We've got some 4:3 SD footage we're needing to uprez for an HD production. We're needing to do a trick that a machine some of the bigger post houses back in the day used to have but is no longer available. This machine would take the SD footage and scale it to fit the HD frame vertically (486 - 1080). Then it would lock in the middle 70% or 80% of the SD footage, and gradually stretch out the remaining 30% or 20% (15 or 10% on each side) to fit the rest of the horizontal frame.
I thought maybe I could do this with a gradient displacement map, but it's not doing the gradual scaling I need.
And by gradual, I mean that the first vertical row of pixels touching the 80% middle is left alone. The next row is stretched out just a little bit (sub-sampled, I'm assuming), and the next row stretches just a little bit more.
Try that, just replace video with yours.
You were right, displacement is a thing to do. However two instances needed for left and right side (one displaces with red, another with green channel).
Some small upscaling on horizontal were still needed, but you can play with displacement amount and reduce that too.