Writing Display Filters

This page covers how to author a new Display Filter using our plugin API. MoonRay currently contains 18 display filters, which themselves can be chained together to achieve new effects. You can find the code for our existing display filter plugins in the moonshine repository. For the DisplayFilter base class, look here.


Each Display Filter plugin requires you to author three files:

  • DisplayFilterName.cc
  • DisplayFilterName.ispc
  • DisplayFilterName.json

Unlike with other plugins, where you must author a .cc file for scalar mode and a .ispc file for vector mode, Display Filters require both regardless of the mode. DisplayFilter filtering operations always occur in ISPC, while the C++ code acts as the intermediary between the DisplayFilterDriver and the ISPC code.

Mix & Mask

By default, all display filters offer the following attributes:

  • mask
  • invert_mask
  • mix

You are responsible for initializing and updating the corresponding mMask, mInvertMask, and mMix attributes, as well as defining how they operate on your new display filter. We typically assume that the mask attribute masks the output to reveal input1, while the mix attribute mixes between input1 and the output. We provide an ISPC function called DISPLAYFILTER_mixAndMask (see code) which takes your mix and mask attributes and outputs a float you can use to lerp between input1 and the result.


The DisplayFilterName.cc file is responsible for processing data and passing it to our ISPC DisplayFilter class to perform the filtering computations. The class declaration will always have these bare bones:

// start the DisplayFilter class definition
RDL2_DSO_CLASS_BEGIN(DisplayFilterName, DisplayFilter)

    // constructor, where you must set the corresponding mIspc attributes
    DisplayFilterName(const SceneClass& sceneClass, const std::string& name);

    // get the user attributes and update the mIspc member variables
    virtual void update() override;

    // process the data inputs (including the mask) and update the inputData
    virtual void getInputData(const displayfilter::InitializeData& initData,
                              displayfilter::InputData& inputData) const override;

    // ISPC DisplayFilter
    ispc::DisplayFilterName mIspc;

// end the DisplayFilter class definition

Two notes on this:

  • At the beginning of the constructor for our existing display filters, you will see:
    mFilterFuncv = (DisplayFilterFuncv) ispc::DisplayFilterClassName_getFilterFunc()

    This is the filter function you will export from ISPC. The DisplayFilterDriver will call this function when processing all of the display filters.

  • getInputData() takes two arguments: InitializeData and InputData. InitializeData is a struct that contains mImageWidth and mImageHeight (padded width and height). The purpose of this function is to fill in InputData, which looks like the following:
    struct InputData 
      // vector of inputs (should include any masks as well)
      SceneObjectVector mInputs;
      // window width required for each input, in the same order as the inputs
      vector<int> mWindowWidths;


The DisplayFilterName.ispc file is responsible for performing filtering on the provided input(s). A barebones ispc file would look like this:

struct DisplayFilterName
    bool mMask;         // was a mask provided in the inputs?
    bool mInvertMask;   // should we invert the provided mask?
    float mMix;         // mix amount between input1 and output
    // insert any other attributes here

// define function that will get const pointer to display filter subclass
export const uniform DisplayFilterName * uniform
DisplayFilterName_get(const uniform DisplayFilter * uniform displayFilter)
    return DISPLAYFILTER_GET_ISPC_CPTR(DisplayFilterName, displayFilter);

static void
filter(const uniform DisplayFilter * uniform me,
       const uniform InputBuffer * const uniform * const uniform inputBuffers,
       const varying DisplayFilterState * const uniform state,
       varying Color * uniform result)
    // get pointer to display filter subclass
    const uniform DisplayFilterName * uniform self = DisplayFilterName_get(me);
    // get first input from our input buffers vector
    const uniform InputBuffer const * uniform inBuffer = inputBuffers[0];
    // get pixel from the input buffer
    varying Color src = InputBuffer_getPixel(inBuffer, state->mOutputPixelX, state->mOutputPixelY);

    // do filtering computations here

    // get the mix amount
    float mix = DISPLAYFILTER_mixAndMask(self->mMix,
                                         self->mMask ? inputBuffers[1] : nullptr,
    // if mix is zero, return the first input
    if (isZero(mix)) {
        *result = src;

    // if mix is one, return result as is; otherwise, lerp between input1 and output
    if (!isOne(mix)) {
        *result = lerp(src, *result, mix);

// export our filter func as DisplayFilterName_getFilterFunc() for use in our cpp 
// DisplayFilterDriver, which processes all of our display filters
DEFINE_DISPLAY_FILTER(DisplayFilterName, filter)

Fill out the filter function to perform whatever filtering you wish. Keep in mind to also do the mixing and masking before outputting the result.


This is simply where all of your attributes are defined. No need to define mask, invertMask, or mix, as these are default attributes that all display filters have in common.