Distortion removal filter

[NotMyFault]

I often have images like scans which contain distorted lines of objects with known geometry like 

• straight lines,
• perpendicular lines,
• defined angles
• circles or ellipse shapes

This is relatively easy to correct in case of only one global distortion is relevant.

Unfortunately in most practical cases you have multiple distortions accumulated, e.g.

• paper which had water damage
• bad lens distortion
• double lens distortion (photo taken from photo)

I suggest to add a new distortion remove filter which allows the following process:

1. you can add multiple alignment helper objects (vector shapes) or grids of those shapes
2. you can then mark with mouse / pen the corresponding lines (marking pixel positions) in the source image. This should be a curve (vector line), to allow later adjustments
3. affinity then creates a transform matrix mapping the marked lines to the alignment helper shapes

• A switch should toggle if the filter effects gets visible or not
• the interpolation between the given objects should be selectable, e.g. linear, gamma, and nodes like gradient tools.
• The helper objects should be semi-transparent with automatically contrasting colors (like mouse cursor)
• you may add a mode where every helper object shows a selectable number of points along its edge, you can then move the corresponding nodes (either source in image or destination in helper, choosable by user) 

The main point is improving the ability to tweak those helper objects for a perfect result. The current mesh warp / perspective / distortion filters are usable mainly to distort straight objects into no longer straight objects, but almost unusable to support the reverse process.

• One wrong click and all your node fine-tuning is gone, e.g. switching between source and destination mode. 
• Impossible to tweak both source and destination grid in one edit
• mesh warp defaults for node handles unsuitable, as it always creates wrong distortions, not allowing to keep lines straight
• only rectangular grid (no support for other grids or shapes)