Dropped frames in Photogrammetry

Posted by Mark Fink on March 15, 2017 at 10:44am

I've been doing more testing lately, specifically for an issue I've noticed with the camera not firing consistently in photogrammetry mode with my Inspire 1 Pro and 25mm lens. I created a simple route with one leg up and back, then ran that in the simulator using different flight speeds.

According to the calculations, this particular route would require 122 images, using the auto by distance option. I'm shooting in Shutter priority and using a fixed shutter speed of 1/1250. The Calc. Photo Interval is how often I expect the camera has to fire. Here are the speed results:

Speed     Frames Taken    Calc. Photo Interval

12 m/s     88                     1 per 1.73"

10 m/s     100                    1 per 2.08"

8 m/s       116                    1 per 2.6"

6 m/s       118                    1 per 3.47"

Even at 6 m/s, it appears to be missing some images. I've done tests with the Inspire 1 on my desk, manually taking photos using the DJI Go app every second without any issues, so I know the camera and my SD card are capable of capturing and storing data at that rate.

I also did another test with the above photogrammetry path, this time using the photo by time option, setting it to one photo every 1.5". That should have yielded 138 photos, but only took 72. I can't figure out where the issue is. The hardware seems perfectly capable. If it were a GPS issue, where delays in calculating the current position and deciding when to take the next photo were at play, then wouldn't that be ruled out with the photo by time test?

Thanks,

Mark

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Replies

  • Mark Fink April 6, 2017 at 6:42am

    I did some test flights manually triggering the camera every 1.5 seconds, (I had a metronome app running in the background at 40 beats per minute), and it worked just fine. There is hope!

  • Mark Fink March 25, 2017 at 8:28am

    Finally got a chance to do some testing, and now I see that the DJI Go app only supports a minimum of one shot every 5 seconds in JPG mode! I was sure that I had seen an option for one every 2 seconds. So, that won't work either. I've tried various settings to see if a faster option shows up, such as full manual exposure, manual focus, JPG (not RAW), but can't get the option for anything faster than one shot every 5 seconds. Does anyone else see a faster option for the Inspire 1 Pro with X5 camera?

    Maybe it's time to start looking at a different vehicle for photogrammetry? Any suggestions from other users on a platform that will give the same or better resolution, but be able to shoot faster and thereby complete large surveys faster?

    Thanks,


    Mark

  • Artūrs March 21, 2017 at 2:17am

    Yes, the time lapse and focusing from DJI GO and then switching to UgCS to fly should work. It would be really interesting if that solved your issue, we will had to add Time Lapse to our app :)

    Good luck!

    Artūrs

  • Mark Fink March 17, 2017 at 9:32am

    Hi Artūrs,

    Wow - that is really surprising! Thanks for the link to their specifications. I wonder why they have such a long time between images. For photogrammetry, I'm shooting in JPG, not RAW, so I would think it would be capable of faster throughput.

    I'm going to see if there might be another way to overcome this. There is a time lapse mode in the DJI Go app that, (in JPG mode), allows one frame every 2 seconds. Would it be possible to start with DJI Go, set focus, exposure AND timelapse every 2 seconds in that app, then connect UGCS and fly the mission? You would end up with many more images than necessary while taking off, flying to the first waypoint, and landing, but those could be deleted after the fact. I'll try some tests this weekend.

    Mark

  • Artūrs March 17, 2017 at 2:52am

    Hello Mark,

    We appreciate your analysis, you have done great job!

    Please do understand, that there is a difference between triggering camera manually (using button in app) and the flight controller triggering camera during auto flight.

    Please check out these values defined by DJI for each of the gimbals (source):

    The time interval during which two photos are taken depends on the photo file format. 
     For X5 and X5R, the range is [5, 2^16 - 1] seconds for all formats. 
     For XT, the range is [1, 60] seconds for all formats. 
     For X4S, X5S and Phantom 4 Pro, when the file format is JPEG, the range is [2, 2^16 - 1] seconds;
     when the file format is RAW or RAW+JPEG, the range is [5, 2^16 - 1] seconds. 
     For other products, when the file format is JPEG, the range is [2, 2^16 - 1] seconds; 
     when the file format is RAW or RAW+JPEG, the range is [10, 2^16 - 1] seconds.

    Meaning that DJI assures that photos can be taken at least every 5 seconds with your gimbal.

    I can suggest to set the trigger according to your test results - with 4 second interval.

    DJICamera
    Javadoc API documentation.
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