Techwiki:Win32k/XCLIPOBJ

Windows XP version of REGION

typedef struct
{
  LONG  scnPntTop;     // top line (inclusive)
  LONG  scnPntBottom;  // bottom line (exclusive)
  LONG  scnPntX[2];    // v-length array of x pairs
} SCAN2;
//
// REGION Object minimum size of 0x70, 3 cScan
typedef struct
{
  LONG  scnPntCnt;     // 038 count of x coordinates
  SCAN2 scnPnt[3];     // 03C
  LONG  scnPntCntToo;  // 108 same as scnPntCnt;
} SCAN1;
//
// REGION Object minimum size of 0x4C, 1 cScan
typedef struct
{
  LONG  scnPntCnt;     // 38    count of x coordinates
  LONG  scnPntTop;     // 3c    top line (inclusive)
  LONG  scnPntBottom;  // 40    bottom line (exclusive)
  LONG  scnPntX[2];    // 44 48 v-length array of x pairs
} SCAN;
//
typedef struct _REGION
{
  BASEOBJECT BaseObject;
  unsigned   sizeObj;
  ULONG      iUniq;      // from Clip object
  DWORD      nRefCount   // inc/dec Ref count if 0 deleteRGNOBJ
  SCAN *     pscnTail;
  unsigned   sizeRgn;
  unsigned   cScans;
  RECTL      rcl;
  //
  SCAN       scnHead[1]; // 038 + 1 = 0x4C
  // 
  SCAN1      scnHead[1]; // 038 + 1 = 0x70
};

sizeRgn reports 2 minimum sizes, 0x4C and 0x70.

Windows XP version of CLIPOBJ or WNDOBJ

typedef struct _CLIPOBJ
{
  ULONG iUniq;
  RECTL rclBounds;
  BYTE iDComplexity;
  BYTE iFComplexity;
  BYTE iMode;
  BYTE fjOptions;
} CLIPOBJ;

typedef struct _WNDOBJ
{
  CLIPOBJ coClient;
  PVOID pvConsumer;
  RECTL rclClient;
  SURFOBJ *psoOwner;
} WNDOBJ, *PWNDOBJ;

// EXtended CLip and Window Region Object
// Multipurpose E/X/CLIPOBJ and E/WNDOBJ structure
typedef struct _XCLIPOBJ
{
 /* 000 */ WNDOBJ  exClipWnd;
 /* 030 */ PVOID   pClipRgn;     // prgnRao_ or (prgnVis_ if (prgnRao_ == z))
 /* 034 */ RECTL   rclClipRgn;   // if ShouldDoAll TRUE: Copy of clip prgn.rcl, FALSE: from
 //                                 exClipWnd.co.rclBounds.
 /* 044 */ PVOID   pscanClipRgn; // based on iDirection, ulUpDown -1 *Tail up, 1 *Head down
 /* 048 */ DWORD   cScans;       // from clip rgn cScans.
 /* 04C */ DWORD   Unknown1;
 /* 050 */ ULONG   ulBSize;      // recomputed buffer size.
 /* 054 */ LONG    lscnSize;     // Add (+-)size for pointer based on size of SCAN & iDirection.
 /* 058 */ ULONG   ulObjSize;    // Specifies the size, in bytes, of the enum output buffer.
 /* 05C */ ULONG   iDirection;   // CD_ANY CD_LEFTDOWN CD_LEFTUP CD_LEFTWARDS CD_RIGHTDOWN
 //                                 CD_RIGHTUP CD_UPWARDS
 /* 060 */ ULONG   ClipType;     // always CT_RECTANGLES aka z
 /* 064 */ DWORD   Unknown2;     // init z
 /* 068 */ LONG    lUpDown;      // -1 up, 1 down, based on "iDirection" of reading SCAN list.
 /* 06C */ DWORD   Unknown3;     // init z
 /* 070 */ BOOL    ShouldDoAll;  // Specifies whether the entire region should be enumerated.
 /* 074 */ DWORD   nComplexity;  // count/mode based on # of rect in regions SCAN.
 /* 078 */ PVOID   pUnknown;     // pointer to a large DDA based drawing structure. See Note.
} XCLIPOBJ, *PXCLIPOBJ; // Size is 0x7C or 124

Signed lUpDown is not used, but is a notification which direction the scan list is read.

Note:

On a raster-based device, such as a screen display or a printer, the line drawing uses a class of algorithms called DDA (digital differential analyzer) to select the pixels to paint to represent a line. The Bresenham algorithm is a classical incremental DDA algorithm. The Windows NT/2000 graphics engine uses 28.4 fixed-point coordinate space on physical device surfaces. Lines are drawn following the so-called GIQ (Grid Intersection Quantization) diamond convention, in which each pixel is thought of as having a 1-by-1 pixel size diamond shape around it. A pixel is painted if its diamond shape touches a line segment. From section 8.4 of Windows Graphics Programming Win32 GDI and DirectDraw.