7.4 – Physical Dynamics – Particle Systems: Hair

Particle systems of the Hair type are used to simulate the presence of hairs, grass, etc. on any surface. Since there is a wide range of possible applications for these systems, let’s limit ourselves to studying the classic example of a hairy ball or sphere. Therefore, we start from the primitive UV Sphere and add a Hair particle system to it, then set the segments to 25 (to get a decent level of detail) and the Hair Length to 4mt (4,000 with the none unit system); this value may initially seem high, but considering that we want to achieve the effect of long, wavy hair, the final volume will be different.

At this point, it is advisable not to activate the next Hair Dynamics panel as it would be quite heavy. Let’s immediately examine a function that is valid for any particle system (both hair and emitter) that allows you to significantly increase the number of elements in the system without excessively weighing down the calculation, by using child particles obtained through different algorithms. The panel responsible for creating child particles is Children.

Here, set everything as in the top image. Now check the Advanced option at the top of the system options to make the new panels appear: Velocity, Rotation, and Physics, located between Hair Dynamics (which should be left disabled) and Render (which we will examine last). In Velocity, set Randomize to 0.8, in order to make our tangle of hair more complex and disordered.

The next step is to find the appropriate shading to simulate what could be hairs, an animal’s fur, or a stuffed animal’s fur. We will start with Cycles, which provides the effective Principled Hair Shader that we will use in its default Direct Coloring mode.

As you can see, we will simply have to enter the RGB color of the hair and the shader will take care of approximating the absorption coefficient in a physically realistic way. The Roughness parameters adjust the reflections of the hair. Coat is used to further refine the shine of the fur/hair. Then we also find a refraction index for light passing through the hair. Offset is a value that should be kept low, especially in the simulation of human hair. To create fur or hair that changes color visibly from the root to the tip, perhaps fading between different shades, we can use the combination of nodes visible in the image.

which will give us the following result

The Principled Hair BSDF is a very advanced shader, originally created by Pixar for the film Zootopia. It does not work in Eevee (it is not even in the list of shaders) where we will have to continue using the Principled Shader and enable Strip in the Render Context Curves panel.

The result, of course, will not be able to compete with what was seen in Cycles, but let’s remember that this is a fairly complex particle system that Eevee is still able to render in real time in the 3D preview.

Even without the Hair Dynamics active, the hair particle system will react to the presence of force fields, which we can use to further “model” the system.

By activating the Hair Dynamics panel, depending on the power of your CPU and the complexity of the particle system, Blender may take a few moments to go from one frame to the next in the timeline, which indicates how heavy the calculation of the dynamics is for this type of systems; a weight that can be supported by CPUs starting from quad-core upwards. All of this will be stored in a temporary memory cache, or on the hard drive through the options of the cache panel already seen in previous cases. What you will notice when starting the physical simulation of the dynamics of a Hair particle system is that these systems, like Cloth objects, with which they share the same physical model, will tend to droop immediately, as in the figure.

Therefore, let’s examine the Hair Dynamics panel and its main options divided into two groups: Structure and Volume.

  • Mass
  • Stiffness, resistance of the hair to bending
  • Random, random variation of stiffness
  • Damping, to quickly dampen oscillations and the motion of the hairs
  • Air Drag, air resistance to motion
  • Internal friction, between the different hairs in the system.
  • Voxel Grid Cell Size, size of the basic volume element for the calculation of dynamics, smaller values correspond to greater precision

With higher Quality Steps values, the processing time increases significantly. Therefore, the computing power of the computer in use is essential in the physical animation of hair particle systems. It is enough to increase some of these parameters to put any CPU in difficulty. The advice is to initially proceed with preliminary tests on systems that are not very rich in particles, and then gradually increase the complexity along the way to the desired result. A first animation that you can experiment with is that of a system similar to the example of the hairy sphere, with a limited action of the gravitational field, greater resistance of the hair to bending, plus a wind-type force field. The final rendering of the animation will be so realistic as to fully convince us of the potential of these systems.

When a particle system of the hair type is applied to a mesh, we will have the following additional mode of the 3D View available

with which you will see the following tools appear in the toolbar:

useful for (from top to bottom):

  • combing
  • smoothing
  • adding mass
  • lengthening
  • puffing
  • cutting

Weight is used to distribute a different weight to different parts of the particle system. The Particle mode is essential for intuitively adjusting the fur or hair in real time and directly in the 3D view. There are three selection methods for this mode: the default is Path, while the remaining Point and Tip allow you to work on the structure of each individual vertex of the visible hairs/hair in real time (many fewer than those that will be finally rendered). When you select one of the tools seen before, its characteristics such as the radius of action and the strength are modifiable in the usual Active Tool of the properties, or in the Tool tab of the sidebar menu (N).”

In appendix A1, Extra Topics, an alternative method to particle systems Hair will be discussed.

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