3-D Review
After SynthEyes has solved your scene, you’ll want to check out the paths
in 3-D, and see what an inserted object looks like. SynthEyes offers several
ways to do this: traditional fixed 3-D views, including a Quad orthogonal
isometric view, camera-view overlays, user-controlled 3-D perspective window,
preview movies, and velocity vs time curves.
Quad View
If you are not already in Quad view, switch to it now on the toolbar:
.
You will see the camera/object path and 3-D tracker locations in each view. You
can zoom and pan around using the middle mouse button and scroll wheel. You can
scrub or play the shot back in real-time (in sections, if there is insufficient
RAM). See the View menu for playback
rate settings.
Camera View Overlay
When you are tracking an object and wish to attach a test object onto it
(horns onto a head, say), switch the coordinate system button on the 3-D Panel
from
to
.
Note: the camera-view overlay is quick and dirty, not anti-aliased like
the final render in your animation package will be (it has “jaggies”), so the
overlay appears to have more jitter than it will then. You can sometimes get a
better idea by zooming in on the shot and overlay as it plays back (use
Pan-To-Follow).
Shortly, we’ll show how to use the
Perspective window to navigate around in 3-D, and even render an
antialiased preview movie.
Checking Tracker Coordinates
If SynthEyes finds any trackers that are further than 1000 times the world
size from the origin, it will not save them as “solved.” You can use the Script
menu's Select By Type script to locate and select Unsolved trackers. You can
change them to
Zero-weighted to see where they might fall in 3-D, and prevent them from
affecting future solves.
Frequently these trackers should either distant horizon points that should
be changed to Far, corrected, or deleted if they are on a moving object or the
result of some image artifact. Such points can also arise when a tracker is
visible for only a short time when the camera is not moving. The
Clean up trackers dialog can do this automatically.
Note: the too-far-away test can cause trouble if you have a small world
size setting but are using measured GPS coordinates. You should offset the
scene towards the origin using the Shift Constraints script.
You should also look for trackers that are behind the
camera, which can occur on points that should be labeled Far,
or when the tracking data is incorrect or insufficient for a meaningful
answer.
After repairing, deleting, or changing too-far-away or behind-camera
trackers, you should use the Refine mode on the Solver panel
to
update the solution, or solve it from scratch. Eliminating such trackers will
frequently provide major improvements in scene geometry.
Checking Tracker Error Curves
After solving, the tracker 3-D error channel will be available in the
graph editor
.
It is important to understand the 3-D error: it is the distance, in pixels, on
each frame, between the tracker's 2-D position, and the position in the image
of the solved 3-D tracker position. Let's work this through. The solver looks
at the whole 2-D history of a tracker to arrive at a location such as X=3.2,
Y=4.5, Z=0.1 for that tracker. On each frame, knowing the camera's position and
field of view, we can predict where the tracker should be, if it really is at
the calculated XYZ. That's the position at which the yellow X is displayed in
the camera view after solving. The 3-D error is the distance between where the
tracker actually is, and the yellow X where it should be. If the tracking is
good, the distance is small, but if the tracking has a problem, the tracker is
away from where it should be, and the 3-D error is larger. Obviously, given
this definition, there's no 3-D error display until after the scene has been
solved.
You should check these error curves using the fundamentals described
earlier in
Pre-Solve Tracker Checking, but looking at the
Error
channel. Here we’ve used isolate mode
to
locate a rather large spike in the blue error curve of one of the trackers of a
shot.
This glitch was easy to pick out—so large the U and V velocities had to be
moved out of the way to keep them clearly visible. The deglitch tool
easily
fixes it.
You can look at the overall error for a tracker from the Coordinate System
panel
.
This is easiest after setting the main menu's View/Sort by Error, unselecting
all the trackers (control/command-D), then clicking the down arrow on your
keyboard to sequence through the trackers from worst towards best. In addition
to the curves in the graph editor, you can see the numeric error at the bottom
of the
tracker panel :
both the total error, and the error on the current frame. You can watch the
current error update as you move the tracker, or set it to zero with the Exact
button.
For comparison, following is a tracker graph that has a fairly large
error; it tracks a very low contrast feature with a faint moving highlight and
changing geometry during its lifespan. It never has a very large peak error or
velocity, but maintains a high error level during much of its lifespan, with
some clearly visible trends indicating the systematic errors it
represents.
And finally, a decent tracker with a typical error level:
The vertical scale is the same in these last three graphs. (Note that in
the 3rd one, the current time is to the left, before frame 160 or
so, hence the blue arrow.)
You can sort the trackers within the graph editor's Active Trackers node
by changing Sort Alphabetic
to
Sort By Error .
Do not blindly correct apparent tracking errors. A spike suggesting a
tracking error might actually be due to a larger error on a different tracker
that has grossly thrown off the camera position, so look around.
Check for a Smooth Camera
Path
You should also check that the camera or object path is satisfactorily
smooth, using the camera nodes in the graph editor. We’ve closed the Active
Trackers node, and exposed the Camera & Objects node and the Camera01 node
within it. We’re looking at subset of the velocities of the camera: the X, Y,
and Z translation velocities.
There's a spike around frame 215-220, to find it, expose the Active
Trackers, select them all (control/command-A), and use Isolate mode
around
that range of frames. The result:
We’ve found the tracker that causes the spike, and can use the deglitch
tool
,
or switch back to the
tracker control panel and
camera viewport, unlock the tracker, correct it, then re-lock it.
Tip: In the capture above, the selected tracker is not visible in the
hierarchy view. You can see where it is in the scroll bar, though—it is located
at the white spot inside the hierarchy view's scroll bar. Clicking at
that spot on the scroll bar will pan the hierarchy view to show that selected
tracker.
If that is the last glitch to be fixed, switch to the Solve control panel
,
and re-solve the scene using Refine mode.
You can also use the
Finalize tool
on
the tracker control panel to
smooth one or more trackers, though significant smoothing can cause
sliding. If your trackers are very noisy, check whether film grain or
compression artifacts are at fault, which can be addressed by
image-preprocessor blur, verify that the interlace setting is correct, or see
if you should
fine-tune the trackers.
Alternatively, you can fix glitches in the object path by using the
deglitch tool
directly
on the camera or moving object's curves, because it works on any
changeable channel. You can also move the object using the 3-D viewports and
the tools on the
3-D control panel ,
by repositioning the object on the offending frame.
Warning #1: If you fix the camera path, instead of the tracker data, then
later re-solve the scene, corrections made to the camera path will be lost, and
have to be repeated. It is always better to fix the cause of a problem, not the
result.
If you have worked on the trackers to reduce jitter, but still need a
smoother path (after checking in your animation package), you can turn up the
Filter Size control on the
Solver panel. A filter size of 2 or 3 should make substantial reductions in
jitter. After adjusting the control, switch to
Refine mode and
hit
Go! again to apply the filtering.
Warning #2: filtering the path this way increases the
real error, and causes sliding. Remember that your
objective is to produce a clean insert in the image, not produce an
artificially smooth camera trajectory that works poorly.