RSL Documentation 0.3 - Chapter 1 - General Knowledge
copyright - Dominik Susmel    
 
What is Renderman?

Actually, Renderman consist of two things, Renderman Interface and Renderman Shading Language. Renderman interface is a set of protocols, which describe how a modeling application translates data into rendering program (note, rendering program can be software implementation, device or rendering algorithm). This means that geometry and scene from modeling program has to be described in a certain way described with this protocol. Main goal of Renderman is to provide compatibility between different modeling programs and rendering programs. Think of it as a Postscript for 3D. As you can print from all your programs to any kind of printer with Postscript, in theory you can render from any 3D application with any rendering program if they are Renderman compatible. Since there are a great variety of rendering methods (i.e. Z-Buffer, Raytracing, Scanline…), Renderman only describes how things should move from 3D application into render program. In a way, Renderman describes what the output should be, but not how it should be done in render program. Renderman compatible programs should follow syntax and idioms written by Pixar. Pixar issues a document, which is called RISpec (Renderman Interface Specification), in which they describe the whole Renderman protocol.

What Renderman consists of?

SCENE DESCRIPTION - RIB
All data translated from 3d application into renderman compatible renderer is kept in something called a scene description. Think of it as a big file (or files) that keep everything you've exported. File that keeps all this info is also referred to as RIB (Renderman Interface Bytestream). RIB files contain metadata, which is a fancy word for "data about data". RIB files are text files so they may (and do) get pretty large, although they can be compressed etc. This scene description file keeps information about "things" (or metadata from now on) in the scene. These things are geometry, lights, cameras, particles and everything else you might have in your scene. Each scene description also holds information about the final output, like what kind of resolution is needed for output, what kind of Anti-Aliasing is setup for output, etc. Important thing to notice is that for each frame in the scene, RIB file starts from the beginning. So, you might say that for each frame exported you get an independent scene description. Without going into too much detail for now, let me just point out that RIB file can be nested, so you can have multiple frames in one RIB file and you can have parts of it in separate RIB files which are referenced in the main file. To explain this a little further, let's say you have a RIB for one frame of the scene, which contains a sphere. This sphere can be in a separate RIB file, and when the scene RIB needs this sphere it will look into RIB that contains that sphere. Enough said, let's look at what RIB can hold:

SCENE ITEMS
Items of the scene are anything not geometry related. This may be camera, lights etc.

DESCRIPTIONS
Each item in RIB file can hold metadata about itself. To simplify, each data (item or geometry) can hold some kind of attachment, which further descript and/or complement the data itself. This may sound confusing, but it's really one of the fundamental things in Renderman and any other programming language. Let's examine a hypothetical situation. You want a sphere in your scene. Sphere as a procedural primitive has its creation parameters, for example radius. So, when you add a sphere into the scene, you may attach description, which tells what sphere's radius is. These kinds of descriptions are called input and/or creation parameters (duh), but there is also one other kind of descriptions, which have spread the fame about Renderman. Enter Shaders. Shader is a description attached to a surface (another word for geometry) or item. Shaders can have their own creation parameters too. With shaders you attach a description, which tells the renderer what will the surface or item behave in the process of rendering. Shaders are not a part of RIB; they are separate pieces of code written with RSL (Renderman shading Language) syntax, which is another part of RISpec. In a word, shaders are separate files, which are not written with language like RIB.

GEOMETRY
Geometry can be anything from low-level primitives like vertices, polygons, and curves to high-level primitives like quadrics, bicubic patches, trimmed NURBS, CSG and others. Primitive objects by themselves are fundamental building blocks of 3D geometry. Difference between low-level and high-level primitives is in that low-level primitives are the lowest form of geometry presentation, and high-level primitives are just a compound "things" of low-level primitives. So, high-level primitive has a special type of binding low-level primitives into a more complex type of primitive object.

SHADING LANGUAGE
Shaders are written with RSL (Renderman Shading Language). These descriptions tell the renderer what to do when it needs to render the surface or light that has a shader attached. In order to use shaders you can give them input parameters (for example what color is the surface) and shader will return it's values to the surface it is attached to in order that renderer can proceed with it's rendering. Input parameters for shaders are something you describe when you write a shader. You can have shader receive color of the surface, but if none is given to it, shader can have its default color, which you can also describe. This is how shaders work. You give them input values, and they give you output values. These output values are what renderer uses for rendering. There are several types of shaders, each one with it's own usage. Shader types are: Surface, Displacement, Volume, Light and Imager Shaders. Let's have a look at what each type of shader is used for.

Shaders

Surface shaders -Surface shaders are attached to geometry. These shaders describe optical properties of geometric object. Simply said, Surface shaders tells the renderer what will the surface look like when it is hit by the light. Consider this shader, as the one that describes what will the object look like

Displacement shaders - These shaders change the topology of the surface. In order to change the topology, Displacement shader can do one of two things. It can do REAL displacement, which moves the actual geometry, or it can move the surface normals, which produce the infamous BUMP effect. These shaders are also attached to the geometry.

Volume shaders - Volume shader changes the color of the light ray as it travels through volume. Volume can be one of several things. For one, it can be geometry. If light ray travels through geometry and it has volume shader attached, this shader will modulate light ray's color in a way you've described in shader. Secondly, volume can be light item. This means that you can have, for example, Spot Light's cone represent the volume through which light ray's will be modulated by a volume shader (volumetric lights). As for the third type of volume, if volume shader isn't attached to anything, it will be considered as atmospheric volume shader. Consider fog, myst and similar effects.

Light shaders - This shader represents the light source. With light shader you describe the emission of light rays from the source (item that has the light shader attached) to the destination point, which is the surface being illuminated. These shaders generally give control over light color, shadows, intensity, falloff with distance, barn doors etc. Light shaders are shaders with which you control light sources. Important to note is that light source to which this shader can be attached to is either light item or geometry.

Imager Shaders - Imager shaders operate on image just prior to the final output. When renderer is finished with calculating pixel colors for the image output, this shader operates on these pixels. Look at this kind of shaders as MAX Render Effects. It's a post-process shader, but with access to scene data (just like MAX Render Effects).

Benefits of Renderman

INDUSTRY STANDARD - This may not be obvious at first, but with lots of studios working with this Standard, general support is widely available. The fact that the world largest CG Animation studio stands behind it saysA lot. Lots of pioneers in CG field worked on the standard, it has a proven track of record, and it is widely regarded as THE render platform.

IT IS A STANDARD - This means that any RISpec compatible renderer can render out the scene from any Rispec compatible exporter. This means that renderers are generally compatible between them, and if one is not suitable for your need, you can switch to other with very small amount of pain. Shaders are generally compatible between renderers also. In this way, you can switch shaders from render to render without too many problems.

PROGRAMABILITY - RISpec defines two languages, RIB description format, and RSL. With both you can achieve anything you like. For example, you can add your own geometric primitive, or you can import your own particle data, for example. With surface shaders, you can describe the surface to behave exactly how would you like it to behave. RISpec offers several ways to access data; one way is via RIB and Shaders. The other way is via C binding API. This means that you can have control over RIB scene and shaders from within a C style program. This offers a great amount of control.

IMAGE QUALITY - With Rispec being a technical document describing what is and what isn't Renderman, you get something you wouldn't expect to see in this kind of document. Rispec actually describes what the final quality of the generated image should be! With this, you can be sure that any RISpec compatible renderer will generate a superb quality image. Quality relates to Anti-Aliasing method, color range etc.