Wednesday, May 20, 2009

The architecture of mental ray

Figure 1 shows the standard mental ray architecture.
Figure 1 standard mental ray architecture.


F1: Application scene file: Application proper file. For example, .max (3dsmax), .ma or .mb (Maya), .scn (XSI).

F2: .mi file: mental ray scene description file. If the DCC software has mental ray plugin, or integrates mental ray, they usually support .mi file export. (A2)

1. Application: DCC (Digital contents creation) software. For example, Maya, 3DSMax, XSI, CAD program, and so on.

2. mental ray core: mental ray core program. There are library version mental ray and stand alone mental ray. The library version mental ray is usually invoked by a DCC application via translator module of the application. A translator is a module of a DCC application which translate the DCC application's internal data structure to the mental ray data structure. The stand alone version mental ray reads a .mi file and performs the rendering. If you have only mental ray core, mental ray does not work. mental ray always needs shaders. It might be easier to understand that if you realize the shaders are plugin software.

3. mental ray base shader: mental ray supports these shaders as default. There are basic light sources and basic materials. For example, one of the simplest shader is the Lambert shader.

4. Application shaders: Each DCC application or CAD software has own proper shaders. For example, 3dsMax has 3dmaxshaders, Maya has mayabase shader package.

5. Custom shaders: User defined shaders or special shaders from mental ray (for example, mia_material. In 3dsMax language, it is Design and Architectural shader).

Although this structure is a standard, some application does not tightly communicate with mental ray and uses .mi files.

The important thing of this figure is that mental ray core itself does not work without (either default or special) shaders. Shaders have an impornat role in the standalone mental ray or an integrated mental ray in DCC application. Actually, mental ray can not generate meaningfull images without shaders. (mental ray core still can generate a black image without shaders.)

Thursday, May 14, 2009

Using stand alone mental ray

If you render a scene using Maya, 3dsMax, XSI, and so on, you can skip this part of article. If you have your own shaders or you are managing a render firm, then this might help you a bit.

References of mental ray

The following mental ray references are the important ones.

  • Thomas Driemeyer, Rendering with mental ray, 3nd Ed. (Mental Ray Handbooks 1), ISBN-13: 978-3211228753
  • Thomas Driemeyer and Rolf Herken, Programming mental ray, (Mental Ray Handbooks 2), ISBN-13: 978-3211838518
  • Andy Kopra, Writing Mental Ray Shaders: A Perceptual Introduction (Mental Ray Handbooks 3), ISBN-13: 978-3211489642

These are advanced documentation and there are some more introductory documents around. Among these three books, I think Andy Korpa's book is easier to read. There is also a support page of the book http://www.writingshaders.com/.


Getting mental ray

In my case, I use mental ray at my company. Therefore, I have no problem to get mental ray. However, if you want to have mental ray in private, buying Maya, 3dsMax, XSI, and so on is the simplest way. I heard one license of stand alone mental ray costs 1000 dollar. But I am not sure about this.

If you just want to get a license of stand alone ray, getting the Andy Kopra's book seems most simple. The CD-ROM of this book contains almost full version of mental ray 3.6+. When I check the book on Amazon.com (2009-5-14(Thu)), the price is 157 dollar. This mental ray is a full version except parallel rendering functions. It means this one can generate the same image as the regular version except network rendering and multi-thread rendering. This one only use one core of your computer. However, 160\$ is a reasonable price.

Wednesday, May 13, 2009

What is mental ray shader?

mental ray shader is C/C++ code that is invoked by the mental ray core when a ray hits an object. The name ``shader'' is used by hysterical reason, I think ``Plugin'' is much better word nowadays. mental ray shader gives us a lot of freedom for users, but, I think we do not need to write a custom shader in most of the cases.

The meaning of 'gives us a lot of freedom' is when a ray hit an object, a user could almost everything by a shader. For example, when a ray hit an object, mental ray can send an email from the shader. (I believe that mental ray designer does not recommended this kind of side effect usage, but, a shader is just a plugin and it should be possible.) You can do anything means also it is difficult to handle the shaders. mental ray was developed for a long time and takes care of backward compatibility, this contributed for its complexity.

Friday, April 24, 2009

What is shader?

Shader is one of the important component of mental ray. Let me tell about it a bit.

A shader is a program code fragment that computes how light interacts with objects. This was the original concept of shader in early computer graphics time and nowadays the role of shader and its meaning are expanded. In one of the common scenario in computer graphics, objects are usually represented as polygons. First, the intensity of each vertex is computed, then inside the polygon is filled by an interpolation method. This becomes more complex as the computer graphics technology developed. There were two shader development directions. One is a hardware rendering that concentrates rendering speed and interactivity. The other is a software rendering that concentrates the image quality.

Nowadays graphics hardware is called GPU (Graphics Processing Unit). The early stage GPU has fixed functionalities. Because fixed functionality can be achieved more performance in practice. The implementation was based on deep pipeline processing which gives us good throughput of pixel generation. However, as the semiconductor technology evolved and hardware had more computational power, GPU also started to consider high quality rendering. GPU becomes programmable. The initial shader programs were written in assembler like language, then shader language for GPU was established, e.g., Cg. Now there are CUDA and OpenCL (2009), these languages are no longer for only shaders, but also for general purpose programming.

On the other hand, mental ray shader started with the purpose to generate high quality rendering results. The main role of shader is to simulate light and object interaction, some of are based on physics and some are based on a good heuristics. GPU is good at if the light and object interaction is local, since parallel processing works better with high local computation. Especially, pixel parallelism works quite well in early days. But the real world problem is global. Any far away object can cast a shadow to other objects. One single object can affects whole scene. Mental ray is designed for this global illumination simulation. Mental ray's each shader still could compute a local interaction between light and objects, but this effect is propagated by ray tracing algorithm to compute global illumination effect.

Recently, these two shader directions are coming to merge. There are several GPU ray tracers. mental images produces a product called mental mill (http://www.mentalimages.com/products/mental-mill.html), that employs general shader language MetaSL. mental mill compiles MetaSL and exports several GPU based languages (e.g., Cg) and also mental ray shaders.

I think GPU shaders still largely take care of speed because of game industry. Software rendering still has some advantages for the quality of image.

(mental images was taken over by NVidia at November, 2007. NVidia is one of the leading company of GPU industory. http://www.nvidia.com/object/mental_images.html)

Tuesday, April 21, 2009

About this blog

This blog is about mental ray, a rendering software.

What is mental ray?

Mental ray is a rendering software produced by a German company (a subsidiary company of NVidia. (2009)) mental images, GmbH. Several DCC (Digital Contents Creation) software, e.g., Maya, 3dsMax, Softimage XSI, employ mental ray as one of a renderer. Mental ray is also used in some CAD software, like Solidworks. The famous usage is special effect of some movies, such as StarWars, Matrix II/III, recently one is The Curious Case of Benjamin
Button.

http://www.mentalimages.com/products/mental-ray.html
http://www.mentalimages.com/gallery/motion-pictures.html

The main sales point of mental ray is a lot of features and you can do a lot of things. But on the other hand, we can do everything means sometimes we can do nothing. It is complex and thus difficult. Especially the customization ability of shaders is flexible, but it is a bit hard for me. However, usually it is suffice to use the provided shaders. One of my Italian friend said, ``mental ray is Ferrari. If you do not know how to drive it, you even can not move. But, if you know that, you can make Matrix.'' As a one user of mental ray, I hope the future direction is simpler.

Many of the mental ray information is in English, I would try to write this in Japanese (see Japanese version pages).