Step 3: Loading a Bitmap on the Back Buffer

After the DirectDrawPalette object is associated with the DirectDrawSurface object, DDEX2 loads the Back.bmp bitmap on the back buffer by using the following code:

// Load a bitmap into the back buffer.

ddrval = DDReLoadBitmap(lpDDSBack, szBackground);

if(ddrval != DD_OK)

// Load failed.

DDReLoadBitmap is another function found in Ddutil.cpp. It loads a bitmap from a file or resource into an already existing DirectDraw surface. (You could also use DDLoadBitmap to create a surface and load the bitmap into that surface. For more information, see Tutorial 5: Dynamically Modifying Palettes.) For DDEX2, it loads the Back.bmp file pointed to by szBackground onto the back buffer pointed to by lpDDSBack. The DDReLoadBitmap function calls the DDCopyBitmap function to copy the file onto the back buffer and stretch it to the proper size.

The DDCopyBitmap function copies the bitmap into memory, and it uses the GetObject function to retrieve the size of the bitmap. It then uses the following code to retrieve the size of the back buffer onto which it will place the bitmap:

// Get the size of the surface.

ddsd.dwSize = sizeof(ddsd);

ddsd.dwFlags = DDSD_HEIGHT | DDSD_WIDTH;

pdds->GetSurfaceDesc(&ddsd);

The ddsd value is a pointer to the DDSURFACEDESC structure. This structure stores the current description of the DirectDraw surface. In this case, the DDSURFACEDESC members describe the height and width of the surface, which are indicated by DDSD_HEIGHT and DDSD_WIDTH. The call to the IDirectDrawSurface::GetSurfaceDesc method then loads the structure with the proper values. For DDEX2, the values will be 480 for the height and 640 for the width.

The DDCopyBitmap function locks the surface and copies the bitmap to the back buffer, stretching or compressing it as applicable by using the StretchBlt function, as shown below:

if ((hr = pdds->GetDC(&hdc)) == DD_OK)

{

StretchBlt(hdc, 0, 0, ddsd.dwWidth, ddsd.dwHeight, hdcImage, x, y,

dx, dy, SRCCOPY);

pdds->ReleaseDC(hdc);

}