How to save ink to png image?

Here I have write an ink app using COM, now I want to save the ink data to a png image. But  only an empty png file is created, maybe there is something wrong in my code, please help me! The code is follow:

HDC hDC = XXX;  // DC handle
IInkOverlay *pInkOverlay = XXX;  // the InkOverlay object
...
IInkRenderer *render = NULL;
pInkOverlay->get_Renderer(&render);

IInkDisp *ink = XXX;
pInkOverlay->get_Ink(&ink);

IInkRectangle *rect = NULL;
int->GetBoundingBox(IBBM_CurveFit, &rect);

long l, t, r, b;
rect->GetRectangle(&t, &l, &b, &r);

long cx = r - l;
long cy = b - t;
render->InkSpaceToPixel((LONG_PTR)hDC, &l, &t);
render->InkSpaceToPixel((LONG_PTR)hDC, &cx, &cy);

IInkStrokes *strokes = NULL;
pInk->get_Strokes(&strokes);

CLSID clsid;
GetImageEncoderClsid(L"image/png", &clsid)

Bitmap bitmap(cx, cy);
Graphics *graphics = Graphics::FromImage(&bitmap);
if (graphics)
{
	graphics->Clear(Color::Transparent);
	render->Draw((LONG_PTR)graphics->GetHDC(), strokes);
	bitmap.Save(L"c:\\ink.png", &clsid);

	delete graphics;
}