An advanced version of "Make Seamless".
When searching for a good tool for making seamless textures I found "Seamless texture maker" by RPG, but it turned out to be very simplistic and a bit glitchy, so I wrote my own version. It produces two additional layers - the horizontal and the vertical overlap.
* Separate width settings for horizontal and vertical overlaps
* Two gradient types to choose: simple and intelligent
* Gradient contrast setting allowing sharper transitions
* Gradient fuzziness option
* Hi-pass pre-filter to equalize spatial brightness
* Manual overlap: just make the overlap layers and crop
Using the "Manual overlap" function:
1. Set "Horizontal overlap" to the value you need and "Vertical overlap" to 0
2. (optionally) Turn on "Hi-pass pre-filter"
3. Click OK
4. Edit the overlap to make it seamless. Don't worry about the top and bottom edge correspondence - this one will be covered by the vertical overlap.
5. Merge down (sic!)
6. Start the plugin again
7. Now set "Vertical overlap" to the value you need and "Horizontal overlap" to 0
8. (optionally) Turn off "Hi-pass pre-filter"
9. Click OK
10. Edit the overlap to make it seamless. This time you MUST care about left and right edge correspondence; in the most cases it has no need be very precise.
11. Merge down or flatten
When used correctly the manual method produces results never possible to achieve using any automatic method. Still, try intelligent gradient with the contrast = 50 first and only if it is not good enough move on to manual stitching.
New in the version 1.16:
* Fixed crash if the hi-pass was on and any of the overlap values was 0 (which is vital for the manual overlap mode)
New in the version 1.15:
* Greatly improved hi-pass filter (now on by default)
* Now overlap values are checked against the image dimensions, if larger the maximum recommended value is displayed
New in the version 1.11:
* Manual overlap mode with both horizontal and vertical overlaps specified simultaneously is now explicitly disallowed (a warning message will be given)
New in the version 1.1:
* Hi-pass pre-filter added (thanks to RPG for the idea)
* Another fuzziness anti-aliasing method (now only a toggle)
Simple gradient, no hi-pass pre-filter
Simple gradient, with the hi-pass pre-filter
Intelligent gradient, gradient contrast = 75, with the hi-pass pre-filter
Written in Python. Download, rename (remove the .txt extension), put it to ~/.gimp-[version]/plug-ins and make it executable (chmod 755). Run from Filters/Map/Make Seamless Advanced...