By name: tune_encoder description: Guide for tuning x264 encoder settings (latency vs quality)
Tune Video Encoder
This skill helps you adjust src/encoder.rs to optimize the H.264 encoding pipeline for the desired balance between latency and image quality.
🧠 Context
The encoder uses libx264 (exposed via C bindings or the x264 crate).
Key parameters:
- Preset: Speed vs Compression efficiency (
ultrafast,superfast,veryfast,medium,slow). - Tune: Optimization target (
zerolatency,film,animation). - Bitrate / CRF: Constant bitrate or Constant Rate Factor (quality).
🛠️ Configuration Guide
1. Achieving Lowest Latency
To minimize delay (Glass-to-glass latency), prioritize speed over compression.
// In src/encoder.rs
// 1. Use 'ultrafast' or 'superfast'
// 2. Set tune 'zerolatency' (CRITICAL)
let mut param = x264_param_t::default();
x264_param_default_preset(&mut param, "superfast\0".as_ptr() as *const i8, "zerolatency\0".as_ptr() as *const i8);
Tradeoff: Higher bitrate required for same quality, or lower quality at same bitrate.
2. Improving Image Quality
If the image is blocky or blurry:
- Lower CRF: (Lower is better quality). Range 0-51. Default ~23. Try 18-20 for high quality.
- Increase Bitrate: If using CBR/ABR.
- Slower Preset: switch to
veryfastorfaster.
// Adjusting Quality (CRF)
param.rc.i_rc_method = X264_RC_CRF;
param.rc.f_rf_constant = 20.0; // Higher quality than default
3. Handling Profiling (High/Main/Baseline)
Some H/W decoders on clients (older phones) only support Baseline.
x264_param_apply_profile(&mut param, "baseline\0".as_ptr() as *const i8);
🚀 Performance Tips
- Threads: x264 is multithreaded. Ensure
param.i_threadsis set to auto (0) or specific count. - Slicing: For strict low latency, use
i_slice_max_sizeto send smaller NAL units, reducing jitter.
🧪 Verification
Monitor the stats endpoint (/stats) to see if frames_captured matches frames_encoded. If dropped frames increase, the encoder is too slow (CPU bottleneck).
Also run the unit tests to ensure configuration validity:
cargo test