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Ella, what size did you choose for the initial image? I made mine too small, apparently, and when I use it to fill a 3600x3600 pixel page (my normal page size), the pattern all but disappears.
@enxio27 , I tested my pattern on a 500x500 pixels file.
If you start with a large file (3600x3600 pixels) then you have to make your pattern larger than 256x256 pixels to have a nice outcome, I think. The starting size of your pattern doesn't matter it only should be square.
@Krikor you're welcome and nice shirts you made. Where can I buy them
@enxio27 , I tested my pattern on a 500x500 pixels file.
If you start with a large file (3600x3600 pixels) then you have to make your pattern larger than 256x256 pixels to have a nice outcome, I think. The starting size of your pattern doesn't matter it only should be square.
Do you start with a larger pattern (larger than 500x500, so that the finished pattern is 500x500) if you are planning to make it diagonal?
@enxio27 I never work with files that large (3600x3600 pixels). But I tried something out for you. I started with a file 1000x1000 pixels and a big brush to make the pattern. Then the same way as in the tutorial with the smooth palette. I obtained a 256x256 pixel pattern. I enlarged it to 1000x1000 pixels. And I edited it again as described in the tutorial. This time I made a square cut of 670x670 pixels. Started a new file of 3600x3600 pixels and this is the result. Hopefully this will help you because I don't know more about it or if you can determine the size of the stripes yourself.
Attachments
diagonal pattern.png (359.1 KiB) Viewed 3184 times
@Ella So the size of each colored area is what controls the width of the stripes? I will try that, as well as the method you described above. The image you posted looks to be about the pattern density I'm aiming for.
I use GIMP mostly for scrapbooking on 12"x12" pages, which are 3600x3600 pixels at 300dpi.
UPDATE: No, the size of the colored area doesn't control the stripe width. It appears that I do have to enlarge the pattern as you described above to get the size density that I want.
I'm still having difficulty getting it seamless. I've used the Filters > Map > Tile Seamless on the final pattern, but there are still visible joints in the corners.
@enxio27 I've never made this pattern in 2.10 always in 2.8.
So I didn't use "Tile seamless" but "Make seamless".
It seems Wallace hadn't a problem to make it in 2.10.
Sorry, but I can't help you any further.
@Ella , would you please test your 2.8 "Make Seamless" on the following image? I'm wondering whether the issue is with my image or with the new 2.10 "Tile Seamless".
@enxio27 I made the pattern seamless in 2.8 and then tested it on a file of 3600x3600 pixels.
And indeed when I look at the large file at 100% size I see errors in the plaid.
@enxio27 I made the pattern seamless in 2.8 and then tested it on a file of 3600x3600 pixels.
And indeed when I look at the large file at 100% size I see errors in the plaid.
That's the same thing I'm seeing using 2.10's "Tile Seamless". I don't know how to fix it, either. Hopefully someone else can help. At least now I know it's not a problem with my image, and that going back to 2.8's "Make Seamless" isn't going to fix it.
and nice shirts you made. Where can I buy them 
It appears that I do have to enlarge the pattern as you described above to get the size density that I want. 