I find myself making fill patterns in Gimp every so often and always having to look up the trick.
Here’s how to use Gimp to make tileable fill patterns, which you can either use inside Gimp, inside other programs, or as web backgrounds.
First, to make an image that will tile smoothly when repeated, start with a source image. This can be a drawing or a photograph.
Be careful about copyrights. If you drew or photographed the image yourself, you’re fine. Or if the image is from 1924 or earlier, you’re fine. If the person who created the image puts it in the public domain, licenses it under terms that permit distribution and modification, or otherwise gives you permission, you’re fine. Under any other circumstances, you may not be so fine. For personal use, nobody’s going to beat down your door, but putting the image on the ‘Net doesn’t exactly constitute personal use, if you know what I mean.
Got that? Got a picture? Good. Open it in Gimp. Actually you’ll probably want to open it twice. You’ll see why shortly.
Crop one of the copies of the image down into something that resembles the pattern you want to make. A lot of patterns aren’t much more than 256×256 pixels and you may be able to accomplish what you want in less than that. If it’s photorealism you seek, you may need to go a bit larger.
If the image isn’t straight or square, crop it slightly oversize (select the region with the mouse, then go to Image, Crop image) then use the perspective tool (Tools, Transform Tools, Perspective; or hit shift-p) and/or the rotate tool (Tools, Transform Tools, Rotate; or hit shift-r) to get the image straight and square. Then crop it.
Now, the magic. To make the image tile smoothly, use the offset command and smooth it out. Go to Layer, Transform, Offset (or hit shift-ctrl-o). Punch the button that says x/2 y/2 and hit OK. Your image will now be a tangled mess, in all likelihood. Smooth in the gaps. If you’re tiling bricks or something similar, you may want to go back to the original, uncropped image and copy and paste bits and pieces from it back into the image to cover up the gaps.
Keep in mind that when you cut/copy and paste, you can also select a region and use the paste into command, also from the edit menu.
You may also find it helpful to blur some gaps. Select the region you want to blur, then go to Filters, Blur. You might also find the Tileable Blur under the same menu helpful. Sometimes I’ve gotten good effects by repeatedly sharpening and blurring a region. It introduces just enough noise to bring it close enough that I can finish retouching by hand. You’ll find sharpen under the Filters, Enhance menu.
Once the image looks smooth, hit shift-ctrl-o to offset it again. You may find you’ve introduced new problems. Fix those, and offset again. Repeat the process until the problems disappear.
I find myself zooming way in and out a lot during this process. It’s often easier to select a precise spot you want to fix when you’re zoomed in.
And that’s the secret to making fill patterns in Gimp. Armed with an image, a copy of Gimp, this knowledge, and some determination and patience, you now have everything you need to make spectacular tileable patterns.