Rey's Leadlight Window by Chin Rey

What is required for this filter to produce seamless tiling?

Unfortunately there doesn't seem to be a way to make a diamond pattern like this seamless with Filter Forge. There is a trick but it requires a bit of post processing in an image editor. I prefer to do it with paint.net but Gimp, Photoshop and GraphicConverter will do jsut as well (GraphicConverter may be the the best option):

+ Set proprotions to 0 and check the Square Pattern and Seamless Tiling checkboxes
+ Save image - preferbly at a higher resolution than you need
+ Open with an image editor
+ Increase the canvas size to twice the original
+ Copy and paste so you have two complete repeats horizontally and two complete repeats vertically.
+ Rotate the image 45 degrees
+ Crop the image down to 1.41 times the original size (square root of two) - give or take a pixel or two.
+ Scale the image to the resolution you want. If you want a stretched diamond shape like the default, set vertical resolution to twice that of the horizontal.