Portfolio presents an external screen separate from the iPad's default mirroring. When not in the viewer, it displays the External Screen image set. Once in the viewer it reflects what is currently displayed on your iPad.
The external screen is filled with the configured background color and the image centered on it.
The size of this image should reflect the resolution of the primary screen you're intending to use. Be sure to account for any overscanning by the display that may crop out the edges.
See also AirPlay Best Practices
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