Curves

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It is possible to render Maya curves with Arnold. An Arnold tab is appended to a Maya curve, which allows the user to define a shader (defaults to a hair shader), curve width and the sample rate used to describe the curve.

To render multiple hair curves using the same hair shader, it is far easier to create an override set. Tutorials on how to render curves can be found here.

The above settings are provided via the Attribute Editor for curves

Currently there is no checkbox to avoid SSS computations in point and curve primitives. However, the use of SSS is not supported with point and curve primitives and is therefore not recommended.

Render Curve

Determines whether or not the curve will render. 

Curve Width

Defines the rendered thickness of the spline.

 

Curve Width 0.01 - 0.1

It is also possible to connect a texture map or shader (e.g. Noise) to the Curve Width attribute.

Noise texture connected to 'Curve Width' attribute

It is not currently possible to map a 3d texture or project a texture to the Curve Width attribute.

It is not possible to displace a spline curve using a displacement shader.

Sample Rate

Sets the number of samples taken to reproduce the maya curve. Note that it is only possible to have more samples along the length direction of the spline and not the width.

 

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 antialias 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. 

 

There are three algorithms for rendering curves in Arnold.  

Ribbon

Ribbon 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 a proper hair shader (perhaps based on a Kay-Kajiya or Marschner specular model).

Thick

Thick 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 especially useful for motion graphics work, but their varying normals make them more difficult to antialias when they are small, so it is recommended to use flat mode in general. You can use any shader with this rendering mode, including Lambert, Phong, etc. 

Oriented

Similar to ribbon mode, except the ribbons do not face the camera. 

 

 

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