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Open Shading Language

Users can now create their own nodes using the Open Shading Language (OSL). Note that these nodes will only work for CPU rendering; there is no support for running OSL code on the GPU. To enable it, select Open Shading Language as the shading system in the render settings.

Note: on Linux, C/C++ compiler tools (in particular /usr/bin/cpp) must be installed to compile OSL scripts.

Script Node

OSL was designed for node-based shading, and each OSL shader corresponds to a node in a node setup. To add an OSL shader, add a script node and link it to a text datablock or an external file. Input and output sockets will be created from the shader parameters on clicking the update button in the node or the text editor.

OSL shaders can be linked to the node in a few different ways. With the Internal mode, a text datablock is used to store the OSL shader, and the OSO bytecode is stored in the node itself. This is useful for distributing a .blend file with everything packed into it.

The External mode can be used to specify a .osl file on disk, and this will then be automatically compiled into a .oso file in the same directory. It is also possible to specify a path to a .oso file, which will then be used directly, with compilation done manually by the user. The third option is to specify just the module name, which will be looked up in the shader search path.

The shader search path is located in the same place as the scripts or configuration path, under:

Linux /home/$user/.config/blender/Version Number/shaders/
Windows C:\Users\$user\AppData\Roaming\Blender Foundation\Blender\Version Number\shaders\
Mac OS X /Users/$user/Library/Application Support/Blender/Version Number/shaders/

(Replace Version Number with the release number of your current Blender installation, e.g. 2.65 or 2.66.)

For use in production, we suggest to use a node group to wrap shader script nodes, and link that into other .blend files. This makes it easier to make changes to the node afterwards as sockets are added or removed, without having to update the script nodes in all files.

Writing Shaders

For more details on how to write shaders, see the OSL specification. Here is a simple example:

shader simple_material(
    color Diffuse_Color = color(0.6, 0.8, 0.6),
    float Noise_Factor = 0.5,
    output closure color BSDF = diffuse(N))
{
   color material_color = Diffuse_Color * mix(1.0, noise(P * 10.0), Noise_Factor);
   BSDF = material_color * diffuse(N);
}

Closures

OSL is different from, for example, RSL or GLSL, in that it does not have a light loop. There is no access to lights in the scene, and the material must be built from closures that are implemented in the render engine itself. This is more limited, but also makes it possible for the render engine to do optimizations and ensure all shaders can be importance sampled.

The available closures in Cycles correspond to the shader nodes and their sockets; for more details on what they do and the meaning of the parameters, see the shader nodes manual.

BSDF
  • diffuse(N)
  • oren_nayar(N, roughness)
  • diffuse_ramp(N, colors[8])
  • phong_ramp(N, exponent, colors[8])
  • diffuse_toon(N, size, smooth)
  • glossy_toon(N, size, smooth)
  • translucent(N)
  • reflection(N)
  • refraction(N, ior)
  • transparent()
  • microfacet_ggx(N, roughness)
  • microfacet_ggx_aniso(N, T, ax, ay)   New in version 2.72  
  • microfacet_ggx_refraction(N, roughness, ior)
  • microfacet_beckmann(N, roughness)
  • microfacet_beckmann_aniso(N, T, ax, ay)   New in version 2.72  
  • microfacet_beckmann_refraction(N, roughness, ior)
  • ashikhmin_shirley(N, T, ax, ay)   New in version 2.72  
  • ashikhmin_velvet(N, roughness)
HAIR
  • hair_reflection(N, roughnessu, roughnessv, T, offset)
  • hair_transmission(N, roughnessu, roughnessv, T, offset)
BSSRDF
  • bssrdf_cubic(N, radius, texture_blur, sharpness)
  • bssrdf_gaussian(N, radius, texture_blur)
Volume
  • henyey_greenstein(g)
  • absorption()
Other
  • emission()
  • ambient_occlusion()
  • holdout()
  • background()

Attributes

Some object, particle and mesh attributes are available to the built-in getattribute() function. UV maps and vertex colors can be retrieved using their name. Other attributes are listed below:

geom:generated Generated texture coordinates
geom:uv Default render UV map
geom:dupli_generated For instances, generated coordinate from duplicator object
geom:dupli_uv For instances, UV coordinate from duplicator object
geom:trianglevertices 3 vertex coordinates of the triangle
geom:numpolyvertices Number of vertices in the polygon (always returns 3 currently)
geom:polyvertices Vertex coordinates array of the polygon (always 3 vertices currently)
geom:name Name of the object
geom:is_curve Is object a strand or not
geom:curve_intercept Point along the strand, from root to tip
geom:curve_thickness Thickness of the strand
geom:curve_tangent_normal Tangent Normal of the strand
path:ray_length Ray distance since last hit
object:location Object location
object:index Object index number
object:random Per object random number generated from object index and name
material:index Material index number
particle:index Particle instance number
particle:age Particle age in frames
particle:lifetime Total lifespan of particle in frames
particle:location Location of the particle
particle:size Size of the particle
particle:velocity Velocity of the particle
particle:angular_velocity Angular velocity of the particle

Trace

We support the trace(point pos, vector dir, ...) function, to trace rays from the OSL shader. The "shade" parameter is not supported currently, but attributes can be retrieved from the object that was hit using the getmessage("trace", ..) function. See the OSL specification for details on how to use this.

This function can't be used instead of lighting; the main purpose is to allow shaders to "probe" nearby geometry, for example to apply a projected texture that can be blocked by geometry, apply more “wear” to exposed geometry, or make other ambient occlusion-like effects.