# projection(3) — light onto a surface that is not flat

## NAME

projection — putting an image where the architect did not plan for one

---

## SYNOPSIS

```
throw ratio = distance / image width
lux at surface = lumens / area           (area grows with the square)
```

## DESCRIPTION

Projection is a budget problem wearing an optics costume. You have a
fixed number of lumens, a surface with a reflectance you did not choose,
and ambient light from a skylight nobody will agree to cover. Everything
else is arithmetic.

```
   projector          throw distance            surface
      [=]  - - - - - - - - - - - - - - - - ->  |
       |\                                      |
       | \  half-angle set by the throw ratio  |
       |  \ - - - - - - - - - - - - - - - - -> |
       |                                       |
       offset (lens shift, not keystone)
```

## FIELD NOTES

- **Keystone correction is not free.** Digital keystone throws away
  pixels and softens the edges you paid for. Lens shift is optical and
  costs nothing but mount geometry. Move the projector before you touch
  the menu.
- **Measure ambient at the surface, at the worst hour.** A gallery at
  9am and the same gallery at 2pm are different rooms. Contrast ratio
  against real ambient is the only number that predicts whether the
  content reads.
- **Blend zones want overlap, and overlap wants matching black level.**
  Two projectors at 50% in the overlap do not produce one projector at
  100% — they produce a brighter black. Choose the blend width for the
  black floor, not the white peak.
- **Every surface has a gain and it is rarely 1.0.** Raw drywall,
  primed drywall, and museum-grade matte paint differ by more than a
  projector upgrade would buy you. Paint is the cheapest lumen.
- **Warp last, calibrate first.** Undistort the lens, solve the
  homography or mesh, then apply the content. Doing it in the other
  order bakes an error into a file someone will re-export in a year.
- **Ten-year installs fail at the lamp schedule, not the math.** Write
  the filter cleaning interval on the mount, in paint pen, where the
  next person will be standing.

## SEE ALSO

`man homography`, `man camera-parameters`