At how many photos is it worth it to use pair preselection?

[driftertravel]

Hi, I'm curious about pair pre-selection. I'm under the impression, and let me know if I'm wrong here, that pair preselection is really only helpful when one has 'a lot' of photos. My question is: "how many is 'a lot'?" At what point does it become beneficial to processing time to use this function? Thanks!

[Wishgranter]

pair preselection is great for aerial or GIS project. if use it for small object no need to use it. The idea is GIS project is very carefully captured with proper overlap and etc so can benefit from preselection  pairs...

[driftertravel]

Cool, that's what I needed to know!

[Marcel]

I always have generic pair preselection enabled. Only in the rare case that a project fails to align I turn it off. I don't see any downsides to having it enabled, and the speedup is huge.

[Wishgranter]

again it depend on project, and experience...

[Marcel]

again it depend on project, and experience...

What do you mean exactly? Can you give an example of a project where you would not use it (and for what reason)?

[driftertravel]

Interesting... I use a stereo camera setup with ~50% overlap, so there would always be pairs to choose from... I guess I'll just have to run some A/B tests

[Wishgranter]

Marcel have experimented "years" back, and  it could NOT align my project data, on sme it was OK, but from speed/results experience i stay on no preselection.  What sort of reconstruction you do ??   

[Arie]

In the manual it is stated that when using generic preselection "the overlapping pairs of photos are selected by matching photos using lower accuracy setting first."

This saves a significant amount of time when matching large image sets, since not every image is matched with high accuracy to every other image available. This speedup is more noticable with systematic aerial imagery but it also applies to arbitrary models.

Here an example for an arbitrary model:
Cameras: 49
Accuracy: High
Pair preselection: Generic
Matching Time: 4 min. 9 sec.

Cameras: 49
Accuracy: High
Pair preselection: None
Matching Time: 5 min. 32 sec.

I personally always use generic preselection. In one or two rare cases some alignment didn't work out, after deactivating preselection it aligned well.

Cheers.

[Marcel]

Marcel have experimented "years" back, and  it could NOT align my project data, on sme it was OK, but from speed/results experience i stay on no preselection.  What sort of reconstruction you do ??

We do mainly aerial style scans (so everything in a flat plane) but also scans of objects. The objects we scan have a lot of fine detail so we never have problems with alignment. I also like to align with max points at 120.000 instead of 40.000, so then the pre-selection is very useful to keep processing times reasonable.