Fix --video producing black frames without --viz kit (issue #5052)

[pascal-roth]

Summary

• Root cause: SimulationContext.render() was not calling omni.kit.app.get_app().update() when no active visualizer pumped the Kit app loop. Replicator render products (used by gym.wrappers.RecordVideo via render_mode="rgb_array") were never refreshed, producing black video frames.
• Fix: Call app.update() in render() when has_kit() is True and no visualizer with pumps_app_update() == True is active. KitVisualizer already calls app.update() in its own step(), so we skip the call to avoid double-rendering when --viz kit is used.
• Tests: Two regression tests added to test_simulation_context.py — one verifying app.update() IS called without a visualizer, one verifying it is NOT called when a visualizer already pumps the Kit loop.

Test plan

• Regression test test_render_pumps_app_update_without_visualizer — fails without fix, passes with fix
• Regression test test_render_skips_app_update_when_visualizer_pumps_it — passes with fix (no double-rendering)
• Pre-commit checks pass (./isaaclab.sh -f)
• Manual: ./isaaclab.sh -p scripts/reinforcement_learning/rsl_rl/train.py --task=Isaac-Velocity-Rough-Anymal-C-v0 --video should produce correct (non-black) video

Closes #5052

🤖 Generated with Claude Code

[nblauch]

Hi @pascal-roth . Thanks for this! I tested it out. It does fix the blank videos, but it seems the camera position is not updated properly according to the ViewerCfg. For the example listed in my issue (#5052 ), the video outputs are from a different perspective for --viz none (headless) vs. --viz kit. I also found this for other custom scripts using a DirectRLEnv setup.

[pascal-roth]

Thanks for testing and reporting this, @nblauch!

The camera perspective mismatch you're seeing (headless --viz none vs --viz kit) is a separate issue from the black frames this PR addresses. In short: this PR only ensures app.update() is called so replicator render products are refreshed — it doesn't set the recording camera's position from ViewerCfg.

Good news: that exact problem is already being fixed in PR #5011 (Add perspective video recording via Newton GL viewer for Kitless backends; Fix for Kitfull backends camera position). That PR explicitly reads viewer.eye / viewer.lookat from the task config and passes them to the video recorder as camera_position / camera_target, so the recording matches the configured viewer perspective regardless of the visualizer backend.

To avoid duplicating the fix here and keeping both diffs focused, I'd suggest we track resolution in #5011. Once that lands, the ViewerCfg perspective will be honoured in all render modes.

Let me know if the scope there differs from what you experienced!