Android, Programming

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The SDL library supports the Android OS quite well. To get started, this guide is a great starting point.

I have a couple of points to add to that tutorial. I didn’t actually use the pre made SDL project, but rather took all the structure from SDL’s Mercurial repository. If you do that, the project will build, but you’ll have problems if you try to open a file via SDL (for example using IMG_Load). SDL seems to have good Asset Manager integration, but there’s a key piece of glue code missing, a static function called getContext that should be in the SDLActivity.java file. This is my fixed SDLActivity.java file, which also includes a workaround to disable OpenGL ES 2.x without recompiling SDL (it’s in the initEGL function).

 

package org.libsdl.app;

import javax.microedition.khronos.egl.EGLConfig;
import javax.microedition.khronos.opengles.GL10;
import javax.microedition.khronos.egl.*;

import android.app.*;
import android.content.*;
import android.view.*;
import android.os.*;
import android.util.Log;
import android.graphics.*;
import android.text.method.*;
import android.text.*;
import android.media.*;
import android.hardware.*;
import android.content.*;

import java.lang.*;

/**
    SDL Activity
*/
public class SDLActivity extends Activity {

    // Main components
    private static SDLActivity mSingleton;
    private static SDLSurface mSurface;

    // Audio
    private static Thread mAudioThread;
    private static AudioTrack mAudioTrack;

    // Load the .so
    static {
        System.loadLibrary("SDL");
        System.loadLibrary("SDL_image");
        System.loadLibrary("mikmod");
        System.loadLibrary("SDL_mixer");
        System.loadLibrary("SDL_ttf");
        System.loadLibrary("main");
    }

    // Setup
    protected void onCreate(Bundle savedInstanceState) {
        //Log.v("SDL", "onCreate()");
        super.onCreate(savedInstanceState);

        // So we can call stuff from static callbacks
        mSingleton = this;

        // Set up the surface
        mSurface = new SDLSurface(getApplication());
        setContentView(mSurface);
        SurfaceHolder holder = mSurface.getHolder();
        holder.setType(SurfaceHolder.SURFACE_TYPE_GPU);
    }

    // Events
    protected void onPause() {
        //Log.v("SDL", "onPause()");
        super.onPause();
    }

    protected void onResume() {
        //Log.v("SDL", "onResume()");
        super.onResume();
    }

    // Messages from the SDLMain thread
    static int COMMAND_CHANGE_TITLE = 1;

    // Handler for the messages
    Handler commandHandler = new Handler() {
        public void handleMessage(Message msg) {
            if (msg.arg1 == COMMAND_CHANGE_TITLE) {
                setTitle((String)msg.obj);
            }
        }
    };

    // Send a message from the SDLMain thread
    void sendCommand(int command, Object data) {
        Message msg = commandHandler.obtainMessage();
        msg.arg1 = command;
        msg.obj = data;
        commandHandler.sendMessage(msg);
    }

    // C functions we call
    public static native void nativeInit();
    public static native void nativeQuit();
    public static native void onNativeResize(int x, int y, int format);
    public static native void onNativeKeyDown(int keycode);
    public static native void onNativeKeyUp(int keycode);
    public static native void onNativeTouch(int action, float x,
                                            float y, float p);
    public static native void onNativeAccel(float x, float y, float z);
    public static native void nativeRunAudioThread();

    // Java functions called from C

    public static boolean createGLContext(int majorVersion, int minorVersion) {
        return mSurface.initEGL(majorVersion, minorVersion);
    }

    public static void flipBuffers() {
        mSurface.flipEGL();
    }

    public static void setActivityTitle(String title) {
        // Called from SDLMain() thread and can't directly affect the view
        mSingleton.sendCommand(COMMAND_CHANGE_TITLE, title);
    }

    public static Context getContext() {
        return mSingleton;
    }

    // Audio
    private static Object buf;

    public static Object audioInit(int sampleRate, boolean is16Bit, boolean isStereo, int desiredFrames) {
        int channelConfig = isStereo ? AudioFormat.CHANNEL_CONFIGURATION_STEREO : AudioFormat.CHANNEL_CONFIGURATION_MONO;
        int audioFormat = is16Bit ? AudioFormat.ENCODING_PCM_16BIT : AudioFormat.ENCODING_PCM_8BIT;
        int frameSize = (isStereo ? 2 : 1) * (is16Bit ? 2 : 1);

        Log.v("SDL", "SDL audio: wanted " + (isStereo ? "stereo" : "mono") + " " + (is16Bit ? "16-bit" : "8-bit") + " " + ((float)sampleRate / 1000f) + "kHz, " + desiredFrames + " frames buffer");

        // Let the user pick a larger buffer if they really want -- but ye
        // gods they probably shouldn't, the minimums are horrifyingly high
        // latency already
        desiredFrames = Math.max(desiredFrames, (AudioTrack.getMinBufferSize(sampleRate, channelConfig, audioFormat) + frameSize - 1) / frameSize);

        mAudioTrack = new AudioTrack(AudioManager.STREAM_MUSIC, sampleRate,
                channelConfig, audioFormat, desiredFrames * frameSize, AudioTrack.MODE_STREAM);

        audioStartThread();

        Log.v("SDL", "SDL audio: got " + ((mAudioTrack.getChannelCount() >= 2) ? "stereo" : "mono") + " " + ((mAudioTrack.getAudioFormat() == AudioFormat.ENCODING_PCM_16BIT) ? "16-bit" : "8-bit") + " " + ((float)mAudioTrack.getSampleRate() / 1000f) + "kHz, " + desiredFrames + " frames buffer");

        if (is16Bit) {
            buf = new short[desiredFrames * (isStereo ? 2 : 1)];
        } else {
            buf = new byte[desiredFrames * (isStereo ? 2 : 1)];
        }
        return buf;
    }

    public static void audioStartThread() {
        mAudioThread = new Thread(new Runnable() {
            public void run() {
                mAudioTrack.play();
                nativeRunAudioThread();
            }
        });

        // I'd take REALTIME if I could get it!
        mAudioThread.setPriority(Thread.MAX_PRIORITY);
        mAudioThread.start();
    }

    public static void audioWriteShortBuffer(short[] buffer) {
        for (int i = 0; i < buffer.length; ) {             int result = mAudioTrack.write(buffer, i, buffer.length - i);             if (result > 0) {
                i += result;
            } else if (result == 0) {
                try {
                    Thread.sleep(1);
                } catch(InterruptedException e) {
                    // Nom nom
                }
            } else {
                Log.w("SDL", "SDL audio: error return from write(short)");
                return;
            }
        }
    }

    public static void audioWriteByteBuffer(byte[] buffer) {
        for (int i = 0; i < buffer.length; ) {             int result = mAudioTrack.write(buffer, i, buffer.length - i);             if (result > 0) {
                i += result;
            } else if (result == 0) {
                try {
                    Thread.sleep(1);
                } catch(InterruptedException e) {
                    // Nom nom
                }
            } else {
                Log.w("SDL", "SDL audio: error return from write(short)");
                return;
            }
        }
    }

    public static void audioQuit() {
        if (mAudioThread != null) {
            try {
                mAudioThread.join();
            } catch(Exception e) {
                Log.v("SDL", "Problem stopping audio thread: " + e);
            }
            mAudioThread = null;

            //Log.v("SDL", "Finished waiting for audio thread");
        }

        if (mAudioTrack != null) {
            mAudioTrack.stop();
            mAudioTrack = null;
        }
    }
}

/**
    Simple nativeInit() runnable
*/
class SDLMain implements Runnable {
    public void run() {
        // Runs SDL_main()
        SDLActivity.nativeInit();

        //Log.v("SDL", "SDL thread terminated");
    }
}

/**
    SDLSurface. This is what we draw on, so we need to know when it's created
    in order to do anything useful.

    Because of this, that's where we set up the SDL thread
*/
class SDLSurface extends SurfaceView implements SurfaceHolder.Callback,
    View.OnKeyListener, View.OnTouchListener, SensorEventListener  {

    // This is what SDL runs in. It invokes SDL_main(), eventually
    private Thread mSDLThread;

    // EGL private objects
    private EGLContext  mEGLContext;
    private EGLSurface  mEGLSurface;
    private EGLDisplay  mEGLDisplay;

    // Sensors
    private static SensorManager mSensorManager;

    // Startup
    public SDLSurface(Context context) {
        super(context);
        getHolder().addCallback(this);

        setFocusable(true);
        setFocusableInTouchMode(true);
        requestFocus();
        setOnKeyListener(this);
        setOnTouchListener(this);

        mSensorManager = (SensorManager)context.getSystemService("sensor");
    }

    // Called when we have a valid drawing surface
    public void surfaceCreated(SurfaceHolder holder) {
        //Log.v("SDL", "surfaceCreated()");

        enableSensor(Sensor.TYPE_ACCELEROMETER, true);
    }

    // Called when we lose the surface
    public void surfaceDestroyed(SurfaceHolder holder) {
        //Log.v("SDL", "surfaceDestroyed()");

        // Send a quit message to the application
        SDLActivity.nativeQuit();

        // Now wait for the SDL thread to quit
        if (mSDLThread != null) {
            try {
                mSDLThread.join();
            } catch(Exception e) {
                Log.v("SDL", "Problem stopping thread: " + e);
            }
            mSDLThread = null;

            //Log.v("SDL", "Finished waiting for SDL thread");
        }

        enableSensor(Sensor.TYPE_ACCELEROMETER, false);
    }

    // Called when the surface is resized
    public void surfaceChanged(SurfaceHolder holder,
                               int format, int width, int height) {
        //Log.v("SDL", "surfaceChanged()");

        int sdlFormat = 0x85151002; // SDL_PIXELFORMAT_RGB565 by default
        switch (format) {
        case PixelFormat.A_8:
            Log.v("SDL", "pixel format A_8");
            break;
        case PixelFormat.LA_88:
            Log.v("SDL", "pixel format LA_88");
            break;
        case PixelFormat.L_8:
            Log.v("SDL", "pixel format L_8");
            break;
        case PixelFormat.RGBA_4444:
            Log.v("SDL", "pixel format RGBA_4444");
            sdlFormat = 0x85421002; // SDL_PIXELFORMAT_RGBA4444
            break;
        case PixelFormat.RGBA_5551:
            Log.v("SDL", "pixel format RGBA_5551");
            sdlFormat = 0x85441002; // SDL_PIXELFORMAT_RGBA5551
            break;
        case PixelFormat.RGBA_8888:
            Log.v("SDL", "pixel format RGBA_8888");
            sdlFormat = 0x86462004; // SDL_PIXELFORMAT_RGBA8888
            break;
        case PixelFormat.RGBX_8888:
            Log.v("SDL", "pixel format RGBX_8888");
            sdlFormat = 0x86262004; // SDL_PIXELFORMAT_RGBX8888
            break;
        case PixelFormat.RGB_332:
            Log.v("SDL", "pixel format RGB_332");
            sdlFormat = 0x84110801; // SDL_PIXELFORMAT_RGB332
            break;
        case PixelFormat.RGB_565:
            Log.v("SDL", "pixel format RGB_565");
            sdlFormat = 0x85151002; // SDL_PIXELFORMAT_RGB565
            break;
        case PixelFormat.RGB_888:
            Log.v("SDL", "pixel format RGB_888");
            // Not sure this is right, maybe SDL_PIXELFORMAT_RGB24 instead?
            sdlFormat = 0x86161804; // SDL_PIXELFORMAT_RGB888
            break;
        default:
            Log.v("SDL", "pixel format unknown " + format);
            break;
        }
        SDLActivity.onNativeResize(width, height, sdlFormat);

        // Now start up the C app thread
        if (mSDLThread == null) {
            mSDLThread = new Thread(new SDLMain(), "SDLThread");
            mSDLThread.start();
        }
    }

    // unused
    public void onDraw(Canvas canvas) {}

    // EGL functions
    public boolean initEGL(int majorVersion, int minorVersion) {
        // Temporarily disable OpenGL ES 2 as the SDL backend is buggy
        if (majorVersion != 1) return false;

        Log.v("SDL", "Starting up OpenGL ES " + majorVersion + "." + minorVersion);

        try {
            EGL10 egl = (EGL10)EGLContext.getEGL();

            EGLDisplay dpy = egl.eglGetDisplay(EGL10.EGL_DEFAULT_DISPLAY);

            int[] version = new int[2];
            egl.eglInitialize(dpy, version);

            int EGL_OPENGL_ES_BIT = 1;
            int EGL_OPENGL_ES2_BIT = 4;
            int renderableType = 0;
            if (majorVersion == 2) {
                renderableType = EGL_OPENGL_ES2_BIT;
            } else if (majorVersion == 1) {
                renderableType = EGL_OPENGL_ES_BIT;
            }
            int[] configSpec = {
                //EGL10.EGL_DEPTH_SIZE,   16,
                EGL10.EGL_RENDERABLE_TYPE, renderableType,
                EGL10.EGL_NONE
            };
            EGLConfig[] configs = new EGLConfig[1];
            int[] num_config = new int[1];
            if (!egl.eglChooseConfig(dpy, configSpec, configs, 1, num_config) || num_config[0] == 0) {
                Log.e("SDL", "No EGL config available");
                return false;
            }
            EGLConfig config = configs[0];

            EGLContext ctx = egl.eglCreateContext(dpy, config, EGL10.EGL_NO_CONTEXT, null);
            if (ctx == EGL10.EGL_NO_CONTEXT) {
                Log.e("SDL", "Couldn't create context");
                return false;
            }

            EGLSurface surface = egl.eglCreateWindowSurface(dpy, config, this, null);
            if (surface == EGL10.EGL_NO_SURFACE) {
                Log.e("SDL", "Couldn't create surface");
                return false;
            }

            if (!egl.eglMakeCurrent(dpy, surface, surface, ctx)) {
                Log.e("SDL", "Couldn't make context current");
                return false;
            }

            mEGLContext = ctx;
            mEGLDisplay = dpy;
            mEGLSurface = surface;

        } catch(Exception e) {
            Log.v("SDL", e + "");
            for (StackTraceElement s : e.getStackTrace()) {
                Log.v("SDL", s.toString());
            }
        }

        return true;
    }

    // EGL buffer flip
    public void flipEGL() {
        try {
            EGL10 egl = (EGL10)EGLContext.getEGL();

            egl.eglWaitNative(EGL10.EGL_NATIVE_RENDERABLE, null);

            // drawing here

            egl.eglWaitGL();

            egl.eglSwapBuffers(mEGLDisplay, mEGLSurface);

        } catch(Exception e) {
            Log.v("SDL", "flipEGL(): " + e);
            for (StackTraceElement s : e.getStackTrace()) {
                Log.v("SDL", s.toString());
            }
        }
    }

    // Key events
    public boolean onKey(View  v, int keyCode, KeyEvent event) {

        if (event.getAction() == KeyEvent.ACTION_DOWN) {
            //Log.v("SDL", "key down: " + keyCode);
            SDLActivity.onNativeKeyDown(keyCode);
            return true;
        }
        else if (event.getAction() == KeyEvent.ACTION_UP) {
            //Log.v("SDL", "key up: " + keyCode);
            SDLActivity.onNativeKeyUp(keyCode);
            return true;
        }

        return false;
    }

    // Touch events
    public boolean onTouch(View v, MotionEvent event) {

        int action = event.getAction();
        float x = event.getX();
        float y = event.getY();
        float p = event.getPressure();

        // TODO: Anything else we need to pass?
        SDLActivity.onNativeTouch(action, x, y, p);
        return true;
    }

    // Sensor events
    public void enableSensor(int sensortype, boolean enabled) {
        // TODO: This uses getDefaultSensor - what if we have >1 accels?
        if (enabled) {
            mSensorManager.registerListener(this,
                            mSensorManager.getDefaultSensor(sensortype),
                            SensorManager.SENSOR_DELAY_GAME, null);
        } else {
            mSensorManager.unregisterListener(this,
                            mSensorManager.getDefaultSensor(sensortype));
        }
    }

    public void onAccuracyChanged(Sensor sensor, int accuracy) {
        // TODO
    }

    public void onSensorChanged(SensorEvent event) {
        if (event.sensor.getType() == Sensor.TYPE_ACCELEROMETER) {
            SDLActivity.onNativeAccel(event.values[0],
                                      event.values[1],
                                      event.values[2]);
        }
    }

}

The other issue I had happened when loading a bunch of files from native code (via de Android NDK). It so happens that there’s object reference stack that the JNI mechanism relies on, and while it gets properly handled by the Java VM internals, if your native code opens a series of files without returning to the Java VM, you run the risk of going over the 512 object reference’s limit and get an exception.

Luckily the solution to this problem is quite simple. In SDL_android.cpp, look for the function: static int Android_JNI_FileOpen(SDL_RWops* ctx)

At the beginning add

mEnv->PushLocalFrame(10);

and before every return expression:

mEnv->PopLocalFrame(NULL);

This will take care of freeing the object references that are no longer in use after the function returns, and then you can load as many images as you need in native code without hitting that limit.