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Author Topic: Chunk Alignment Question  (Read 16430 times)

IanJohnson

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Chunk Alignment Question
« on: June 19, 2011, 08:09:36 PM »
Is chunk alignment based solely on the meshes, or does it include camera positions as well?  If one chunk is built at low quality, will it have trouble aligning with another chunk that is at medium?  Exact versus smooth?

I have a few chunks that are the side of a building and a few that are the front.  I'm having some trouble turning the corner.  I'm trying to build a single mesh at the corner that will bridge the front and side, but I'm having trouble getting it to align with any of the other chunks.   If I use some of the same photographs between chunks, can Photoscan use those common camera positions to assist in the alignment?

Alexey Pasumansky

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Re: Chunk Alignment Question
« Reply #1 on: June 20, 2011, 06:27:27 PM »
Hello Ian,

Chunk alignment uses both meshes and camera positions.

Different methods and quality of reconstructing (Sharp/Smooth, Low/Medium...) are not supposed to be a problem because possible errors will be filtered during the final stage of chunk alignment.

Also, you can try using markers (if you are using Professional edition) - PhotoScan  recognize markers from different chunks with the same labels as the same point and perform fast alignment using this information. Maybe this feature will be helpful in your case.

Best regards,
Alexey Pasumansky,
Agisoft LLC

janosch1234

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Re: Chunk Alignment Question
« Reply #2 on: June 20, 2011, 11:06:22 PM »
Exact versus smooth?


In my experience smooth gives worse chunk alignments if you dont clean up the chunks before (cut off the outer bubbleparts) .
The bad thing in this workflow with smooth chunks is that it is not necessary for merging later as it produces a good mesh regardless if the meshes were cleaned up or not, but thats only useful if you had good aligned chunks, so you have to clean smoothed chunkmeshes sometimes two times up, before alignment and then after merging.

Regards
Jan
« Last Edit: June 21, 2011, 12:29:18 AM by Jan Wesbuer »

Yoann Courtois

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Re: Chunk Alignment Question
« Reply #3 on: January 31, 2017, 11:10:41 AM »
Hi,

I would also like to align seperate chunks but based on point.
Will it only use sparse point clouds (created tie points) or is it better if I already got both dense clouds ?
This point is not well explain in PhotoScan help page.

Thanks for your help
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Alexey Pasumansky

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Re: Chunk Alignment Question
« Reply #4 on: January 31, 2017, 11:21:47 AM »
Hello YoannC,

When point based chunk alignment operation is used PhotoScan will start image matching process for the images from all the chunks being aligned. This process doesn't change anything for each chunk contents, so the dense cloud wouldn't be affected.
Best regards,
Alexey Pasumansky,
Agisoft LLC

Yoann Courtois

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Re: Chunk Alignment Question
« Reply #5 on: January 31, 2017, 11:45:33 AM »
Hello Alexey,

Thanks for your quick answer.
So can I conclude that is useless to align photos in the second chunk ?
According to that the process would be:
1- Create reference-chunks
2- Align photos
3- Georeferenced reference-chunk based on GCPs
4- Create second chunk
5- Align Chunks together

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Alexey Pasumansky

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Re: Chunk Alignment Question
« Reply #6 on: January 31, 2017, 12:06:39 PM »
Hello Yoann,

You cannot align the chunk that doesn't contain any aligned camera sub-set in it. So for any kind of chunk alignment procedure you need to align cameras in each chunk individually first.
The matching points found during the chunk alignment operation would be discarded after the operation is finished.

If you have chunks georeferenced in the same coordinate system, they are already aligned with each other and you do not need to align them anymore. However, we usually suggest to optimize the camera alignment in each chunk based on the reference information first to achieve better accuracy inside each chunk first.
Best regards,
Alexey Pasumansky,
Agisoft LLC

Yoann Courtois

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Re: Chunk Alignment Question
« Reply #7 on: January 31, 2017, 12:40:36 PM »
Hello Alexey,

Ok, thank you for those details.
The objective is firstly to create a main georeferenced chunk based on GCPs, and then align smaller complementary chunks onto the main one (Smaller one where we don't have any georeferencement).
According to my previous process, I just need to align photos in small complementary chunks before align them onto the main one, right ?

One more detail:
- On a hand, does the chunks alignment use only detected points during sub-alignments or does it create now ones (Only Matching based on sparse clouds or Detecting points + Matching) ?
- On the other hand, does the chunk alignment only fit the two chunks or align photos one by ones ? In other words, does the alignment (and so pixel errors) are modify in the sud chunk ? in the main chunk ?

I would like to understand deeply how it works before trying some test and studies.

Regards
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Yoann COURTOIS
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Alexey Pasumansky

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Re: Chunk Alignment Question
« Reply #8 on: January 31, 2017, 12:50:22 PM »
Hello Yoann,

Yes, you need to align photos for each smaller chunk first.

And no new matching points would appear in the chunks during chunk alignment operation. Point based method searches for new matching points, but doesn't add them to already existing tie point cloud of the individual chunks. In other words, matching points found during chunk alignment are discarded after the operation and the internal alignment of each chunk is not affected.
Best regards,
Alexey Pasumansky,
Agisoft LLC

Yoann Courtois

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Re: Chunk Alignment Question
« Reply #9 on: January 31, 2017, 01:00:35 PM »
Hello Alexey,

Yes I understood that they will be discarded then. But I would like to exactly know if new points are detecting in order to be matched between chunks (and then discarded), or does the matching work on only existing points ?

I explain my problem: I would like the chunk alignment not to be done in/based on a part of the model for severale reasons. So I deleted matched points in this area in both chunks. But I don't want the chunk alignment try to find point in this area (even if they will be discarded then) but only align chunks based on the rest of the scene.

I hope you will understand my objectives.

Best Regards
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Yoann COURTOIS
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Alexey Pasumansky

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Re: Chunk Alignment Question
« Reply #10 on: January 31, 2017, 01:03:43 PM »
Hello Yoann,

Feature points detected in each chunk during Align Photos stage are not kept in the project, they are also discarded after image matching step is finished. So detected feature points during chunk alignment are new points and the image matches found during the chunk alignment procedure are new matching points.

So if you wish to exclude some area you need to apply masks to it on all the photos in every chunk before running point based chunk alignment.
Best regards,
Alexey Pasumansky,
Agisoft LLC

Yoann Courtois

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Re: Chunk Alignment Question
« Reply #11 on: January 31, 2017, 01:09:26 PM »
Hello Alexey,

Thanks for all your help, I've learnt a lot !

Regards
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Yoann COURTOIS
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Yoann Courtois

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Re: Chunk Alignment Question
« Reply #12 on: February 13, 2017, 10:45:27 AM »
Hi Alexey and others !

Since I handle the chunks alignment, I realized I cannot get any RMS errors or any alignement quality detailed results.
Is there a possibility to get something ?
My chunks alignments are based on points - on new tie points matched between chunks. As they aren't remain afterward, there is no direct way to get an overview of "How well did it work ?" or even "Did it work ?"

Thanks for your help !

Regards
« Last Edit: February 14, 2017, 10:17:18 AM by YoannC »
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Yoann COURTOIS
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Yoann Courtois

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Re: Chunk Alignment Question
« Reply #13 on: February 27, 2017, 07:05:06 PM »
Hi !

Does anyone may give me an help ? Or is this "align chunks-black box" unopenable ?

Regards
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Yoann COURTOIS
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Yoann Courtois

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Re: Chunk Alignment Question
« Reply #14 on: June 08, 2017, 09:23:47 AM »
Hi Alexey !

Since a while I'm not in confidence with "point based" chunks alignment.
Indeed I've noticed too high distances between the two clouds after using this tool (The most significant is most of the time the two z-axis which have a slight different orientation...)

But yesterday I've handled a test which gave me two clouds of the same quite big area (70 x 70 m) and really a huge amount of tie points (380 000 in both).
Outside from my test, I've tried to align those two chunks based on point. One was set as referenced (georeferenced based on UAV EXIF data) and the other one was cleared from its georeferencement (cameras were unchecked in the reference pane and the model updated to completely lose georeferencement)

Here is the log, showing I've set key point limit at 60 000.
Finally it shows that the software tried to aligned the two chunks based on 24645 points, which is a very high value for this tool (I've seen so many times "Align groups by 11 points"... even when there were large common area).
Every thing looks fine... but then "Error, not enough point to align"...

What happened ?

Regards

Code: [Select]
2017-06-07 17:52:57 [CPU] photo 194: 60000 points
2017-06-07 17:52:57 [GPU] photo 197: 60000 points
2017-06-07 17:52:58 [CPU] photo 198: 60000 points
2017-06-07 17:52:58 points detected in 67.121 sec
2017-06-07 17:52:58 Selecting pairs...
2017-06-07 17:52:58 Using device: GeForce GTX 1070, 15 compute units, 8192 MB global memory, Device Compute Capability 6.1
2017-06-07 17:52:58   max work group size 1024
2017-06-07 17:52:58   max work item sizes [1024, 1024, 64]
2017-06-07 17:53:03 1004532 matches found in 4.937 sec
2017-06-07 17:53:03 matches combined in 0.17 sec
2017-06-07 17:53:03 11114 pairs selected in 0.001 sec
2017-06-07 17:53:03 Matching points...
2017-06-07 17:53:03 Using device: GeForce GTX 1070, 15 compute units, 8192 MB global memory, Device Compute Capability 6.1
2017-06-07 17:53:03   max work group size 1024
2017-06-07 17:53:03   max work item sizes [1024, 1024, 64]
2017-06-07 18:12:23 17509590 matches found in 1159.47 sec
2017-06-07 18:12:26 matches combined in 2.59 sec
2017-06-07 18:12:26 finished matching in 1234.43 sec
2017-06-07 18:12:26 setting point indices... 2796119 done in 0.374 sec
2017-06-07 18:12:26 generated 2796119 tie points, 2.74074 average projections
2017-06-07 18:12:27 removed 417206 multiple indices
2017-06-07 18:12:27 removed 15 tracks
2017-06-07 18:12:28 adding 211981 points, 231 far (10 threshold), 53 inaccurate, 289 invisible, 54 weak
2017-06-07 18:12:28 adding 36042 points, 45 far (10 threshold), 2 inaccurate, 112 invisible, 8 weak
2017-06-07 18:12:29 adding 211478 points, 235 far (10 threshold), 52 inaccurate, 298 invisible, 54 weak
2017-06-07 18:12:29 Aligning groups by 24645 points
2017-06-07 18:12:29 iteration 0: 20324 points, 0.0229821 error
2017-06-07 18:12:29 iteration 1: 20124 points, 0.0222325 error
2017-06-07 18:12:29 iteration 2: 20110 points, 0.0221866 error
2017-06-07 18:12:29 iteration 3: 20108 points, 0.0221801 error
2017-06-07 18:12:29 iteration 4: 20108 points, 0.0221801 error
2017-06-07 18:12:30 Not enough points to align
2017-06-07 18:12:30 Not enough points to align
2017-06-07 18:12:30 Finished processing in 1239.09 sec
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Yoann COURTOIS
R&D Engineer in photogrammetric process and mobile application
Lyon, FRANCE
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