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To interpret these parameters in the .mi file, mental ray needs a declaration of parameter names and types that is equivalent to the C struct that the shader later uses to access the parameters. The declaration in the .mi file must be exactly equivalent to the C struct, or the shader will mis-interpret the parameter data structure constructed by mental ray. (Using the mkmishader utility ensures that both declarations agree, see section mkmishader.) This means that three parts are needed to write a shader: the .mi declaration, the C parameter struct, and the C source of the shader. The .mi declaration is normally stored in a separate file that is included into the .mi scene file using a $include statement.

The syntax of .mi shader declarations is fully described in section declaration. Here, only a brief overview is given. Shaders must be declared with shader name, return type, and the types and names of all parameters. Options such as the shader version may be specified also:

 declare shader
  [ type ] " shader_name " (
   type " parameter_name ",
   type " parameter_name ",
   ...
   type " parameter_name "
  )
  [ version versionint ]
  [ options ]
 end declare

For example, a simple material shader containing ambient, diffuse, and specular colors, transparency, an optional array of bump map textures, and an array of lights could be declared in the .mi file as:

 declare shader
  color "my_material" (
   color "ambient",
   color "diffuse",
   color "specular",
   scalar "shiny",
   scalar "reflect",
   scalar "transparency",
   scalar "ior",
   vector texture "bump",
   array light "lights"
  )
  version 1
 end declare

If there is only one array, there is a small efficiency advantage in listing it last. It is recommended that the largest array (arrays of large structs are larger than arrays of primitives) is given as the last parameter. The material shader declared in this example can be used in a material statement like this:

 material "mat1"
  "my_material" (
   "specular" 1.0 1.0 1.0,
   "ambient" 0.3 0.3 0.0,
   "diffuse" 0.7 0.7 0.0,
   "shiny" 50.0,
   "bump" "tex1",
   "lights" [ "light1", "light2", "light3" ],
   "reflect" 0.5
  )
 end material

Note that the parameters can be defined in any order that does not have to agree with the order in the declaration, and that parameters can be omitted. Omitted parameters default to 0. This example assumes that the texture tex1 and the three lights have been defined prior to this material definition. Again, be sure to use the names of the textures and lights, not the names of the texture and light shaders. All names in the above two examples were written as strings enclosed in double quotes to disambiguate names from reserved keywords, and to allow special characters in the names that would otherwise be illegal.

When choosing names, avoid double colons and periods, which have a special meaning when accessing structured shader return values and interface parameters in phenomenon subshaders.

When the shader my_material is called, its third argument will be a pointer to a data structure built by mental ray from the declaration and the actual parameters in the .mi file. In order for the C source of the shader to access the parameters, it needs an equivalent declaration in C syntax that must agree exactly with the .mi declaration. The type names can be translated according to the following table:

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