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Can anyone explain to me how to display part of a surface that is larger than the full screen size using direct draw? I want to scroll a large background and place a backdrop on the screen that is clipped from a large image. Thanks

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When you call the surface's Blt() / BltFast() function, you can pass in a RECT which gives the region of the surface to transfer, so you can select which part of your backdrop to show on the screen at any one time.

From the MSDN docs:

LPRECT lpDestRect,
LPRECT lpSrcRect,
DWORD dwFlags,

Points to a RECT structure that defines the upper left and lower right points of the rectangle on the source surface which is to be blted from.

So, for example:

// Define the backdrop offset
int backdropX = 240;
int backdropY = 200;
// Create a rectangle
RECT rcSrc = {backdropX, backdropY, backdropX + screenX, backdropY + screenY};
// Blit to the screen using the source rectangle
m_pSurface->Blt(... &rcSrc ...);

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