Photogrammetry tutorial add-on: The consequences of optimizing a sparse point cloud

If you build a photogrammetric model following the workflow I suggest in this post, one of the steps is optimizing the sparse point cloud (tie point cloud) via gradual selection and deletion of points. Here’s why that matters.

Below you see a screenshot of a model of a sauropod bone. The sparse point cloud is shown as produced in-program by aligning the 190 images (one image doesn’t count, as it shows the label, and accordingly wasn’t aligned).

01

(click to enlarge)

Note that there are close to 200,000 tie points connecting the images.
Now, I used Gradual Selection to select lower-confidence tie points, and deleted them. After optimizing the alignment the point cloud looks like this:
01

(click to enlarge)

In this case, I deleted nearly 50% of the tie points, which are down to just over 100,000. Now compare the errors shown in the screenshots! The very same two scale bars of 25 cm length that differed by 0.000557 m (you need to add the two errors to get the total discrepancy), slightly over half a millimeter, now differ by only 0.00032 m, i.e. 57% of the previous error!

Ok, admittedly this almost-doubling of accuracy doesn’t make much difference in this case, but imagine you’re modelling a specimen with fine details. Sauropod vertebrae with their fine laminae come to mind. Decreasing the error by half means that your chances of getting the fine details modelled without too much error and messy edges doubles!

Let me end this with a view of the derived mesh. Ain’t sauropod bones beautiful? (oh, and this is medium density only! The mesh could be much better resolved).

01

(click to enlarge)

About Heinrich Mallison

I'm a dinosaur biomech guy working at the Museum für Naturkunde Berlin.
This entry was posted in Dinosauria, How to, photogrammetry, Sauropoda, Sauropodomorpha, Tendaguru. Bookmark the permalink.

8 Responses to Photogrammetry tutorial add-on: The consequences of optimizing a sparse point cloud

  1. Pingback: Photogrammetry tutorial 11: How to handle a project in Agisoft Photoscan | dinosaurpalaeo

  2. Pingback: PaleoNews #18 (National Fossil Day 2015 Special Edition) | An Odyssey of Time

  3. dobermunk says:

    Hi Heinrich,
    might want to give CapturingReality a try.
    (https://www.capturingreality.com/)

    It’s in beta, not sure what the price will be, but tests (http://www.pi3dscan.com/index.php/instructions/item/agisoft-vs-capturingreality) show promise in quality and performance.

    • LOL
      this is really neat – I am waiting for the time where I find time to sort through my data and find a really juicy big project so that the people at edico (who are behind capturing reality) run a test for me, to compare to Photoscan🙂

  4. dobermunk says:

    They’d certainly love to have a dinosaur (coughRexcough) to flaunt their wares, so … free license? If you bargain hard, you may get a free license for me, too!

  5. Colour me officially impressed – although I must say that the comparison you linked to may be unfair. It is unclear how the Agisoft file was run. If the sparse cloud was not optimized, then it is no surprise that there is a lot of floating nonsense data, and that there are rough edges and seams.

  6. David McGahan says:

    Hello Heinrich,

    I am trying to find literature comparing the accuracy of scaling/metrics between Agisoft and NextEngine laser scans. Are you aware of any I should read?

    Cheers

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s