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changing bit-count of a surface

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Hi, I loaded a bitmap with the function "LoadBitmap" into an DirectDraw-Surface. However, the surface had an bit-depth of 24, and I just wanted 16 bpp. So I opened the help file and looked for some solution. After a few hours I had this code (which doesn''t work) :
	DDSURFACEDESC2	ddsdOld; // Alte Surface-Beschreibung
	DDSURFACEDESC2  ddsdNew; // Neue Surface-Beschreibung mit 16-Bit-Daten
	HRESULT		ddrval;

	// Alte Surface-Beschreibung laden
	ZeroMemory (&ddsdOld, sizeof (ddsdOld));
	ddsdOld.dwSize = sizeof (ddsdOld);
	ddrval = lpDDSurf->GetSurfaceDesc (&ddsdOld);
	if (ddrval !=  DD_OK)
		return FALSE;

	// Ändern der Daten für 16-Bit
	ZeroMemory (&ddsdNew, sizeof (ddsdNew));
	ddsdNew.dwSize = sizeof (ddsdNew);
	ddsdNew.ddpfPixelFormat = ddsdOld.ddpfPixelFormat;
	ddsdNew.ddpfPixelFormat.dwFlags = ddsdOld.ddpfPixelFormat.dwFlags | DDPF_RGB;
	ddsdNew.ddpfPixelFormat.dwRGBBitCount = 16; // Für 16 Bit
	ddsdNew.lPitch = ddsdOld.dwWidth*2; // 16Bit = 2 Byte = Breite * 2

	// Einstellen der neuen Surface-Beschreibung
	ddrval = lpDDSurf->SetSurfaceDesc (&ddsdNew, 0);
	if (ddrval != DD_OK)
		MessageBox (NULL, "testwindow 1", NULL, MB_OK);
		return FALSE;
	MessageBox (NULL, "testwindow 2", NULL, MB_OK);
I figured out, that "SetSurfaceDesc" doesn''t work correctly, because neighter testwindow1 nor testwindow2 has been showed. thanks for helping

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