Thank you grayOlorin, very useful information!

Does anybody have tips on how to further improve the quality of the projected UVs? Under certain circumstances I get rather distorted UVs and I would like to select those and relax them.

Hey guys, I need help. Following your discussion I get it to work with grayOlorins example file, but I got extrange seam lines in the edges around the UV-island .
I attach an image with the problem. The line is reducted if I increase the number of polygons, but it remains there.

Your UVs are possibly not quite correct. Make sure to enable “fix boundary seams” when unwrapping.

The UV is overlaped. The scene is a photogrametry rock (with mapped UV and textures). Then, inside Houdini, is programed to perform a “cluster” of rocks changing their position/rotation/size.
The problem here, as its duplicated n times, the polygon count is increased a lot in the final cluster. For that I need to reduce the polygon number from high to low and maintain the UV positions.
I dont konw if the method exposed by L.Kruel only works with padding&flating uv islands (not overlaped).
I dont want to recreate a new uv shell because the textures are done(payment asset). Using simplebaker way takes long to render for the thing “procedural cluster” I want and always get artifacts.
And finally, using bool-intersect got pretty results but sometimes are problems with normals and overlaping prims.

If I remember this right, I also had these seams and could not get rid of them. Have you tried to just combine the rocks with booleans?

As I said before , using boolean-intersection works very well for that particularly scene. But the question here is how to transfer UV from high to low (simple object), that is the problem.
Inside Maya there is an automated tool Maya>Mesh>Transfer attributes > UV, that does this thing perfect,.. but i plan to abandon autodesk software and stay with houdini.
¿why there is no such tool in houdini?

Sorry, I should have paid more attention.
I think this is a valid RFE.

I also have an idea:
1. Transfer the class attrib to lowpoly
2. Run for each class on the low-poly.
In that loop, in the high-poly save the uvs of the corresponding class. Grow the size of the patch of the corresponding class on the high-poly mesh.
Redo the UVs for that grown area. Restore the saved uv positions for the class.
Relax the resulting island, but keep the original UVs in place.
Isolate that new bigger UV island (by deleting everything else).
Transfer UVs from that bigger UV island.

Something along those lines.