Windows Store App - SwapChainPanel DrawLine Performance - windows-runtime

Question

I am developing a Windows Store App using XAML / C#. The app also has a Windows Runtime Component, which is used for showing a Chart ouput using DirectX.
I am using SwapChainPanel approach for drawing the lines (x-axis, y-axis and waveform).
I chose this approach from the below MSDN sample (refer scenario 3 - D2DPanel)
http://code.msdn.microsoft.com/windowsapps/XAML-SwapChainPanel-00cb688b
Here is my question,
My waveform contains a huge number of data (ranging from 1,000 to 20,000 set of points). I am calling DrawLine continuously for all these points during each Render function call.
The control also provides panning and zooming but keeps the StrokeWidth constant irrespective of zoom level, hence the visible area (render target) might be much less than the lines I am drawing.
Does calling DrawLine for the area which are going to be off-screen cause performance issues?
I tried PathGeometry & GeometryRealization but I am not able to control the StrokeWidth at various zoom level.
My Render method typically resembles the below snippet. The lineThickness is controlled to be same irrespective of zoom level.
m_d2dContext->SetTransform(m_worldMatrix);
float lineThickness = 2.0f / m_zoom;
for (unsigned int i = 0; i < points->Size; i += 2)
{
double wavex1 = points->GetAt(i);
double wavey1 = points->GetAt(i + 1);
if (i != 0)
{
m_d2dContext->DrawLine(Point2F(prevX, prevY), Point2F(wavex1, wavey1), brush, lineThickness);
}
prevX = wavex1;
prevY = wavey1;
}
I'm kind of new to DirectX, but not to C++. Any thoughts?

Answer 1

Short answer: It probably will. It's good practice to push a clip before drawing. For instance, in your case, you'd do a call to ID2D1DeviceContext::PushAxisAlignedClip with the bounds of your drawing surface. That'll ensure no drawing calls attempt to draw outside the surface's bounds.
Long answer: Really, it depends on a handful of factors, including but not limited to what target the device context is drawing to, the display hardware, and the display driver. For instance, if you're drawing to a CPU-backed ID2D1Bitmap, it's probably fair to assume that there won't be much of a difference.
However, if you're directly drawing to some hardware-backed surface (a GPU bitmap, or a bitmap created from an IDXGISurface), it can get a little hairy. For example, consider this comment from an excellently documented MSDN sample. Here, the where the code is about to Clear an ID2D1Bitmap created from an IDXGISurface:
// The Clear call must follow and not precede the PushAxisAlignedClip call.
// Placing the Clear call before the clip is set violates the contract of the
// virtual surface image source in that the application draws outside the
// designated portion of the surface the image source hands over to it. This
// violation won't actually cause the content to spill outside the designated
// area because the image source will safeguard it. But this extra protection
// has a runtime cost associated with it, and in some drivers this cost can be
// very expensive. So the best performance strategy here is to never create a
// situation where this protection is required. Not drawing outside the appropriate
// clip does that the right way.