Re: make seamless horizontaly

[Rebecca Jean Pedersen]

can you just copy it and paste it into a canvas twice as wide, paste
again, flip horizontal, and there you go.

Re: make seamless horizontaly

[James Smaby]

You could, of course, do it manually:
copy the image into a second layer,
flip this horizontally,
add a layer mask,
give the layer mask a linear gradiant.
I beleive the make seamless plugin works in a similar way, only
more complicated because it needs to tile in two dimentions.
-James Smaby