User Manual

things in pixel shifts, this can cause problems with Proxy and AutoProxy. Fusion follows the
convention that, for proxied images, these channels store unscaled pixel shifts valid for the
full-sized image. So if you wish to access the Disparity values in a script or via a probe, you
need to remember to always scale them by (image. Width/image. OriginalWidth, image. Height/
image. OriginalHeight).
Viewing of Disparity and
Vector Channels
Aux channels can be displayed directly in the viewers through the Channel viewer
button’s menu.
The CopyAux node is used to copy those channels directly into the RGB channels for viewing
or further processing. The advantage of using the CopyAux node is that it does static
normalization, which reduces a lot of flicker that the viewer’s time-variant normalization causes.
When viewing long sequences of aux channels, the CopyAux node has the option to kill off aux
channels and keep only the current RGB channels, freeing up valuable memory so you can
cache more frames.
TIP: Although you can use the Channel Booleans to copy any aux channel into RGBA,
it involves a few additional clicks when compared to CopyAux.
One thing to be aware of is that aux channels tend to consume a lot of memory. A float-32
1080p image containing just RGBA uses about 32 MB of memory, but with all the aux channels
enabled it consumes around 200 MB of memory.
Stereo and Optical Flow Best Practices
How you create your composition, the images you are using, and the type of shot you are
working on can all have an impact on the success of the Disparity generation and Optical Flow
analysis. Below, we’ll look at some of the situations to be aware of and how you can avoid some
pitfalls when dealing with optical flow.
Semi-Transparent Objects
The Optical Flow and Disparity generation algorithms Fusion uses assume there is only one
layer per pixel when tracking pixels from frame to frame. In particular, transparent objects and
motion blur will cause problems. For example, a shot flying through the clouds with the semi-
transparent clouds in the foreground and a distant landscape background will confuse the
Optical Flow/Stereo algorithms, as they do not recognize overlapping objects with different
motions. Usually the optical flow will end up tracking regions of one object or the other. If the
transparent object and the background are near the same depth and consequently have the
same disparity, then it is not a problem.
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