abner

A/B testing for videos. Point it at two (or more) video files — different encodes, different upscalers, different grades of the same content — and flip between them while they play in frame-locked sync.

abner original.mp4 encoded.mp4
abner --view delta original.mp4 encoded.mp4
abner a.mp4 b.mp4 c.mp4          # three-way works too
abner                            # launch window — drag clips onto it

--view takes overlay, sbs, delta, split, checker or blend, so every mode is reachable from the command line.

Drag and drop

Run abner with no arguments and drag clips onto the launch window. One file fills slot A and the window keeps waiting; two land as A and B and start playing; more add C, D… — in the order the platform hands them over, which for a Finder multi-select is the order they appear in that window, so drop them one at a time if it matters which is A. Dropping onto a running comparison adds streams — hold ⌘ while dropping to replace the whole set instead. A single path on the command line (abner reference.mp4) opens the same half-filled window.

Every load rewinds all streams to 0, so an arrival is frame-locked with what was already playing rather than joining mid-flight.

Born out of the switchblade project's graphics stack: in-process libav decode (VideoToolbox for h264/hevc/prores), a wgpu renderer with mip-chained video textures (no minification shimmer on 4K sources), idle-throttled render loop, and mpv-style fake fullscreen.

Sync model

One master clock drives every stream. Players decode into small bounded queues; each frame the app pops everything due and shows the newest. Flipping the displayed video (Enter) switches textures, not players — the other stream was already decoding the same instant, so the flip is seamless and time never jumps. Pause stops the clock (backpressure stalls every decoder for free); framesteps are exact seeks whose landing frame's true pts is adopted back into the clock, so stepping can't accumulate drift.

Keys

Key	Action
Enter	flip to the next video (in overlay mode)
Space	pause / play
< > (or , .)	frame-step back / forward
← →	seek ±1s
1	overlay — videos stacked, flip with Enter (the classic A/B)
2	side-by-side — all videos in a row
3	delta — amplified |A−B| difference (-/= adjusts gain)
4	split — vertical wipe, divider follows the pointer
5	checker — checkerboard mix (-/= adjusts tile size)
6	blend — dissolve between A and B (-/= adjusts mix)
pinch	photo-style zoom on the pointer — every video pans/zooms to the same spot
drag / scroll	pan while zoomed (synced across videos)
Z	reset zoom
[ ]	slow down / speed up playback (0.25×–4×; Backspace resets)
F	fullscreen (borderless, same Space, instant)
Tab	toggle the info overlay (filename, path, res, fps, codec, bitrate, size, duration)
Q	quit
Esc	leave fullscreen, else quit

In compare modes (delta/split/checker/blend) the pair is the active video vs the next one; Enter rotates which pair you're looking at. Big letter badges mark what you're seeing — A hugs the left edge, B the right. With the overlay hidden, Enter still flashes the letter briefly so you know where you are.

The HUD

Corner brackets frame the active stream, a centre A|B toggle shows what's on screen, and the top-left block lists every clip's filename, resolution, fps, codec, bitrate, size, duration and path. Along the bottom sits a transport — prev / play-pause / next, a seek bar you can click and drag, the mode and timecode readout, and a keycap legend. The transport is hover-revealed: it fades out after a few seconds of stillness and any pointer movement brings it back. Tab hides the whole HUD.

Ideas for more views

• Loupe — a magnifier following the pointer showing A|B split at 4–8× inside the ring
• Flicker — auto-alternate A/B every N frames (temporal delta your eyes compute)
• Vertical split / horizontal wipe
• Heatmap delta — false-color per-pixel error with a scale
• Signed delta — grey = equal, warm = A brighter, cool = B brighter

Requirements

• macOS (first target; the shader/loop are portable, fullscreen + font paths are mac-specific)
• ffmpeg 8.x — the ffprobe CLI for metadata and the dev libraries the in-process decoder links against (brew install ffmpeg provides both)

Build

cargo build --release
./target/release/abner --help
./packaging/build-app.sh --open      # self-contained Abner.app (bundles the ffmpeg dylibs, ad-hoc codesigns)
./packaging/build-app.sh --install   # …and copy it to /Applications

The bundle carries its own copies of the ffmpeg libraries, so it runs on a machine without Homebrew; the ffprobe CLI it shells out to at startup is still looked up on PATH (--with-cli-tools copies that in too). The app icon is assets/app-icon.png — drop a new square PNG there and both the bundle's .icns and the bare binary's Dock icon follow. Double-clicking a video on the bundle doesn't load it yet (see TASKS.md).

cargo test runs the regression suite: master-clock draining, exact seek, two-player sync, framestep adoption, reader-thread cleanup (including a reader wedged in libav I/O on a FIFO), redraw cadence. It generates tiny test clips with ffmpeg under $TMPDIR.

A startup ffprobe that doesn't return within 30s (a file on a volume that has gone away) is an error rather than a hang; a decoder whose seek fails is marked failed in the HUD rather than silently drifting out of sync.

License

MIT