Raven lays two snapshot cards side by side. Each one is a 5 by 5 grid of brightness numbers.
"One number is different," she says. "Find it."
Rocket squints at the grids. "They look exactly the same. This is going to take forever."
Nova dims her lights so the numbers are easy to read. "Remember, a picture is pixels," she says.
"And each pixel is just a number. Change one number and you change the picture, even if nobody notices."
Rocket picks up a pencil and starts checking, square by square.
Remember from Week 3: pixel values go from 0 (black) to 255 (white).
Here are the brightness values for the center of two snapshot cards.
| Card A | A | B | C | D | E |
|---|---|---|---|---|---|
| Row 1 | 20 | 35 | 40 | 35 | 20 |
| Row 2 | 35 | 90 | 150 | 90 | 35 |
| Row 3 | 40 | 150 | 250 | 150 | 40 |
| Row 4 | 35 | 90 | 150 | 90 | 35 |
| Row 5 | 20 | 35 | 40 | 35 | 20 |
| Card B | A | B | C | D | E |
|---|---|---|---|---|---|
| Row 1 | 20 | 35 | 40 | 35 | 20 |
| Row 2 | 35 | 90 | 150 | 90 | 35 |
| Row 3 | 40 | 150 | 250 | 150 | 40 |
| Row 4 | 35 | 90 | 150 | 80 | 35 |
| Row 5 | 20 | 35 | 40 | 35 | 20 |
Here is one Sky Sorter feature row, before and after someone edited the snapshot.
| Version | Brightness | Size | Streak? |
|---|---|---|---|
| Original snapshot | 210 | 2 | yes |
| Edited snapshot | 210 | 2 | no |
| Lab idea | True or false? |
|---|---|
| Changing one pixel value changes the picture. | ? |
| A change you cannot see by eye cannot change a computer's decision. | ? |
| Erasing one feature can change Sky Sorter's label. | ? |
Excellent detective work. Tomorrow you build a slow-down checklist for anything that seems too wild to be true.