docs: private Bitmap savePixels(int x, int y, int w, int h) {
  		int b[] = new int[w * (y + h)];
  		int bt[] = new int[w * h];
  		IntBuffer ib = IntBuffer.wrap(b);
  		ib.position(0);
  		GLES20.glReadPixels(x, 0, w, y + h, GLES20.GL_RGBA,
  				GLES20.GL_UNSIGNED_BYTE, ib);
  
  		for (int i = 0, k = 0; i < h; i++, k++) {
  			// remember, that OpenGL bitmap is incompatible with Android bitmap
  			// and so, some correction need.
  			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 - k - 1) * w + j] = pix1;
  			}
  		}
  
  		Bitmap sb = Bitmap.createBitmap(bt, w, h, Bitmap.Config.ARGB_8888);
  		return sb;
  	}