I am routinely processing images from the DJI Phantom 4 and have noticed a re-occurring issue. I am also routinely using the DJI Phantom 4 Pro, often at the same location, and need to be sure I can produce comparable models without GCPs.
I’m finding that using Pix4D’s default cameras parameters for the DJI Phantom 4 (FC330_3.6_4000x3000) using the “All Prior” Internal Camera Parameter option is producing models are that dramatically distorted. Both the camera alignment and model shape are very convex. Turning the “Linear Rolling Shutter” parameter on alleviates this slightly.
If I allow Pix4D to optimize all internal camera parameters, the model is less convex, but the curve is still very noticeable. This produces a large Quality Report “Camera Optimization” relative difference of ~30%. For example, the focal length has been optimized at ~4.6 mm in comparison to the input 3.61. Turning the “Linear Rolling Shutter” parameter on gives similar quality report values, but there is a visual improvement; the model is only very slightly convex.
Setting all distortion values to “0” and setting “All Prior” produces a better (flatter) result, but the model still appears distorted (concave this time).
My current hypothesis is that A) some rolling shutter effect is present, and B) the default radial and tangential distortion values for the DJI Phantom 4 are significantly different from my own camera, although not 0.
If we can assume that the input focal length & principal point camera parameters are stable and correct, is there a way to have Pix4D optimize only lens distortion values and correct for the rolling shutter effect?
What is the source of Pix4D’s radial & tangential distortion values for DJI’s camera models? Is it recommended to measure lens distortion for each camera to get the most accurate results?
Do you have any other advice on how to get results as close to reality as possible?