Two arrows on a flat floor, each a list [across, up]. The direction marks a line through the start; light falls on it at a right angle, so every point of arrow drops straight onto that line. Write shadow(arrow, direction): how far the arrow’s shadow reaches along the direction. A shadow that falls backward, against the direction, counts as negative.
| across | up |
|---|---|
| 3 | 4 |
| across | up |
|---|---|
| 1 | 0 |