Computer Science Principles Unit 2 Chapter 1 Lesson 3: Encoding B&W Images (2018)

The class begins by asking students to invent their own image encoding protocol in order to familiarize themselves with some of the subtle complications of encoding images, namely the need for other data, called metadata, that describes properties of the image necessary for rendering it. Students will learn about pixels, raster images, and what an image file format is. Students will encode binary image data using a widget in Code Studio.

The main purpose of this lesson is for students to exhibit some creativity while getting some hands-on experience manipulating binary data that represents something other than plain numbers or text. Connections to abstraction in data can be made here. Connections can be made back to file sizes and file formats here as well - e.g. how many bytes does it take to store an image v. text? If you want to broach the subject, the concept of data compression can come in here too - it is interesting to think about how a black and white image might be compressed. You should be aware that this lesson largely acts a stepping stone to the next lesson which addresses how RGB colors are represented in binary.

Image file types have some similarities to data packets we saw in the Internet unit -- because images must include metadata or data about the data. The data of a black-and-white image is the list of bits that represent whether each pixel is on or off. To create the image, however, we must also know how wide and tall the image is in order to recreate it accurately. This necessitates the creation of a file format that clearly defines how this metadata will be encoded since it is crucial for interpreting the subsequent data of the image. It is similar to how an internet packet doesn't only contain the data you need to send, but must also include metadata like the to and from addresses and packet number.

Digital images can be stored in many formats, but one of the most common formats is "raster". Raster images store the image as an array of individual pixels, each of which has a particular color. Higher-quality images can be obtained by decreasing the size of the pixels (resolution). While full color will be addressed in the next lesson, an important idea here is that images on computer screens are created with light by illuminating pixels on the screen. This is why it is typical in a black and white image for the value 1 to represent white - it means to turn the light on - and 0 represents black - light off. If you were drawing on paper you might do the inverse.

Students will be able to:
- explain how images are encoded with pixel data.
- describe a pixel as an element of a digital image.
- encode a B&W image in binary representing both the pixel data (intensity) and metadata (width, height).
- create the necessary metadata to represent the width and height of a digital image, using a computational tool.
- explain why image width and height are metadata for a digital image.