how to use glreadpixels with android — usually get zeros

Question

I'm trying to detect the opengl object under the cursor... I have read it referred to as picking. Here is my code:

public int makeBuffer(GL10 gl, int x, int y) {

    ByteBuffer PixelBuffer = ByteBuffer.allocateDirect(4);
    PixelBuffer.order(ByteOrder.nativeOrder());
    PixelBuffer.position(0);
    int mTemp = 0;
    gl.glReadPixels((int)x, (int) y, 1, 1, GL10.GL_RGBA, GL10.GL_UNSIGNED_BYTE, PixelBuffer);
    Log.e("Picking", " xy: x" + x + " y"+ y);
    byte b [] = new byte[4];
    PixelBuffer.get(b);
    Log.e("Picking", " rgba: r"+ PixelBuffer.get(0) + " g" + PixelBuffer.get(1) + " b" +
            PixelBuffer.get(2) + " a" + PixelBuffer.get(3));
    Log.e("Picking", " rgba: r"+ b[0] + " g" + b[1] + " b" +
            b[2] + " a" + b[3]);

    //mTemp = PixelBuffer.get(0);
    mTemp = b[0];

    Log.e("Picking", "result:" + mTemp );

    return mTemp;
}

See that most of the code above is logcat statements. My code prints zeros to the screen for r,g, and b. For alpha it prints '-1' which is translatable to 255 (unsigned) as 'full alpha'. I'm trying to detect a color on the screen at the given x/y position. I would be happy with a red value that's somewhere between 1 and 15, as that's the color that should be below the touch. I would expect that if I was doing it entirely wrong I would get all zeroes, but I must be doing it at least partially right, as I'm getting alpha. I have also included lines in my manifest that tell the phone that I use the permissions for the 'surface flinger' and the 'read frame buffer'. I don't know if these lines are working.

<uses-permission
android:name="android.permission.ACCESS_SURFACE_FLINGER" />
<uses-permission android:name="android.permission.READ_FRAME_BUFFER" /

any help would be appreciated.

Answer 1

These permissions are only available for signature apps. Apps that were signed when the Android ROM was compiled. So they do nothing for regular apps.

android:name="android.permission.ACCESS_SURFACE_FLINGER"
android:name="android.permission.READ_FRAME_BUFFER" 

Answer 2

I also faced similar issues when creating an openGL ES app on android.

My workaround so far was to change the app from RGB565 to RGBA8888. Insert these lines before setting the renderer:

getHolder().setFormat(PixelFormat.RGBA_8888);
setEGLConfigChooser(8, 8, 8, 8, 0, 0);

You must be aware that this solution will only work on devices that support RGBA8888.