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I am attempting to recombine the two halves of a large photograph from a flatbed scanner without leaving an obvious seam.

Each image is 7206 x 9450 (and is a separate layer below). There is about a 150 pixel overlap.

I can move each layer, confirming the position by toggling visibility. However, the rotation is a bit off. Rotating messes up the alignment of course and does not undo quickly.

Then, I got the idea of matching the alignment of the grey lines from the photograph being in the frame for so many years (see image). I hope that if I get the grey lines perfectly horizontal, then the layers will be rotated correctly. I would then be able to simply move one layer over the other and all features would match.

Questions:

  1. Does GIMP have a feature in which I could select two points, then rotate the layer so a line between the points would be a straight line?
  2. Would this approach likely remove the visible seam between the scanned images?

enter image description here

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With the ofn-layer-aligner script:

  • have the two images loaded as layers in the same Gimp image
  • (recommended) make the top image partially transparent
  • using the Path tool, create a path with 4 points:
    • the first two points over the reference image (the one that won't move)
    • the next two points over the adjusted image
  • this path can be either a single stroke with 4 points, or two strokes of two points each
  • make sure the layer with the adjusted image is selected
  • call the script: Layer>Realign layer
  • restore the full opacity of the top layer
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This might be something easier done with dedicated image stitching software. If you have a Windows PC, Microsoft ICE (Image Composite Editor) is free. https://www.microsoft.com/en-us/research/product/computational-photography-applications/image-composite-editor/ The advantage of this software is that the stitching takes place automatically, and the alignment and blending is done for you.

However, if you insist on doing it in GIMP, it's possible if you were to create a layer mask, and fill the mask with a gradient - so that when the images overlap, there is a partially transparent gradient, to help make the seam look invisible.

Screenshot of GIMP showing use of layer mask

The above image shows how a layer mask has been applied to the second image and under that, it shows the join without a seam when combining the two.

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