Documentation

Plugins API

This guide consists of multiple sections, first we discuss Wikitude SDK Plugins in general, than we talk about platform specifics and how to register a plugin with the Wikitude SDK and then we go through each of the sample plugins included with the Wikitude Example Applications.

  1. About Wikitude SDK Plugins
  2. Platform Specifics
  3. Registering Plugins
  4. QR & Barcode Plugin
  5. Face Detection Plugin

About Wikitude SDK Plugins

Technically a plugin is a class, either written in C++ or Java, that is derived from the Wikitude Plugin base class. Beside lifecycle handling and options to enable and disable the plugin, the Plugin class has four main methods that you can override cameraFrameAvailable which is called each time the camera has a new frame, update which is called after each image recognition cycle as well as startRender and endRender which are called before and after the Wikitude SDK does its rendering.

The most important thing to remember when working with plugins is that they need to have a unique identifier! If the attempt is made to register a plugin with an identifier that is already known to the Wikitude SDK, the register method call will return false.

Plugin Base Class

class Plugin {
   public:
      Plugin(std::string identifier_);
      ~Plugin();
      string getIdentifier() const; // returns a unique plugin identifier
      bool processesColorCameraFrames(); // returns true if the plugins wants to process color frames instead of bw

      void setEnabled(bool enabled_);
      bool isEnabled();

      string callJavaScript(string javaScriptSnippet); // evaluates the given JavaScript snippet in the currently loaded ARchitect World context.

   protected:
      void initialize(); // called when the plugin is initially added to the Wikitude SDK
      void pause(); // called when the Wikitude SDK is paused e.g. the application state changes from active to background
      void resume(uint pausedTime_); // called when the Wikitude SDK resumes e.g. from background to active state. pausedTime represents the time in milliseconds that the plugin was not updated.
      void destroy(); // called when the plugin is removed from the Wikitude SDK

      void cameraFrameAvailable(const Frame&; cameraFrame_); // called each time the camera has a new frame
      void update(const vector<RecognizedTarget> recognizedTargets_); // called each time the Wikitude SDK renders a new frame

      void startRender(); // called before any Wikitude SDK internal rendering is done
      void endRender(); // called right after any Wikitude SDK internal rendering is done

   protected:
      string      _identifier;
      bool        _enabled;
};

With those methods in place your plugin will be able to read the full camera image for your own purpose, where the YUV image is also processed in wikitude’s computer vision engine.

Information about Recognized Targets

In case you have the wikitude SDK running with ongoing image recognition, the plugin API will populate the RecognizedTarget in the update method once an image has been recognized. The plugin can then work with class RecognizedTarget, which wraps the details of the target image in the camera view. With that you can read out the pose of the target image and use it for your purposes. Additionally the call contains the calculated distance to the recognized target

class RecognizedTarget {
   public:
      const string&    getIdentifier() const; // the identifier of the target. The identifier is defined when the target is added to a target collection
      const Mat4&      getModelViewMatrix() const; // the model view matrix that defines the transformation of the target in the camera frame (translation, rotation, scale)
      const Mat4&      getProjectionMatrix() const;
      const float      getDistanceToCamera() const; // represents the distance from the target to the camera in millimeter
};

Passing values from within the plugin to the JavaScript part of your augmented reality experience is done via the addToJavaScriptQueue() method of the Plugin class. Using this function will execute any JavaScript code in the context of your augmented reality experience.

Platform Specifics

To be able to use a C++ Wikitude plugin on Android, it is necessary to create a binary from the C++ code for each supported CPU architecture. To make this process as easy as possible we prepared an Android NDK make file and and some template code which passes your plugin to the Wikitude SDK. In the following section we discuss how you need to adapt these templates so they'll work for your plugin.

Please note that if you would like to use multiple C++ plugins in your app, you will need to package all plugins in one shared library. This is necessary because we use JNI to register C++ plugins with the Wikitude SDK and the symbol to do that has to be unique.

Android C++ Wikitude Plugin Library Build

All files needed are located under the folder PluginBuilder in the Wikitude SDK Android package. If you didn't setup the Android NDK yet, please follow the official guide.

Let's take a look at the Android.mk file, located under SDKPackageRoot/PluginBuilder/jni. The first thing we do is declare a variable containing the path to the source files relative to the make files location and set LOCAL_PATH to this location. We define where all include files are located and which files need to be compiled. Since our Example Plugin uses Android log we link android native log.

LOCAL_PATH := $(call my-dir)/..
SRC_DIR := $(LOCAL_PATH)/src

include $(CLEAR_VARS)

LOCAL_PATH := $(SRC_DIR)
include $(CLEAR_VARS)

LOCAL_MODULE := samplePlugin

LOCAL_C_INCLUDES := $(SRC_DIR)
LOCAL_SRC_FILES := __YOUR_PLUGIN__.cpp JniRegistration.cpp Plugin.cpp

LOCAL_LDLIBS += -llog

include $(BUILD_SHARED_LIBRARY)

The PluginLoader/src folder contains various src files which are needed so your plugin will compile and link correctly. Please don't modify any of them except the __YOUR_PLUGIN__.h and __YOUR_PLUGIN__.cpp. One other file that needs some slight modifications is JniRegistration.cpp shown below. Adapt the include directive and the constructor call to your plugin name and if you would like to use multiple C++ plugins, feel free to add more plugins to the cPluginsArray array but adjust the numberOfPlugins count accordingly.

If you packaged multiple plugins in one shared library but would like to instantiate only a subset of those plugins you can pass an identifier to this method when loading the library from Java. You can then decide which plugins to create depending on the value of jPluginName.

#include <jni.h>

#include "Plugin.h"
#include "__YOUR_PLUGIN__.h"

extern "C" JNIEXPORT jlongArray JNICALL Java_com_wikitude_common_plugins_internal_PluginManagerInternal_createNativePlugins(JNIEnv *env, jobject thisObj, jstring jPluginName) {

    int numberOfPlugins = 1;

    jlong cPluginsArray[numberOfPlugins];
    cPluginsArray[0] = (jlong) new __YOUR_PLUGIN__("com.example.plugin");

    jlongArray jPluginsArray = env->NewLongArray(numberOfPlugins);
    if (jPluginsArray != nullptr) {
        env->SetLongArrayRegion(jPluginsArray, 0, numberOfPlugins, cPluginsArray);
    }

    return jPluginsArray;
}

To build the plugin binary files, navigate to the jni folder and call ndk-build. A libs folder will be created containing libraries for arm7, arm64 and intel. Copy the contents of the libs folder to YourProjectRoot/app/src/main/jniLibs.

Should you desire to rebuild the plugins distributed with the Wikitude example application, you are required to add OpenCV to the project as it is used by the marker tracking plugin and the face detection plugin. To do so, download the 3.0.0 OpenCV Android package from the official OpenCV download page and place the content of the archive into the SDKExamplePlugins folder, next to the include, jni, lib and src folders.

Registering Plugins

On Android plugins can either be written in C++ or Java. The registration process between C++ and Java is a little different in the following sections we first show how to register a C++ plugin and then how to register a Java plugin.

Register C++ Plugin

To register a C++ plugin with the Wikitude SDK, call the registerNativePlugins method of the ArchitectView instance, passing the name of your plugin library. Do not add lib in front of the name or add the .so extension. Please make sure that the onCreate method of the ArchitectView was already called.

@Override
protected void onPostCreate(final Bundle savedInstanceState) {
    super.onPostCreate(savedInstanceState);
    this.architectView.registerNativePlugins("pluginLibraryName");
}

If you packaged multiple plugins into your library and would like to decide at runtime which plugin(s) to load, you can pass an identifier on which you are able to react to in native code.

@Override
protected void onPostCreate(final Bundle savedInstanceState) {
    super.onPostCreate(savedInstanceState);
    this.architectView.registerNativePlugins("pluginLibraryName", "plugin_1");
}

Register Java Plugin

To register a Java plugin with the Wikitude SDK, call the registerPlugin method of the ArchitectView instance passing an instance of your Plugin.

@Override
protected void onPostCreate(final Bundle savedInstanceState) {
    super.onPostCreate(savedInstanceState);
    this.architectView.registerPlugin(new MyPlugin());
}

Barcode and QR code reader

This samples shows a full implementation of the popular barcode library ZBar into the Wikitude SDK. As ZBar is licensed under LGPL2.1 this sample can also be used for other projects.

ZBar is an open source software suite for reading bar codes from various sources, such as video streams, image files and raw intensity sensors. It supports many popular symbologies (types of bar codes) including EAN-13/UPC-A, UPC-E, EAN-8, Code 128, Code 39, Interleaved 2 of 5 and QR Code.

First let's have a look at the BarcodePlugin.h file. To create the bar code plugin we derive our BarcodePlugin class from the wikitude::sdk::Plugin class and override initialize, destroy, cameraFrameAvailable and update. We also declare the following member variables: _worldNeedsUpdate, _image and _imageScanner. The _worldNeedsUpdate variable will later be used as an indicator if we need to update the ArchitectView, _image and _imageScanner are classes from zBar which we'll use to scan for bar codes.

class BarcodePlugin : public wikitude::sdk::Plugin {
public:
    BarcodePlugin(int cameraFrameWidth, int cameraFrameHeight);
    virtual ~BarcodePlugin();

    virtual void initialize();
    virtual void destroy();

    virtual void cameraFrameAvailable(const wikitude::sdk::Frame& cameraFrame_);
    virtual void update(const std::list<wikitude::sdk::RecognizedTarget>& recognizedTargets_);

protected:
    int                             _worldNeedsUpdate;

    zbar::Image                     _image;
    zbar::ImageScanner              _imageScanner;
};

We now go through each method of the BarcodePlugin class, starting with the constructor and destructor. In the constructor we set _worldNeedsUpdate to zero indicating that there is no update necessary and initialize the zBar::Image member variable passing its constructor the width and height of the camera frame, the image format of Y800, set its data pointer to null and the data length to zero. Since we didn't allocate any data dynamically, there is nothing to do in the destructor.

BarcodePlugin::BarcodePlugin(int cameraFrameWidth, int cameraFrameHeight) :
Plugin("com.wikitude.ios.barcodePluign"),
_worldNeedsUpdate(0),
_image(cameraFrameWidth, cameraFrameHeight, "Y800", nullptr, 0)
{
}

BarcodePlugin::~BarcodePlugin()
{
}

In the initialize method we configure the zbar::ImageScanner by calling setConfig, enabling all supported bar codes. If you are only interested in one or some particular types of bar codes, first disabling all bar code types and manually enabling each particular type would be the better idea. That way performance could be greatly improved.

void BarcodePlugin::initialize() {    
    _imageScanner.set_config(zbar::ZBAR_NONE, zbar::ZBAR_CFG_ENABLE, 1);
}

We react to the destroy event by setting the current data pointer of the zbar::Image member to null and length to zero.

void BarcodePlugin::destroy() {
    _image.set_data(nullptr, 0);
}

The last but most interesting methods are cameraFrameAvailable and update. In the cameraFrameAvailable method we set the data of our previously initialized zbar::Image member variable to the frame data we just received and the length of the data to frame width * frame height by calling set_data. We then start the scanning process by calling the scan method of our zBar::ImageScanner passing the zBar::Image member instance. The zBar::ImageScanner::scan method returns the number of detected bar codes in the image frame, we save this number in a local variable n. If n is not equal to the result of the last frame, which we saved to _worldNeedsUpdate member variable, we know there was a new bar code detected (meaning there was no bar code in the last frame) or that there was a bar code in the last frame and now there isn't. When that's the case, we do another check if there really was a bar code detected this frame and if there was we create a javascript code fragment which we pass to the addToJavaScriptQueue method from the sdk::Plugin base class. The javascript fragment contains code to set the html contents of the loadingMessage div element which is placed in the top of the view. We use the zbar::Image::SymbolIterator to get the first detected symbol and retrieve its data. That means even if there were multiple bar codes detected in the last frame we only use the first detected.

Since we aren't interested in the results of the Wikitude SDK image recognition and aren't rendering anything with OpenGL in this example we implement the update method empty.

void BarcodePlugin::cameraFrameAvailable(const wikitude::sdk::Frame& cameraFrame_) {
    int frameWidth = cameraFrame_.getSize().width;
    int frameHeight = cameraFrame_.getSize().height;

    _image.set_data(cameraFrame_.getLuminanceData(), frameWidth * frameHeight);

    int n = _imageScanner.scan(_image);

    if ( n != _worldNeedsUpdate ) {
        if ( n ) {
            std::ostringstream javaScript;
            javaScript << "document.getElementById('loadingMessage').innerHTML = 'Code Content: ";

            zbar::Image::SymbolIterator symbol = _image.symbol_begin();
            javaScript << symbol->get_data();

            javaScript << "';";

            addToJavaScriptQueue(javaScript.str());

        }
    }

    _worldNeedsUpdate = n;
}

void BarcodePlugin::update(const std::list<wikitude::sdk::RecognizedTarget>& recognizedTargets_) {
}

Face Detection

This samples shows how to add face detection to your Wikitude augmented reality experience using OpenCV.

The Face Detection Plugin Example consists of the C++ classes FaceDetectionPlugin and FaceDetectionPluginConnector as well as the Java class FaceDetectionPluginActivity. We will use OpenCV to detect faces in the current camera frame and OpenGL calls in Java to render a rectangle around detected faces.

The FaceDetectionPluginConnector acts as our interface between native code and Java and contains some JNI code, since JNI is not the focus of this example we won't go into detail about the implementation. If you would like to have a look at the complete code feel free to browse the source code in the Wikitude SDK release package.

We implement two Java native methods initNative and setFlipFlag. First will be used to initialize the plugin with the path to an OpenCV database, second will be used to notify the Plugin about orientation changes of the device. The other methods faceDetected, faceLost, projectionMatrixChanged and renderDetectedFaceAugmentation will be called by the Plugin to update the Java Android Activity, which controls the rendering.

extern "C" JNIEXPORT void JNICALL
Java_com_wikitude_samples_FaceDetectionPluginActivity_initNative(JNIEnv* env, jobject obj, jstring databasePath_)
{
    ...
}

extern "C" JNIEXPORT void JNICALL
Java_com_wikitude_samples_FaceDetectionPluginActivity_setFlipFlag(JNIEnv* env, jobject obj, jint flag)
{
    ...
}

... ctor / dtor ...

void FaceDetectionPluginConnector::faceDetected(const float *modelViewMatrix)
{
...
}

void FaceDetectionPluginConnector::faceLost()
{
...
}

void FaceDetectionPluginConnector::projectionMatrixChanged(const float *projectionMatrix)
{
...
}

void FaceDetectionPluginConnector::renderDetectedFaceAugmentation() {
...
}

Next let's have a look at the FaceDetectionPlugin class. Again we we will leave out implementation details and focus on how we use the plugin itself. In the cameraFrameAvailable method we use OpenCV to detect faces in the current camera frame which the Wikitude SDK passes to the plugin. We call the observer which is an instance of the FaceDetectionPluginConnector to notify the Java activity about the result. The plugin base class defines startRender and endRender, depending on if you would like to render on top of or below of all rendering the Wikitude SDK does, you choose one of them or both to override. To render below all Wikitude rendering we choose startRender and again call the FaceDetectionPluginConnector instance which in turn calls the Android activity. Since we do not react on the result of the Wikitude SDK image recognition we leave update blank.


... ctor/dtor ...

void FaceDetectionPlugin::cameraFrameAvailable(const wikitude::sdk::Frame& cameraFrame_) {

    ... Control Open CV ...

    if ( _result.size() ) {
        convertFaceRectToModelViewMatrix(croppedImg, _result.at(0));
        _observer->faceDetected(_modelViewMatrix);
    } else {
        _observer->faceLost();
    }
}

void FaceDetectionPlugin::startRender() {
    _observer->renderDetectedFaceAugmentation();
}

void FaceDetectionPlugin::update(const std::list<wikitude::sdk::RecognizedTarget> &recognizedTargets_) {
}

... other internally used methods ...

In the FaceDetectionPluginActivity Java class we override onPostCreate and initialize the Plugin by calling the initNative native method, passing the path to the database file. We also override onConfigurationChanged to get notify about device orientation changes and again notify the Plugin about it by calling the setFlipFlag native method. To render a rectangle around detected faces we created a class called StrokedRectangle which contains OpenGL calls. When the plugin detects, looses or recalculated the projection matrix it will call the appropriate Java methods which we use to update the StrokedRectangle instance. If the Plugin decides to render a rectangle around a detected face it will call renderDetectedFaceAugmentation. Since the plugin will only call this method in the startRender method, we know the current thread is the OpenGL thread and are able to dispatch OpenGL calls.


... imports ...

public class FaceDetectionPluginActivity extends SampleCamActivity {

    private File _cascadeFile;
    private StrokedRectangle rectangle = new StrokedRectangle(StrokedRectangle.Type.FACE);

    @Override
    protected void onPostCreate(final Bundle savedInstanceState) {
        ...
        initNative(_cascadeFile.getAbsolutePath());
        ...
    }

    private void setInterfaceOrientationInPlugin() {
        ...
        setFlipFlag(...);
        ...
    }

    @Override
    public void onConfigurationChanged(Configuration newConfig) {
        super.onConfigurationChanged(newConfig);
        setInterfaceOrientationInPlugin();
    }

    public void onFaceDetected(float[] modelViewMatrix) {
        rectangle.setViewMatrix(modelViewMatrix);
    }

    public void onFaceLost() {
       rectangle.onFaceLost();
    }

    public void onProjectionMatrixChanged(float[] projectionMatrix) {
        rectangle.setProjectionMatrix(projectionMatrix);
    }

    public void renderDetectedFaceAugmentation() {
        rectangle.onDrawFrame();
    }

    private native void initNative(String casecadeFilePath);
    private native void setFlipFlag(int flag);

}

If you are interested in the implementation details of the FaceDetectionPluginActivity or the StrokedRectangle class, you can find both classes in our Wikitude SDK Example Application.