4.6 Keystone
Tilt a projector and its footprint stops being a rectangle: the far edge is further away, so it is wider. Throwcast reports the true trapezoid rather than a planar approximation.
Worked example — the demo project’s cyc machine
| Throw ratio / tilt | 1.44 · 12° down |
| Throw distance | 7.54 m |
| Nominal image | 5.24 × 3.27 m |
| Top edge (further away) | 5.12 m |
| Bottom edge (nearer) | 5.62 m |
| Difference | 0.50 m — about 9 % |
Half a metre of taper across a 5 m image. In a blend that is the difference between a seam you cannot see and one you can.