[Bug Report] --video does not work in headless mode

[nblauch]

If you are submitting a bug report, please fill in the following details and use the tag [bug].

Describe the bug

--video used to be capable of recording videos without enabling a particular GUI visualizer. This is now not the case.

Steps to reproduce

Please try to provide a minimal example to reproduce the bug. Error messages and stack traces are also helpful.

Compare

./isaaclab.sh -p scripts/reinforcement_learning/rsl_rl/train.py --task=Isaac-Velocity-Rough-Anymal-C-v0 --video

vs.

./isaaclab.sh -p scripts/reinforcement_learning/rsl_rl/train.py --task=Isaac-Velocity-Rough-Anymal-C-v0 --video --viz kit

The later produces correct video, whereas the former produces a black screen. This makes video recording impossible in headless cluster training settings, which hinders research where seeing developing policies during training can be very informative.

System Info

Describe the characteristic of your environment:

• Commit: [e.g. 8f3b9ca]
• Isaac Sim Version: [e.g. 5.0, this can be obtained by cat ${ISAACSIM_PATH}/VERSION]
• OS: [e.g. Ubuntu 22.04]
• GPU: [e.g. RTX 5090]
• CUDA: [e.g. 12.8]
• GPU Driver: [e.g. 553.05, this can be seen by using nvidia-smi command.]

Additional context

Add any other context about the problem here.

Checklist

• [x ] I have checked that there is no similar issue in the repo (required)
• I have checked that the issue is not in running Isaac Sim itself and is related to the repo

Acceptance Criteria

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• Criteria 1
• Criteria 2