BUGFIX: Adds a fix for parsing gridlayout

[wpumacay]

• Resets the gridlayout string to zeros with memset to avoid taking the extra characters that are not copied when using memcpy with a shorter string
• Adds a test for this when using mjSpec

Here is a short Python script that uses mjSpec. Note that after saving the model the gridlayout string in the XML file contains some extra dots, instead of the value LFRB.D.. it gets the value LLFRB.D.......

from pathlib import Path

import mujoco as mj

CURRENT_DIR = Path(__file__).parent

SKYBOX_NAME = "SkyOakland"

XML_STRING = """
<mujoco model="test-scene">
  <visual>
    <headlight diffuse="0.6 0.6 0.6" ambient="0.3 0.3 0.3" specular="0 0 0"/>
    <rgba haze="0.15 0.25 0.35 1"/>
    <global azimuth="120" elevation="-20"/>
  </visual>

  <asset>
    <texture type="skybox" builtin="gradient" rgb1="0.3 0.5 0.7" rgb2="0 0 0" width="512" height="3072"/>
    <texture name="SkyOakland" type="skybox" file="{texname}" gridsize="2 4" gridlayout="LFRB.D.."/>
    <texture type="2d" name="groundplane" builtin="checker" mark="edge" rgb1="0.2 0.3 0.4" rgb2="0.1 0.2 0.3"
      markrgb="0.8 0.8 0.8" width="300" height="300"/>
    <material name="groundplane" texture="groundplane" texuniform="true" texrepeat="5 5" reflectance="0.2"/>
  </asset>

  <worldbody>
    <light pos="1 -1 1.5" dir="-1 1 -1" diffuse="0.5 0.5 0.5" directional="true"/>
    <geom name="floor" size="0 0 0.05" type="plane" material="groundplane"/>
  </worldbody>
</mujoco>
"""


def main() -> int:
    skybox_texture_path = CURRENT_DIR / f"{SKYBOX_NAME}.png"
    model_str = XML_STRING.format(texname=skybox_texture_path.as_posix())

    spec: mj.MjSpec = mj.MjSpec.from_string(model_str)
    print(f"gridlayout: {spec.texture(SKYBOX_NAME).gridlayout}")
    print(f"gridsize: {spec.texture(SKYBOX_NAME).gridsize}")

    _ = spec.compile()
    test_model_path = CURRENT_DIR / "test_model.xml"
    with open(test_model_path, "w") as fhandle:
        fhandle.write(spec.to_xml())

    try:
        _ = mj.MjModel.from_xml_path(test_model_path.as_posix())
    except Exception as e:
        print(f"Got error while opening the saved model: {e}")

    return 0


if __name__ == "__main__":
    raise SystemExit(main())

I ran into this issue when using mjSpec to convert a scene that contained a skybox. Here's an example in C++. When printing the gridlayout field it prints the string with the extra . characters