Graphics Reference
In-Depth Information
To connect these two track segments, I can simply go to frame 18, enable the track, and position the marker
as shown in
Figure 10-15
.
Figure 10-15
Enabling the marker on frame 18
Using alternate motion models is also a powerful way to get accurate tracks.
Figure 10-16
shows a feature
from the street used as a tracking marker. Because the street is oblique to the camera, foreshortening and per-
spective cause the pattern to warp on the screen. Using the Perspective motion model, the shape of the pattern
area is automatically updated from frame to frame to keep the tracking pattern as uniform as possible. In the
figure, you can see frames 18 and 72. The pattern area shape has followed the correct shape of the feature.
There are several other important ways to optimize tracking. If your footage is moving fast and the pattern
is getting out of the search area from frame to frame, try enlarging the search area. A larger pattern area can be
helpful in adding context, but be careful that the pattern area does not contain background noise that changes
from frame to frame. The algorithm will try to identify the entire pattern, so if only the middle of the pattern
staysconsistent,forexample,thealgorithmwillfail.Trytoenclosethebiggestareapossiblethatremainsstable
and visible throughout the footage.
The Enable check box is very important. If a trackable feature goes offscreen or is obscured by another ob-
ject, you should disable the track for that feature for the frames where the feature is not visible and then enable
the track again for the frames where the feature is visible. It's okay to have a single track switch from disabled
to enabled multiple times. When a track is disabled, it doesn't matter where it is; it will not contribute to calcu-
lating the camera movement. But on frames where the track is enabled, it
must
be accurately placed, or it will
increase the camera movement error.
Finally,ifallelsefails,youcanplacethemarkerbyhand.TheBlendercameratrackerisextremelypowerful,
andwiththerightsettingsitwilloftengetgoodtracksautomaticallyeveninquiteadverseconditions.However,
for some cases of extreme motion blur, or when a marker goes briefly out of view or is otherwise a problem for
the algorithm, you may be able to identify the correct location yourself. In this case, you can simply enable the
marker and drag it to the correct point. If you find yourself doing a lot of hand marker placement, though, you
