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EECS 442 Computer Vision: Homework 5 solution

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EECS 442 Computer Vision: Homework 5
Instructions

• We are going to use Google Colab to run our code. For more information on using Colab, please see
our Colab tutorial.
• The submission includes two parts:
1. To Canvas: submit a zip file of all of your code.
We have indicated questions where you have to do something in code in red.
We have indicated questions where we will definitely use an autograder in purple.
Please be especially careful on the autograded assignments to follow the instructions. Don’t
swap the order of arguments and do not return extra values.
If we’re talking about autograding a filename, we will be pulling out these files with a script.
Please be careful about the name.

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EECS 442 Computer Vision: Homework 5
Instructions

• We are going to use Google Colab to run our code. For more information on using Colab, please see
our Colab tutorial.
• The submission includes two parts:
1. To Canvas: submit a zip file of all of your code.
We have indicated questions where you have to do something in code in red.
We have indicated questions where we will definitely use an autograder in purple.
Please be especially careful on the autograded assignments to follow the instructions. Don’t
swap the order of arguments and do not return extra values.
If we’re talking about autograding a filename, we will be pulling out these files with a script.
Please be careful about the name.
Your zip file should contain a single directory which has the same name as your uniqname. If I
(David, uniqname fouhey) were submitting my code, the zip file should contain a single folder
fouhey/ containing all required files.
What should I submit? At the end of the homework, there is a canvas submission checklist
provided. We provide a script that validates the submission format here. If we don’t ask you for
it, you don’t need to submit it; while you should clean up the directory, don’t panic about having
an extra file or two.
2. To Gradescope: submit a pdf file as your write-up, including your answers to all the questions
and key choices you made.
We have indicated questions where you have to do something in the report in blue.
You might like to combine several files to make a submission. Here is an example online link
for combining multiple PDF files: https://combinepdf.com/.
The write-up must be an electronic version. No handwriting, including plotting questions.
LATEX is recommended but not mandatory.
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1 Fashion-MNIST Classification [35 pts]
In this part, you will implement and train Convolutional Neural Networks (ConvNets) in PyTorch to classify
images. Unlike HW4, backpropagation is automatically inferred by PyTorch, so you only need to write code
for the forward pass.
Figure 1: Convolutional Neural Networks for Image Classification1
Figure 2: Example images from Fashion MNIST dataset [1]
The dataset we use is Fashion-MNIST dataset, which is available at https://github.com/zalandoresearch/
fashion-mnist and in torchvision.datasets. Fashion-MNIST has 10 classes, 60000 training+validation
images (we have splitted it to have 50000 training images and 10000 validation images, but you can change
the numbers), and 10000 test images.
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Image comes from https://medium.com/data-science-group-iitr/building-a-convolutional-neural-network-in-python-withtensorflow-d251c3ca8117
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Open the part1.ipynb notebook in Google Colab and implement the following:
• The architecture of the network (define layers and implement forward pass)
• The optimizer (SGD, RMSProp, Adam, etc.) and its parameters. (weight decay is the L2 regularization strength)
• Training parameters (batch size and number of epochs)
You should train your network on training set and change those listed above based on evaluation on the
validation set. You should run evalution on the test set only once at the end.
Complete the following:
1. Submit the notebook (with outputs) that trains with your best combination of model architecture,
optimizer and training parameters, and evaluates on the test set to report an accuracy at the end. (15
pts)
2. Report the detailed architecture of your best model. Include information on hyperparameters chosen
for training and a plot showing both training and validation accuracy across iterations. (10 pts)
3. Report the accuracy of your best model on the test set. We expect you to achieve over 85%. (10 pts)
Hints: Read PyTorch documentation for torch.nn at https://pytorch.org/docs/stable/nn.html and pick
layers for your network. Some common choices are
• nn.Linear
• nn.Conv2d, try different number of filters (out channels) and size of filters (kernel size)
• nn.ReLU, which provides non-linearity between layers
• nn.MaxPool2d and nn.AvgPool2d, two kinds of pooling layer
• nn.Dropout, which helps reduce overfitting
Your network does not need to be complicated. We achieved over 85% test accuracy with two convolutional
layers, and it took less than 5 mins to train on Colab. You will get partial credits for any accuracy over 70%,
so do not worry too much and spend your time wisely.
2 Pre-trained NN, Saliency Map and Adversarial Attack [35 pts]
2.1 Task 1: Pre-trained NN (8 pts)
In order to get a better sense of the classification decisions made by convolutional networks, your job is now
to experiment by running whatever images you want through a model pretrained on ImageNet. These can be
images from your own photo collection, from the internet, or somewhere else but they should belong to one
of the ImageNet classes. Look at the idx2label dictionary in part2.ipynb for all the ImageNet
classes.
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For this task, you have to find:
• One image (img1) where the pretrained model gives reasonable predictions, and produces a category
label that seems to correctly describe the content of the image
• One image (img2) where the pretrained model gives unreasonable predictions, and produces a category label that does not correctly describe the content of the image.
You can upload images in Colab by using the upload button on the top left. For more details on how to
upload files on Colab, please see our Colab tutorial. Submit the two images with their predicted classes
in your report.
2.2 Task 2: Saliency Map (12 pts)
When training a model, we define a loss function which measures our current unhappiness with the model’s
performance; we then use backpropagation to compute the gradient of the loss with respect to the model
parameters, and perform gradient descent on the model parameters to minimize the loss.
In the next two tasks will do something slightly different. We will start from a convolutional neural network
model which has been pretrained to perform image classification on the ImageNet dataset. We will use
this model to define a loss function which quantifies our current unhappiness with our image, then use
backpropagation to compute the gradient of this loss with respect to the pixels of the image. We will
then keep the model fixed, and perform gradient descent on the image to synthesize a new image which
minimizes the loss. As a part of this task we will first explore saliency maps and then perform an adversarial
attack in the next task.
A saliency map tells us the degree to which each pixel in the image affects the classification score for that
image. To compute it, we calculate the gradient of the unnormalized score corresponding to the correct
class (which is a scalar) with respect to the pixels of the image. If the image has shape (3, H, W) then this
gradient will also have shape (3, H, W); for each pixel in the image, this gradient tells us the amount by
which the classification score will change if the pixel changes by a small amount. To compute the saliency
map, we take the absolute value of this gradient, then take the maximum value over the 3 input channels;
the final saliency map thus has shape (H, W) and all entries are nonnegative. More specifically, we will
compute the saliency map as described in the section 3.1 of [2].
Implement the compute saliency maps function in part2.ipynb to build the saliency maps for
the two images from for the previous task (sec 2.1) and submit the saliency maps in the pdf.
2.3 Task 3: Adversarial Attack (15 pts)
We can also use image gradients to generate ”adversarial attacks” as discussed in [3]. Given an image and
a target class (not the correct class), you have to perform gradient descent over the image as we did in
the previous task to maximize the target class, stopping when the network classifies the image as the target
class.
Implement the make adversarial attack function in part2.ipynb to fool the network for the
correctly predicted image from the first task (sec 2.1). Submit the original image, the attacked image,
as well as the difference between them which you can get by running the last cell of the notebook.
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3 Semantic Segmentation [30 pts]
Besides image classification, Convolutional Neural Networks can also generate dense predictions. A popular
application is semantic segmentation. In this part, you will design and implement your Convolutional Neural
Networks to perform semantic segmentation on the Mini Facade dataset.
Mini Facade dataset consists of images of different cities around the world and diverse architectural styles
(in .jpg format), shown as the image on the left. It also contains semantic segmentation labels (in .png
format) in 5 different classes: balcony, window, pillar, facade and others. Your task is to
train a network to convert image on the left to the labels on the right.
Note: The label images are plotted using 8-bit indexed color with pixel value listed in Table 1. We have
provided you a visualization function using a “jet” color map.
Table 1: Label image consists of 5 classes, represented in [0, 4].
class color pixel value
others black 0
facade blue 1
pillar green 2
window orange 3
balcony red 4
Open up the part3.ipynb notebook in google colab and implement the following:
• A convolution network which takes an RGB image of size H × W × 3 and returns a segmentation
map of size H × W × 5. There is a dummy network already present which uses a 1×1 convolution to
convert 3 channels (RGB) to 5 channels, i.e. 5 heatmaps for each class, with cross-entropy loss.
• The optimizer (SGD, RMSProp, Adam, etc.) and its parameters. (weight decay is the L2 regularization strength)
• Training parameters (batch size and number of epochs)
We will evaluate your model on its average precision (AP) on the test set (higher the better). We have
provided you the code to evaluate AP and you can directly use it. An introduction to AP is available at
https://scikit-learn.org/stable/modules/model evaluation.html#precision-recall-f-measure-metrics.
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Complete the following:
1. Report the detailed architecture of your model. Include information on hyperparameters chosen for
training and a plot showing both training and validation loss across iterations. (10 pts)
2. Report the average precision on the test set. The function cal AP will calculate the average precision
on the test set. All hyperparameter tuning should be done on the validation set. We expect you to
achieve over 0.45 AP on the test set. (10 pts)
3. Submit the notebook which contains your best combination of model, optimizer and training parameters. (5 pts)
4. Find a satisfactory result in the output test folder generated by the get result function and
paste it in your pdf. There should be three images for each input – gt<num.png (ground truth
label), x<num.png (input RGB image) and y<num.png (predicted label). Below is an example
of a satisfactory result. (5 pts)
Figure 3: Semantic Segmentation Result
Hints: You should still choose layers from classes under torch.nn. In addition to layers mentioned in
the previous part, you might want to try nn.Upsample that upsamples the activation map by a simple
interpolation, and nn.ConvTranspose2d that upsamples by performing transpose convolution. Any
test AP over 0.25 will receive partial credits. You will easily get a high test AP if you use the U-net [4]
based architecture outlined in the notebook or you can try to implement a network from [5] [6], but they can
take a little longer to train.
Canvas Submission Checklist
In the zip file you submit to Canvas, the directory named after your uniqname should include the following
files:
• Code files: part1.ipynb, part2.ipynb and part3.ipynb
All plots and answer to questions should be included in your pdf report submitted to Gradescope. Run all
the cells of your Colab notebooks, and do not clear out the outputs before submitting. You will only get
credit for code that has been run.
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References
[1] H. Xiao, K. Rasul, and R. Vollgraf, “Fashion-mnist: a novel image dataset for benchmarking machine
learning algorithms,” 2017.
[2] K. Simonyan, A. Vedaldi, and A. Zisserman, “Deep inside convolutional networks: Visualising image
classification models and saliency maps,” CoRR, vol. abs/1312.6034, 2014.
[3] C. Szegedy, W. Zaremba, I. Sutskever, J. Bruna, D. Erhan, I. J. Goodfellow, and R. Fergus, “Intriguing
properties of neural networks,” CoRR, vol. abs/1312.6199, 2014.
[4] O. Ronneberger, P. Fischer, and T. Brox, “U-net: Convolutional networks for biomedical image segmentation,” ArXiv, vol. abs/1505.04597, 2015.
[5] E. Shelhamer, J. Long, and T. Darrell, “Fully convolutional networks for semantic segmentation,” IEEE
Transactions on Pattern Analysis and Machine Intelligence, vol. 39, pp. 640–651, 2017.
[6] A. Newell, K. Yang, and J. Deng, “Stacked hourglass networks for human pose estimation,” in ECCV,
2016.
Acknowledgement
The Mini Facade dataset are modified from CMP Facade Database by Radim Tylecek and Radim ˇ Sˇ ara. ´
Please feel free to similarly re-use our problems while similarly crediting us.
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