Preserving shading after cutting a hole

Question

I'm trying to cut a hole in a mesh to create torpedo tubes on a spacecraft, and am running into a bit of trouble. I can get the hole cut out just fine using Knife Project, the problem, however, is that it completely screws with the shading of the mesh.

Here are the target and pattern meshes:

Here's a detail of the pattern that creates the shape of the outer tube:

and the target mesh:

Now, as I said I can cut the hole in the target mesh just fine. The problem is, I get this:

As you can see, cutting the hole in the mesh throws off the smoothing of the mesh, leading to some odd shading which is just unacceptable.

I'm trying to do this with as few extra polys as possible.

Answer 1

Prior to cutting the new geometry the transitions are smooth because the shading is a function of the average face normals connected to each vertex. Because each of those adjacent face normals is different as you go around the surface of the object, the transition is relatively smooth. It's a cheap but effective trick.

Once you start cutting into a face, you get new geometry which has a number of vertices which are surrounded by faces which have the same face normals, therefore the shading is 'flat' on those sections. The transition from surrounding faces onto that flat area is also abrupt, and more prominent with transitions between quads and triangles.

Read Blender docs about custom normals

possible solutions

1. A way out of that is to carefully adjust the positions of the new vertices such that they follow the inferred curvature, this doesn't have to be exact. as long as you get away from the faces all having the same normals on that spot.

   The dark area just below the blue arrow indicates the flow that your geometry would need to have (seen from the side of that edited face). If you can move the internal vertices to match the curvature you'll get reasonable smoothing.

2. or adjust the vertex normals to follow the curvature, but (May 2015) this is not exposed to the UI, but can be scripted. Manual changes might be overwritten by operations such as remove doubles / recalc normals.

   

3. Normal Edit modifier. It takes a vertex group, and an object, and modifies the normals of the vertices in the vertex group to follow as if they eminated from the center of the object (radial) or directionally if you use a mesh object to help guide the normals.

Blender's Proportional Editing feature can help get nice curvature, but you'll have to experiment with it.

Answer 2

Zeffi mentioned surface normals and shading. Perhaps you want sharp edges where the hull meets the torpedo launcher. This effect is often achieved using an Edge Split modifier for Sharp edges. You have to leave the Sharp Edges checkbox checked, and then mark those edges in the mesh editor (Ctrl-E > Mark Sharp).

Another thing that sometimes causes strange shading is if the surface normals are flipped. You can check that using the 3d View's N-panel Mesh Display > Normals (pick the Face icon).

Answer 3

I ended up turning on Auto-Smooth and then making the transition between the sides of the hull and the bottom (specifically, the edge along the bottom of the poly in which the torpedo tube was cut) sharp, and that fixed it. It's not exactly how I wanted the contour to look, but it works.