When digitising art paintings by panning the camera - taking multiple shots - and then stitching them together with Photomerge, the resulting image usually contains significant concave distortion along the long edges (especially when the art has a high aspect ratio of 3:1 or greater).
In ACR (as a filter) the distortion tool can straighten these curved horizontal edges well, but because it works in both planes simultaniously, this then results in the vertical sides (that were relatively straight to begin with) taking on a convex curvature.
The only way I've found to adjust the image is to firstly get the "best compromise" to all edges in ACR using the distortion tool and then attempt to "fine-tune" the image using warp transforms. If the art being digitised is already framed then this technique doesn't work well because it's virtually impossible to get an edge perfectly straight using the warp transform (no matter how many adjustment handles I add). And even if the art isn't framed it's usually still necessary to crop a bit of the image off to straighten the edge out which is less than ideal.
A simple (and near perfect) solution would be the addition of a control adjacent to the ACR (as a filter) distortion tool that allowed the adjustment to be applied to (*) Horizontal Only, (*) Vertical Only, or (*) Both.
A second (and more complex) solution would be to introduce "square-up and straighten" functionality; perhaps in the same vein as the existing "select and mask" where (optionally) the 4 corners of the object are identified and the high-contrast edges "traced" to identify the edge to the algorithm that then adjusts the entire image in such a way that the edges are parallel and straight.
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