CS Unplugged activity

Pixel Pictures: lesson plan

A short teacher-led plan for Pixel Pictures: how a computer turns a black-and-white picture into numbers, and how a fax machine squeezes those numbers down. About 50 minutes. Leaving it for a sub? Use the box below.

For a sub: the 2-minute version

Skip everything below this box. Nothing to set up.

Optional, if you can project: type https://learn.porttack.com/go/pixel into the browser’s address bar (not a search box), press Enter, then use the right arrow key to step through. Stop at the slide titled On Your Sheet, then hand out the sheets. Unsure about a slide? Skip the slides: the sheet explains everything.

Hand out the instructions, one per student. Then hand out Decode & Encode and Make Your Own right behind it, in that order. Names go on those two; nothing needs to be stapled.
Say: “Today you’re learning how a computer stores a picture as numbers. Read the first sheet, then follow the rule on your own. Shade lightly in pencil first.”
If the teacher left more puzzles, hand the next one to each student who finishes: a new set of mystery pictures to decode and encode.
Restless room? Let neighbors work in pairs. Talking about the puzzle is fine.
Collect Decode & Encode and Make Your Own from every student, plus at least one keep-going sheet from anyone who got that far (its Set # shows it’s their own work, not a copy of a neighbor’s). The instructions sheet doesn’t need to come back.

Before the sub day (teacher, or whoever preps the sub folder): print one copy of each sheet per student: (1) Instructions, (2) Decode & Encode, (3) Make Your Own. Then print the keep-going set: a ready class set of 30 already has 30 different 6-picture sets built in (one of the six is an encode question), two pages each (front and back), so print that one large page once for the whole class, not once per student. Different class size? Open the New set page and change Copies there instead.

Students will be able to

Explain that a picture on a screen is a grid of pixels, stored as numbers.
Decode and encode a black-and-white picture with run-length encoding.
Say when this kind of compression saves space and when it doesn’t.

Before class

Print one copy of each sheet per student, handed out in order: (1) Instructions, (2) Decode & Encode, (3) Make Your Own. Only the second two come back from students, so there’s no need to staple all three together.
Print the keep-going set once for the whole class, not once per student: a ready class set of 30 already has 30 different 6-picture sets built in (one of the six is an encode question), two pages each (front and back). Different class size? Open the New set page and change Copies there.
Have any photo ready to project, or use the slides instead.

The lesson

1. Hook (5 minutes)

Project a photo and zoom in until the squares show. Ask: “What’s under the squares?” (Nothing. Every picture is just a grid of pixels.) Then: “A computer can only store numbers. How could it store this?”

2. The sheets (25 minutes)

Students read the rule on the first sheet, then decode and encode on the second. Walk the room: the most common mistake is forgetting that every row starts with a white count, even when that count is 0.

When most students reach the third sheet, Make your own, pair them up to trade and decode each other’s pictures.

3. Keep going (as students finish)

Hand out keep-going sheets. Nobody sits idle and nothing is optional. Collect at least one from anyone who gets this far, along with their other two sheets.

4. Wrap-up (5 minutes)

Ask: “Why does a picture with big blocks of one color shrink a lot, and a checkerboard hardly at all?” (Long runs need few numbers; a checkerboard changes color every pixel.)

Going further

6th grade, first time through: the three sheets are already sized for this group. If time is short, cut Your partner’s pictures rather than Make your own: drawing and encoding their own picture is where the idea actually clicks.
9th grade or faster finishers: after all three sheets, hand out Pixel Pictures: how much do you save?. It has them count the numbers their own completed codes actually used and work out a real compression ratio, instead of taking “it saves space” on faith.
AP CSP: this activity is lossless compression (nothing is thrown away; the exact picture comes back). Use the down arrow on the slides’ On Your Sheet slide, or just ask in discussion: a real photo has thousands of colors, so what would a computer have to give up to compress one the same way? That trade-off is lossy compression (JPEG), and it’s a useful contrast to draw before or after the AP exam’s data compression questions.

Text Compression uses the same idea (store the pattern, not every piece) for words instead of pixels.