I am modeling a character and I used the shrinkwrap modifier to combine two parts of his head. It all looks fine until you render it and you can clearly see the difference between the two separate parts. (they are both in the same color. Is there a way to get a smoother transition or get rid of this shadow? When I render it you can see this nasty shadow.
What causes this problem:
I have tried my best to make sure that they transition very smoothly into each other. I have even applied a few subdivision. But then too, I can see the border between them. The last row of faces in one mesh don't point in the same direction as the other one. So there will be a visible difference.
So there are two ways you can solve this:
Do it manually:
If you are planning to animate this thing then having a proper topology will help. Just take a UV sphere, extrude the bottom faces and adjust a little. This shape is not too complicated.
Transfer Custom Normals:
You can use a Vertex Weight Proimity modifier with a Data Transfer Modifier on one of the meshes, perferably the one above, and make use of custom normals to smoothen the transition of direction of the normals from one mesh to the next.
Create a new vertex group for this and use it for the transfer. This setup creates a gradient based on how close the faces of the two meshes are, outputs it as a vertex group, and uses this vertex group for transfer the normals of the nearest face(corner).
This is tutorial form Pierrick Picaut where he goes into more details about this technique. https://www.youtube.com/watch?v=99zHzN-Rqro
This is the result after using this: