CSCI5561-Assignment 3 Scene Recognition Solved

Figure 1: You will design a visual recognition system to classify the scene categories.

The goal of this assignment is to build a set of visual recognition systems that classify the scene categories. The scene classification dataset consists of 15 scene categories including office, kitchen, and forest as shown in Figure 1 [1]. The system will compute a set of image representations (tiny image and bag-of-word visual vocabulary) and predict the category of each testing image using the classifiers (k-nearest neighbor and SVM) built on the training data. A simple pseudo-code of the recognition system can found below:



Algorithm 1 Scene Recognition



1: Load training and testing images

2: Build image representation

3: Train a classifier using the representations of the training images 4: Classify the testing data.

5: Compute accuracy of testing data classification.



For the knn classifier, step 3 and 4 can be combined.

1           Scene Classification Dataset
You can download the training and testing data from here: http://www.cs.umn.edu/~hspark/csci5561/scene_classification_data.zip

The data folder includes two text files (train.txt and test.txt) and two folders (train and test). Each row in the text file specifies the image and its label, i.e., (label) (image path). The text files can be used to load images. In each folder, it includes 15 classes (Kitchen, Store, Bedroom, LivingRoom, Office, Industrial, Suburb, InsideCity, TallBuilding, Street, Highway, OpenCountry, Coast, Mountain, Forest) of scene images.

2           VLFeat Usage
Similar to HW #2, you will use VLFeat (http://www.vlfeat.org/install-matlab. html). You are allowed to use the following two functions: vl_dsift and vl_svmtrain.


3           Tiny Image KNN Classification


                                                   (a) Image                                       (b) Tiny Image

Figure 2: You will use tiny image representation to get an image feature.

function [feature] = GetTinyImage(I, output_size)

Input: I is an gray scale image, output_size=[w,h] is the size of the tiny image. Output: feature is the tiny image representation by vectorizing the pixel intensity in a column major order. The resulting size will be w×h.

Description: You will simply resize each image to a small, fixed resolution (e.g., 16×16). You need to normalize the image by having zero mean and unit length. This is not a particularly good representation, because it discards all of the high frequency image content and is not especially invariant to spatial or brightness shifts.

function [label_test_pred] = PredictKNN(feature_train, label_train, feature_test, k) Input: feature_train is a ntr × d matrix where ntr is the number of training data samples and d is the dimension of image feature, e.g., 265 for 16×16 tiny image representation. Each row is the image feature. label_train∈ [1,15] is a ntr vector that specifies the label of the training data. feature_test is a nte ×d matrix that contains the testing features where nte is the number of testing data samples. k is the number of neighbors for label prediction.

Output: label_test_pred is a nte vector that specifies the predicted label for the testing data.

Description: You will use a k-nearest neighbor classifier to predict the label of the testing data.



Figure 3: Confusion matrix for Tiny+KNN.

function [confusion, accuracy] = ClassifyKNN_Tiny

Output: confusion is a 15×15 confusion matrix and accuracy is the accuracy of the testing data prediction.

Description: You will combine GetTinyImage and PredictKNN for scene classification. Your goal is to achieve the accuracy 18%.

4           Bag-of-word Visual Vocabulary




 
 


Figure 4: Each row represents a distinctive cluster from bag-of-word representation.

function [vocab] = BuildVisualDictionary(training_image_cell, dic_size) Input: training_image_cell is a set of training images and dic_size is the size of the dictionary (the number of visual words).

Output: vocab lists the quantized visual words whose size is dic_size×128. Description: Given a set of training images, you will build a visual dictionary made of quantized SIFT features. You may start dic_size=50. You can use the following built-in functions:

•    vl_dsift from VLFeat.

•    kmeans from MATLAB toolbox.

You may visualize the image patches to make sense the clustering as shown in Figure 4.



Algorithm 2 Visual Dictionary Building



1: For each image, compute dense SIFT over regular grid

2: Build a pool of SIFT features from all training images 3: Find cluster centers from the SIFT pool using kmeans algorithms.

4: Return the cluster centers.



Figure 5: Confusion matrix for BoW+KNN.

function [bow_feature] = ComputeBoW(feature, vocab)

Input: feature is a set of SIFT features for one image, and vocab is visual dictionary. Output: bow_feature is the bag-of-words feature vector whose size is dic_size. Description: Give a set of SIFT features from an image, you will compute the bag-ofwords feature. The BoW feature is constructed by counting SIFT features that fall into each cluster of the vocabulary. Nearest neighbor can be used to find the closest cluster center. The histogram needs to be normalized such that BoW feature has a unit length.

function [confusion, accuracy] = ClassifyKNN_BoW

Output: confusion is a 15×15 confusion matrix and accuracy is the accuracy of the testing data prediction.

Description: Given BoW features, you will combine BuildVisualDictionary, ComputeBoW, and PredictKNN for scene classification. Your goal is to achieve the accuracy 50%.

5           BoW+SVM
function [label_test_pred] = PredictSVM(feature_train, label_train, feature_test) Input: feature_train is a ntr × d matrix where ntr is the number of training data samples and d is the dimension of image feature. Each row is the image feature. label_train∈ [1,15] is a ntr vector that specifies the label of the training data. feature_test is a nte × d matrix that contains the testing features where nte is the number of testing data samples.

Output: label_test_pred is a nte vector that specifies the predicted label for the testing data.

Description: You will use a SVM classifier to predict the label of the testing data. You don’t have to implement the SVM classifier. Instead, you can use VLFeat vl_svmtrain. Linear classifiers are inherently binary and we have a 15-way classification problem. To decide which of 15 categories a test case belongs to, you will train 15 binary, 1-vs-all SVMs. 1-vs-all means that each classifier will be trained to recognize ‘forest’ vs ‘nonforest’, ‘kitchen’ vs ‘non-kitchen’, etc. All 15 classifiers will be evaluated on each test case and the classifier which is most confidently positive “wins”. For instance, if the ‘kitchen’ classifier returns a score of -0.2 (where 0 is on the decision boundary), and the ‘forest’ classifier returns a score of -0.3, and all of the other classifiers are even more negative, the test case would be classified as a kitchen even though none of the classifiers put the test case on the positive side of the decision boundary. When learning an SVM, you have a free parameter ’lambda’ which controls how strongly regularized the model is. Your accuracy will be very sensitive to lambda, so be sure to test many values.



Accuracy: 0.629333

Figure 6: Confusion matrix for BoW+SVM.

function [confusion, accuracy] = ClassifySVM_BoW

Output: confusion is a 15×15 confusion matrix and accuracy is the accuracy of the testing data prediction.

Description: Given BoW features, you will combine BuildVisualDictionary, ComputeBoW, PredictSVM for scene classification. Your goal is to achieve the accuracy 60%.