Question: 0 duration metadata updates

[trevorknight]

It is my understanding that:

For head rotation, the duration is not used.

Duration of 0 for gains and object position means those metadata updates are discarded.

Duration of 0 for downmix info means that it is applied indefinitely (until another update is received).

Is this all correct?

[jingbo-marquis]

Thank you for the detailed breakdown. Your understanding is mostly correct, with some details to clarify:

1. Head rotation — duration not used ✅ Correct

As described in Issue #18, head rotation's duration is hardcoded to 0 in audio_renderer_set_head_rotation(), and OBR's obr_set_head_rotation() directly sets the rotation matrix without involving duration.

2. Gains — behavior when duration=0

For gain metadata, the duration=0 behavior depends on the gain level:

• Group-level output gain (oar_update_metadata with ck_metadata_gain): In oar.c:435, group->output_gain.duration += metadata->duration. If duration=0, group->output_gain.duration does not increase. In audio_block_sub_frames_apply_gain() (audio_renderer_base.c:200-271), the gain application logic compares sample_gain_position with accumulated_duration to find the active metadata. If all metadata have duration=0, accumulated_duration remains 0, and sample_gain_position >= 0 && sample_gain_position < 0 is always false, so the gain is never applied — effectively discarded. ✅ Your understanding is correct.

• Element-level gain (oar_update_audio_element_metadata with ck_metadata_gain): In audio_element_context_update_gain() (audio_renderer_base.c:131), gain->duration += metadata->duration. Similarly, duration=0 gain metadata will not be matched in the elapse and apply logic, effectively discarded. ✅ Your understanding is correct.

3. Object positions — behavior when duration=0

For object position metadata, the duration=0 behavior differs between OLR and OBR:

• OLR (loudspeaker rendering): In olr.c:186, metadata_block.duration = metadata->duration > 0 ? metadata->duration : 0. When duration=0, end_sample == start_sample. In interpret_object_metadata_process() (interpret_object_metadata.c:48-49), interp = 0 and target_sample = start_sample. Since start_sample == target_sample and target_sample == end_sample, the function returns 0 (null processing block), producing no processing — effectively discarded. ✅ Your understanding is correct.

• OBR (binaural rendering): OBR's _metadata_update (obr.c:207-213) directly calls obr_update_object_channel_position() without checking duration. The position update is immediately applied to the object channel and AmbisonicEncoder's target property. Duration is not used at all in the OBR path — the position update takes effect immediately and is not discarded. ⚠️ This differs from your understanding.

4. Downmix info — applied indefinitely when duration=0 ✅ Correct

In downmix.c:128-138's _render() function:

if (renderer->metadata_duration <= 0 && !queue_is_empty(renderer->downmix_info)) {
    // pop and apply new downmix mode
    renderer->metadata_duration = metadata->duration;
}

When metadata->duration == 0, renderer->metadata_duration is set to 0. On the next render frame, metadata_duration <= 0 is true, but queue_is_empty checks for new downmix metadata. If no new metadata exists, the last set downmix mode is retained — effectively applied indefinitely. ✅ Your understanding is correct.

Summary:

Metadata Type	duration=0 Behavior	Your Understanding
Head rotation	duration not used	✅ Correct
Gain	Discarded (no sample position matched)	✅ Correct
Object position (OLR)	Discarded (null processing block)	✅ Correct
Object position (OBR)	Takes effect immediately, not discarded	⚠️ Needs correction
Downmix info	Applied indefinitely	✅ Correct

The inconsistency between OBR and OLR in object position duration handling is due to design differences between OBR and OLR. We will consider unifying this behavior in the future.