Avoiding creases when applying smoothing groups to holes/non-quads?

[Anonymous]

I'm poly modeling (without meshsmooth/turbosmooth) and am running into major problems when trying to cut holes into surfaces.

As depicted in the attached file (PNG image), if I cut a simple hole in the face of a cylinder by removing some of the quad faces, I'm left with creases along the edge faces, despite all faces being perfectly planar.

The problem is made worse if I decide to create a couple triangular faces in the corner, thus making certain quads into N-gons. The creases become even worse, although all faces are still perfectly planar!

How exactly does one poly model shapes with holes cut into the surface if creases immedaitely appear, and only get worse as additional articulation is added? This is a very simple technique-- I'm directly creating the final mesh (like I said, no smoothing modifiers), and I'm working entirely with planar faces, yet I find the creases thus far inescapable.

[spacefrog_]

the faces on the left of your hole are definitely not planar - it looks like you deleted the edge going vertically from the top to the bottom without deleting the corresponding vertices.
So you got several 6 vertice-polys ( non-quad ) which are NOT planar -
you can verify this by going into vertex-subobject mode.... now you will see the leftover vertices.
I would recommend to recreate the missing edges to get rid of this "seam" - or if you removed the edges by purpose, remove the leftover vertices (select them - press backspace )

to delete the vertices automaticly wshen removing the edges, press "CTRL-Backspace" instead of "backspace" only

[Anonymous]

Planar simply means that all vertices (no matter how many there are) exist on the same plane in 3space. All of the quads, tris, and N-gons in the depicted object are planar (I can provide the scene file to prove this if necessary). It doesn't refer to the number of vertices.

As for missing edges within the polys, whether or not I create additional edges to turn everything into quads and tris, the creases remain.

The problem simply appears to be that smoothing groups crease whenever a mesh contains a face that has more than 4 vertices, or whenever a face is part of the edge of a hole. How does one poly model under such circumstances? Both holes and N-gons are going to be necessary for most types of models. For instance, the cylinder depicted above is a very common, simple type of object, and yet it's severely hampered by creases.

Edit - I should note that even in the second image, when all faces are quads, the creases are still a problem. So this is an issue whether or not N-gons are involved.

[ekahennequet]

Josef is right. You have removed edges that run vertically.

[ekahennequet]

Planar simply means that all vertices (no matter how many there are) exist on the same plane in 3space. All of the quads, tris, and N-gons in the depicted object are planar (I can provide the scene file to prove this if necessary). It doesn't refer to the number of vertices.

As for missing edges within the polys, whether or not I create additional edges to turn everything into quads and tris, the creases remain.

The problem simply appears to be that smoothing groups crease whenever a mesh contains a face that has more than 4 vertices, or whenever a face is part of the edge of a hole. How does one poly model under such circumstances? Both holes and N-gons are going to be necessary for most types of models. For instance, the cylinder depicted above is a very common, simple type of object, and yet it's severely hampered by creases.

Edit - I should note that even in the second image, when all faces are quads, the creases are still a problem. So this is an issue whether or not N-gons are involved.

You have n-gons that are non-planar even in the second image. You need to cut/create edges.

[Anonymous]

Whoops, I understand the confusion. In my original image, the edge lines happen to be rendered in grey-- in the second image, by coincidence, the grey edge color blends in with the lighting on the faces and appears to vanish. The edge is there, however, I assure you. All faces are quads and no edges are missing. This is evident in the attached scene file.

Jospef - I misinterpted which faces you were refering to as non-planar. Sorry if my response sounded condescending. 🙂

Sorry for the mix-up.

274_RElWeAUh2wl1z8vzHpGB.zip

[Anonymous]

Also, attached is an updated image that uses pink edge lines. The creases exist within planar quads, N-gons and tris alike. It should be noted that this appears in the render as well, so it's not just a viewport artifact.

[spacefrog_]

ah okay - now i see the problem

short answer is :
the smooth surface is faked - you can't expect a perfect shaded surface because of that.
To fix the problem you have to give the object more geometry, which allows the shading algorithms
to be more precise.
Introducing non-quads makes the problem even more obvious , cause the algos have more complex interpolations between each vertex to compute.
If you look at the problematic face's vertices and their lightlevels, you quickly will see why this slight different shading is generated - the problematic vertices do not have a partner to the right ( cause there is the hole) - so no light from the right vertex can be calculated into these vertices.
The vertices WITH a partnervertex have a rather high lightlevel in these partners - these high values
get interpolated throughout to the left - so these vertices produce brighter shading

Additionally keep in mind that everything is tri-based in Max - even Polys are .
The visible crease in shading happens to be exactly where these hidden edges sit....

try to make a second ring of edges arround the cutout, but keep all geometry strictly planar quads
the problem is very subtile IMHO - even less visible if a material is applied -
what kind of application do you have in mind for these super lowpoly objects ?

[Anonymous]

It's the n-gons that are messing up the smoothing (nothing to do with whether they're planer or not) The rule of thumb with polys is quads, quads, quads.

If the cylinder on the right is your desired result, then you need to cut horizontal and vertical edges that pass through the selected verts - the ones where the tri meets the edge - and continue all the way around the object. Then you need to add a vert to the hypotenuse of the tris to make them triangle shaped quads. This way everything will be a quad and the smoothing should work.

If you need me to, I can upload another file...

Edit: you also need to adjust the new verts, the ones where the tri meets the edge of the hole so that they're not planer, but consistent with the curvature of the cylinder (that is, equidistant from the center). This will give you the smoothest result.

On second thought. Your best bet is to start with a cylinder with twice as many height and side segments so you don't have to add any verts except the ones on the hypotenuse of the tris...

[Anonymous]

That's what I was afraid of. 🙂

These particular objects are just examples; what I'm working on right now is a model of a cell phone that I'm poly modeling instead of using sub-D because I got sick of dealing with all the drama you get when trying to do hard-edged holes in curved surfaces, but it looks like I'm running into similar problems. 🙂

The phone is much more high-poly of course and looks great overall, but is actually having even more serious creasing problems in a particular area where I'm cutting a button hole into the side of the phone. The material itself will simply be matte plastic, so there won't be a lot of texture detail available to help hide the creasing.

Thanks for the insight, though. I think my strategy now is simply to find a way to work as many of the weird edges in the hole on my phone model into quads, and try to add edge-loops around the hole itself to minimize the creasing. I thought I could forget about concerns like quad faces and edge loops when I stopped using sub-D, but I guess they're going to follow me everywhere. 🙂

[Anonymous]

Yes, quads suck. Well, quads are fine, but being forced to use them exclusively sucks. It would be great if someone could come up with a sub-div/smoothing scheme that could handle tris and n-gons with ease. I haven't looked at the math, so it may be impossible, but it sure would make life easier...