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The structure of your polygonal mesh can be thought of like the structure of a bridge or tower. In both cases you want to get the most use out of the fewest pieces. The arrangement of the polygons in a model, like the arrangement of beams in a tower, needs to relate to the shapes and forms that they're contributing to. The way in which edges relate to anatomy and to each other is called edge flow, and it's a crucial concept for making characters that bend and move in correct ways. So I'm just showing this image of the Eiffel Tower to illustrate the point that these different struts aren't just positioned at random angles.
They're positioned in such a way that they relate to the overall shapes that the architect had in mind when he designed the Eiffel Tower. So how do we know where our edge flow should go? In 3D character animation modelers and riggers use a principle called edge flow to determine what the polygonal structure of a model should look like. Edge flow is a way of relating the topology to the anatomy. In this exercise file, I have color coded different parts of the body according to their flow zones. That's the term I just made up. It sounds fun to say.
Flow zones are sections of a model that all have the same edge-flow pattern. I'm going to go over some of the things to take into consideration when planning out flow zones for a character. Look for where creases form. Look for lines and edges in the model. When the model animates, having the edge flow follow creases will help maintain their shape. So let's look at Hank here. I'm going to zoom in at his shoulders and his pec region, and we've got edges that are following right here underneath the pectoral muscles and going up, following this crease that goes up to the shoulder.
If the polygon edges went against the grain of the anatomy, it would be very difficult to animate, let alone model correctly. Another thing to take into consideration is the directional movement of flesh and muscle. When the body moves the skin pulls in various directions. Understanding what direction the skin moves in will help you place edge flow. Let's zoom in on the face. Notice the eyes and mouth. The skin is going to need to stretch and deform in certain directions to make smiles, squints, and other expressions.
The edges flow in the direction of the most common movements. So you see here on the mouth all of these edges are looping around. That's because common expressions that the mouth takes pulls the skin either up towards the cheekbones or pursing the lips moves them down towards the lips and so when the edges are laid out in this direction, it's very easy to form all of these different expressions. Another thing to look for is obvious structures. Anatomy often has a clear direction to it.
Bones, limbs, and muscles can be indicators of where to place edge flow. So let's zoom out and look at the arms. Limbs are usually easy to figure out because they're based on cylinders. Unless there's some specific reason to make it more complicated, limbs on cartoony characters can be very straightforward. So we see this flow zone here for the forearm. I'm just going to look around at all different angles, and you can see the arm is really just a cylinder with some extra little lumps modeled into it here and there.
So you can really have a very straightforward flow zone, with edges just going straight down the arm and edges looping around the arm. Something else you can do is play Connect the Dots. Some parts of the anatomy will be harder to figure out than others. Do the simpler parts first and then see if you can bridge the harder spaces between them. We're going to model the head and the torso separately, so if you're not sure what the edge flow for the neck should look like, you can do the parts above it and below it and then just bridge the gap between the two.
Another good tip is to get ideas from looking at other people's models. Lots of professionals post their work online with edge flow visible. Study what they do, but understand that there's not just one right way to do it. In general, understand that there often isn't one single edge flow pattern that will work the best. You may have to test out the model with a rig or experiment with putting the character in different poses to see how well it works. I often go back to the drawing board because the edge flow that I've chosen needs a little tweaking.
The edge flow that we're using in this course is a generally good one that works for most humanoid characters. However, every character model is different and will require some variations to what I show here. Luckily for you, I'll be showing various ways of thinking about edge flow and tricks that will help you adapt what you learn here to many different situations. As you get better at character modeling, you'll certainly discover your own techniques, tricks, and ways to improve on the basics.
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