How it works
Three things decide what the video looks like: where the clicks are, how many photos you picked, and two settings you will probably never touch.
Where the clicks come from
Every bundled ratchet has been analysed once, and the exact time of each click is shipped with it. The app reads that list — it never listens to the audio while you wait. A sound you record or upload goes through the same detector, right on the phone, and from then on behaves like a bundled one.
The analysis is an onset detector: the recording is high-passed to leave the click's edge, its energy is measured in 6 ms windows, and a jump above the local level marks an attack. Each click is then refined to the exact sample where it starts, which is why the cuts are tight rather than approximately right.
Photos are the master
Twenty photos produce exactly twenty frame changes. If the ratchet has more clicks than that, the extra ones are simply not used; if it has fewer, the recording repeats — and the app says so before you render, because the seam is audible.
This is the rule everything else bends around. A ratchet at its peak emits twenty to thirty clicks a second, and no one can read a photo shown for thirty-five milliseconds. The obvious fix — show fewer photos — would break the rule, so SnapClack skips the *click* instead. The frame holds through it, the sound is untouched, and every cut is still on a beat.
The two settings that matter
Shortest frame is the floor above. At its default of 80 ms it turns a dense burst into something the eye can follow. Lower it for a strobe, raise it for a slideshow.
Cut against the click is why the cuts feel right. Vision lags hearing by a few tens of milliseconds, so a frame that changes 40 ms *before* its click reads as exactly on the beat, and one that changes on the click reads as slightly late. The default is −40 ms; the range is a tenth of a second either way and it is set by ear.
Questions
Does it work with any number of photos?
Two or more. There is no upper limit — the photos are downsampled as they come in, so forty cost about what four do.
What happens to photos that are not 9:16?
They are fitted whole, over a blurred and dimmed copy of themselves. Nothing is cropped. You can switch the frame to 1:1 or 16:9 in Settings, or replace the blur with black or white.
Can I use my own sound?
Yes. Record one with the microphone or bring an audio file — the clicks are measured right on the phone, by the same detector the bundled sounds went through. Trim the take, adjust the sensitivity, and it works like any built-in ratchet.
What if I change my mind?
Every edit on the making screen undoes and redoes. Projects have names and search; a deleted one waits in the trash until you empty it, and any project can branch into a variant — another sound, another effect — without touching the original.
Can I get the video without the soundtrack?
Yes — Settings has a switch for it. You keep the cuts and add your own music in a video editor.
Do my photos go anywhere?
No. There is no server, no account and no upload. The only network traffic the app makes at all is crash reporting, and that carries no photo data.
Why a ratchet and not a metronome?
A metronome is even, and even is boring. A cranked ratchet accelerates and dies away on its own, and photos following that curve look edited rather than generated.