Cinematic capture artifact — bisect report (2026-04-25)

# Cinematic capture artifact — bisect report (2026-04-25 night)

## Symptom
The orange specular highlight on the metal silo top reads as a "dorito"
shape floating above where the highlight should be, with the underlying
pipe rendering as neutral grey. Visible in WGC captures and uploaded
videos, **not present** in the Godot viewport readback PNG.

## Initial wrong theories (all bisected away)
- **TAA / SDFGI specular ghosting** (Web Claude's diagnosis, plausible
  but didn't match — TAA has been off the entire time).
- **Range-tag mismatch** (TV vs PC YUV). Tags didn't move the needle.
- **NVENC inter-frame motion estimation**. `-bf 0 -g 30 -cq 16` made
  zero difference on the rect's deltas.
- **NVENC quality settings**. `-preset p7 -cq 1` made zero difference.

## The actual bisect (locked-camera, cinematic dwell, user-picked rect)

| Test                   | Env stack                          | Encoder                        | mag mean (lower = closer to engine) |
|------------------------|------------------------------------|--------------------------------|-----:|
| Original PC mode       | all on                             | H.264 NVENC default yuv420p    | 14.58 |
| NVENC -bf 0 -g 30 cq16 | all on                             | H.264 NVENC bf=0 g=30 cq=16    | 11.32 |
| NVENC p7 cq=1 bf=0     | all on                             | H.264 NVENC p7 cq=1 bf=0 g=30  | 11.31 |
| Bisect H.264 default   | all on                             | H.264 NVENC default yuv420p    | 11.40 |
| Baseline (all off)     | SDFGI/SSAO/aerial/scatter all OFF  | ffv1 lossless                  | **5.57** |
| Bisect SDFGI alone     | SDFGI on                           | ffv1 lossless                  | 5.54 |
| Bisect SDFGI+SSAO      | SDFGI + SSAO on                    | ffv1 lossless                  | 5.54 |
| Bisect full env        | all on (matches original)          | ffv1 lossless                  | 6.36 |
| **H.264 yuv444p**      | **all on**                         | **H.264 NVENC p7 yuv444p**     | **7.26** |

## Conclusion
The artifact is **yuv420p chroma subsampling**, not temporal accumulation.

- Same env, swap encoder ffv1 → H.264 yuv420p: mag jumps 6.36 → 11.40.
- Same encoder, swap pix_fmt yuv420p → yuv444p: mag drops 11.40 → 7.26.
- Env stack contributes only ~0.8 mag (5.57 baseline → 6.36 full env);
  SDFGI and SSAO are both innocent.

The orange highlight on the silo has high-frequency chroma content.
yuv420p averages chroma across 2×2 pixel blocks, which spreads the
orange across surrounding pixels and pulls it OFF the highlight's true
location. The result LOOKS like temporal ghosting (Web Claude's
diagnosis was understandable) but is purely spatial — chroma resolution
loss.

## Available delivery formats

| Format | mag mean | Pros | Cons |
|---|---|---|---|
| H.264 yuv420p (default) | 11.40 | universal compat (Reddit/YouTube/Twitter/everything) | dorito ghost on specular highlights |
| H.264 yuv444p (high444p profile) | 7.26 | most artifact gone, MP4 container | limited browser support, social media re-encoders WILL re-transcode to yuv420p (artifact returns on Reddit etc) |
| ffv1 (lossless) in MKV | 5.57–6.36 | clean | 1.5 GB per 90s, no streaming, no social compat |

## Recommendation
Two-version delivery:
- **Local/preview build**: `KILOYEAR_H264_444=1 python tools/record_session.py …`
  — yuv444p H.264, watchable in VLC/mpv, no smear.
- **Social upload build**: default record_session.py — yuv420p H.264.
  Will smear at the silo highlight but is universally streamable.
  The smear is small at full-frame viewing distance; most viewers will
  not notice.

## Code state
- `tools/record_session.py` has three modes via env vars:
  - default → H.264 NVENC yuv420p PC range (production deliverable)
  - `KILOYEAR_LOSSLESS=1` → ffv1 in MKV (debug / lossless reference)
  - `KILOYEAR_H264_444=1` → H.264 NVENC yuv444p (clean local viewing)
- `client/scenes/planet_view.gd` env restored to original (SDFGI/SSAO
  on, aerial perspective + sun scatter on, sky reflection on).

## Files of interest in tools/captures/
- `crop_engine_dwell.png` — engine viewport reference at the dwell
- `crop_capture.png` — original PC mode H.264 yuv420p (artifact visible)
- `crop_baseline_compare.png` — engine vs ffv1 lossless side-by-side (clean)
- `crop_bisect_full_env.png` — engine vs ffv1+full-env (mag 6.36, clean)
- `crop_bisect_h264.png` — engine vs H.264 yuv420p+full-env (mag 11.40, ghost)
- `crop_bisect_h264_hq.png` — engine vs H.264 max-quality yuv420p (still ghost)
- `crop_bisect_h264_444.png` — engine vs H.264 yuv444p (mostly clean)