Inconsistent Lighting Behavior Between Textured and Untextured Geoms

[jjyyxx]

Intro

Hi!

I am a graduate student at HKU, I use MuJoCo for my research on robotic manipulation.

My setup

MuJoCo 3.3.0, Python, Linux

What's happening? What did you expect?

When comparing two geoms, A and B, under MuJoCo's GL_MODULATE setting:

• Geom A uses material MA with rgba = (0.5, 0.5, 0.5, 1) (a non-white color).
• Geom B uses material MB with rgba = (1, 1, 1, 1) and applies a pure-color texture (0.5, 0.5, 0.5, 1).

Under diffuse lighting, both geoms render the same color, as expected. However, when ambient or specular lighting is introduced:

• Geom A's rendered color changes according to ambient/specular intensity.
• Geom B's rendered color remains invariant to ambient/specular lighting.

This inconsistency makes it impossible to seamlessly transition between textured and untextured faces under non-diffuse lighting.

Additional Clarification Request: Color Space Handling

OpenGL's Fixed Function Pipeline (FFP) operates in linear RGB unless explicitly configured otherwise (e.g., via GL_SRGB8_ALPHA8 or GL_FRAMEBUFFER_SRGB). Since MuJoCo does not appear to enable either:

1. Are all input/output colors (including textures) treated as linear RGB?
2. If textures are typically authored in sRGB, does the lack of gamma correction lead to incorrect brightness after lighting?

I would appreciate any corrections or insights into these behaviors.

Steps for reproduction

Run script to generate textuer, then launch model.

Minimal model for reproduction

<mujoco>
    <visual>
        <headlight active="0" />
    </visual>
    <asset>
        <texture name="test" type="2d" file="test.png" />
        <material name="test" texture="test" rgba="1 1 1 1" specular=".5" />
        <material name="test2" rgba="0.4980392156862745 0.4980392156862745 0.4980392156862745 1" specular=".5" />
    </asset>

    <worldbody>
        <light directional="true" ambient="0.8 0.0 0.0" diffuse="1 1 1" specular="0.0 0.0 0.0" pos="0 0 5" dir="0 0 -1"/>
        <geom type="plane" size="0.1 0.1 1" pos="-0.1 -0.1 0" material="test"/>
        <geom type="plane" size="0.1 0.1 1" pos=" 0.1  0.1 0" material="test"/>
        <geom type="plane" size="0.1 0.1 1" pos=" 0.1 -0.1 0" material="test2"/>
        <geom type="plane" size="0.1 0.1 1" pos="-0.1  0.1 0" material="test2"/>
    </worldbody>
</mujoco>

Code required for reproduction

import PIL.Image
import numpy as np

image_array = np.zeros((16, 16, 3), dtype=np.uint8)
image_array[...] = (0x7F, 0x7F, 0x7F)
PIL.Image.fromarray(image_array).save("test.png")

Confirmations

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