You can render C4D's native Hair object as Arnold curves using C4DtoA. To create a Hair object, add some geometry to the scene and select Simulate > Hair Objects > Add Hair from the menu. The Hair object has a default Hair Material. All features of the material which are related to the topology of the curves are supported. The look of the hair can be defined by using the Arnold hair shader (Arnold > Surface > hair in the Material Manager). If you don't add any Arnold shaders to the Hair object it will render using a default hair shader.
To add Arnold related parameters to the geometry you must add an Arnold Parameters tag to it.
Refer to the splines page to view more information about these specific hair attributes.
Hair ids are exported to a curve_id user parameter. Curve index is used as the id.
UV coordinates of curves are exported to uparamcoord and vparamcoord user parameters. More information and workflow example can be found on the hair shader UV Parameters page.
If this value is non-zero, curves with a small on-screen width will be automatically enlarged so that they are at least the specified size in pixels. The enlargement fraction is then used in the hair shader to adjust the opacity so that the visible thickness of the hair remains the same. For a given number of AA samples, this makes it a lot easier to anti-alias fine hair, at the expense of render time (because of the additional transparency/depth complexity). Good values are in the range 0.2 to 0.7. Values closer to 0 are faster to render but need more AA samples. So if your scene already uses very high AA settings, you should use a low value like 0.1. For best results, you may need to increase the auto-transparency depth, and/or lower the auto-transparency threshold, but watch the effect on render times. Note that this parameter currently works with the ribbon mode only and Opaque must be disabled.
There are two algorithms for rendering curves in Arnold, 'Ribbon' and 'Thick'.
This mode is recommended for fine geometry such as realistic hair, fur or fields of grass. These curves are rendered as camera-facing flat ribbons. For secondary and shadow rays, they face the incoming ray direction. This mode doesn't look so good for very wide hairs or dramatic zoom-ins because of the flat appearance. This mode works best with the built-in hair shader, that uses a Kay-Kajiya specular model.
This mode resembles spaghetti. It has a circular cross section, and a normal vector that varies across the width of the hair. Thick hairs look great when zoomed in, and are specially useful for effects work, but their varying normals make them more difficult to anti-alias when they are small. You can use any shader with this rendering mode, including lambert, phong, etc.
The Thick mode currently does not work well with transparency nor refraction.
The hair shader is not designed for hairs that are much bigger than the pixel size.
Hair thickness can be defined using the Thickness tab of the native Hair Material. You can setup root / tip width, a random variation and scaling thickness along hair guides by a curve (e.g. make hair thinner in the middle, etc.).
Hair thickness settings
Texture is not supported.