Splitting Scans
The heart of Winder is the splitter. This page explains what it does and every knob you can turn.
Why scans need splitting
A half-frame camera exposes two 18×24mm shots inside one 24×36mm frame. Labs scan the full frame, so every file you get back holds two photographs separated by a strip of unexposed film — a black bar a couple of millimetres wide, not a thin line.
How detection works
Winder walks the luminance profile across the middle of the scan looking for that dark valley:
- It samples brightness column by column in the central ±18% of the scan — the divider is always near the middle, so a dark subject at the edge can't fool it.
- It looks for a contiguous run of columns markedly darker than the median: the valley.
- The valley's center becomes the proposed cut, and its width becomes the bar width. Both are shown in the review screen.
Rotated scans are handled too. Winder scores both axes and splits along the stronger valley, so it doesn't matter whether your lab scanned the strip horizontally or vertically.
When detection fails
Very dark frames — night shots, heavy underexposure — can hide the bar. Winder then falls back to a center cut using the fallback divider width from Settings, and says so in the review screen. Nothing is ever cut without you seeing it first.
The review screen
- Drag the bar to move the cut. Both half previews update live.
- Rotate either half in 90° steps.
- Keep both / left / right — sometimes one half is a lens cap test.
- Discard scans that shouldn't be imported at all.
- Auto-straighten (Pro) suggests the upright orientation for every frame in the batch.
Review runs against a 1280px preview so panning stays instant. The cut you approve is applied to the original at full resolution when you tap DONE.
Settings that affect splitting
| Setting | Default | What it does |
|---|---|---|
| Scan size | 3130 × 2075 px | The pixel size of a full 35mm scan from your lab |
| Divider fallback width | 140 px | Bar width assumed when detection fails |
| Trim frame edges | On | Shaves the dark rebate off each frame after the cut |
| Output quality | High | JPEG quality for the saved frames — High or Medium |
All of it lives in Settings → Film & scanning, and all of it is stored in the library folder rather than on the device — see settings.json.
Provenance
Every split frame remembers where it came from. Its sidecar records the original scan's filename, which half it was, the cut position and the rotation applied — so no cut is ever a mystery later. See Frame sidecars.
Updated 2026-08-06