Scanned images have 'stepped' curved lines
 

I've recently been sent some images for cleaning up prior to being used in high-quality with printing. However I've noticed that curved edges in the images are not smooth but are made up of a series of 'steps'. See attached screen grabs.

Does anyone know how these might have been generated? The original scans were at 400dpi (annoyingly in Index Color space instead of grayscale). I believe they were done on an Epson flatbed scanner.

Anyone have any idea?

Found it!

The original scans were fine. However, if I tried to straighten the image (wonky horizon etc) while still in Index Color space, the stepped effect appears. If I convert to Grayscale first and then straighten, there isn't an issue.

Hope that proves useful to someone else out there!