After displaying the overlay surface, you might not need to do anything else. However, some software might need to reposition the overlay surface. The Mosquito sample uses the IDirectDrawSurface3::SetOverlayPosition method to reposition the overlay, as shown in the following example:
// Set x- and y-coordinates
.
.
.
// We need to check for any alignment restrictions on the x-position
// and align it if necessary.
if (g_dwOverlayXPositionAlignment)
dwXAligned = g_nOverlayXPos - g_nOverlayXPos % g_dwOverlayXPositionAlignment;
else
dwXAligned = g_nOverlayXPos;
// Set the overlay to its new position.
ddrval = g_lpddsOverlay->SetOverlayPosition(dwXAligned, g_nOverlayYPos);
if (ddrval == DDERR_SURFACELOST)
{
if (!RestoreAllSurfaces())
return;
}
The preceding example starts by aligning the rectangle to meet any destination rectangle boundary alignment restrictions that might exist. The global variable that it checks, g_dwOverlayXPositionAlignment, was set earlier to equal the value reported in the dwAlignBoundaryDest member of the DDCAPS structure when the application previously called the IDirectDraw2::GetCaps method. (For details, see Step 4.1: Determining the Minimum Display Requirements). If destination alignment restrictions exist, the example adjusts the new x-coordinate to be pixel-aligned accordingly. Failing to meet this requirement will cause the overlay surface not to be displayed.
After making any requisite adjustments to the new x-coordinate, the example calls IDirectDrawSurface3::SetOverlayPosition method to reposition the overlay. For the call, the first parameter is the aligned x-coordinate, and the second parameter is the new y-coordinate. These values represent the new location of the overlay's top-left corner. Width and height information are not accepted, nor are they needed because DirectDraw already knows the dimensions of the surface from the IDirectDrawSurface3::UpdateOverlay method made to initially display the overlay. If the call fails because one or more surfaces were lost, the example calls an application-defined function to restore them and reload their bitmaps.
Note Take care not to use coordinates too close to the bottom or right edge of the target surface. The IDirectDraw2::SetOverlayPosition method does not perform clipping for you; using coordinates that would potentially make the overlay run off the edge of the target surface will cause the method to fail, returning DDERR_INVALIDPOSITION.