What Are Structural and Center Lines?

Structural lines and center lines are a key part of the sketching process for both traditional animation and standard drawing and are used to help create balanced, symmetrical figures with proper distribution of weight and perspective views. While not everyone uses them, they help at the base sketch level to help block out the figures, especially when working with people or animals - though they apply to pretty much anything with mass and depth, and can be extremely useful with buildings or objects such as cars. For the sake of this discussion, though, we'll focus on the center and structural lines in the context of character drawing for animation.

The center line is exactly what it sounds like: a line that divides your line down the center. I usually start off with stick figures before creating full character designs and determine my center line starting at the circular head. The line drawn over the circular head not only adds depth for me and cements the direction of the head, but tells me where the facial features will be, since the center line should pass right between the eyes, over the exact tip of the nose, and through the central peak of the lips.

If I'm drawing a character that's standing facing perfectly forward, the center line would be a straight line perfectly bisecting the head into two vertical hemispheres. For a 3/4 shot, though, I'd use a curving line; it would start and end in the exact same place as the straight line for a frontal shot, but it would curve outwards to show the turn of the head, leaving a crescent to one side and an ovoid to the other. The crescent would account for about 25% of the circle's area, while the ovoid would account for 75%. Even though the distribution is uneven, this is still a center line, as we're showing where the center of the face would be if the head was half-turned away and we were viewing it in perspective. The visual effect is almost similar to 2.5D animation.

The same accounts for the body's center line. When starting with a stick figure, the center line represents the body itself, but you'll end up building around it as you add rough shapes of your figure atop it. Your center line may be a straight line from head to hips, or it may be a shorter line showing the center line of the neck, another from the neck to the waist, and another from the waist to the groin. You can even use more fluid curves to show the distribution of weight and posture of the animation frame you intend to draw. The important thing is that you keep in mind the perspective view and draw the center line accordingly in relation to the position of the head.

Structural lines aid the center line in creating the appearance of natural posture. Once you have your basic center lines, you can add structural lines to represent the hips, shoulders, arms, and legs, focusing mainly on perspective and angle. If your character is facing the camera head-on, the structural lines for their shoulders and hips will be the same horizontal length to either side of the center line. The legs and arms may differ depending on if they're standing straight at attention or slouched more casually with one or both bent - which will affect the angle, but not the length, of the structural lines for the shoulders and hips. The planes of the body constantly shift to counter-balance each other; if one leg is bent, cocking the right hip up, the left shoulder will rise to compensate for it and properly distribute weight. It's always important to keep this distribution of weight in mind when plotting character poses.

From a perspective view, structural lines will appear to shorten and taper off as they recede into the distance. The structural line representing the shoulders will be shorter on the side further away from the camera than the line on the side closer to the camera, and depending on the pose will often appear to slope either down or up. The lines representing the arms and legs will be shorter on the farther side as well because the distance makes the limbs appear shorter.

The important thing to remember when animating is to work with your center lines and structural lines from frame to frame and make sure as the character moves these lines flow smoothly as you draw your in-betweens. If you start off using these lines in preliminary sketches, you'll find that as you build the character animation on top of it, you'll have much more natural, a believable motion that gets rid of any problems with wooden, awkward movement.