User Manual

Picking from Aux Channels
Some nodes, like StereoAlign, allow one to drag pick from the Z or Disparity auxiliary channels.
You must pick from a node upstream of the StereoAlign, not from the output of the StereoAlign.
If you try to pick a disparity from the output of a StereoAlign node, you will get nothing because
StereoAlign consumes/destroys the Disparity aux channel (and even if it did not destroy the
Disparity channel, you would still be picking the wrong value since you would be picking from
the aligned result).
The typical workflow for picking is:
1 View StereoAlign in the left view.
2 View the node upstream of StereoAlign in the right view.
3 Pick the Disparity value from the left eye in the right view.
Although this picking functionality does not operate any differently from normal picking of color
channels, this issue may cause some confusion. If it helps, the analogous workflow mistake with
color nodes would be a user trying to pick a gradient color for a Background node from a view
showing the Background node itself (you are trying to pick a color for a node from its
own output).
Another issue that you need to be aware of is which eye you are picking. To avoid problems, it’s
a good idea to always pick from the left eye. The reason is that the Disparity channels for the
left and right eyes are different, and when you pick from a horizontal/vertical stereo stack,
Fusion has no way of knowing whether you picked the Disparity value from the left or right eye.
The above are not hard and fast rules; rather, they are guidelines to prevent foot shootings. If
you understood the above reasoning fully, you’ll realize there are exceptions, like picking
disparity from the left output of DisparityToZ and Z from the left/right output of ZToDisparity,
where everything is okay.
Vector and Disparity Channels
The Vector and BackVector channels store the forward and reverse optical flow.
The Vector channel might be better named “forward vector” or “forward flow,” since the name
“Vector” to describe a channel is “not technically correct,” as the more mathematically-inclined
user might recognize that all the channels except the scalar channels Z/ID are technically
“vector” channels. A frames Vector aux channel will store the flow forward from the current
frame to the next frame in the sequence, and the BackVector aux channel will store the flow
backward from the current frame to the previous frame. If either the previous or next frames do
not exist (either not on disk or the global range of a Loader does not allow OpticalFlow to
access them), Fusion will fill the corresponding channels with zeros (transparent black).
The Disparity channel stores the displacement vectors that match pixels in one eye to the other
eye. The left image’s Disparity channel will contain vectors that map left > right and the right
image’s Disparity channel will contain vectors that map right > left.
For example:
(xleft, yleft) + (Dleft. x, Dleft. y) -> (xright, yright) (xright, yright) +
(Dright. x, Dright. y) -> (xleft, yleft)
You would expect for non-occluded pixels that Dleft = -Dright, although due to the disparity
generation algorithm, this is only an approximate equality. Note that Disparity stores both X and
Y values because rarely are left/right images perfectly registered in Y, even when taken through
a carefully set up camera rig.
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