Here are some tips to improve on the default Pixel Motion result with Timewarp:

Here are some tips to improve on the default Pixel Motion
result with Timewarp:

. Matte Layer and Matte Channel menus can point to
a matte channel used to isolate foreground and background
motion, as in Figure 2.22. This can be the
source alpha channel, or a matte channel from a separate
layer.
. Warp Layer control analyzes a separate layer (as specifi
ed in the menu) and applies the result. Sometimes
you need to adjust footage (contrast, luminance, key)
in order to get a better result, but you don’t want those
adjustments in your shot; apply them to a duplicate
layer instead and set that.
The Tuning section is where you trade render time and
accuracy, but don’t assume that greater accuracy always
yields a better result—it’s just not so. These tools make use
of Local Motion Estimation (LME) technology, which is
thoroughly documented in the Furnace User Guide.
The following settings control how Timewarp analyzes
footage:
. Raising Vector Detail certainly increases accuracy; a
setting of 100 assigns one vector per pixel. Not only
does this drastically increase render time, it will in some
cases simply increase artifacts with fast motion. This is
because it is analyzing too much detail, similar to setting
a motion track to too small (and noisy) a search region.
. Smoothing relates directly to Vector Detail; vectors
must join similar pixels of adjacent frames but not be

noisy. You can raise Global Smoothness (all vectors),
Local Smoothness (individual vectors) and Smoothing
Iterations in order to combat the noise, but go very far
and lots of detail will be missed. The Foundry claims
that the defaults, which are balanced, work best for
most sequences.
. It typically takes two adjacent images—before and
after—to make a vector, but you can choose Build From
One Image to use only one if that helps.
. LME depends on object brightness remaining consistent
throughout the shot. Toggling Correct Luminance
Changes on warns Timewarp that there will be fl uctuations,
whether sudden (image fl icker) or gradual (moving
highlights).
. Error Threshold evaluates each vector before letting it
contribute; raise this value and more vectors are eliminated
for having too much perceived error.
. Block Size determines the width and height of the area
each vector tracks; like Smoothing, lower values generate
more noise, higher values less detail. The Foundry
documentation indicates that this value should “rarely
need editing.”
. Weighting lets you control how much a given color
channel is factored. The defaults may seem odd, but as
you’ll learn in Chapter 5 they correspond to how the
eye weights each color and so they produce a perfectly
monochrome image. If one channel is particularly
noisy—usually blue—you might consider lowering its
setting.
. Filtering applies to the render, not the analysis; it
increases the sharpness of the result. It will cost you
render time, so if you do enable it, wait until you’re
done with your other changes and are ready to render.
Despite that Timewarp is included free, many knowledgeable
After Effects artists still depend on Twixtor (RE:Vision
Effects); like 3D trackers, keyers, and other automated
solutions, this is more a question of what works for you
than of one being better than another. A demo of Twixtor
can be found on the book’s disc.