2
$\begingroup$

I think my question relates to a fundamental of 3D modeling that I haven't picked up on yet. It's very simple:

Let's say I have a flat quad. A plane on the XY plane inserted into am empty blender file is a great example. Now let's say I start moving each of the 4 vertices around on in any direction, especially the Z axis.

At this point, there would still be one face, but its extremely unlikely that the face would be planar (I just learned this word). So blender does its best to make sense of what this should look like.

The problem I'm having is that it seems like my quads are always like this, but they never look the way I want. There are edges and seams that look like they are dangling in space, and what tends to happen is that my mesh looks great when viewed from most angles, but if you look at it from just the right angle, you notice the dangling edges or sometimes even some kind of impossible geometry. And so I might try to nudge the problematic vertices around, but this often seems to either make it worse, or just hide the bad geometry until you see it from a different angle.

For me, this always seems to happen whenever I'm trying to move vertices to add roundness to something that is originally square.

There is another question on here about how to actually flatten the quads. That's great, and I might use some of the techniques mentioned there. My question is:

  • Why does this happen? (Can someone explain this better?)
  • What are some tips and tricks to avoid this situation?
  • What are some tips and tricks to mitigate this situation?

EDIT Here is an illustration:

The overall mesh as viewed from a distance looks decent (for my skill level)

good picture

But if you get closer and look from a certain angle...

enter image description here

Super close up:

enter image description here

The "edge" that is pointed to by the arrow is not actually an edge, but some kind of artifact created when blender tries to show the quad. The way the vertices are positioned, it shouldn't actually be possible to see the sliver of the face that I marked in red, since this face is supposed to be around the corner from the edges. The edges should form the silhouette on the right side of my figure.

enter image description here

Here is the side view. Maybe it is the angled topology that skews the face in a funny way?

But I'm not looking for help with this one particular issue, but rather a better understanding of why this happens, and ideas to prevent this in future models.

$\endgroup$
  • 1
    $\begingroup$ It would be easier if you could add images to your question to explain your point. $\endgroup$ – Ray Mairlot Jun 27 '16 at 20:31
  • 1
    $\begingroup$ If faces start becoming non-planar after shape of mesh is edited then it's most likely that there are too few faces. $\endgroup$ – Mr Zak Jun 27 '16 at 20:38
5
$\begingroup$

As you already suspect, in three-dimensional digital graphics computer basically only know how to draw two things: Lines and Triangles

There are a few other primitives (point clouds, volumes, voxels etc.) but the question at hand this is what matters.

Historically speaking, in the first 3D modelling software you could only model with triangle based geometry because of this, it's the only unequivocally definable 3D structure with three points. Even Blender itself a few years ago as of versions pre 2.63 was still only able to define faces with 3 or 4 sides, not more.

As technology evolved it started being practical to define faces with more than 3 sides.

A quad, as you correctly stated is not unequivocally defined by those four points, it is not however mathematically undefined. Since computer only know how to draw triangles in the background, the software is trying to triangulate the shapes how best it can, as you can see bellow.

Triangulate

More complex shapes (when supported by the software) are even more difficult to triangulate, and may often generate artifacts from unexpected or uncontrollable tessellation. That is one of the reasons why you will often hear people say you should strive to have a dominant topology.

Blender doesn't explicitly allow tessellation control, but other software or mesh engines some times have tools that allow controlling how a shape is triangulated.

Tesselation

TL;DR Should you be worried by this? No, not at all, this is normal behavior and non-planar shapes are totally expected everywhere in your mesh. Some extreme cases could cause issues, like non convex faces or very high distortion quads, but most others should be fine.

There are tools in Blender like the Mesh > Cleanup > Make Planar Faces to help you if you experience issues.

What you seem to be missing is the concept of subdivision. Except for hard-edge models most organic shapes are frequently modeled using a Subdivide modifier, that will take care of automatically smoothing your angles, so that quads appear to have a smoother appearance.

Subdivisions

$\endgroup$
  • $\begingroup$ Thanks. I understand subdivision. The magic words i was looking for are "Should you be worried by this? No... this is normal". My fear was that I was continuing to fall into some sort of anti-pattern and making bad meshes. Ultimately I want to learn how to rig, animate, and get this stuff into the Unity game engine. I've gleaned that there are certain best practices when it comes to modelling, so I want to make sure I am building good habits from the beginning. $\endgroup$ – parker.sikand Jun 28 '16 at 0:33
3
$\begingroup$

As per Mr Zak's comment, You need to add more quads in your geometry.

IN EDIT MODE - Do one of the following:

To get the extra geometry needed I would recommend using loop cuts Ctrl+R, then hover your mouse over the mid-point of one of your edges.

Once you get a highlighted loop, you can either specify the number of loops you want to equally divide it into that number of cuts, or alternatively you can scroll your mouse wheel up and down until you are satisfied. I would probably at least do this in two directions, if not all three (if needed).

As an alternative to this, you can just use the subdivide command W>>S. To do this make sure your whole object is selected (every face otherwise your geometry is prone to getting even more out of control) then type W then hit S. Repeat this until you are satisfied.

enter image description here

$\endgroup$
  • $\begingroup$ Thanks. The little video you made perfectly illustrates my question. In your image, isn't the representation of the large center face mathematically undefined? Is it accurate to say that blender makes an educated guess to render the quad as two triangles? $\endgroup$ – parker.sikand Jun 27 '16 at 22:18
  • $\begingroup$ So I guess to refine my question, is more quads usually the proper way to solve this kind of situation? Are there techniques or ideas while modeling to avoid funny quads like this in the first place? $\endgroup$ – parker.sikand Jun 27 '16 at 22:33
  • $\begingroup$ I'm accepting the other answer because it gives a nice little blurb about the bigger picture of what is going on here, but I really appreciate your answer and I think most people will find it very helpful, just as I have. $\endgroup$ – parker.sikand Jun 28 '16 at 0:43
  • $\begingroup$ @parker.sikand, in regards to there being techniques to solve these kind of issues. The answer is yes there are, however it is a really big discipline that some follow like a religion of sorts. It doesn't really have a name other than "Good Topology Practises" or similar variants. If you are really interested in this and want a good place to learn it, I know that the guys over at CG-Cookie are a little on the religious side of this, and they have a good learning platform for Blender. Also you could go the more social route and search around over at Blenderartists.org $\endgroup$ – Rick Riggs Jun 28 '16 at 5:45
  • $\begingroup$ @parker.sikand. I also wanted to point out that even the subdivided mesh with more topology has the same errors. They are just being minimized be the smaller domain of the cuts, and as a result the error is way less noticeable. There are reasons to not over-due the topology as well, and that's what makes this a discipline worthy to at least spend some time in. As far as why you picked someone elses answer, that's just the way the site works, I won't hold it against you. Pick the post that is most helpful, and vote. :) $\endgroup$ – Rick Riggs Jun 28 '16 at 5:53
3
$\begingroup$

I think the issue is best described as - blender does not draw concave quads accurately.

a quad being edited to be concave

This is similar to the extra edge that you see coming from behind your model, the quad located just behind the visible faces is getting deformed inwards giving you the extra bit.

This mostly comes from the fact that a quad is turned into two triangles for drawing, while the method used is stable (and faster), it is not always the best way. If we deform the same quad as above but on opposite diagonals we get a better result showing that the quad is always split between the same vertices.

a quad being edited the opposite way

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.