Finding the right context to take a screenshot with glReadPixels()

Question

I'm trying to read directly from the frame buffer via glReadPixels() so that I can take a screenshot from whatever is on my screen.
I'm not using a GUI, instead it should print the raw content of the screen whenever I touch the screen (which is sufficient for the beginning).

I'm now stuck in getting the right context for my gl.glReadPixels() function.
Because I think the context I get at the moment has nothing to do with what is actually on the screen.

When I run the application it prints an libEGL error which says

call to OpenGL ES API with no current context (logged once per thread)

and afterwards I get my array filled with zeros.

To hide my application I'm using the following line in my manifest.

android:theme="@android:style/Theme.Translucent.NoTitleBar"

And here is the code:

public class ScreenshotActivity extends Activity {

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        if (event.getAction() == MotionEvent.ACTION_UP) {   
                savePixels();
        }
        return true;
    }


    EGL10 egl = (EGL10) EGLContext.getEGL();
    GL10 gl = (GL10) egl.eglGetCurrentContext().getGL();

    public void savePixels() {
         w= getWidth(this);
         h= getHeight(this);
         int b[] = new int[w * h];
         int bt[] = new int[w * h];
         IntBuffer ib = IntBuffer.wrap(b);
         ib.position(0);

         gl.glReadPixels(0, 0, w, h, GL10.GL_RGBA, GL10.GL_UNSIGNED_BYTE, ib);


         for (int i = 0; i < h; i++) {
             for (int j = 0; j < w; j++) {
                 int pix = b[i * w + j];
                 int pb = (pix >> 16) & 0xff;
                 int pr = (pix << 16) & 0x00ff0000;
                 int pix1 = (pix & 0xff00ff00) | pr | pb;
                 bt[(h - i - 1) * w + j] = pix1;
                 }
             }

             Log.w("debug", Arrays.toString(bt));
             moveTaskToBack(true);
    }
}

Help is highly appreciated :)

score 1 · Answer 1 · answered Mar 27 '13 at 10:44

1

Could people please stop trying to abuse OpenGL for taking screenshots? It's fine if you want to capture the rendering output of the the program that did the glReadPixels call, but as soon as other programs are the intended pixel source it becomes highly unreliable.

The only reliable way to take full screen screenshots is to directly read from the GPUs videobuffer memory. Android is based on Linux and Linux offers the fbdev device for this. Since working with all those ioctls is a bit cumbersome some tested library should be used. There's a library for Android that offers a dedicated API for taking screenshots: http://code.google.com/p/android-screenshot-library/wiki/DeveloperGuide

answered Mar 27 '13 at 10:44

datenwolf

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At first it looked promising but there is a problem. At least when I run the asl-demo.apk it just reads what is in /dev/graphics/fb0, which on my device is only black. I have read that it has something to do with the tegra 3 chip which doesn't use the fb0 framebuffer. To be sure I filled fb0 with /dev/urandom and had this random noise image afterwards as a screenshot. So it doesn't seem as it would read directly from the GPU videobuffer, does it? – derasd Mar 27 '13 at 13:57
Do you know how screencap.cpp is working? Because this function is able to take screenshots. I would just need to know where it gets the data for a screenshot. https://github.com/android/platform_frameworks_base/blob/master/cmds/screencap/screencap.cpp – derasd Mar 28 '13 at 10:27
1

@user2207148: It's talking to the display compositor to retrieve a composited image. Apart from reading directly from the framebuffer memory, this is the only other reliable method in grabbing a screenshot in composited graphics systems. – datenwolf Mar 28 '13 at 11:49

score 0 · Answer 2 · answered Mar 27 '13 at 09:35

0

mGLSurfaceView.queueEvent(new Runnable() {
  public void run() {
    savePixels();
  }
});

you have to change mGLSurfaceView to your SurfaceView Instance.

answered Mar 27 '13 at 09:35

dmnlk

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But what is my SurfaceView when I'm not going to create one? My app is hidden and should capture the content of the screen (which doesn't belong to my app). – derasd Mar 27 '13 at 10:24

score 0 · Accepted Answer · answered Apr 19 '13 at 12:28

0

Finally found an open source program which is not using /dev/graphics/fb0 to capture the framebuffer.

The package fb2png includes a C program and a python script which are both working perfectly.

answered Apr 19 '13 at 12:28

derasd

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12

Finding the right context to take a screenshot with glReadPixels()

3 Answers3