Root cause of the Mac "session ended" at 880 Mbps / 1.3 Gbps: the host requests a
bitrate NVENC can't express at any codec level and `avcodec_open2` returns EINVAL
("Invalid argument"), so the pipeline build fails after 4 identical retries and the
session dies at encoder init — before a single video packet (which is why the
client's UDP counters never moved). The ceiling is GPU/driver-specific: an RTX 4090
caps HEVC at ~800 Mbps (Level 6.2 High tier) and rejects above it, while an RTX
5070 Ti accepts 1.3 Gbps.
Rather than hard-cap every build to a conservative guess (which would needlessly
throttle capable cards), open_video now PROBES: open at the requested bitrate, and
step down (codec spec ceiling, then 0.75x to a 50 Mbps floor) ONLY when this GPU
returns EINVAL. Each GPU runs at its own real maximum — the 5070 Ti keeps 1.3 Gbps,
the 4090 lands at 800 Mbps and streams instead of dying. Non-EINVAL failures (no
GPU, bad mode, OOM) still surface immediately rather than being masked by retries.
Codec::max_bitrate_bps is now just the first step-down candidate, not a clamp.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
enricobuehler
40a7b2c835