Configurable Lighting in Meshcat

[yu-fz]

I was looking at the drake meshcat API to see if I could customize the default meshcat scene lighting. The Meshcat class documentation states that adding lights is possible: You can modify these elements, create new lights/cameras, and even delete these elements (one at a time). However, a look through the meshcat methods and the meshcat js source code suggests that this may not be implemented. I can change certain parameters for the default scene lights in meshcat such as intensity,position, and castShadow via a meshcat.SetProperty(...) call, but it seems that I am unable to change the light color or add any new lights.

A related issue is that the meshcat path /Lights/DirectionalLight appears deprecated, but this is still provided as an example for the meshcat.SetProperty() documentation, and may have been remapped to /Lights/FillLight instead.

Suggested solution: Provide a SetLight() method for meshcat that takes as its arguments a path, light type, RGB color, intensity, and position. Any additional properties specific to certain light types such as decay, distance, castShadow can be modified after construction via meshcat.SetProperty()

[SeanCurtis-TRI]

I've been poking into this as well, and generally agree.

I'd take this a step further and suggest that we need to add light configuration to a meshcat configuration struct (and a corresponding yaml) so that the lights can be configured easily. That configuration struct should also be consumed by model_visualizer and meldis.

[jwnimmer-tri]

I can change certain parameters for the default scene lights in meshcat such as intensity,position, and castShadow via a meshcat.SetProperty(...) call, but it seems that I am unable to change the light color or add any new lights.

FYI see https://github.com/RobotLocomotion/drake/pull/20920, where the user can set properties at construction-time now. As you say, that only allows edits to the existing lights, not adding new ones.

For adding new lights, I recommend this approach: #20941.

If we allow an advanced user to call Meshcat::SetObject(path : str, object_json : str) with an arbitrary JSON object, then the user can place whatever kind of light they want into that description. That's a kind of general solution that unlocks a lot more use cases than just this one.

Of course it would also be a kindness to add "syntactic sugar" to our Meshcat class where the user can specifically add a light and we have nice kwargs to choose its settings, but at the end of the day it just desugars down to a set_object call.

[jwnimmer-tri]

Closing as duplicate of #20941.

After we implement #20941, if we find that things are still too difficult we can circle back here and re-open / file a new ticket.