Mesh flow

"Mesh flow" refers to the polygonal arrangement within a mesh and to what degree it displays the intended shape without error or waste.

The idea of a well "flowing" mesh can mean a few things depending on the kind of object you have. In previous sections we've talked about how wasted polygons should be avoided or when polygons should or should not be connected. This is similar but the idea of "flow" relates directly to not only how many, or where the polygons of a model are, but which direction they are facing.

As you'll see this is more of a worry with animated meshes, but is still an important consideration for static objects, especially models with poylgon count constraints.

Let's look at a few conditions under which problems can occur with bad mesh flow.

Subdivision problems

When using subdivision surfaces it won't be long before you start to notice certain oddities. Especially in mesh areas made of either non-quads or non-continuous quads (places where 3 or 5 quads meet at one corner). The first place you might have seen this is after applying a smoothing subdivision to a standard sphere shape where the lines converge at one point forming a circle of triangles.

Below are images of 3 spheres. Examine the wireframe versions and then the rendered version.

Wireframes of various sphere types. From left to right the standard 3d spehere, an icosahedron, and a hexahedron.
Wireframes of the same spheres with smoothed subdivision. Notice how the triangles at the top of the standard sphere have to create distortions so that all the edge loops can meet at one point.
A rendered view of the subdivided version. Note the distortions in areas that had triangles or non-continuous quads.

This problem won't only happen with just spheres. Any subdivided surface is susceptible to the same problem. Depending on the object there are typically two different solutions.

  1. Converge loops on a flat surface
  2. If your object has a flat surface somewhere you can try to lead lines to converge at points on a flat plane. The flatter the surface the less noticeable the distortion you see above will be.

  3. Turn loops
  4. Sometimes this problem can be avoided just by "turning" the loops so that they are guided around the object. In the above image of the three spheres notice how the sphere at the back right doesn't really have a point where many lines converge, causing triangles, but instead has loops that go all around the object. This leads the third spehere to have some of the least amount of distortion.

This brings us to a singular idea that will be critical for anyone who plans to model organic shapes. The importance of the loops. How important are loops?

Edge looping and pathing

Even if you don't plan on subdividing an organic object, as you probably won't with most game-ready models, it can help with your flow to make use of loops.

The importance of edge loops, and loops in general, is tied to the importance of quads for certain model types.

Particularly in models that are either animated or have the possibility of being deformed.

Examine another series of images. These are of two planes that have been subdivided and edited so that a single cylinder can be extruded out from the center. Note how the arrangement of the points on the plane itself are different.

In the second image below the only difference is that the object has been subdivided to create a smooth bend from the base of the object to the extrusion (though the subdivision itself is not the problem in this case, it merely exasperates it). Notice how the corner quads are stretched because of their initial layout.

The third image (which is the same as the second but without the wireframe) makes the problem inherent in the left object clear. The flow of the mesh is causing a distortion in the specular highlight that gives the surface a "shiny" appearance. Instead of a smooth circular highlight we see a jagged malformed highlight.

In this case the object does have clear loops that circle around the base of the extrusion. What's different is the way that the lines from the outside of the plane lead into the main extrusion.

The left object has lines that stop or converge at each edge ring as they go. The right object has been edited in a way to allow the lines to continue on straight into the extrusion.

On this particular model they are not "loops" but still maintain the idea that the lines of the mesh should lead on to somewhere without abruptly ending.

Convergence points

As you've seen, carefully chosen convergence points are important, and will help in how light can fall upon your model. In any model, organic or synthetic, you'll probably reach a point in the initial modeling process where many polygons need to flow into one or one needs to flow into many. Finding the right points will allow you to add edges that radiate out from that point but that doesn't have to go all the way around a model.

For instance if I am making a dog, horse, or bird, then I can have a convergence point where the eye is on the side of the head. If I find that I want to make the head curvature smoother and need to add edges then I can just have those new cuts lead into the eyeball instead of having to go all the way down the body to loop around a leg.

Of course it's possible to crowd a convergence point just like any part of a model so you can't just end as many edge loops as you want within one. But the option is there.

There are a few common points on organic models where you'll find 3D artists including such convergence points.

  • The eye socket (behind the eye).
  • Bottom of the foot.
  • The arm pits.
  • Finger tips.
  • The ear canal.
  • Exposed nipples (yeah...) LOOK WE GET LONELY SOMETIMES.
  • Any hidden area (such as a leg into a boot or the tip of a pin stuch into a pin cushion).

You must take into consideration how these points might affect any animated models however. Especially if they make use of smoothed subdivisions in some way.