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 veryfast or faster.

// 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_threads is set to auto (0) or specific count.
• Slicing: For strict low latency, use i_slice_max_size to 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