There are cases where you want to remove the paper texture when you scan. An often mentioned technique is to do two scans, rotating the paper 180° between the two. Then in an image editor you load both images, and rotate one 180° so that they overlap again, and take advantage of the different shadows to remove them (“Lighten only” mode or else).
At least, this is the theory. But if I try this on my Epson V200 with a detached sheet of squared paper, vertical lines (in portrait mode) are shifted, and I cannot have them all overlap across the paper: if they overlap around the center, they won’t on the edges, and vice-versa, and there is about as much as 1mm of discrepancy across an A4 sheet. On the other hand horizontal lines can be made to overlap across the whole height of the paper.
Made a test sheet. Here is what I get when I use Gimp to show the difference between two scans (if the images a identical, the result is all black)
- 1st scan: Reference
- 2nd scan, without moving the sheet in the scanner:
- 3nd scan, after shifting the sheet in the scanner:
- 4th scan, after rotating the sheet 180° in the scanner, and then rotating the result 180° in Gimp. The little arrows were added to the rotated scan to identify lines in the layer, so in the differences below they show where the lines for the rotate scan are). Two differences:
- Optimizing for center:
- Optimizing for edges:
- Optimizing for center:
So, the result isn’t too bad (but far from perfect) when scanned in the same direction, but there is an obvious problem with the
Digging in to this a bit more, I find that a side of the sheet is slightly compressed, in other words, on the
Original scan the center mark on the paper is not centered on the scan. The distance between the vertical lines of the pattern that correspond to my edges is 4822px, but the vertical line of the center is 2396px from left and 2426px from right in the
Original scan. Of course, on the
Rotated scan I find similar values but in the opposite direction, the left half is slightly wider that the right. So this is a 0.6% inaccuracy. Should I expect better? Do you see something similar with your scanner?
Any clues for the cause of the problem?
- Incorrect paper handling (but then why only vertical lines)
- Common problem with home-grade scanners, yours does that too…
- Scanner is old technology
- Scanner is old, period
- Scanner is broken/out of specs
My surprise is that the scan head transport (timing belt/stepper motor) is still accurate, but whatever happens in the scanning head (I thought there were no moving part there) is not.