In Windows XP the Win32 API renders the controls using GDI/GDI+. Now I'm on 7, so if I use the API's functions, will the rendering automatically be handled by the DWM/WDDM (so by DirectX)? or will it continue to render with GDI?
Or likewise, will an old app written with WinAPI, be rendered with GDI also in Windows 7?
Thank you in advance for the help :)
If your application is written to use GDI, then it will continue to use GDI. The underlying implementation has changed quite a bit (as I recall, most hardware acceleration was removed in Vista, and put back in, in a new form, in Win7)
But it won't magically be transferred to Direct2D, no.
In my experience, if the Aero display is on everything will render via that system, it just won't be obvious to your application. You'll still render in GDI, but it will be to a back buffer and not directly to the screen buffer (in fact it's more complicated then that). That way your older app can get the benefits of the new features, like the live preview effects, without having to be aware of them.
Really though, your application doesn't really notice a difference. The API is still the same API as before and works as you expect it. There are ways to take advantage of this, but you have to opt in to really use it.
On performance front, it is the same old farce and as per usual 'Do Your Own Benchmarking' across OS-es. But, almost a decade old OS for anything GDI still slams Windows7 and plenty of apps will be incredibly more responsive on XP.. Sadness.. especially when you consider that translation of such an old API (as a compatible interface) should be trivial to any new driver or display tech.
Apparently some ops like blitting are supported but look at the result of your target 2D GDI app and judge for yourself..
Here are some observations, but please do your own for a dozen scenarios and compare, especially if it is GDI intensive like a million audio/video apps out there (no wonder iFruit boys are gaining share).
http://www.passmark.com/forum/showthread.php?t=2233
And while you will find all the typical and fluffy MSDN blog explanations that get 'technical' (ie. surface-style nonsense) the reason is very simple: buy a new OS, pull the carpet on old API and complicate it further by allowing new APIs to work with it (but not benefit the old one, easily doable by providing a new dll and .lib ).
For one history lesson and how hard it was to translate GDI calls to Direct2D or use libgdiplus from Mono and derive ideas etc, here is the propaganda:
http://blogs.msdn.com/directx/archive/2009/05/12/2d-drawing-apis-in-windows.aspx