universal-video-transcode/references/hevc-conversion-policy.md

HEVC conversion policy — never bigger, same look, no judder

Owner's standing requirements for a HEVC conversion. Read this before starting a library job; the default settings in transcode.py do not satisfy them.

1. Size: shrink where there is headroom, growth is acceptable where there is not

Playability on iPad + Android is the primary goal. Size is secondary and conditional — it is NOT a universal "must never grow" rule. Two regimes, and they need opposite handling:

regime	detector	expectation	-maxrate	output assertion
headroom — an ordinary H.264 encode that is not yet at the transparency floor	bpp > ~0.030, or a sampled pre-flight encode needs fewer bits than the source	output should be smaller. If it grows, that is a recipe bug, not an acceptable outcome	1.0x the source video bitrate	out <= src (hard)
already compressed — YouTube / streaming downloads, near-optimal 2-pass sources	bpp < ~0.030, or a sampled pre-flight encode needs >= the source bits	growth is acceptable. The file already sits at the transparency floor; only playability matters	1.5x the source video bitrate	out <= 1.25x src (runaway-bloat guard only)

The authority is the sampled pre-flight (section 2): encode a 60-120 s window at the target quality and compare bits. bpp is just the cheap prior that tells you which way you expect it to go.

Do not clamp to 1.0x on an already-compressed source. Capping at the source bitrate forces the encoder below the transparency point, which is a visible quality loss on top of the second-generation loss — strictly worse than accepting a slightly larger file. A hard cap is only correct in the headroom regime, where shrinking is the actual goal.

Why an already-compressed source can legitimately grow: HEVC's ~40% efficiency advantage is measured against H.264 at equal encoding effort. A near-optimal 2-pass AVC stream is already closer to the floor than a single-pass hardware HEVC encode is, so at matched perceived quality there may simply be no bits left to win.

An H.264 source with headroom is expected to come out smaller

When the source is ordinary H.264 with real headroom and the output still grows, the recipe is wrong, not the source. The two measured causes, both in this history:

• a quality target above the source (-cq 27 on NVENC = SSIM 0.9971 while the source is already at ~0.9963-equivalent), landing the video at 109% of the source bitrate;
• audio re-encoded to a fixed 256k when the source ships mp3 at 128k: ~+10% for no audible gain.

HEVC is not what makes a file playable

H.264 + AAC in MP4 already plays on both iPad and Android — no conversion needed. HEVC is only required for sources that are not already in that shape: Matroska, Opus/Vorbis audio, DivX/Xvid, MPEG-TS, or a broken frame cadence. For an already-playable, already-lean H.264 file the only benefit is size; where there is no headroom a re-encode buys nothing and costs a generation, so -c copy (or leaving it alone) is the better answer. Say this to the owner before spending days of GPU/CPU on files that are already fine.

The two derived values every rule below needs, from one ffprobe of the source:

src_video_bps  = format_bit_rate - audio_stream_bit_rate   # NOT the file's total bitrate
src_audio_kbps = max(96, min(256, audio_stream_bit_rate // 1000))

Comparing against the file's total bitrate instead of the video's is the specific mistake to avoid: it turned a real 109% growth into a false 86% shrink in one analysis.

2. Same picture quality — pin it to a number

"Same quality" is meaningless unless it is measurable, and with an already-lossy source and no master, the only self-consistent definition is SSIM against the source.

• Target SSIM >= 0.996 on a sampled window. Measured on 1080p25 H.264: this lands the video near 70% of the source bitrate — that is where the saving is. cq 27 on NVENC gives 0.9971 and grows; cq 30 gives 0.9963 at 76%; cq 32 gives 0.9955 at 58%.
• Never compare cq/crf numbers across encoders. -cq 27 (NVENC) = SSIM 0.9971 while -crf 27 (x265) = 0.9952. Match on SSIM, then compare bitrate.
• Software x265 is 9-16% more bit-efficient at matched SSIM, not 2x — most of the raw difference between cq27 and crf27 is a quality difference, not an efficiency difference.
• Verify the claim per file (or per class): encode a 60-120 s window, measure SSIM vs the same source window, and fail the item if it is under target.

3. Classify the source before choosing a setting

bpp = video_bitrate / (w * h * fps), readable in milliseconds, tells you how much fat is left:

bpp	verdict
< 0.015	already soft — re-encoding gains nothing, use -c copy
0.015-0.025	lean
0.025-0.040	normal
> 0.040	real headroom — worth a real re-encode

One library: 131 files < 0.025 vs 243 files > 0.040 out of 572. A single global quality setting cannot be right for both — that is precisely how a job ends up growing some files and shrinking others. Calibrate per class on 1-2 representatives (60-120 s, sweep 2-3 quality levels).

4. Same sound quality

• Source audio already AAC -> -c:a copy, bit-identical. Always preferable.
• Source audio mp3/opus/vorbis -> must be re-encoded; those are not valid in MP4.
• -b:a must track the source, never a fixed 256k. A weekly series shipping mp3 at 128 kbps re-encoded to 256k AAC added ~128 kbps for zero audible gain: ~+10% file size, enough to cancel the video saving by itself. AAC is more efficient than mp3, so matching the source bitrate is already an upgrade. Clamp to a sane floor (96k).

5. Playable on iPad AND Android

MP4 + -tag:v hvc1 (Apple rejects hev1) + -pix_fmt yuv420p + AAC. No Matroska, never Opus. HEVC 1080p is Main profile Level 4.0 — every modern iPhone/iPad (A9+) and Android 5+ handles it, but a cheap/old Android device is only guaranteed Main Level 3 (~SD).

6. No judder — three defect classes, same symptom

-fps_mode cfr is mandatory on every re-encode: with the default auto, ffmpeg drops frames it cannot place (0.5-1.3% measured), leaving doubled frame intervals that play as judder. CFR duplicates a frame instead, which costs almost nothing. Never pass -r unless you want a different playing speed.

class	signature	detector	fix
source-inherited VFR	r_frame_rate != avg_frame_rate	ffprobe header, gap > 0.01	re-encode (remux cannot repair)
encoder-dropped	doubled intervals (0.040 + 0.080)	histogram packet=duration_time shows 2+ values	-fps_mode cfr
flat composition offsets	r == avg exactly, uniform stts, clean decode, still drops ~23%	ctts entry count <= 1 on a B-frame stream, or 0 PTS inversions over 60 s	real re-encode; no lossless repair exists

The third class is the trap: every cadence test passes and the file still stutters on real hardware. Gate it explicitly (see scripts/transcode.py: _ctts_entries(), pts_inversions()).

Never decode with a *_cuvid hardware decoder. Measured on this hardware (RTX 3060, ffmpeg 9.0.1): av1_cuvid emitted the wrong picture on 1.8% of frames (a frame from a different moment, not a corrupt one). The container stayed perfect — exact intervals, correct ctts, clean decode — so every structural check passed while the output visibly jumped. Use software decode (4K AV1 ran ~150 fps on the i9-13900K) or the modern -hwaccel cuda -hwaccel_output_format cuda path, and if you ever must use cuvid, compare its frames against the software decode with per-frame SSIM (>=0.99 everywhere).

Judge the picture, not the container, when the complaint is "it jumps".

7. Verification — yes, mandatory, and it must assert on the output

Per item, in this order:

1. Local staging. Fetch the source off the share first; never encode over SMB/WebDAV/Dokan.
2. Structural gate on the encode: codec/tag/duration/resolution/pixel format, r vs avg within 0.01, ctts/PTS inversion check, one distinct frame duration.
3. Full decode pass (ffmpeg -v error -i OUT -f null -) — the only check that proves the payload is not truncated or corrupt.
4. SSIM vs the source window >= target (section 2).
5. Size assertion: output video bitrate <= source (section 1).
6. Upload to a temp name, read the share copy back end-to-end, compare SHA-256 + byte count, then replace atomically. A duration probe is not enough: with +faststart the moov is written first, so a truncated upload still reports the full duration.
7. Re-probe on the share after the replace.

Cheap confidence check on a healthy file, one distinct value expected:

ffprobe -v error -select_streams v:0 -read_intervals "%+60" \
  -show_entries packet=duration_time -of csv=p=0 FILE | sort -u | wc -l

Do not hand-sample videos before converting — nothing here is spec-dependent, so the guarantee belongs in the output assertion. Chrome cannot arbitrate HEVC either: canPlayType('...hvc1...') returns '', so an HEVC file reports 0 decoded / 0 dropped and looks perfect while nothing played. HEVC smoothness is confirmable only on the Apple device, via the frame-duration histogram, or on an Android player. For an owner-facing A/B, serve the original and the variant side by side in a real browser and let them compare (the H.264-only variant is the one Chrome can measure).

8. CPU (software) encoding — every knob, and what each one costs

Remind the owner before starting a CPU job and quote the wall-clock time. Software x265 runs 3.5-4.4x realtime at 1080p depending on cores and preset (vs 6.4x for hevc_nvenc), so a 520 h library is days of CPU. Say so before starting, not after — see the measured table in the topology section and the tier timings in section 9.

Efficiency first, and the knobs do different jobs:

goal	knob	effect
better bits-per-quality	`-preset medium	slow
fewer cores	-x265-params pools=N (N threads) — for libx265, ffmpeg's -threads N maps to frame threads	fewer cores, roughly linear slowdown, no efficiency gain
fewer frame threads	-x265-params frame-threads=2	less parallelism, marginal efficiency gain, slower
CPU affinity	start /affinity <hexmask> ffmpeg ..., or PowerShell (Get-Process ffmpeg).ProcessorAffinity	pins to chosen cores; keeps the machine responsive
low priority	PowerShell (Get-Process ffmpeg).PriorityClass='BelowNormal'	yields to everything else at no efficiency cost
no high frequency	powercfg /setacvalueindex SCHEME_CURRENT SUB_PROCESSOR PROCTHROTTLEMAX 60 then powercfg /setactive SCHEME_CURRENT	caps clocks: best perf/watt, quietest, slowest — match pools to the capped cores

Preset buys efficiency; thread count does not. If the priority is efficiency rather than speed, prefer a slower preset at a lower core count over a faster preset with all cores. Never run two encoders of the same kind at once: they compete for the same unit and drop frames — the exact defect the gates exist for. One CPU encoder running alongside one GPU encoder is fine (different units), see section 9.

This host's measured topology and the P-core-avoidance recipe

Windows reports the i9-13900K VM as "13 cores / 26 logical" — a misreport of a heterogeneous topology. A per-logical-CPU microbenchmark settles it in 30 seconds:

logical CPU	relative single-thread throughput
0-15 (8 P-cores x HT)	0.43-0.52 s for a fixed workload
16-25 (10 E-cores)	0.79-0.91 s — 56% of a P-core

Pin to the E-cores only (best perf/watt, keeps the P-cores free for the desktop):

# affinity mask = bits 16-25 set = 0x3FF0000 (= 67,043,328)
start /affinity 3FF0000 ffmpeg ...          # cmd

or set the mask on the driving Python process (children inherit it):

ctypes.windll.kernel32.SetProcessAffinityMask(
    ctypes.windll.kernel32.GetCurrentProcess(), ctypes.c_size_t(0x3FF0000))

Match the thread count to the mask (-x265-params pools=10). pools does not choose cores — the affinity mask does; pools only caps threads.

Measured cost of E-core-only encoding (x265 preset medium, 1080p25, 120 s sample):

cores	speed	output
all 26 threads	4.37x realtime	0.76 Mbps
E-cores only (10 threads)	3.51x realtime	0.76 Mbps

Only 20% slower, not the ~3x that per-core throughput suggests: the 10 E-cores are real cores while half of the P-core threads are SMT siblings, and x265 does not scale linearly anyway. So E-cores-only is the right default when the priority is efficiency over speed.

9. CPU and GPU tiers in parallel

Different tiers can encode at the same time — NVENC is a fixed-function ASIC, x265 is CPU. Pair an E-core-pinned CPU job with a GPU job and the P-cores stay free for the user. Wall clock becomes max(cpu_tier, gpu_tier) instead of the sum:

tier	files	GiB	content h	recipe	time
fat (bpp > 0.040)	252	153	219	x265 crf24, E-cores	65 h
mid (0.025-0.040)	198	117	183	hevc_nvenc cq30	31 h
lean (< 0.025)	131	54	118	-c copy + audio match	<1 h

serial 96 h (4.0 days) -> parallel 66 h (2.7 days). Requirements to run it safely:

• a lock per tier, not one global lock, and a separate local staging dir per instance;
• the share is the shared resource: two instances double the WebDAV/Dokan traffic (staging read + upload + read-back hash). Test a small batch in parallel before committing the library;
• run the fast -c copy tier on its own first, so it is not contending while the two big jobs settle;
• -c copy only where the timing gate passes — a file in the flat-ctts class must be re-encoded whatever its bpp says.

10. The three recipes, ready to run

SRC = the local staged source, SRC_V = src_video_bps from section 1, A = src_audio_kbps (section 4). Affinitize the process to the E-cores (0x3FF0000) before the tier-C command.

# ---- tier A: lean (bpp < 0.025) — no video re-encode at all, size cannot change
ffmpeg -y -i SRC -map 0:v:0 -map 0:a:0? -map_metadata 0 \
  -c:v copy -c:a copy -movflags +faststart OUT.mp4     # -c:a aac -b:a A when the source is mp3

# ---- tier B: mid (0.025-0.040) — GPU, quality pinned to SSIM >= 0.996
ffmpeg -y -i SRC -map 0:v:0 -map 0:a:0? -map_metadata 0 \
  -c:v hevc_nvenc -preset p6 -tune hq -rc vbr -cq 30 -b:v 0 \
  -maxrate SRC_V -bufsize <2.5x SRC_V> \
  -profile:v main -pix_fmt yuv420p -tag:v hvc1 -fps_mode cfr \
  -c:a copy -movflags +faststart OUT.mp4               # -c:a aac -b:a A when the source is mp3

# ---- tier C: fat (bpp > 0.040) — CPU, E-cores only, efficiency first
ffmpeg -y -i SRC -map 0:v:0 -map 0:a:0? -map_metadata 0 \
  -c:v libx265 -preset medium -crf 24 -x265-params pools=10 \
  -maxrate SRC_V -bufsize <2.5x SRC_V> \
  -pix_fmt yuv420p -tag:v hvc1 -fps_mode cfr \
  -c:a copy -movflags +faststart OUT.mp4

Why these exact numbers: cq 30 (NVENC) and crf 24 (x265) both land near SSIM 0.9963 — the matched-quality point where the video comes back at ~70-76% of the source bitrate. Neither is cq 27/crf 27: those are different qualities from each other, and cq 27 is above an already-efficient source, which is what made a library grow.

Every tier still runs the whole verification chain from section 7 — tier A included: a -c copy can still originate a truncation, and a flat-ctts source cannot be copied at all.