Unexpected Light behavior plane vs box

[fremaxter]

Intro

Hi!

My setup

mujoco 3.2.6

My question

When using a simple world with a plane and a spotlight (directional="false") the render looks as expected. But for other objects, for example a box or mesh as a ground, the spotlight does not fade off equally in every direction as expected. I have included the code and example image for both cases.

I would expect and like for the box to interact the same as the plane with the light. Is there a reason for the observed behavior?

Expected light behavior:

<worldbody>

  <geom name="large_box_geom" type="plane" size="5 5 0.2" pos="0 0 0"/>

  <light name="main_light" pos="0 0 5" dir="0 0 -1" diffuse="1 1 1"/>

</worldbody>

Unexpected light behavior:

<worldbody>

  <geom name="large_box_geom" type="box" size="5 5 0.2" pos="0 0 0"/>

  <light name="main_light" pos="0 0 5" dir="0 0 -1" diffuse="1 1 1"/>

</worldbody>

Minimal model and/or code that explain my question

No response

Confirmations

• I searched the latest documentation thoroughly before posting.
• I searched previous Issues and Discussions, I am certain this has not been raised before.

[yuvaltassa]

This is explained somewhere in the docs, but I forget where. Reducing the third number in the plane size spec will fix it (you currently have 0.2). Also you can use wireframe ('F' in simulate) to better understand what is happening. Basically lighting is interpolated between plane vertex subdivisions.

[fremaxter]

Thanks for the fast reply! I'm not sure I fully understand your answer.

The first code snippet with: <geom name="large_box_geom" type="plane" size="5 5 0.2" pos="0 0 0"/> does already produce the desired behavior.

My main question is: why does it not work with type="box" (as shown in the second image and code I attached)? Is it possible to use a box-type object and achieve the same smooth circular spotlight lighting as with type="plane"?

The real objective is: I would like to import a mesh, but the spotlight does not look correct for it. I then boiled it down to the fact that even a simple box does not produce the expected spotlight fade-out. So, my question is: can I use a box or a mesh-type object to achieve the same spotlight fade-out effect that I can reproduce using type="plane"?

[yuvaltassa]

Ah sorry, I misread your question. You want to increase https://mujoco.readthedocs.io/en/latest/XMLreference.html#visual-quality-numquads

[fremaxter]

No problem :). I can confirm that increasing this value does indeed produce the expected behavior for type="box".

Looking at the wireframe like you mentioned, I can also see the difference in resolution for these objects.

This works for using type="box." What would you recommend as the best approach or setting (in terms of performance) if one imports a mesh (for example, also a simple box)? I just checked, and it does not apply to meshes.

(I just tried adding a similar vertices-grid to the top face of a cube in Blender and then imported it as a mesh to MuJoCo. After this, it does also create the expected light behavior (compared to without the added vertices; no light is absorbed by the mesh cube). Is it the same in terms of performance if it’s part of the mesh compared to the grid used by a plane or box?)

[yuvaltassa]

Well the actual mesh you collide with is the convex hull, so if the added vertices are coplanar they should not have any effect. You can verify by looking at the wireframe (W) of the convex hulls (H), note that whether a vertex is coplanar or not can depend on very tiny numerical differences, and mesh vertices are float32, not float64...

[fremaxter]

Okay. Thanks for these answers and quick explanations.

Checking the large cube created in blender with the added vertice grid does indeed only show 2 triangle top faces when looking at the convex hulls (W and H).

• Checking the same for the builtin type="box" with increased numquad does still show the high-res grid for convex hulls. Would this mean that the increased numquads type="box" collision calculation are slower compared to the mesh box that only has 2 convex hull top faces?
• Just to make sure, would you recommend another approach for having multiple box type objects with smooth lighting? (Or does, based on these checks, increasing face/vertices count in blender for the cube seem currently the best option?)

[yuvaltassa]

1. No. The subdivisions are purely visual. A box is just a box 🙂
2. Adding coplanar vertices faces should be a good solution, as long as the convex hull does get more complicated.