Measuring and correcting for lens distortion

[Chris Smith]

Any 3D tracking softwre does this already. Since you need to get a track anyways, I’d use the software, it’s internal pipeline is everything you need, to undistort the image and get a proper track.

Check out
SynthEyes
Boujou
Matchmover
PF Track

Chris Smith
https://www.sugarfilmproduction.com

[Mylenium]

I concur with Chris. Those softwares have far more sophisticated algorithms than anything you could achieve within AE by e.g. using the Lens Compensation effect.

Mylenium

[Pour Myl

[Tamanegi65]

Ah, ok – makes sense. Thank you.

So, is there any use to shoot a calibration card with a grid on it during the shoot?

And also – if I re-write out my footage after being corrected in the tracking software and render my CG stuff with the camera it creates – do I really need to reverse correct the CG elements to composite on the original plates? Couldn’t I just use the corrected footage? Wouldn’t it look a bit cleaner without the distortion? Or does the correction of the distortion create more artifacts due to smooshing pixels around?

[Mylenium]

That mostly depends on what 3D software you have access to and what type of shots you have. If you use standard cameras in most 3D programs you don’t get correct distortions on your CG elements anyway (as they are simple pinhole cameras as in AE), only things such as Mental Ray’s lens shaders will do that. Also on wide shots you will barely notice any difference between distorted and undistorted footage except at the outermost edges, so there is usually little point in trying to match the lens curvature perfectly. I would only do that on tight/ close-up shots where distortions are very noticable.

Mylenium

[Pour Myl

[Chris Smith]

Don’t need a card. Assuming you have any straight line along the X, Z, or/and Y axis, you create a couple user lines over where the hard edges are and it will figure out the correction.

Chris Smith
https://www.sugarfilmproduction.com

[Tamanegi65]

I was just thinking some more about this – what about a green screen shoot? Yea, I put in tracking markers, but I don’t usually make sure they are in a line. I usually measure their location and take some picts on set for reference later – but typically there won’t be anything in frame that has a straight line….????

[Chris Smith]

I gotta tell you that I’ve done a good handful of 3D tracks in my time and have never fixed the lens distortion as part of the process. My tracks are still dead on. But that being said I make sure to use great quality lenses. Unless yours is insanely distorted, you may not notice a difference.

Chris Smith
https://www.sugarfilmproduction.com

[Tamanegi65]

I really haven’t noticed it too bad either – but there was one outdoor shot where we composited a building into a field and the shot began with a long dolly/pan to reveal the building. I was tracking the ground and other FG objects, as well as large tracking markers we put where the building was to go about 80-150 feet away, as well as far distant (300-1000feet) buildings in the background. I had a real hard time getting a clean track and I’ve been wondering ever since if it was due to lens distortion. It was shot with a VariCam – just stock lens I guess (not a prime, etc.)

So in general do you just track and not really worry about lens correction unless it seems to be causing problems?

[Chris Smith]

[DHenion] “So in general do you just track and not really worry about lens correction unless it seems to be causing problems?”

That’s what I’ve done. Haven’t had to compenaste for distortion yet.

Chris Smith
https://www.sugarfilmproduction.com

[Matthew Lundstrom]

Apple Shake, and PF Hoe (an entry level version of PF Track) both do lens distortion correction out of the box. You do need a straight line reference however. If you shoot a gray card / color card on set that would work perfectly. It’s a good habit to get into anyways.

MattLundstrom
https://www.mattlundstrom.net