How Do I Unwrap Only Certain Verticies/Faces in An Already Baked Texture

Question

I have a Blender file and I'm adding greeble to the walls of a corridor. I have an object from another spot in the file that I copied over to the new wall. I already baked the texture for the object but in order to get the object to fit in the new wall I had to add a few new faces and extrude a few already existing faces. The trouble is that these new faces/extruded faces are horribly tangled and oversized in the UV map. This is the UV map with my added faces and all of the vertices selected.

This is the UV map with just the new verticies/faces selected.

I want to just unwrap the new vertices/faces. Is there any way to unwrap the new vertices/faces into untangled shapes so I can reposition them in the old baked texture map in a way that looks somewhat natural?

Answer 1

Is there any way to unwrap the new vertices/faces into untangled shapes so I can reposition them in the old baked texture map in a way that looks somewhat natural?

Yes, probably.

Select the vertices with a good UV map, where you don't want to change the UV coordinates. In the UV editor, 'p' pin these vertices. Select all of your vertices and unwrap. (Or, pin only the border good faces, and unwrap the bad faces plus a margin of 1 face.) The unpinned vertices will be unwrapped in a way that stitches them to your pinned vertices.

This isn't guaranteed to give you a good unwrap: if you have a lot of distortion in your base unwrap, you'll have even more distortion in your new vertices, probably leading to overlap. And you'll need to seam new edges appropriately, to match any existing seams. But this is basically what you want. (And is a pretty useful technique to know about anyways, even if it isn't.)