"Keep keypoints" confusion - not working as expected

[andyroo]

I understood the "keep keypoints" option to work with incremental alignment, but when we try to align a subset of photos (e.g. adjacent lines) with a previous subset, we don't get as good alignment as if we re-run from scratch.

I also expected that with "keep keypoints" and the same keypoint threshold, we would not need to regenerate keypoints with a new alignment, but I see that through the GUI that step is performed anyway.

What are the best practices for working with "keep keypoints" enabled, both for incremental image alignment, and if we want to restart the alignment process, but with the same keypoints?

[Alexey Pasumansky]

Hello andyroo,

The expected workflow should contain these steps:
- enable Keep Key Points option in the Advanced Preferences tab,
- add images and run Align Photos command (which contains of key point detection, image matching and camera alignment steps),
- add new subset of images,
- run Align Photos dialog with "reset current alignment" option disabled;
 you will see that the feature points are being detected for the newly added images only.

If you remove key points from the Workspace pane (the option is available in the Tie point cloud context menu), there will be no way to use "incremental alignment approach".

In case you are following this procedure, but the result is considerably worse than the results of complete re-alignment for the joint set of images, please share the example sub-sets to support@agisoft.com, so that we could re-produce the problem.

[andyroo]

OK I just did a workflow that reproduced this issue (keypoints regenerated for each photo after they've been done once already)

1) align a set of images
2)copy with keypoints to a new chunk
3)reset alignment and load new camera models (precalibrated --> fixed)
4)align images with reset alignment checked unchecked [edited]

keypoints are regenerated rather than reused.

Is that expected behavior?

[JMR]

Hello, Andyroo:
I think you are doing the opposite as explained by Alexey.
You should not reset existing alignment and to do so you should make sure to unckeck=disable "reset current alignment".

Regards

[andyroo]

Thanks JMR - that was a typo - I meant *reset alignment unchecked*

[JMR]

what does "reset alignment" in your step 3 mean? and why do you do so if you are trying incremental alignment... if you reset current alignment, then you are aligning from scratch... aren't you?

[andyroo]

Sorry to confuse, JMR. I'm doing too many different things at once. I think I understand now.

I was under the impression that once I matched images, if "keep keypoints" is selected, I don't have to go through the matching step again (this can take awhile with a project that has 50-100k images). I will sometimes reset alignment for large swaths of images to realign by seeding with a modified camera model. It frustrates me that I need to spend time re-matching keypoints on the same images, when I've already generated those keypoints previously - regardless of whether I reset alignment or not.

[andyroo]

I guess this is the part that I am still confused about - If key points are extracted for each image based on some variant of a SIFT algorithm, and I don't change the matching parameters (number of key points, alignment quality) then why do I have to regenerate key points when I reset alignment, if I have "save key points" selected?

I would at least like the option to re-use existing key points and reset alignment (destroy tie points but not key points). This would save time when realigning after resetting the camera model and recalculating positions.

[Alexey Pasumansky]

Hello andyroo,

Currently it is not possible to split key point detection and image matching stages, they are grouped into Match Photos task. Keep key points feature has been introduced to allow the incremental matching, when new images are added to the already matched and aligned set of images.

As for the name of "reset current alignment" is meant to reset all the results obtained with running Align Photos operation, which include key points, tie points and EO/IO parameters.

[andyroo]

Thank you! this clears it up for me.