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 .msl
or
.xmsl
file extension. If such a file name is announced
to mental ray, for example with the $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
(like shader.h
and others), as well as MetaSL C++
header files (in the directory rooted at
mi/metasl_runtime/*
). 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
({_MI_REG_INCLUDE}
).
On Windows platforms, shaders need to be linked against
shader.lib
or ray.lib
. If
shader.lib
is not present in the compiler library
search path, it needs to be specified explicitly (like with
command-line options -ld_libs
and/or
-ld_path
).
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
{_MI_REG_METASL_BACKEND}
mental ray registry variable
need to be set to the value "llvm"
. 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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