To understand UV Mapping more I created a UV Sphere and a material which is totally black. I also created a yellow material and assigend it to some faces. Then I baked three textures (high 2048x2048, mid 1024x1024, low 512x512) and saved the uv sphere as a wavefront obj file. I also set the margin to 0 because I don't want to waste space.
Then I opened blender new, imported the obj file (with the uv coordinates of course) and added a new material. I added a texture to that material and choose as Type Image or Movie. I choose the low image and the right UV Map. I thought it's working like it should but when I let Blender render that image I have ugly white lines between the "parts" where the smart uv map "cut" through the mesh.
When I change the method how the viewport is displayed as textured - then there are no lines.
What is the problem? Do I need to set margin to 1?