Minimal example: Meshcat opacity with GLTF files

iiwa.dmd.yaml

directives:
- add_model:
    name: iiwa
    file: package://drake_models/iiwa_description/sdf/iiwa7_no_collision.sdf # Drake 1.28.0
    # file: package://drake/manipulation/models/iiwa_description/iiwa7/iiwa7_no_collision.sdf # Drake 1.27.0
- add_weld:
    parent: world
    child: iiwa::iiwa_link_0

meshcat_opacity_example.py

import os

from pydrake.all import (
    AddMultibodyPlantSceneGraph,
    BodyIndex,
    DiagramBuilder,
    MeshcatVisualizer,
    MultibodyPlant,
    Simulator,
    StartMeshcat,
    Parser,
)


def main():
    model_path = "./iiwa.dmd.yaml"
    alpha = 0.5

    # Create plant
    builder = DiagramBuilder()
    plant: MultibodyPlant
    plant, scene_graph = AddMultibodyPlantSceneGraph(builder, time_step=0.0)
    parser = Parser(plant)
    parser.AddModels(model_path)
    plant.Finalize()

    # Create meshcat
    meshcat = StartMeshcat()
    MeshcatVisualizer.AddToBuilder(builder, scene_graph, meshcat)

    diagram = builder.Build()

    simulator = Simulator(diagram)
    simulator.set_target_realtime_rate(1.0)
    simulator.Initialize()

    # Make existing visual geometries transparent
    for i in range(plant.num_bodies()):
        body = plant.get_body(BodyIndex(i))
        if body.name() == "world":
            continue
        meshcat_body_path = os.path.join(
            "visualizer", *body.scoped_name().to_string().split("::")
        )
        for idx in plant.GetVisualGeometriesForBody(body):
            meshcat_path = os.path.join(
                meshcat_body_path, str(idx.get_value()), "<object>"
            )
            meshcat.SetProperty(meshcat_path, "opacity", alpha)

    # Simulate
    while True:
        simulator.AdvanceTo(simulator.get_context().get_time() + 1.0)


if __name__ == "__main__":
    main()