I have been learning to model in Blender using the subdivision modifier, cuts, and support loops. (For an example of what I mean, see this video from Arrimus 3D. I find this technique is quite good for generating smooth and interesting geometries while retaining a pretty low poly mesh to work with.

I find sometimes I would like to add a high-poly region within an otherwise low-poly mesh. Consider the following simplified example: enter image description here

Here I have a somewhat low-poly cylinder, with two levels of Catmull-Clark subdivision modifier applied. I would like to add an extruded panel (seen in the bottom left of the picture). Do do this, I extrude out some faces and then beveled the corners with a few segments. The beveling tightens up the corners nicely, in effect forcing the sub-div modifier to interpolate over that smaller region.

At first glance it seems ok, but if you zoom in there are noticeable artifacts (most visible w/ the red racecar matcap, but also visible with studio lighting): enter image description here

I understand that this is likely the consequence of the ngon being generated in those regions. Converting to quads is tricky in this case, because the bevel has higher geometry density than the rest of the cylinder (although I am also new to this and possibly just missing the obvious solution). If I try cutting across the corner quad I seem to get different (but still visible) visual artifacts.

I realize this is a specific instance, but there are other simpler examples of what I mean. For example, consider a scenario like this: enter image description here In the image above, I start with a low poly plane and then slice one of the interior quads. I extrude the sliced quad, convert its inner region to a circle, and then extrude once more. At first glance it looks ok, but the normals have some odd artifacts where the high-cut and low-cut regions intersect: enter image description here The effects are less pronounced in this example.

My question is: are there some general retopology techniques or guidelines I can follow in instances like this? It seems like a relatively common thing - wanting to create a more detailed region within a low-poly mesh and avoiding artifacts at the boundaries.

You can download the blend file here if you want to take a closer look: Download (800KB)

Thanks for the assistance.

  • 2
    $\begingroup$ The short answer is subdivision surface doesn't like N Gons. Try to work only with quads,and try to preserve mesh density along the mesh $\endgroup$ – Gorgious Feb 7 at 21:12

So the rules for good subdivision topology, which you're violating and which are responsible for your artifacts, are:

  1. Quads only-- no ngons, and minimal triangles.
  2. No 6+ poles.
  3. Put 3 and 5 poles on the most planar parts of your mesh-- definitely not at places where your mesh has a right angle.

There are other rules. The rules only really matter for non-planar faces-- for faces that are out-of-plane with their neighboring faces-- because any verts created in the middle of a planar neighborhood will still be in-plane, and it doesn't much matter where exactly in the face they get created. (As long as you don't have any concave faces-- when you subdivide a concave face, the subdivision-generated vert might not actually be in the bounds of your face, leading to double of the mesh and z-fighting.)

One way that good subdiv topology is sometimes communicated is, "Follow your edge flow." If you look at the top of your circle in N2RPm.png, you can see that you're just laying a grid over your circle and bridging to the edge. "Follow the edge flow" isn't always very useful advice for beginners, because it's something that doesn't make sense for a while, maybe not until you're good at topo anyways, but that's a good example of not following your edge flow.

Now, given those rules, how do we bridge areas of dense verts with areas of sparse verts? Something like this:

enter image description here

Notice something especially important: I bridge the higher vert count to the lower count in a planar part of the mesh. Not where the mesh bends 90 degrees. Notice, no ngons, no triangles. All 3 and 5 poles are placed on planar parts of the mesh.

There are limits to bridging like this. You can drop two verts, or multiples of that, every time you use a new edge loop. Here, I'm bridging 16 verts to 8 verts. But you cannot ever drop one vert-- you cannot ever, using pure quads, bridge an even number of verts to an odd number of verts.


In regards to your cylinder: creating holes or extrusions on curved surfaces are no doubt tricky. One standard way of controlling subdivision modifier are holding edges:

This will change the shape of the cylinder also where you don't want it to be changed:

The change will be the smallest if after pressing CTRL + R, and clicking, you right-click to let the loop-cut sit in the middle of the faces. Then you can adjust only the part outside of the curved surface:

Then you could divide with loop-cuts all other parallel segments, so they all have equal dimensions, but you will now no longer have a perfect cylinder, which is not unfixable, but in this case it's easier to just start over with twice as many segments on the cylinder.

Another technique is creasing. Select an edge and press SHIFT + E then drag like with a bevel tool. You will not see a difference unless you have subdivision enabled for the edit mode. The effects are quite bad for low levels of subdivision, because there's just not enough geometry to get a smooth surface. On the GIF below, after selecting the edge, I disable the overlay, then crease, and at the end I increase subdivision levels from 2 to 4.


I tried Nathan's solution above on some planar examples. The first two do not use a bevel, and the third is a working solution the case of the bevel. Note that these solutions do feature triangles at the border, which can be removed after the application of the mirror modifier. The bevel variant looks a little smoother w/ the subdivision applied.

enter image description here enter image description here enter image description here


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