All color, displacement, contour, and other computation in mental ray is based on shaders. There are various types of shaders for different situations, such as material shaders to evaluate the material properties of a surface, light shaders to control the light-emitting properties of a light source, lens shaders to specify camera properties other than the default pinhole camera, and so on. Much of the power of mental ray relies on the possibility to write custom shaders and call them during rendering in mental ray. Traditional custom shaders are written in C/C++, using all language features and the complete shader interface in mental ray for advanced effects. On the other hand, MetaSL shaders may be used to write functions for common shading tasks like defining material behavior in a standardized form independent of the computing platform or the final renderer.
Vendors and integrators of mental ray typically provide custom shader libraries which implement compatibility with their products, like Autodesk 3ds Max, Maya, and Softimage, Dassault Systèmes CATIA and SolidWorks, and others.
MetaSL shaders are provided in files with
file extension. If such a file name is announced to mental ray, for example
$include statement in a
.mi file), it is
treated as MetaSL shader source code. mental ray will read, parse, and convert
it to an internal representation. This internal representation is then used to
create a shader declaration, as well as intermediate shader code in a native
language of the current platform for final compilation/linking and execution
in mental ray. The shader code generation is performed by so-called MetaSL
back-ends. mental ray currently provides two back-end alternatives :
The C++ back-end is using an external C++
compiler and linker that is automatically invoked on intermediate source files
to create a dynamically linked shared library in the directory containing
temporary files. That resulting shared object .dll or .so is then loaded into
mental ray. In order for C++ compilation to succeed, the C++ compiler needs
to be pointed to include directories containing mental ray headers files
shader.h and others), as well as MetaSL C++ header files
(in the directory rooted at
For convenience, mental ray now applies the common mental ray include path
also to the compiler invocations automatically. In other words, the include
paths may be specified on the mental ray standalone command line (option
-I) or with a registry variable (
On Windows platforms, shaders need to be linked against
shader.lib is not present in the compiler
library search path, it needs to be specified explicitly (like with command-line
Note, that the C++ compilation and linkage is done on demand. For shader declaration, parsing and analysis of the MetaSL source code is done when the MetaSL file is loaded.
The LLVM back-end is using internal compilation
technology, and is executing the generated code using a built-in low-level
virtual machine. No intermediate files are created and no external tools
are required to use this back-end. To enable it the
mental ray registry variable need to be set to the value
This will automatically disable the C++ MetaSL back-end.
Here are the steps necessary to create a native shader:
Note that if the shader is expected to work on Windows NT, all four function definitions (the shader, the version function, and the init and exit shaders) must be preceded with DLLEXPORT. This is a pseudo type specifier that makes the functions visible to users of the generated shader library. On Unix systems DLLEXPORT evaluates to an empty word.
The shader is then ready to be used in the scene. These steps are described in detail below.
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