Non-square pixels (aspect ratio) from flatbed scanner

[John Eklund]

I'm scanning maps in patches for later stitching and realized that many flatbed scanners give slightly non-square pixels. (When scanning a circle I get an oval). Hugin has no ability to optimize for this parameter. My solution would be to manually adjust each image in Photoshop before loading into Hugin. Is there no elegant way of correcting this? Any ideas?

Some old HP models are very poor in this regard. My Canon Canoscan 4400F gives perfect square pixels but I had to replace it because when painstakingly dismounting the lid, I introduced dust somewhere which resulted in colored lines that I have been unable to get rid of despite cleaning repeatedly. I then bought a used Canoscan 8800F. It outputs very slightly non-square pixels, though not much at all so I figured I can live with it. Still, I assume it should be impossible to get a perfect stitch.

Are we really certain that all cameras have square pixels?

[John Eklund]

When I said "impossible to get a perfect stitch" I was considering that it's inevitable to have the map slightly rotated between scans. The problem would probably manifest clearest if rotating some scans 90 degrees from the others. Very small rotations won't be a problem but the stitched map will have the same aspect error as the scans. And I need to overlay the stitched historical map over new maps later, so it has to have the right aspect...

[Bruno Postle]

I seem to remember that a combination of the g&t (shear) and r (roll) parameters will correct for squashed images.

But introducing more optimisation variables is not always helpful. If you can calibrate the distortion by scanning a ruler or similar, you will get much more consistent results by pre-processing all your scans with an image editor before using Hugin.

--
Bruno

[Abrimaal]

The most of scanners store info about the scanner model in EXIF data. Hugin can connect to a camera lens database, but I don't know if any scanner database exists. It depends also on the orientation of scanner paper. It is important that all parts of the map should be scanned in the same direction.
This is only an idea, don't ask me for details. :)