Compensating scanner distortion - looking for volunteers with a scanner

[Ofnuts]

(03-04-2022, 07:54 PM)Krikor Wrote:

(03-04-2022, 03:50 PM)Ofnuts Wrote:

• A 7px error isn't bad if you are scanning a 600PPI (my scanner is 76px). If you used a lower definition, it's proportionally worse (7px @300PPI would translate to 14px@6OOPPI
  

• What is the picture above? The anchors are not on the lines, so it is after you ran the "fix" part?
  

In the previous attempt, the image skewed during printing. I scanned the image without noticing this deviation (0.16º).

So I redid the process again, but this time I scanned with a resolution of 600ppi.

Generating an output of : Scanner map 600PPI (5, 0.513, 0.551)

But I still don't quite understand what was done or how to take advantage of it.
I guess I'll have to read the manual again...

Forgetting the script for  a while so if you do:

• Scan image upright ->Image1
  
• Scan image rotated ->Image2
  
• Load image1 in Gimp
  
• Load image2 as layer over image1
  
• Rotate image2 layer
  
• Set image2 layer to Difference mode
  
• Find best alignment
  

You never get more that 5px of distance between the two images? If so, you scanner is good enough and doesn't need the fix.

Otherwise, the vertical bands in your displace map aren't expected, the resulting map should look like a uniform gradient (even if looking up close it is a sequence of gradients):

   

Your anchors could be off by one  pixel but with a max delta of 5px this  is relatively important.

PS: What paper did you scan? The numbers in your displace map says it is based on a 14mm grid?