how to manually edit a 3d camera track?

[djmattyz]

im working on a 3d camera track here , but it doesnt seem to be working out

i took the shoot using a sony A7s camera, full frame  with a  24mm lens

the 3d camera track came up with : 36mm sensor and 44.53mm lense , i left a beer matt on the table as a guide, and i create a solid from that, and oriented the solid so that it fitted perfectly over the beermat.  however when i extend the solid out the full size of the table, it appears as though the perspective is different from the footage.  it looks like a wider lense or something.  however when i attempt to change the focal length or focal distance to rectify this i loose the track position of my solid , as in, it doesnt stay stuck to the table anymore.

how can i rectify this?

[Rick Gerard]

The first thing you need to do is correct for lens distortion. AE's lens is perfect, there isn't a 24 MM lens that costs under about 15K that is distortion free. It's best to set up a test shot filming a grid that is perfectly square to the camera then figure out what correction you need. The edges will be the worst. It is really too bad that the Lightroom lens profiles cannot be applied to video. They are amazingly good.

Once you remove the distortion either render a suitable DI or pre-compose and run camera tracker again. Make sure that you do detailed analysis, set an origin and ground plane, remove any tracker points stuck to anything that is moving in the shot and run your test again. Adding the Effects>Generate>Grid effect to the solid will really help you position the reference solid perfectly. It's very easy to include track points that are not on the plane you are referencing and this will foul up your placement. That plain green surface probably does not have many track points on it so you will probably have to use the edges of the table and then move the placed solid down a little bit to get it to perfectly match the shot.

It is really easy to get frustrated when you don't perfectly pick the track points you use to define the plane and even more difficult if the plane you pick is not the one you want to modify. It is also impossible to perfectly match the edges of the frame unless you remove lens distortion.

[djmattyz]

thanks very much rick

one other related question

if i do a camera track, and afterwards i realise its too short, and i need to camera track another part of the clip. 

is it possible  without starting over and losing all my work up until that point?

[Rick Gerard]

You can cut the clip and run different camera tracks on each piece. I do that all the time on long shots where something comes in and out of frame. You actually get better results.

On the other hand, if the plane (surface) you want track stays in the shot it is much better if you track it all at the same time. I usually pre-edit my sequences in Premiere Pro and then only use the footage I'm actually going to have in the final edit in a comp. Most of my comps are under 7 seconds and only one shot. That's the most efficient way to work.

If you are going to cut up that shot into several pieces and insert a different angle then do your pre-edit, make sure you like it, and then make a new comp for each shot. It will take you less time then trying to track a 2 or 3-minute shot, the chance of tracking errors or complete failure goes way down, and you'll tell a better story in the long run. Camera tracker is a resource hog and the quality of the track depends entirely on the shot. I probably would have shot your scene a little differently to give myself more information to work with.

Hope this helps.