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GoPro 5.4mm non-fisheye lens replacement (for mapping mostly)

Posted by W Hayden on July 13, 2015 at 9:43pm

Hello,

so, I'm an archaeologist by trade, and we're (generally speaking) sort of technology bottom-feeders, so my apologies for the lack of familiarity with what seems like should be a straightforward situation.

I have a 5.4mm replacement lens on a 3+ Silver and it works extremely well:

http://peauproductions.com/store/index.php?main_page=product_info&cPath=124_126_127&products_id=685

I've noticed jello/chop on video in 15+ mph wind, but the stills I've gotten at 0.5s interval look great.

I'm running into problems understanding the specs though, or at least how to use them to:

  • create camera calibrations for Photoscan and Pix4D
  • create a custom camera profile xml for 3DRServices/Tower

I know how to create and place an xml so that Tower recognizes it *as detailed elsewhere on this site), and I know how to do camera calibrations for Photoscan and Pix4D, but I don't really understand what's going on with the specs and how to convert them.

Performing a 3D Calibration with Pix4D, tells me the focal length of the new lens om the GoPro is "2.6333", as opposed to the stock which reads as 2.77. I'm ashamed to admit, I would have expected that number to be 5.4, but I know there's a relationship between the 5.44mm and the f/2.5 that I'm not getting.

To further complicate things, the camera profile .xml for 3DR/Tower looks like this:

gopro%20silver%203%20stock.xml

At the end of the day, I'd really like to know:

  • In Photoscan and Pix4D, do I change the "focal" length to 2.633 (as calculated) rather than 5.44, (which is what I originally assumed, but now think is wrong)
  • I don't have the faintest idea why the 3DR/Tower xml focal length value is set to 14 for a stock GoPro, but what on earth would I want to set it to based on the Peau replacement lens?

thanks for any insights anyone might have the time to provide!

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Replies

  • W Hayden July 16, 2015 at 1:53pm

    Huh - so whatever is going on with Photoscan, it seems to be working OK.

    Yesterday, I reflew a structure scan that just was not working well.

    Here's the medium resolution (for alignment, cloud, and mesh):

    https://drive.google.com/file/d/0B4GwYrFMW6TPRW9aLVg5RlZOMUk/view?u...

    There are obviously some errors in there, but for a first pass with no masking or cleanup, I was really pleasantly surprised.

    BTW - one thing I've noticed that I think may have made a huge difference. I changed my flight speed on both waypoint and circle modes to about 1/2 the original speed.

    Whether it's an issue with a stock GoPro or the replacement lens, I was definitely getting motion blur at full speed, knocking that down makes a huge difference in image quality! That scan was about 5 minutes of flight time at 1/2-speed.

    Hoping the high quality version will turn out similarly, but it's going to take a day or two with my computer...

  • Buck B July 14, 2015 at 3:24pm

    I am using a silver 4 with the same questions.  I have been getting a focal length error in Pix4d with 104%  Can you post your setting you have for tower and what you are using for pix4d.

     

    Thanks

    • W Hayden > Buck B July 16, 2015 at 10:07am

      Hi - so I am still trying to figure out what's going on with Photoscan, PhotoModeler, and Pix4d results, but I did look at the camera parameters here:

      https://github.com/ne0fhyk/3DRServices/tree/master/Android/assets/C...

      and double check the sensor size (though yours will be different on the 4 I think), then changed the focal length to 5.4mm, which seems to work perfectly in Tower (attached).

      In case you hadn't already found this:

      https://github.com/DroidPlanner/Tower/issues/1420

      It's instructions for where to put the custom xml

      GPSilver3_5_4mm.xml

      • W Hayden > W Hayden July 16, 2015 at 2:22pm

        Huh - so whatever is going on with Photoscan, it seems to be working OK.


        Yesterday, I reflew a structure scan that just was not working well.


        Here's the medium resolution (for alignment, cloud, and mesh):

        https://drive.google.com/file/d/0B4GwYrFMW6TPRW9aLVg5RlZOMUk/view?u...

        (NOTE: you need save the PDF locally and then zoom out, I also hadn't setup scale in PhotoScan when I saved it since it was a rough cut)

        There are obviously some errors in there, but for a first pass with no masking or cleanup, I was really pleasantly surprised.


        BTW - one thing I've noticed that I think probably made a difference: I changed my flight speed on both waypoint and circle modes to about 1/2 the original speed.


        Whether it's an issue with a stock GoPro or the replacement lens, I was definitely getting motion blur at full speed -  knocking that down makes a huge difference in image quality! That scan was about 5 minutes of flight time at 1/2-speed.


        Hoping the high quality version will turn out similarly, but it's going to take a day or two with my computer...

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