World planes

at puts a screen primitive somewhere on the screen. onPlane puts it in the 3D scene instead, flat on a plane, so it turns with the camera and stays on whatever it is labelling.

show << Formula::Add("\\mathbf{n}")->onPlane("wall");

The name works like a persistent label, except that it names a transform rather than a screen position: press T to place the plane with the gizmos.

Explicit construction

You can also give the plane parameters yourself, as an origin, an in-plane direction and a normal:

onPlane(const vec& origin,const vec& u,const vec& normal,scalar alpha = 1)

The item is as wide as u is long, and its height follows from its own aspect ratio.

show << fig->onPlane(vec(0,0,0), vec(0.6,0,0), vec(0,0,1));

Deck format

on: is the deck form of both.

- image: chart.png
  on: wall              # a plane placed with the T gizmo
  two_sided: true       # also drawn from behind
- formula: \mathbf{n}
  on: {origin: p, u: e1, normal: nor}

In the second form each of the three is either three numbers, or the name of a snippet variable re-read every frame.

on: takes no other placement

The item lives in the scene, so there is no screen position left to set: at:, follow: and below:/above:/right_of:/left_of: are refused next to it.