Writing Shaders

This page covers some general information about writing plug-ins (aka. shaders, DSO’s, procedurals) for MoonRay. At the time of this writing, MoonRay includes about 163 plug-ins, (see scene-objects) but additional plug-ins can be authored to extend MoonRay’s functionality.

Probably the best way to quickly understand what is required to write a new plug-in of a given type is to have a look at the source code for existing plug-ins of that type. This page covers some of the general topics that are common to all MoonRay plugin types. See the links at the bottom of this page for type-specific information.

The moonray, moonshine and moonshine_usd repositories all contain various plug-ins which can be referenced as examples and provide more information than can be conveyed in this document.

Plug-in Types

MoonRay supports several types of plug-ins. Each plug-in derives from one of the scene_rdl2 types and implements its corresponding interface.

For some plug-in types a public API (using this term loosely) is provided which allows for authoring new plug-ins using only the installed public header files. This allows them to be built separately from MoonRay itself – in another repository, for example. For other types, a public API has not yet been created. It is still possible to author new plug-ins of these types (this is open source, after all), but for now it will be necessary to build them alongside MoonRay inside the moonray source repo codebase.

Type Derives from Has Public API?
Camera scene_rdl2::rdl2::Camera no
Displacement scene_rdl2::rdl2::Displacement yes
DisplayFilter scene_rdl2::rdl2::DisplayFilter yes
Geometry scene_rdl2::rdl2::Geometry yes
Light scene_rdl2::rdl2::Light no
LightFilter scene_rdl2::rdl2::LightFilter no
Map scene_rdl2::rdl2::Map yes
Material scene_rdl2::rdl2::Material yes
NormalMap scene_rdl2::rdl2::NormalMap yes
VolumeShader scene_rdl2::rdl2::VolumeShader no

All plug-in types are authored as C++ classes, but some plug-in types also include functions that are written in ISPC.

Naming Your Plug-in

Our convention is to append the plug-in type to the class name to make it easy to identify what type of SceneObject it is when it appears in an RDLA file. For example, the class for a new Map shader might be named ZebraMap, and the class for a new Light shader might be named TorusLight. This makes it possible to infer the plug-in type from the class name. In an RDLA file they might appear as:

-- scene.rdla
ZebraMap("/some/name/my_zebra") {
    ["num_stripes"] = 42,

TorusLight("/some/name/foo_17") {
    ["inner_radius"] = 4.2,
    ["outer_radius"] = 5.2,

The Plug-in’s Files and Directory Structure

Most MoonRay plug-ins comprise several source files, as follows:

File Required Purpose
<ClassName>.cc always The C++ source file which defines the class and scalar functions
<ClassName>.ispc for some plug-in types An ISPC source file which implements one or more vector functions
attributes.cc or <ClassName>.json always A C++ or JSON file (dependending on plug-in type) which defines a list of user-facing attributes
CMakeLists.txt always Used to build the plug-in with CMake
SConscript never Used only for legacy internal DWA builds, not needed for new plug-ins. Existing SConscript files will be removed in a later release

All of the above files should be placed within their own source directory to satisfy the one-dso-per-directory requirement of the current build system.

For example, the source files for a new “Zebra” map shader might be:

├── ZebraMap.cc
├── ZebraMap.ispc
├── ZebraMap.json
└── CMakeLists.txt

Defining the Plug-in’s Attributes

Attributes are how a SceneClass exposes control to the end-user. Attributes come in many types, including:

Bool Int Long Float Double String Rgb Rgba Vec2f Vec2d Vec3f Vec3d Vec4f Vec4d Mat4f Mat4d BoolVector IntVector LongVector FloatVector DoubleVector StringVector RgbVector RgbaVector Vec2fVector Vec2dVector Vec3fVector Vec3dVector Vec4fVector Vec4dVector Mat4fVector Mat4dVector SceneObject* SceneObjectVector SceneObjectIndexable SceneObjectSet ConstSceneObjectSet

Each attribute can have a default value, a label to use for display in a UI, a useful comment to describe its purpose to the user, as well as one or more flags that affect their behavior, eg. FLAGS_BINDABLE FLAGS_BLURRABLE FLAGS_ENUMERABLE FLAGS_FILENAME etc.

Attributes can also have additional arbitrary metadata associated with them to control where and how they appear or behave when displayed in a user-interface, or for any other purpose, really.

The build system expects a SceneClass’s attributes to be declared in a C++ file named attributes.cc or in a JSON file named <ClassName>.json, depending on the plug-in type.

The table below shows where the attributes are defined for each plug-in type:

Type Attributes file
Camera attributes.cc
Displacement <ClassName>.json
DisplayFilter <ClassName>.json
Geometry attributes.cc
Light attributes.cc
LightFilter attributes.cc
Map <ClassName>.json
Material <ClassName>.json
NormalMap <ClassName>.json
VolumeShader attributes.cc

For plug-in types that use JSON to declare their attributes an extra step in the build process reads the JSON and automatically generates the attributes.cc file at build time. In either case, the attributes.cc file should ultimately be included near the top of the plug-in’s <ClassName>.cc file.

// ClassName.cc
#include "attributes.cc"

The attributes.cc file

This file declares the attributes for the DSO type, including their default values and any metadata. It contains no other code than RDL2 attribute declarations. It consists of two sections (the declarations and the definitions), delimited by 3 macros. A bare bones attributes.cc file might look like this:

// attributes.cc
#include <scene/rdl2/rdl2.h>
using namespace arras;
    rdl2::AttributeKey<rdl2::Bool>  attrShowLid;
    rdl2::AttributeKey<rdl2::Float> attrSpoutRadius;
    attrShowLid =
        sceneClass.declareAttribute<rdl2::Bool>("show lid", true);

    attrSpoutRadius =
        sceneClass.declareAttribute<rdl2::Float>("spout radius", 1.0f, rdl2::FLAGS_BINDABLE);

In the declare section, you simply declare an AttributeKey<T> for each attribute. By convention, we prefix these with “attr” to avoid name conflicts. They will exist as variables in an anonymous namespace within our MyTeapotGeometry.cc file. You are in a namespace scope, so you can’t write arbitrary code here. You really shouldn’t anyway. Just declare your AttributeKey<T> variables.

In the define macro, we specify rdl2::Geometry. This indicates that, in addition to the attributes we’re defining here, we expect to have all the attributes that rdl2::Geometry and its parent classes define as well.

In the define section, you are in a function scope, with a local variable named sceneClass, which is the SceneClass being defined. You should call the declareAttribute<T>() method (see lib/scene/rdl2/SceneClass.h for documentation) for each attribute and assign it to its corresponding AttributeKey<T> that you declared.

vector types

Since you’re in a function scope, you can also write short snippets of code that that make things more convenient. For example, to provide a default value for a FloatVector attribute, this is perfectly acceptable:

// attributes.cc
    rdl2::FloatVector myFloats;
    attrMyFloats =
        sceneClass.declareAttribute<rdl2::FloatVector>("my floats", myFloats);

enum types

RDL2 does not have any native enum type. Instead, we represent enums as an rdl2::Int. However, since the attributes.cc file can’t include any outside dependencies, these can be somewhat opaque. As a convention, we recommend that enum attributes add metadata which links the integer value (metadata key) to a user-friendly string (metadata value). This lets content tools and scene introspection tools display more useful information for both the SceneClass and any SceneObjects stamped out from it.

For example:

// attributes.cc
    attrGamma =
        sceneClass.declareAttribute<rdl2::Int>("gamma", 2);
    sceneClass.setMetadata(attrGamma, "0", "off");
    sceneClass.setMetadata(attrGamma, "1", "on");
    sceneClass.setMetadata(attrGamma, "2", "auto");

The <ClassName>.json file

For plug-in types that use JSON to declare their attributes, the attributes.cc file will be generated automatically at build time.

Here’s an example of a JSON file for a Map shader that defines several attributes of differing types:


    "name": "RampMap",
    "type": "Map",
    "attributes": {
        "attrPositions": {
            "name": "positions",
            "type": "FloatVector",
            "default": "{0.0, 0.25, 0.75, 1.0}",
            "comment": "Color ramp",
            "group": "Ramp Knot"
        "attrColors": {
            "name": "colors",
            "type": "RgbVector",
            "default": "{Rgb(0.0), Rgb(0.25), Rgb(0.75), Rgb(1.0)}",
            "comment": "List of colors on the ramp",
            "group": "Ramp Knot"
        "attrRampType": {
            "name": "ramp_type",
            "label": "ramp type",
            "type": "Int",
            "default": "0",
            "flags": "FLAGS_ENUMERABLE",
            "enum": {
                "v": "0",
                "u": "1",
                "diagonal": "2",
                "radial": "3",
                "circular": "4",
                "box": "5",
                "uxv": "6",
                "four corner": "7",
                "input": "8"
            "group": "Ramp properties"
        "attrSpace": {
            "name": "space",
            "type": "Int",
            "default": "0",
            "flags": "FLAGS_ENUMERABLE",
            "enum": {
                "render": "0",
                "camera": "1",
                "world": "2",
                "screen": "3",
                "object": "4",
                "reference": "5",
                "texture": "6"
            "group": "Ramp properties"
        "attrCamera": {
            "name": "camera",
            "type": "SceneObject*",
            "flags": "FLAGS_NONE",
            "interface": "INTERFACE_CAMERA",
            "comment": "Camera used to define camera and screen space",
            "group": "Ramp properties"

Refer to the SceneClass source code, the scene_rdl2 page, or the existing plug-ins for more examples of how to declare SceneClass attributes and their metadata.

The Plug-in’s Class Definition

This is where you flesh out the actual C++ class that extends the parent RDL2 type. A bare bones MyTeapotGeometry.cc file might look like this:

// MyTeapotGeometry.cc
#include <scene/rdl2/rdl2.h>
#include "attributes.cc"
using namespace arras;
RDL2_DSO_CLASS_BEGIN(MyTeapotGeometry, rdl2::Geometry)

You may include other third party libraries in the MyTeapotGeometry.cc file. Notice that we explicitly include attributes.cc, to bring in the attribute variables in the anonymous namespace.

The class begin and end macros establish a class scope. Each macro requires the name of the class, which is MyTeapotGeometry in this case. Additionally, the begin macro requires the name of the parent class you’re extending, which is rdl2::Geometry in this case.

Many DSOs don’t have anything to do in the constructor. If this is the case, you can use the default ctor macro to stamp out a default constructor. This is not a C++ “default” constructor that takes no arguments. It’s a constructor that takes 2 arguments, but does nothing but pass them along to the parent class constructor.

If you do have something to do in the constructor, it must have a specific signature. For example:

// MyTeapotGeometry.cc
RDL2_DSO_CLASS_BEGIN(MyTeapotGeometry, rdl2::Geometry)
    MyTeapotGeometry(const rdl2::SceneClass& sceneClass, const std::string& name);
    int mMyPrivateVariable;
MyTeapotGeometry::MyTeapotGeometry(const rdl2::SceneClass& sceneClass, const std::string& name) :
    Parent(sceneClass, name),
    // Do initialization-y things, but for private variables only.
    // Your attribute values, accessible via get(attrMyAttribute),
    // HAVE NOT been set yet (use the virtual update() function to
    // initialize based on attribute values).

At this point, you can declare and define any functions you’d like, including virtual functions that you must override for your custom type to be complete. For example, since our teapot extends rdl2::Geometry we must override the createProcedural() method, which is pure virtual.

// MyTeapotGeometry.cc
RDL2_DSO_CLASS_BEGIN(MyTeapotGeometry, rdl2::Geometry)
    geom::Procedural* createProcedural() const override;
MyTeapotGeometry::createProcedural() const
    // Call into the geometry library to get a geom::Procedural...
    return magicTeapotProceduralGenerator();

The SceneObject::update() Function

In RDL2, attribute values can be updated between renders. (They cannot change during a render – you get to keep your sanity.) If you have any data that is built from, or dependent upon, the values of your attributes, you can react to those changes by overriding the virtual update() method. This method is available to all SceneObjects.

// MyTeapotGeometry.cc
RDL2_DSO_CLASS_BEGIN(MyTeapotGeometry, rdl2::Geometry)
    void update() override;
MyTeapotGeometry::update() const
    if (hasChanged(attrShowLid) && get(attrShowLid)) {
        // Show lid change from off to on.
    if (hasBindingChanged(attrSpoutRadius)) {
        // Spout radius attribute has had its binding changed.
        // Check the base value with just hasChanged().

You cannot assume your attributes have been set until you receive the first call to update(). Multiple attributes may have changed for a single call to update(), so use the hasChanged() and hasBindingChanged() functions to determine how to react. Lastly, you cannot assume that bound objects have had their attributes set or updated just because the binding changed. (This applies to attributes of type SceneObject* as well. You can’t assume the other object has valid attributes yet.)

In fact, RDL2 should be allowed to call update() on each SceneObject in parallel if it wants to. You can’t assume anything about the world outside of your SceneObject.

The Plug-in Interfaces

Depending on the plug-in type, each plugin class will potentially:

  • implement/override the virtual interface members declared by the base class
  • implement one or more of the static functions prototyped in scene_rdl2/lib/scene/rdl2/Types.h
    and set the associated inherited function pointer members

The table below lists the plug-in types with name of the function(s) that comprise that plug-in type’s “interface” and the languages involved in writing a plug-in of the given type.

Note: This table is for example purposes only - it may be incomplete. Refer to the documentation or examples for each specific plug-in type for more information.

Plug-in Type Functions (incomplete list) Language(s)
Camera setFocalLength() setFilmApertureWidth() computeProjectionMatrix() C++
Displacement DisplaceFunc DisplaceFuncv C++ ISPC
DisplayFilter getInputData() DisplayFilterFuncv C++ ISPC
Geometry createProcedural() destroyProcedural() deformed() resetDeformed() C++
Light canIlluminate() eval() intersect() sample() C++ ISPC
LightFilter canIlluminate() eval() sample() C++ ISPC
Map SampleFunc SampleFuncv C++ ISPC
Material ShadeFunc Shadefuncv PresenceFunc C++ ISPC
NormalMap SampleFunc SampleFuncv C++ ISPC
VolumeShader albedo() anisotropy() emission() extinct() C++

For example, Map shader plug-ins implement functions based on the SampleFunc and SampleFuncv prototypes and are responsible for computing a Color result.

// SampleFunc
// This is C++ code
static void
sample(const scene_rdl2::rdl2::Map *self,
       moonray::shading::TLState *tls,
       const moonray::shading::State &state,
       scene_rdl2::math::Color *result);
// SampleFuncv
// This is ISPC code
static Color
sample(const uniform Map * uniform self,
       uniform ShadingTLState * uniform tls,
       const varying State &state);

Material shader plug-ins implement functions based on the ShadeFunc and ShadeFuncv protoypes and are responsible for configuring a Bsdf via the BsdfBuilder API.

// ShadeFunc
// This is C++ code
static void
shade(const scene_rdl2::rdl2::Material *self,
      moonray::shading::TLState *tls,
      const moonray::shading::State &state,
      moonray::shading::BsdfBuilder &bsdfBuilder);
// ShadeFuncv
// This is ISPC code
static void
shade(const uniform Material * uniform self,
      uniform ShadingTLState * uniform tls,
      const varying State &state,
      varying BsdfBuilder &bsdfBuilder);

Building with CMake

MoonRay includes a CMake module called MoonrayDso which defines two functions to facilitate the building of DSOs, depending on the plug-in type: moonray_dso_simple() and moonray_ispc_dso().

ZebraMap’s CMakeLists.txt file might contain the following:

# CMakeLists.txt

Tip: if you are developing a new plug-in within the moonray or moonshine codebase, you’ll also need to edit the CMakeLists.txt file in the plug-in’s parent directory and add a line so that your plug-in’s CMakeLists.txt file can be discovered by the build system:

# parent directory's CMakeLists.txt

Writing New Plug-ins

See the links below for information specific to each plug-in type.

Writing Camera Plug-ins
Writing Displacement Plug-ins
Writing DisplayFilter Plug-ins
Writing Geometry Plug-ins
Writing Light Plug-ins
Writing LightFilter Plug-ins
Writing Map Plug-ins
Writing Material Plug-ins
Writing NormalMap Plug-ins
Writing Volume Shader Plug-ins