name = "littletechbird/dirac-belt"
version = "1.0.0"
runtime = "steve"
license = "MIT"
summary = "Dirac belt (belt trick): a cube tied to anchor pads by ribbons, posed at any moment of a 720 degree untangle."
description = """Dirac's belt trick as a solid model. A cube is tied by two (or six) elastic ribbons to anchor pads that \
never move. Turning the cube 360 degrees leaves the ribbons twisted in a way no sliding can undo; after 720 degrees \
they can be untangled without turning the cube or the pads. It is the topology behind spin-1/2 particles, which \
need two full turns to come back to themselves.

`time` picks the moment in a 37.5 s sequence: rest, a 360 degree turn, Hanson's 360 loop, a second turn, the 720 \
untangle, four more ribbons grow out, and a six-ribbon finale in which the cube turns another 720 degrees. Each sphere around the cube turns as one rigid \
piece by an angle set only by its distance from the centre, so the ribbons never pass through each other or the \
cube at any moment, and each keeps its exact volume. Ribbons are B-spline solids fitted through the exact mapped \
points. Cube at the origin, ribbon +z up, the cube's +x face marked red. Units mm."""
tags = ["physics", "topology", "dirac-belt", "belt-trick", "spinor", "quaternion", "education", "animation"]

[parameters.time]
type = "number"
default = 24.0
min = 0
max = 37.5
step = 0.01
unit = "s"
label = "Moment in the motion"
description = "0-3 rest, 3-9 first 360 degree turn, 9-15 Hanson 360 loop, 15-21 second turn, 21-28 the 720 untangle, 28-29.5 side ribbons grow, 29.5-37.5 six-ribbon finale. Same timeline as the video, frame = time x 60."

[parameters.ribbons]
type = "choice"
default = "as in the video"
choices = ["as in the video", "1", "2", "6"]
label = "Ribbons"
description = "As in the video: the two vertical ribbons, then all six from 28 s. Or a fixed count: 1 (+z), 2 (+z, -z) or 6 (all faces) at any moment."

[parameters.cube_size]
type = "number"
default = 40
min = 5
max = 1000
unit = "mm"
label = "Cube size"

[parameters.ribbon_width]
type = "number"
default = 18
min = 0.5
max = 600
unit = "mm"
label = "Ribbon width"
description = "At most 0.5 x cube size."

[parameters.ribbon_thickness]
type = "number"
default = 0.8
min = 0.01
max = 50
unit = "mm"
label = "Ribbon thickness"
description = "At most 0.02 x cube size (as in the video), and at most a quarter of the ribbon width."

[parameters.ribbon_length]
type = "number"
default = 108
min = 12.5
max = 10000
unit = "mm"
label = "Ribbon length"
description = "From the cube face to the anchor pad; at least 2.5 x cube size."