ID 350081 - ZDefocus node using 'use input channels' creates artifacts in the processed alpha channel is both filter and image input have pure white alphas

Problem summary
Within the ZDefocus node you have the ability to filter based upon an input image, you can do this through changing the filter type to 'image' and connecting the source image to the ZDefocus 'filter' inputs.

One option also available within this filter type is the 'use input channels' checkbox, this will allow both the filter and image inputs to use the same channels.

An issue has been discovered that if your 'filter' input and your 'image' input both contain a pure white alpha and you are using the 'use input channel' option, you will get artifacting in the resulting ZDefocus alpha.

Customer reported version
nuke.11.1v3

Customer reported platform
suse

Steps to reproduce

Open attached script and view the result of the ZDefocus node

or

1) Create a new Nuke script at HD resolution​

2) Read in an image that has a depth pass embedded.

3) Create a Roto node, set the input to 256x256 and draw a small shape (to use as the filter image)

4) Create a Shuffle node and shuffle the alpha into pure white.(see image below)

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5) Create a ZDefocus node and plug the Read/Ramp node combination into the 'image' input

6) Connect the Shuffle into the 'filter' input (see image below)

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7) Ensure the 'depth' channel is set to 'depth.Z' in the ZDefocus node and set a focal point to anywhere in the viewer using the viewer manipulator

8) View the ZDefocus nodes alpha channel
Result: ZDefocus alpha channel contains vertical stripe artifacts.
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Workaround
Avoid using the 'use input channels' option if you have two pure white alphas coming into the ZDefocus node. Turning off the checkbox will allow you to pick an independent channel which works correctly.