How To: Prepare seemless bitmap textures in Photoshop

For all kinds of 3D material channels, it sometimes benefits the project to have a small, tileable image that will fill the surface of the object without an obvious seam. Of course, a tileable image will show the duplication if the instance shows up enough times. The trick is to avoid any hard lines where the image might change dramatically as it tiles across the face of the 3D object. Here's one method to avoid those nasty straight seams. (In two parts).