[Bf-vfx] Footage for Tracking

[Andrew Hunter]

Hey Keir,

Just thought I'd chime in on the point about the focal length changing
during focusing on.

That isn't a function of the lens being a cine lens but rather the lens
design being poor. That effect is called breathing.

Good lenses, master primes etc., don't breath.

Cheers,

Andrew
On 2011-07-17 11:42 PM, "Keir Mierle" <mierle at gmail.com> wrote:
> 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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