Tangent Space Normal maps are infamous for being wracked with seams along UV borders. What most artists don't know is that setting up your UVs has a profound effect on your normal map! Take the following example. I make a slightly irregular sphere (so that we can easily see where the equator-cut-area is), and I cut the model's UV projection right along the center:
When I take this mesh into Zbrush, I can sculpt whatever I want, here a few random stars will do. But when I make a normal map, we are going to have some big problems:
These Problems arise because Normal maps are lit as so... Red from the right, Cyan from the Left, Green from the Top, Violet from the Bottom, Yellow away from the viewer, Blue Towards:
If the UV shells are rotated when we create the map, then we are going to run into seams when we try and render the map, or view it in Maya's high quality renderer:
But..... In certain cases, we can fix this by editing the channels of the map in photoshop. Since the Lower shell in this case is rotated 90 degrees, what we can do is take the Red Channel for the Lower shell and swap it with the Green Channel ... FOR THE LOWER SHELL ONLY... and then invert the colors to flip it correct. Follow me here:
First I take the Red Channel and make a dupliacte. This will store it for later since I am going to override the real Red in just a second.
Use the Marquee Select tool to select the rotated lower shell in the Green Channel.
Copy that selection, then Paste it into the Red Channel.
Here is the Red Channel, now correct after having been pasted:
Now, leaving the same selection Marquee in place, lets select the Red Copy Channel, and Paste it onto the Green Channel
We no longer need the Red Copy once we use it to duplicate to Green. So at this point, right click on it and choose Delete:
Finally, we need to invert the direction of the green channel. In the green channel, what we see as White is actually Green when we view as RGB, and what we see as Black is actually Violet. If we invert the color, 50% gray will not change, but all the other values will. Black goes White and visa versa. Thus, Green goes Violet and visa versa. Soooooo..... on the model, slope up becomes slope down (and visa.... you get the idea).
Save this out, and load it in as your normal map.... and:
I hope you enjoed this one! Make some use out of it.