Demonstrate nested VizGroup hierarchies

Keywords: scenes · VizGroup · hierarchy · nested

Builds a two-link planar arm out of nested scene-graph groups::

arm1  (top-level group — pivot at the world origin)
├── rod1     segment from arm1's origin to its far end
└── arm2     (group *nested inside* arm1, offset to the end of rod1)
    ├── rod2 segment from arm2's origin to its tip
    └── tip  point at the end of rod2

Each rod rotates about its own origin:

• arm1 rotates rod1 (and everything nested under it) about pivot 1.
• arm2 rotates rod2 about pivot 2 — the end of rod1 — which is itself carried around by arm1's rotation, so the two rotations compose.

Only the two group transforms change per frame (the transform aspect), so the rod geometry is never re-serialized.

Each rod's label is anchored to the center of the line via LabelStyle(along=0.5) — along is the fraction along the line's extent (0 = start, 0.5 = midpoint, 1 = end).

Run

uv run python py/examples/viz/scenes/nested_groups.py

Source

viz/scenes/nested_groups.py

Code

# SPDX-License-Identifier: Apache-2.0
# Copyright 2021 Christian Perwass

"""nested_groups.py — Demonstrate nested ``VizGroup`` hierarchies.

Builds a two-link planar arm out of *nested* scene-graph groups::

    arm1  (top-level group — pivot at the world origin)
    ├── rod1     segment from arm1's origin to its far end
    └── arm2     (group *nested inside* arm1, offset to the end of rod1)
        ├── rod2 segment from arm2's origin to its tip
        └── tip  point at the end of rod2

Each rod rotates about its *own* origin:

* ``arm1`` rotates ``rod1`` (and everything nested under it) about pivot 1.
* ``arm2`` rotates ``rod2`` about pivot 2 — the end of rod1 — which is itself
  carried around by ``arm1``'s rotation, so the two rotations compose.

Only the two group transforms change per frame (the ``transform`` aspect), so
the rod geometry is never re-serialized.

Each rod's label is anchored to the *center* of the line via
``LabelStyle(along=0.5)`` — ``along`` is the fraction along the line's extent
(``0`` = start, ``0.5`` = midpoint, ``1`` = end).

Run with:  uv run python py/examples/viz/scenes/nested_groups.py

Keywords: scenes, VizGroup, hierarchy, nested
"""

import math

from pytanga.geometry import Line, Point
from pytanga.viz import LabelStyle, LineStyle, PointStyle, Visualizer

L1 = 2.0  # length of rod 1
L2 = 1.5  # length of rod 2

viz = Visualizer(title="Tanga — Nested Groups")
viz.show()

# ── arm1: pivot at the world origin ─────────────────────────
arm1 = viz.add_group("arm1")
arm1.new(Point(0, 0, 0), color="#ffaa00", label="pivot1", style=PointStyle(size=0.10))
arm1.new(
    Line.from_points(Point(0, 0, 0), Point(L1, 0, 0)),
    color="#ff5555",
    label="rod1",
    label_style=LabelStyle(along=0.5),  # anchor label at the line's midpoint
    style=LineStyle(thickness=3.0),
)

# ── arm2: nested inside arm1, offset to the end of rod1 ─────
arm2 = arm1.add_group("arm2")
arm2.set_transform(position=(L1, 0.0, 0.0))
arm2.new(Point(0, 0, 0), color="#ffaa00", label="pivot2", style=PointStyle(size=0.09))
arm2.new(
    Line.from_points(Point(0, 0, 0), Point(L2, 0, 0)),
    color="#5599ff",
    label="rod2",
    label_style=LabelStyle(along=0.5),  # anchor label at the line's midpoint
    style=LineStyle(thickness=3.0),
)
arm2.new(Point(L2, 0, 0), color="#44ff44", label="tip", style=PointStyle(size=0.10))

viz.flush()

print("Animating two nested rods until Ctrl+C...")
t = 0.0
for dt in viz.animate(fps=50):
    t += dt

    # arm1 rotates rod1 + arm2 + rod2 about pivot 1 (its own origin).
    arm1.set_transform(rotation=(0.0, 0.0, math.sin(t) * 1.2))

    # arm2 rotates rod2 about its own origin (pivot 2), compounding with arm1.
    arm2.set_transform(rotation=(0.0, 0.0, math.sin(1.7 * t) * 1.5))

    viz.flush()

print("Animation stopped.")