5.0 / 5
We will cover the following tasks in 1 hour and 1 minute:
Loading the Data and Performing K-Means Clustering
We begin by loading the digits from the MNIST dataset and then finding the KMeans clusters. The digits consist of 1,797 samples with 64 features, where each of the 64 features is the brightness of one pixel in an 8×8 image
Plotting the Cluster Centers
In the previous task, we noticed that the cluster centers can be interpreted as a digit within the cluster. Here, we plot the cluster centers to see what the look like. We will find that even without any label information, k-means is able to find clusters whose centers are recognizable digits.
The k-means algorithm is blind to the true cluster assignment. So, the class labels from 0-9 can be permuted, resulting in incorrect labeling of the digits.
In the first half of this task, we solve the above issue by matching each learned cluster assignment with the true labels found in them.
Next, we evaluate our model using the accuracy score. This metric tells us how accurate our k-means clustering is in finding similar digits within the data. You’d be surprised to find that running a simple k-means on the data is sufficient to discover almost 80% of the correct grouping of the input.
Interpreting the Confusion Matrix
We plot the confusion matrix of the cluster centers we visualized before. Following from that insight, we observe that our model is confused between 8 and 1.
Even with its limitations, we will have shown that we can build a good digit classifier, using k-means, without using any known class labels!
Loading a Sample Image for Color Compression
With this task, we begin our journey into applying k-means for color compression within images.
We use Scikit-Learn’s
datasets module to load a sample image and explore its attributes. Through the rest of this project will work with the same image and compress the original 16 million colors to just 16 colors!
From 16 Million to 16 Colors
In this task, we first normalize the data. We then use k-means across the pixel space to reduce the 16 million colors in our sample image to just 16 colors.
After visualizing these pixels in the color space, and comparing the original to the reduced representation, we find that the result is a recoloring of the original pixels, where each pixel from the sample image is assigned the color of its closest cluster center.
Plotting the Results
In the last task, we visualized the pixels in the color space. Given the abstract nature of color space, let us now plot our result from k-means in the image space. This let’s us compare our sample image of 16 million colors to our compressed image of just 16 colors, achieving a compression factor of around 1 million!
About the Host (Snehan Kekre)
Snehan Kekre is a Machine Learning and Data Science Instructor at Coursera. He studied Computer Science and Artificial Intelligence at Minerva Schools at KGI, based in San Francisco. His interests include AI safety, EdTech, and instructional design. He recognizes that building a deep, technical understanding of machine learning and AI among students and engineers is necessary in order to grow the AI safety community. This passion drives him to design hands-on, project-based machine learning courses on Rhyme.