 1, 2, 1 1, 1 = 4 1, 1, 1 = 2 2, 1, 2, 1 1, 1 2, 2 = 1 1, 1001, 1006: 1 2 3 2, 1002, 1007, 1008: 1, 1004, 1007: 3, 1006, 1004: 3, 1004, 1005, 1009: 1001 1002 1003 1004 1005 1006 1007 1008 1009 A Training Example 2, 2, 2, 2 2, 2 2, 2, 2 1001, 1005, 1007: Update rule: Winnow α = 2, β = ½,θ = 3.5

16. Detection Hypothesis using Learned Classifier • Classifier Activation Map – for single scale • Neighborhood Suppresion: Based on nonmaximum suppression. • Repeated Part Elimination: Greedy algorithm, uses windows around highest activation points.

17. Detection:Classifier Activation Pyramid • Scale the input image a number of times to form a multi-scale image pyramid • Apply the learned classifier to fixed-size windows in each image in the pyramid • Form a three-dimensional classifier activation pyramid instead of the earlier two-dimensional classifier activation map.

18. Evaluation Criteria • Test Set I consists of 170 images containing 200 cars of same size and is tested for single scale case. In this case for each car in the test images, the location of best 100 x 40 window containing the car is determined. • Test Set II consists of 108 images containing 139 cars of different sizes and is tested for multi scale case. In this case for each car in the test images, the location and scale of the best 100 x 40 window containing the car is determined.

19. Performance Measures • Goal is to maximize the number of correct detections and minimize the number of false detections. • One method for expressing the trade-off between correct and false detections is to use the receiver operating characteristics (ROC) curve. This curve plots the true positive rate vs. the false positive rate. # of true positive (TP) True positive rate = -------------------------------------------------- Total # of positives in the data set (nP) # of false positive (FP) False positive rate = ------------------------------------------------- Total # of negatives in the data set (nN) This measures the accuracy of the system as a “classifier” rather than a “detector”.

20. Performance Measures (contd.) • We are really interested in knowing how many of the objects it detects (given by recall), and how often the detections it makes are false (given by 1-precision). This trade-off is thus captured very accurately by (recall) vs. (1-precision) curve; where TP TP Recall = ------------- ; 1 – Precision = --------------- nP TP + FP The threshold parameter that achieves the best trade-off between the two quantities is measured by the point of highest F-measure, where 2 * Recall * Precision F-measure = --------------------------- Recall + Precision

21. Experimental Results Single-scale detection with Neighborhood Suppression Algorithm Single-scale detection with Repeated Part Elimination Algorithm

22. Experimental Results (contd.) Multi-scale detection with Neighborhood Suppression Algorithm Multi-scale detection with Repeated Part Elimination Algorithm

23. Some Graphical Results

24. Analysis: A. Performance of Interest Operator

25. Analysis: B. Performance of Part Matching Process

26. Analysis: C. Performance of Learned Classifier

27. Conclusion • Automatic vocabulary construction from sample images • Methodologies for object detection • Detector from Classifier • Standardizing evaluation criterion • Good for classification of objects with distinguishable parts

28. Questions? Slides adapted from http://www.cs.uga.edu/~ananda/ML_Talk.ppt and http://l2r.cs.uiuc.edu/~cogcomp/tutorial/SNoW.ppt