The three libavcodec backends each set the identical low-latency rate-control
block on the not-yet-opened encoder context: fixed time_base/frame_rate, CBR
(bit_rate == max_bit_rate), B-frames off, and a tight ~1-frame VBV/HRD buffer
written through the raw rc_buffer_size field. Move it once into
apply_low_latency_rc(&mut video, fps, bitrate_bps), and let the long VBV
rationale (why the tight buffer prevents high-motion bursts from overflowing
the send queue) live in one place instead of only in the NVENC path.
Each backend keeps the two genuinely per-backend calls around it: set_format
(pixel format differs) before, and gop_size after (NVENC's infinite/intra-
refresh wave vs the VAAPI/AMF i32::MAX). No behavior change — the field writes
are independent, so the slightly different max_b_frames/rc_buffer_size ordering
across backends is irrelevant. Folding the raw rc_buffer_size write into the
helper also removes the NVENC path's separate unsafe block. Drops the now-unused
ffmpeg::Rational import from all three.
Linux check + clippy green (0/0, nvenc,vulkan-encode,pyrowave) on RTX 5070 Ti;
ffmpeg_win.rs is Windows-cfg, pending .173 compile.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
enricobuehler
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