[Bf-vfx] Footage for Tracking

[Keir Mierle]

On Sun, Jul 17, 2011 at 4:59 PM, mindrones <mindrones at gmail.com> wrote:

> Hi all,
>
> a couple of weeks ago me and Corrado Zacchi have shot some footage with
> a HD CCD camera, to see if we could help providing some imagery not
> affected by rolling shutter.


> Enrico Gusso has been so kind to lend us a Panasonic AJ-HDX900P
> (https://panasonic.ca/english/broadcast/broadcast/hd/aj_hdx900.asp) and
> Gianni Centonze helped on the capture part :)
>
> http://download.blender.org/ftp/incoming/libmv/mindrones/


Very cool!


> Feel free to use this footage to test libmv integration as you like, and
> please report any problems related to this footage, so that next time we
> can shoot some better test.
>
>
> We were a bit in a hurry that day so we've done mistakes for sure, but
> this was a first test to hear from libmv devs. Hopefully we'll do more
> later.
>
> Some examples of mistakes to learn from:
>
> * make sure the calibration pattern is *really* flat, taping the grid is
> never enough flat (we didn't have time nor a place where to glue the
> grid, sorry)
>

It's probably fine.


> * as I've learned today chatting with Matthias: let the paper grid fill
> the frame:
>
> <MatthiasF> also, next time you shoot calibration, don't hesitate to
> have the paper fill the frame
> <mindrones> you mean also borders going out?
> <MatthiasF> yes
> <MatthiasF> we detect the inner corners of the checkerboard
> <MatthiasF> so you can have those corners at the corners of your frame
> <mindrones> oh crap! didn't know that!
>
> * increase the shutter speed to reduce the motion blur (not always
> possible in a real situation this is let to the director of photography
> to decide this parameter)
>

The shutter speed doesn't change the calibration, so it should be fine to
set this temporarily. However, it also works to just hold the calibration
grid still momentarily when waving it in front of the camera in a few
positions.


> * remember to remove any gain to avoid noisy grain (same as above)
>
> * (to self) remember to check the iris more often, damn! :)
>
> * also, as seen in the camera specs, the sensor effective pixels are
> 1280x720 and instead we've shot at 1080/25P, which means that the camera
> does some image manipulation to pump the image to 1440x1080: next time,
> best shooting also at native resolution to check if this helps the tracking
>
>
> As an interesting test that we had no time to make, very often, even on
> cine lenses, while focusing you also end up varying the field of view,
> hence zooming a bit because of lenses mechanical and optical
> imperfections. For this reason we didn't focus during these shots and
> hence we had to choose a fixed focus distance, but this forced us to
> have unfocused parts, which I'm sure won't help the tracking much.
>
> What to do in real situations when you have to control the focus? Will
> libmv take in account this factor one day?
>

This is interesting. I didn't realize cine lenses vary the focal length with
focus, but it makes sense. I suspect the calibration would still be good
enough to work in practical cases.

There is also my autocalibration code which I am still working on.

This is great stuff! Thanks for making a contribution. When I get back to
the states, I'll check it out.

Keir


>
> (Not to mention real zoom of course)
>
>
> Regards,
> Luca
>
> _____________________________
>
> http://www.mindrones.com
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