How Vixlo recognises a set
Vixlo decides that frames are a set by measuring them. This page gives the rules for each kind, in plain numbers, so you can tell why a bracket or a sweep of yours was found or was not. What a set is, and how it shows on screen, is on Sets kept whole.
How Vixlo finds them
It looks as a library opens and after every import, in the background, at the frames it has not yet read on this Mac. For each one it reads the shutter speed, aperture and ISO from the file, and a small version of the picture. Nothing leaves your Mac. What it measured is kept in the library's Sets folder, so the next time you open the library there is nothing left to read; see What a library is on disk.
Vixlo shows no progress while it does this. Sets appear in Frames, each with its chip, when the reading has finished, so on a big library they can take a while. A frame with no capture time, or whose picture cannot be read, is never part of a set.
It looks for sweeps first, then brackets among the frames left, then a sweep of brackets, so no frame is in two sets.
Brackets
Frames are a bracket when all of this is true:
- there are three or more, one straight after another in capture order;
- each was taken within three seconds of the one before. A bracket rolled by hand off the shutter-speed dial takes a second or two a frame, and counts;
- they are framed the same. Vixlo compares the pictures in a way a change of brightness does not upset, so the same scene at three exposures matches and a new composition does not;
- from darkest to brightest they are at least a stop apart, and the steps between neighbouring exposures are even, to within about a third of a stop.
The exposure is the light each frame actually received, worked out from its shutter speed, aperture and ISO. The camera's exposure-compensation setting is used only for a file that carries none of those three. So a compensation dial rolled over three identical exposures is not a bracket, and neither is the small drift of auto-ISO. The order does not matter: under, normal, over or the other way about, Vixlo sorts the exposures before it judges them.
Panoramas
Frames are a panorama when all of this is true:
- there are four or more, one straight after another in capture order, each within 30 seconds of the one before;
- each frame is the one before it shifted sideways by between a fifth and about two thirds of the frame's width, so neighbours overlap by at least a third;
- every shift goes the same way, left to right or right to left.
Vixlo measures each shift by lining two pictures up against each other. It believes a shift only when they line up clearly at one place and nowhere else, and two pictures that merely look alike do not make a sweep. A little drift up or down from holding the camera by hand does not stop one.
A shift of less than a fifth of a frame reads as you nudging the composition between shots, not as a sweep. Three frames are never enough: three frames each moved by a third also describe reframing a subject, and at four the other explanations run out.
HDR panoramas
A sweep is an HDR panorama in either of two cases:
- its exposures run to at least a stop and a half from darkest to brightest, because the meter followed the sky across the scene;
- it is a sweep of brackets: four or more brackets in a row, one at each position, each position shifted from the one before as above. Three exposures at each of six positions is one set of eighteen frames, not six brackets.
When Vixlo does not see one
A set is missed when a condition is not met. A bracket shot more than three seconds apart, a file that carries no exposure, a frame with no capture time, a sweep whose steps are under a fifth of a frame or over about two thirds: none of these is recognised, and their frames are ordinary frames.
Vixlo's rules for sets were set against a real shoot that held no bracket and no panorama, and made strict enough to find none in it. No real set has yet confirmed they are not too strict, so a bracket or sweep of yours may go unrecognised. Nothing is lost and nothing is decided when that happens: its frames are ordinary frames, which you cull one by one, and which the Similar photos slider may stack if they look alike.