Computer Vision PS4

CS4476: Computer Vision
PS4
Setup
• Download and extract the data from the link data.zip. Place the data folder inside the project folder.
• Create a conda environment for this assignment as in previous assignments. Follow the instructions
below to set up the environment. If you run into import module errors, try pip install -e . again,
and if that still doesn’t work, you may have to create a fresh environment.
• Install Miniconda. It doesn’t matter whether you use Python 2 or 3 because we will create our own
environment that uses 3 anyway.
• Create a conda environment using the appropriate command. On Windows, open the installed Conda
prompt to run the command. On MacOS and Linux, you can just use a terminal window to run the
command. Modify the command based on your OS (linux, mac, or win):
conda env create -f proj4_env_<OS>.yml
• This should create an environment named cs4476 proj4. Activate it using the Windows command,
activate cs4476_proj4 or the MacOS/Linux command, source activate cs4476_proj4.
• Install the project package, by running pip install -e . inside the project folder.
• Run the notebook using jupyter notebook ./proj4_code/proj4.ipynb
• Ensure that all sanity checks are passing by running pytest either inside the proj4_unit_tests folder,
or directly in the project directory.
• Generate the zip folder for the code portion of your submission once you’ve finished the project using
python zip_submission.py --gt_username <your_gt_username> and submit to Gradescope.
• Start the assignment early, it will take some time to generate the results.
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Dataset
The dataset to be used in this assignment is the 15-scene dataset, containing natural images in 15 possible
scenarios like bedrooms and coasts. It was first introduced by Lazebnik et al, 2006. The images have a
typical size of around 256 by 256 pixels, and serve as a good milestone for many vision tasks. A sample
collection of the images can be found below:
Part 1: Tiny Image Representation and Nearest-Neighbor Classification
Introduction
In this part we will setup the workflow for nearest-neighbor classification. It is easy to understand, implement, and runs very quickly for our experimental setup (less than 10 seconds).
We will use tiny image representations for this part. The tiny image feature, inspired by the work of the
same name by Torralba, Fergus, and Freeman, is one of the simplest possible image representations. It
simply resizes each image to a small, fixed resolution. The representations are then made to have zero mean
and unit length for a slightly better performance. 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.
Torralba, Fergus, and Freeman propose several alignment methods to alleviate the latter drawback, but we
will not worry about alignment for this project. The feature representaion has been implenented for you.
Check get_tiny_images() in utils.py for implementation details.
The k nearest-neighbors classifier is equally simple to understand. When tasked with classifying a test feature
into a particular category, one simply finds the k nearest training examples (L2 distance is a sufficient metric;
you’ll be implementing this in pairwise_distances(X, Y)) and assigns the test case the most common label
among the k nearest neighbors. The k nearest neighbor classifier has many desirable features – it requires no
training, it can learn arbitrarily complex decision boundaries, and it trivially supports multi-class problems.
The voting aspect also alleviates training noise. K-nearest neighbor classifiers also suffers from the curse of
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dimensionality, because the classifier has no mechanism to learn which dimensions are not relevant for the
decision. The k neighbor computation also becomes slow for many training examples.
Part 1.1: Pairwise Distances (10 points)
• Implement pairwise_distances() in student_code.py.
• Use the Euclidean distance metric. You’ll be using this implementation of pairwise_distances() in
nearest_neightbor_classify(), kmeans() and kmeans_quantize().
Note that you are NOT allowed to use any library functions like pairwise_distances from sklearn, pdist
or scipy to help you do the calculation.
Part 1.2: K Nearest-Neighbors Classification (20 points)
• In student_code.py, implement the function nearest_neightbor_classify().
• Given the training images features and labels, together with the testing features, classify the testing
labels using the k nearest neighbors found in the training set. Your k nearest neighbors would vote
on what to label the data point. The pipeline in the Jupyter Notebook will also walk you through the
performance evaluation via a simple confusion matrix.
A confusion matrix is an N x N matrix used for evaluating the performance of a classification model, where
N is the number of target classes. The matrix compares the actual target values with those predicted by the
model.
Together, the tiny image representation and nearest neighbor classifier will get about 15% to 25% accuracy
on the 15 scene database. For comparison, chance performance is ∼ 7%. Paste the results for experiments
in the report. Note that you need to tune your parameters and reach 15% to get full credits for this part.
Part 1.3 Experiment and Report
Now we will perform experiments with the values of tiny image size and k in kNN.
(a) Report the performance of the standard parameters (image size = 16×16, k = 3).
(b) Test the following image sizes: 8×8, 16×16, 32×32, and the following k: 1, 3, 5, 10, 15. Report the
performance
(c) In the writeup reflect on the difference in performance and processing time between the standard
parameters and the experimental parameters you tried.
Part 2: Bag-of-words with SIFT Features
Introduction
Learning Objectives:
1. Understanding the concept of visual words
2. Set up the workflow for k-means clustering to construct the visual vocabulary.
3. Combine with the previous implemented k nearest-neighbor pipeline for classification.
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After you have implemented a baseline scene recognition pipeline it is time to move on to a more sophisticated image representation – bags of quantized SIFT features. Before we can represent our training and
testing images as a bag of feature histograms, we first need to establish a vocabulary of visual words. We
will form this vocabulary by sampling many local features from our training set (10s or 100s of thousands)
and then clustering them with k-means. The number of k-means clusters is the size of our vocabulary and
the size of our features. For example, you might start by clustering many SIFT descriptors into k = 50
clusters. This partitions the continuous, 128-dimensional SIFT feature space into 50 regions. For any new
SIFT feature we observe, we can figure out which region it belongs to as long as we save the centroids of
our original clusters. Those centroids are our visual word vocabulary. Because it can be slow to sample and
cluster many local features, the starter code saves the cluster centroids and avoids recomputing them on
future runs. See build_vocabulary() for more details.
Now we are ready to represent our training and testing images as histograms of visual words. For each
image we will densely sample many SIFT descriptors. Instead of storing hundreds of SIFT descriptors, we
simply count how many SIFT descriptors fall into each cluster in our visual word vocabulary. This is done
by finding the nearest k-means centroid for every SIFT feature. Thus, if we have a vocabulary of 50 visual
words, and we detect 220 SIFT features in an image, our bag of SIFT representation will be a histogram of
50 dimensions where each bin counts how many times a SIFT descriptor was assigned to that cluster and
sums to 220. A more intuitive way to think about this is through the original bag-of-words model in NLP:
assume we have a vocab of [”Messi”, ”Obama”], and article A contains 15 occurrences of ”Messi” with 1
”Obama”, while article B with 2 ”Messi” and 10 ”Obama”, then we may safely assume that article A is focusing on sports and B on politics, relatively speaking (unless Messi actually decides to run for the president).
The histogram should be normalized so that image size does not dramatically change the bag of feature
magnitude. See get_bags_of_sifts() for more details.
You should now measure how well your bag of SIFT representation works when paired with a k nearestneighbor classifier. There are many design decisions and free parameters for the bag of SIFT representation
(number of clusters, sampling density, SIFT parameters, etc.) so accuracy might vary from 40% to 60%.
Part 2.1: k-means (10 + 10 points)
• In student_code.py, implement the function kmeans() and kmeans_quantize().
• kmeans() is used to perform the k-means algorithm and generate the centroids given the raw data
points.
• kmeans_quantize() will assign the closest centroid to each of the new data entry by returning the
indices.
For the max_iter parameter, the default value is 100, but you may start with small value like 10 to examine
the correctness of your code, 10 is also sufficiently good to get you a decent result coupling with proper
choice of k in kNN.
Note that in this part you are NOT allowed to use any clustering function from scipy or sklearn to perform
the k-means algorithm.
Hint: When implementing k-means, you will need to handle the case where you end up with no points for a
particular cluster.
Part 2.2: Build the Visual Vocab (10 points)
• In student_code.py, implement the function build_vocabulary().
• For each of the training images, sample SIFT features uniformly using generate_sample_points in
utils.py with a stride size of 20 for both horizontal and vertical directions.
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• Concatenate all features from all images, and perform k-means on this collection, the resulting centroids will be the visual vocabulary. We have provided a working edition of SIFTNet for you to use
in this part.
Part 2.3: Put it together with kNN Classification (10 points)
Now that we have obtained a set of visual vocabulary, we are now ready to quantize the input SIFT features
into a bag of words for classification.
• In student_code.py, implement the function get_bags_of_sift(), and complete the classification
workflow in the Jupyter Notebook.
• Sample uniformly from both training and testing images for their SIFT features (you may want to
sample with a smaller stride size), and quantize the features according to the vocab we’ve built (which
centroid should be assigned to the current feature?), and by computing the vocab histogram of all the
features for a particular image, we are able to tell what’s the most important aspect of the image.
• Given bag of SIFT features for both the training and testing images, since we know the ground truth
for the training features, we can use nearest-neighbor to identify the labels for the testing feature.
Again, in this part, you need to tune your parameters and obtain a final accuracy of at least 45% to
get full credits for this part.
• In the writeup reflect on the Tiny Image Representation vs the Bag of words with SIFT features.
Why do you think that the tiny image representation gives a much worse accuracy than bag of words?
Additionally why do you think Bag of Words is better in this case?
Part 2.4 Experiment and Report
(a) Using stride(build_vocab) = 20, stride(bags_of_sift) = 5, max_iter(kmeans)=10, play around
with different values for the following parameters:
(a) vocab_size: Try 50, 100 and 200
(b) k in kNN: Try 1, 3, 5, 10, 15.
Paste the confusion matrix with your best result onto the report, and make sure to note your parameter
settings.
(b) Compare the difference here versus the k value experiment in Part 1: what can you tell from it?
Extra Credit
Part 3.1 Additional Experimentation
This is an extension of 2.4. Play around with the different values for the following parameters. Paste the
confusion matrix with your best result onto the report, and make sure to note your parameter settings.
1. max_iter in kmeans()
2. stride or step_size in both build_vocabulary() and get_bags_of_sifts()
Part 3.2 Performance
Use the standard parameters and try a different distance metric for your kNN and report your accuracy.
You should be able to achieve higher accuracy.
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Part 3.3 Model
Set up the experiment with SVM, and reach the accuracy of 65%. Hints: you may take a look at the
SVC classifier in sklearn; also note that common case of SVC classifies object following the one-against-one
scheme, so you may need to do some extra work to make it fit in the multi-class problem. If you manage to
get an accuracy above 60%, add that to your report!
Writeup
For this project (and all other projects), you must do a project report using the template slides provided
to you. Do not change the order of the slides or remove any slides, as this will affect the grading process
on Gradescope and you will be deducted points. In the report you will describe your algorithm and any
decisions you made to write your algorithm a particular way. Then you will show and discuss the results of
your algorithm. The template slides provide guidance for what you should include in your report. A good
writeup doesn’t just show results, it also tries to draw some conclusions from the experiments. You must
convert the slide deck into a PDF for your submission.
Rubric
• +70 pts: Code
– 30 pts: Part 1
– 40 pts: Part 2
• +30 pts: PDF report
– 15 pts: Part 1
– 15 pts: Part 2
• +11 pts: Extra Credit
– 4 pts: Additional Experimentation
– 2 pts: Performance
– 5 pts: Model
Submission Instructions
You will have two submission files for this project:
• your gt username proj4.zip via Gradescope containing: proj4 py/ - directory containing all your code
for this assignment
• your gt username proj4.pdf report via Gradescope
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