Screenshot on android OpenGL ES application

Question

I have a basic openGL ES 20 application running with on a GLSurfaceView that has been added:

    GLSurfaceView view = new GLSurfaceView(this);
    view.setRenderer(new OpenGLRenderer());
    setContentView(view);

Basically I am trying get a screenshot with the following method:

private static Bitmap getScreenshot(View v)
    {
        Bitmap b = Bitmap.createBitmap(v.getWidth(), v.getHeight(),
                Bitmap.Config.ARGB_8888);
        Canvas c = new Canvas(b);
        v.draw(c);
        return b;
    }

But it seems the bitmap is transparent. The view I am passing in is:

View content = m_rootActivity.getWindow().getDecorView().getRootView();

Anyone has a solution on how to get screenshot on openGL ES without resorting into going into the DrawFrame method which I have seen in other solutions.

Maybe pass in the reference of the renderer? Any help would be appreciated.

Update: I was exploring in rendering the bitmap from the onDrawFrame (Display black screen while capture screenshot of GLSurfaceView)

However, I was wondering if there is a better solution since I won't have access to the renderer nor the surfaceview. I can pass in their reference but would like a solution where we can just capture the entire view like what was mentioned earlier.

Answer 1

See this question.

You can get a screenshot with:

@Override
public void onDrawFrame(GL10 gl) {

    GLES20.glClear(GLES20.GL_COLOR_BUFFER_BIT);
    // draw ...

    if (takeScreenshot) {

        int screenshotSize = width * height;
        ByteBuffer bb = ByteBuffer.allocateDirect(screenshotSize * 4);
        bb.order(ByteOrder.nativeOrder());
        GLES20.glReadPixels(0, 0, width, height, GLES20.GL_RGBA, GLES20.GL_UNSIGNED_BYTE, bb);
        int pixelsBuffer[] = new int[screenshotSize];
        bb.asIntBuffer().get(pixelsBuffer);
        bb = null;

        for (int i = 0; i < screenshotSize; ++i) {
            // The alpha and green channels' positions are preserved while the      red and blue are swapped
            pixelsBuffer[i] = ((pixelsBuffer[i] & 0xff00ff00)) |    ((pixelsBuffer[i] & 0x000000ff) << 16) | ((pixelsBuffer[i] & 0x00ff0000) >> 16);
        }

        Bitmap bitmap = Bitmap.createBitmap(width, height, Bitmap.Config.ARGB_8888);
        bitmap.setPixels(pixelsBuffer, screenshotSize-width, -width, 0, 0, width, height);
        // save bitmap...
    }
}

Answer 2

You can not by any chance at all get the buffer data to the CPU from the GPU without reading the pixels. You should understand that this is not the same pipeline as is with views, the data in the buffer are filled on the GPU and are then sent directly to the display or nowhere.

So that being said the answer is no. You can not simply get a screenshot as a concept of screenshot does not even exist in this matter. There are only raw (usually RGBA) data on the GPU buffer. And those data must be filled with what you draw to get all you have drawn, if you were to simply read those data at any time the buffer might just be cleared, it might be half drawn or if you are lucky fully drawn.

So that is the reason why you make those screenshot in the drawing pipeline as you must assure the buffer is filled with the data.

There are generally 2 smart ways of intercepting the drawing pipeline best done just before presenting the buffer. One is to pass a certain flag that a screenshot should be done where then the engine itself creates a screenshot which is nice since it has all the data of the buffer on the fly. The second is to create a callback handle where the engine will notify the owner on every frame being fully drawn, in this case the owner can do some additional drawing or creating a screenshot or count frames per second... this again has many bonuses but you do need to at least pass the buffer dimensions to do anything with the buffer.

Also note that reading the pixels is extremely slow and in some cases the image you will receive will be upside-down.