how to avoid straight lines in normal maps i've been having some trouble with baking normals.
I make a hi poly mesh and retopo it. Since i need to uv unwrap, i just mark some seams into the low poly model. 
The problem is i get some odd straight lines into the normal map in the exact places i marked the seams in.
I've been trying to search for a solution but it keeps happening and adds some strange shadings.
Did you use a margin when baking and did you leave enough space between the UV-Islands? Blender is approximating the map at the seams and if your image doesn't reach all around the UV-maps (e.g. your UV-coordinates are right at the edge of the UV-grid) Blender has problems approximating this edge smoothly.