Clipping – (overhead)

If the source does not fit on the destination surface, a Blit operation will trash memory locations outside the destination unless you clip.

To set up the clipper:

· create a DD clipper object

· create a clipper list

· send list to the clipper w/ SetClipList()

· attach clipper to the window or surface w/ SetClipper()

CreateClipper (0, <addr of ptr to interface>, NULL);

The interface ptr will be set after a successful call to CreateClipper.

Example:

// create a clipper

LPDIRECTDRAWCLIPPER lpddclipper = NULL; // clipper ptr

CreateClipper (0, &lpddclipper, NULL);

Clipper Lists – a list of RECTs indicating valid regions to Blit to.

typedef struct _RGNDATA

{ RGNDATAHEADER rdh; // header info

char Buffer[1]; // var len list of RECTs

} RGNDATA;

To create one:

· new memory to create a data struct to hold your list

o RGNDATA HEADER

o RECT 1 // first entry in buffer

o RECT 2 …

· Fill in header date

Typedef struct _RGNDATAHEADER

{ DWORD dwSize; // header size sizeof(RGNDATAHEADER)

DWORD iType; // region type (Rectangle)

DWORD nCount; // # of regions

DWORD nRgnSize;// buffer size – sizeof(RECT) * nCount

DWORD rcBound; // bounding RECT for all regions

} RGNDATAHEADER

· Cast it to a RGNDATA object

· Send it to the clipper with
lpddclipper->SetClipList(<ptr to list>, 0);

· Attach it to the surface (usually back buffer) with
lpddsurface->SetClipper(<<ptr to clipper>);