Spring 2025 CS 444

Assignment 3: Self-supervised and transfer learning, object detection

Due date: Wednesday, April 2, 11:59:59 PM

Starter Code

Download the starter code here -- and see setup instructions below under Assignment Setup.

Self-Supervised Learning by Rotation Prediction on CIFAR10

You will use PyTorch to train a model on a self-supervised task, fine-tune a subset of the model’s weights, and train a model in a fully supervised setting with different weight initializations. You will be using the CIFAR10 dataset, which is a dataset of small (32x32) images belonging to 10 different object classes. For self-supervised training, you will ignore the provided labels; however, you will use the class labels for fine-tuning and fully supervised training.

The model architecture you will use is ResNet18. We will use the PyTorch ResNet18 implementation, so you do not need to create it from scratch.

The self-supervised training task is image rotation prediction, as proposed by Gidaris et al. in 2018. For this task, all training images are randomly rotated by 0, 90, 180, or 270 degrees. The network is then trained to classify the rotation of each input image using cross-entropy loss by treating each of the 4 possible rotations as a class. This task can be treated as pre-training, and the pre-trained weights can then be fine-tuned on the supervised CIFAR10 classification task.

The top-level notebook (mp3_rotation.ipynb) will guide you through all the steps of training a ResNet for the rotation task and fine-tuning on the classification task. You will implement the data loader, training and fine-tuning steps in PyTorch based on the starter code. In detail, you will complete the following tasks:

1. Train a ResNet18 on the Rotation task. Based on the CIFAR10 dataloader, you will first generate the rotated images and labels for the rotation task. You will train a ResNet18 on the rotation task, report the test performance and store the model for the fine-tuning tasks. For the test performance, find the lowest test loss and report the corresponding accuracy. The expected rotation prediction accuracy on the test set should be around 78%.


2. Initializing from the Rotation model or from random weights, fine-tune only the weights of the final block of convolutional layers and linear layer on the supervised CIFAR10 classification task. Report the test results and compare the performance of these two models. The expected test set accuracy for fine-tuning the specified layers of the pre-trained model should be around 60%.


3. Initializing from the Rotation model or from random weights, train the full network on the supervised CIFAR10 classification task. Report the test results and compare the performance of these two models. The expected test set accuracy for fine-tuning the whole pre-trained model should be around 80%.


4. Now use a more advanced architecture (either custom-created or off-the-shelf) together with any combination of "bells and whistles" (optimization, data augmentation, normalization, etc.) to try to get the highest accuracies you can on tasks #1 and #3 above. For #3, use the training setting (fine-tuning vs. training from scratch) of your better-performing ResNet18 model. Be sure to specify all the details of your architecture and hyperparameters. You should aim for at least 3% improvement in accuracy over your respective ResNet18 models.

Extra Credit

• In Figure 5(b) from the Gidaris et al. paper, the authors show a plot of CIFAR10 classification performance vs. number of training examples per category for a supervised CIFAR10 model vs. a RotNet model with the final layers fine-tuned on CIFAR10. The plot shows that pre-training on the Rotation task can be advantageous when only a small amount of labeled data is available. Using your RotNet fine-tuning code and supervised CIFAR10 training code from the main assignment, try to create a similar plot by performing supervised fine-tuning/training on only a subset of CIFAR10.
• If you have a good amount of compute at your disposal, try to train a rotation prediction model on the larger ImageNette dataset (still smaller than ImageNet, though).

If you will be working on the assignment on a local machine then you will need a python environment set up with the appropriate packages. We suggest that you use Conda to manage python package dependencies (https://conda.io/docs/user-guide/getting-started.html). Unless you have a machine with a GPU, running this assignment on your local machine will be very slow and is not recommended.

Data Setup

Submission Instructions:

The assignment deliverables are as follows. If you are working in a pair, only one designated student should make the submission. You should indicate your Team Name on Kaggle Leaderboard and team members in the report.

1. You must submit your output Kaggle CSV files (generated by "mp3_rotation_report_kaggle.ipynb" or "mp3_classify_report_kaggle.ipynb") for each part to their corresponding Kaggle competition webpages:
1. Part 1
2. Part 2
3. Part 3
4. Part 4 (a) rotation prediction
5. Part 4 (b) classification
2. Upload five files to Canvas:
1. netid_mp3_output.pdf: Your IPython notebook with output cells converted to PDF format
2. netid_mp3_code.zip: All of your code (python files and ipynb file) in a single ZIP file
3. netid_mp3_report.pdf: Your assignment report (using this template) in PDF format

Please refer to course policies on collaborations, late submission, etc.