FlashbackTurbo

A Fabric addon for Flashback that drastically speeds up video export — without quality loss.

Proven in the field

Validated across hundreds of real anonymous exports on Windows (primary platform), macOS and Linux. Export failure rate ~1%, and every observed failure was external (not enough RAM allocated for very long replays, or the game closed mid-export) — never a regression introduced by FlashbackTurbo itself.

What it does

Patches Flashback's export pipeline via Mixin to replace serial, low-tuned operations with parallel ones:

• Parallel PNG writer (N-1 threads) + configurable zlib level + RGB color-type when transparency is off
• FFmpeg threading tunes per encoder (h264_nvenc, h264_qsv, h264_videotoolbox, libopenh264)
• Removes the silent 4K downscale cap
• Fragmented MP4 muxer on hardware encoders → ~10× faster finalize
• Animated "Saving…" overlay during the post-export phase
• Fixes a Flashback crash on mid-replay exports (null mc.level race)
• Auto-promotes software → GPU encoding (0.5.0) — when an export would use the slow libopenh264 encoder, redirects it to a hardware encoder (NVENC/QSV) when one is available, for the ~13% of exports on software-only configs. Fail-safe, SSIM ≥ 0.99.
• Silent auto-update (0.6.0) — keeps your Zeffut mods up to date from Modrinth automatically: background check on launch, verified download, swap at game exit. Opt out with "autoUpdate": false in the config.

All hooks are opt-in via config/flashbackturbo.json and default to safe values.

Performance

Controlled benchmark (PNG sequence)

Real 1.21.11 replay, 10-second slice at 1920×1080 (603 frames):

| Mode | Total | Per frame |
|---------|-----------|-----------|
| Vanilla | 96 081 ms | 159 ms |
| Turbo | 8 774 ms | 14 ms |

10.95× end-to-end.

Real-world field data (MP4 export)

Median export time across real exports (~1080p clips), by encoder:

| Platform · encoder | Median export |
|---------------------------------|--------------------|
| Windows · NVIDIA h264_nvenc | ~8.5 s (60 fps) |
| Windows · Intel h264_qsv | ~23 s (60 fps) |
| macOS · h264_videotoolbox | ~26 s (30 fps) |
| Software fallback libopenh264 | ~55 s |

Fastest on Windows + an NVIDIA GPU. A coming release auto-promotes pure-software exports to the GPU encoder when one is available.

Quality

Lossless PNG output — zlib level only changes file size, decoded pixels are bit-identical. The FFmpeg threading tunes don't change the encoded bitstream.

Privacy

Since 0.4.0, FlashbackTurbo sends anonymous usage telemetry (PostHog) to prioritise improvements: a random local UUID, no username / IP / file paths, sanitised exception messages. Fully fail-safe (it can never slow down or break an export) and opt-out — set "enableTelemetry": false in the config.

Requirements

• Flashback and Fabric API
• Minecraft 1.21.9 / 1.21.10 / 1.21.11 (Java 21) — or 26.1.x (Java 25)