# Writing a filter

Author: | Bradley Chambers
Contact: | brad.chambers@gmail.com
Date: | 10/26/2016

PDAL can be extended through the development of filter functions.

See also

For more on filters and their role in PDAL, please refer to PDAL Architecture Overview.

Every filter stage in PDAL is implemented as a plugin (sometimes referred to as a “driver”). Filters native to PDAL, such as filters.ferry, are implemented as _static_ filters and are statically linked into the PDAL library. Filters that require extra/optional dependencies, or are external to the core PDAL codebase altogether, such as filters.pmf, are implemented as _shared_ filters, and are built as individual shared libraries, discoverable by PDAL at runtime.

In this tutorial, we will give a brief example of a filter, with notes on how to make it static or shared.

## The header

First, we provide a full listing of the filter header.

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    // MyFilter.hpp

    #pragma once

    #include <pdal/Filter.hpp>
    #include <pdal/Stage.hpp>

    #include <memory>

    namespace pdal
    {

    class Options;
    class PointLayout;
    class PointView;

    class PDAL_DLL MyFilter : public Filter
    {
    public:
        MyFilter() : Filter()
        {}

        static void * create();
        static int32_t destroy(void *);
        std::string getName() const;

    private:
        double m_value;
        Dimension::Id m_myDimension;

        virtual void addDimensions(PointLayoutPtr layout);
        virtual void addArgs(ProgramArgs& args);
        virtual PointViewSet run(PointViewPtr view);

        MyFilter& operator=(const MyFilter&); // not implemented
        MyFilter(const MyFilter&); // not implemented
    };

    } // namespace pdal

This header should be relatively straightforward, but we will point out three methods that must be declared for the plugin interface to be satisfied.

        static void * create();
        static int32_t destroy(void *);
        std::string getName() const;

In many instances, you should be able to copy this header template verbatim, changing only the filter class name, includes, and member functions/variables as required by your implementation.

## The source

Again, we start with a full listing of the filter source.

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    // MyFilter.cpp

    #include "MyFilter.hpp"

    #include <pdal/Options.hpp>
    #include <pdal/pdal_macros.hpp>
    #include <pdal/PointTable.hpp>
    #include <pdal/PointView.hpp>
    #include <pdal/StageFactory.hpp>
    #include <pdal/util/ProgramArgs.hpp>

    namespace pdal
    {

    static PluginInfo const s_info =
        PluginInfo("filters.name", "My awesome filter",
                   "http://link/to/documentation");

    CREATE_STATIC_PLUGIN(1, 0, MyFilter, Filter, s_info)

    std::string MyFilter::getName() const
    {
        return s_info.name;
    }

    void MyFilter::addArgs(ProgramArgs& args)
    {
        args.add("param", "Some parameter", m_value, 1.0);
    }

    void MyFilter::addDimensions(PointLayoutPtr layout)
    {
        layout->registerDim(Dimension::Id::Intensity);
        m_myDimension = layout->registerOrAssignDim("MyDimension",
                Dimension::Type::Unsigned8);
    }

    PointViewSet MyFilter::run(PointViewPtr input)
    {
        PointViewSet viewSet;
        viewSet.insert(input);
        return viewSet;
    }

    } // namespace pdal

For your filter to be available to PDAL at runtime, it must adhere to the PDAL plugin interface. As a convenience, we provide the macros in `pdal_macros.hpp` to do just this.

We begin by creating a `PluginInfo` struct containing three identifying elements - the filter name, description, and a link to documentation.

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    static PluginInfo const s_info =
        PluginInfo("filters.name", "My awesome filter",
                   "http://link/to/documentation");

PDAL requires that filter names always begin with `filters.`, and end with a string that uniquely identifies the filter. The description will be displayed to users of the PDAL CLI (`pdal --drivers`).

Next, we pass the following to the `CREATE_STATIC_PLUGIN` macro, in order: PDAL plugin ABI major version, PDAL plugin ABI minor version, filter class name, stage type (`Filter`), and our `PluginInfo` struct.

    CREATE_STATIC_PLUGIN(1, 0, MyFilter, Filter, s_info)

To create a shared plugin, we simply change `CREATE_STATIC_PLUGIN` to `CREATE_SHARED_PLUGIN`.

Finally, we implement a method to get the plugin name, which is primarily used by the PDAL CLI when using the `--drivers` or `--options` arguments.

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    std::string MyFilter::getName() const
    {
        return s_info.name;
    }

Now that the filter has implemented the proper plugin interface, we will begin to implement some methods that actually implement the filter. First, `getDefaultOptions()` is used to advertise those options that the filter provides. Within PDAL, this is primarily used as a means of displaying options via the PDAL CLI with the `--options` argument. It provides the user with the option names, descriptions, and default values.

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    void MyFilter::addArgs(ProgramArgs& args)
    {
        args.add("param", "Some parameter", m_value, 1.0);
    }

The `addArgs()` method is used to register and bind any provided options to the stage. Here, we get the value of `param`, if provided, else we populate `m_value` with the default value of `1.0`.

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    void MyFilter::addDimensions(PointLayoutPtr layout)
    {
        layout->registerDim(Dimension::Id::Intensity);
        m_myDimension = layout->registerOrAssignDim("MyDimension",
                Dimension::Type::Unsigned8);
    }

In `addDimensions()` we make sure that the known `Intensity` dimension is registered. We can also add a custom dimension, `MyDimension`, which will be populated within `run()`.

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    PointViewSet MyFilter::run(PointViewPtr input)
    {
        PointViewSet viewSet;
        viewSet.insert(input);
        return viewSet;
    }

Finally, we define `run()`, which takes as input a `PointViewPtr` and returns a `PointViewSet`. It is here that we can transform existing dimensions, add data to new dimensions, or selectively add/remove individual points.

We suggest you take a closer look at our existing filters to get an idea of the power of the `Filter` stage and inspiration for your own filters!

## StageFactory

As of this writing, users must also make a couple of changes to `StageFactory.cpp` to properly register static plugins only (this is not required for shared plugins). It is our goal to eventually remove this requirement to further streamline development of add-on plugins.

Note

Modification of StageFactory is required for STATIC plugins only. Dynamic plugins are registered at runtime.

First, add the following line to the beginning of `StageFactory.cpp` (adjusting the path and filename as necessary).

    #include <MyFilter.hpp>

Next, add the following line of code to the `StageFactory` constructor.

    PluginManager::initializePlugin(MyFilter_InitPlugin);