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Correcting Vertical Convergence

Perspective works both ways. There's this rather pretty effect:

Railway line in Headingley (1996; Yashica compact)

Less appealing to some is vertical perspective distortion:

Vertically challenged (Leeds City Square, 1996; Yashica compact)

Here's how to straighten the Leaning Towers of Leeds. Photodesk doesn't provide much help for this process, but it's not too hard.

• Load the above image into Photodesk.
• Click on the Scissors, then the Distort toolbar icon (sheared rectangles)
• Distortion requires us to define a four-sided source area, followed by a destination area. Everything in the source area will be mapped to the destination area.
• Click on the lower left of the picture, then drag up and right to form a rectangle over the whole image. A circle will appear at each corner, with the lower left one inverted to mark the starting point.
• Ensure all the circles are in the corners of the image. You can start and end the drag in the border area, which places the circles correctly.
• Now, look at the 'verticals' in the image. Drag the top left circle to the right until the slope matches the lean of the red crane. If your image has a convenient 'vertical' line starting at the lower left corner you can align exactly to this, otherwise you'll have to do it by eye. Unfortunately Photodesk doesn't provide angle measurements when dragging out distortion areas, so you can't use the technique described earlier for horizontally straightening images.
• Repeat for the upper right circle, dragging it leftwards until it matches the slope of the other crane and the tower block (magnify at x2 and align to the tower's windows). You should get something like this:

  

  Defining the source quadrilateral

• When you're satisfied, drag out another rectangle (the destination area) over the entire image so the circles are again jammed into the corners. Important: start from the lower left and drag up and right, as in the first one. You will find it easier to drag out a small rectangle inside the first one, then adjust each corner individually, as two of the circles overlap.
• You should now have this (these demonstration pictures are half-size, to save space; you would normally work at full size):

  

  Ready for distortion

• Note that due to the way Photodesk draws its circles, the lower pair appear to have vanished. They are there though, and this 'feature' is useful when you want to align circles exactly.
• Click on the Tick. Distortion may take some time, so be patient.
• And here's the result:

  

  Straight up

Notes

• Hint: check if your verticals are correct by popping up a menu and moving it around, aligning the edge against various objects.
• You're unlikely to get a perfect result, especially if the original camera lens has any spherical distortion (common on cheap ordinary lenses and expensive ultra-wide-angle lenses). Spherical aberration is much harder to fix, especially as Photodesk doesn't have a 'pinch' filter (though it may be possible to use the Lens special effect for crude correction).
• If the distortion is severe, you may find the image quality degrades significantly at the top. If this happens it may be better to 'shrink' the lower part rather than 'stretch' the top:
  • Define the source quadrilateral as the whole image, with a circle in each corner
  • Now adjust the bottom circles on the destination quadrilateral, dragging them inwards by a similar amount to that used above
  • This is much harder to get right, and you'll need to crop the image after. And as most images have more detail and interest in the lower part of the picture, the 'squishing' may be noticeable.