Question: head_rotation update can be applied to either the group or element, yes?

[trevorknight]

The head tracking metadata can be applied to either the oar_update_metadata or oar_update_audio_element_metadata, but it is applied universally, regardless, right? And duration is ignored, yes?

Maybe it's too late for it, but what if it was its own method:

oar_update_head_rotation(quaternion_t head_rotation);

?

[jingbo-marquis]

Thank you for raising this question. Your understanding is mostly correct. Here are the details:

1. head_rotation can be updated via two APIs, both globally effective:

• oar_update_metadata(oar, gid, metadata) — Updated at the group level (oar.c:437-456). The head rotation is stored in oar->head_rotation, then propagated to all renderers with ctx.out == ck_rio_id_binaural within that group via set_head_rotation.
• oar_update_audio_element_metadata(oar, id, metadata) — Updated at the element level (audio_renderer_base.c:434-451). The head rotation is passed to the underlying renderer via the metadata_update interface (OBR's _metadata_update handles ck_metadata_head_rotation at obr.c:216-223).

Both paths ultimately call OBR's obr_set_head_rotation(), making them equivalent. Head rotation is global — it affects the Ambisonic rotation matrix of all binaural renderers, not limited to a specific element or group.

2. duration is indeed ignored for head_rotation:

In audio_renderer_set_head_rotation() (audio_renderer_base.c:446), metadata.duration is hardcoded to 0:

metadata.duration = 0;  // Not used for head rotation

In OBR's _metadata_update (obr.c:216-223), head rotation directly calls obr_set_head_rotation() without any duration processing. Head rotation is an immediate state update, not a time-based parameter animation.

3. Regarding the standalone API suggestion:

Your proposal of oar_update_head_rotation(quaternion_t head_rotation) is a reasonable design suggestion. The current API is designed this way to be compatible with IAMF rendering functionality — IAMF's metadata system passes all parameter types (gain, object positions, head rotation, downmix mode, etc.) through a unified oar_metadata_t structure, with head rotation embedded as one metadata type.

However, head rotation is semantically different from other metadata — it is a global rendering state rather than an element/group-level parameterized metadata. For new API design, we will fully consider this suggestion.