Regions

    
HRGN BitmapToRegion (HBITMAP hBmp, COLORREF cTransparentColor , COLORREF cTolerance  )
{
	HRGN hRgn = NULL;

	if (hBmp)
	{
		// Create a memory DC inside which we will scan the bitmap content

		HDC hMemDC = CreateCompatibleDC(NULL);
		if (hMemDC)
		{
			// Get bitmap size

			BITMAP bm;
			GetObject(hBmp, sizeof(bm), &bm);

			// Create a 32 bits depth bitmap and select it into the memory DC 

			BITMAPINFOHEADER RGB32BITSBITMAPINFO = {	
					sizeof(BITMAPINFOHEADER),	// biSize 

					bm.bmWidth,					// biWidth; 

					bm.bmHeight,				// biHeight; 

					1,							// biPlanes; 

					32,							// biBitCount 

					BI_RGB,						// biCompression; 

					0,							// biSizeImage; 

					0,							// biXPelsPerMeter; 

					0,							// biYPelsPerMeter; 

					0,							// biClrUsed; 

					0							// biClrImportant; 

			};
			VOID * pbits32; 
			HBITMAP hbm32 = CreateDIBSection(hMemDC, (BITMAPINFO *)&RGB32BITSBITMAPINFO, DIB_RGB_COLORS, &pbits32, NULL, 0);
			if (hbm32)
			{
				HBITMAP holdBmp = (HBITMAP)SelectObject(hMemDC, hbm32);

				// Create a DC just to copy the bitmap into the memory DC

				HDC hDC = CreateCompatibleDC(hMemDC);
				if (hDC)
				{
					// Get how many bytes per row we have for the bitmap bits (rounded up to 32 bits)

					BITMAP bm32;
					GetObject(hbm32, sizeof(bm32), &bm32);
					while (bm32.bmWidthBytes % 4)
						bm32.bmWidthBytes++;

					// Copy the bitmap into the memory DC

					HBITMAP holdBmp = (HBITMAP)SelectObject(hDC, hBmp);
					BitBlt(hMemDC, 0, 0, bm.bmWidth, bm.bmHeight, hDC, 0, 0, SRCCOPY);

					// For better performances, we will use the ExtCreateRegion() function to create the

					// region. This function take a RGNDATA structure on entry. We will add rectangles by

					// amount of ALLOC_UNIT number in this structure.

					#define ALLOC_UNIT	100
					DWORD maxRects = ALLOC_UNIT;
					HANDLE hData = GlobalAlloc(GMEM_MOVEABLE, sizeof(RGNDATAHEADER) + (sizeof(RECT) * maxRects));
					RGNDATA *pData = (RGNDATA *)GlobalLock(hData);
					pData->rdh.dwSize = sizeof(RGNDATAHEADER);
					pData->rdh.iType = RDH_RECTANGLES;
					pData->rdh.nCount = pData->rdh.nRgnSize = 0;
					SetRect(&pData->rdh.rcBound, MAXLONG, MAXLONG, 0, 0);

					// Keep on hand highest and lowest values for the "transparent" pixels

					BYTE lr = GetRValue(cTransparentColor);
					BYTE lg = GetGValue(cTransparentColor);
					BYTE lb = GetBValue(cTransparentColor);
					BYTE hr = min(0xff, lr + GetRValue(cTolerance));
					BYTE hg = min(0xff, lg + GetGValue(cTolerance));
					BYTE hb = min(0xff, lb + GetBValue(cTolerance));

					// Scan each bitmap row from bottom to top (the bitmap is inverted vertically)

					BYTE *p32 = (BYTE *)bm32.bmBits + (bm32.bmHeight - 1) * bm32.bmWidthBytes;
					for (int y = 0; y < bm.bmHeight; y++)
					{
						// Scan each bitmap pixel from left to right

						for (int x = 0; x < bm.bmWidth; x++)
						{
							// Search for a continuous range of "non transparent pixels"

							int x0 = x;
							LONG *p = (LONG *)p32 + x;
							while (x < bm.bmWidth)
							{
								BYTE b = ((RGBQUAD *) p)->rgbRed; // GetRValue(*p);

								if (b >= lr && b <= hr)
								{
									b = ((RGBQUAD *) p)->rgbGreen; // GetGValue(*p);

									if (b >= lg && b <= hg)
									{
										b = ((RGBQUAD *) p)->rgbBlue ;  // GetBValue(*p);

										if (b >= lb && b <= hb)
											// This pixel is "transparent"

											break;
									}
								}
								p++;
								x++;
							}

							if (x > x0)
							{
								// Add the pixels (x0, y) to (x, y+1) as a new rectangle in the region

								if (pData->rdh.nCount >= maxRects)
								{
									GlobalUnlock(hData);
									maxRects += ALLOC_UNIT;
									hData = GlobalReAlloc(hData, sizeof(RGNDATAHEADER) + (sizeof(RECT) * maxRects), GMEM_MOVEABLE);
									pData = (RGNDATA *)GlobalLock(hData);
								}
								RECT *pr = (RECT *)&pData->Buffer;
								SetRect(≺[pData->rdh.nCount], x0, y, x, y+1);
								if (x0 < pData->rdh.rcBound.left)
									pData->rdh.rcBound.left = x0;
								if (y < pData->rdh.rcBound.top)
									pData->rdh.rcBound.top = y;
								if (x > pData->rdh.rcBound.right)
									pData->rdh.rcBound.right = x;
								if (y+1 > pData->rdh.rcBound.bottom)
									pData->rdh.rcBound.bottom = y+1;
								pData->rdh.nCount++;

								// On Windows98, ExtCreateRegion() may fail if the number of rectangles is too

								// large (ie: > 4000). Therefore, we have to create the region by multiple steps.

								if (pData->rdh.nCount == 2000)
								{
									HRGN h = ExtCreateRegion(NULL, sizeof(RGNDATAHEADER) + (sizeof(RECT) * maxRects), pData);
									if (hRgn)
									{
										CombineRgn(hRgn, hRgn, h, RGN_OR);
										DeleteObject(h);
									}
									else
										hRgn = h;
									pData->rdh.nCount = 0;
									SetRect(&pData->rdh.rcBound, MAXLONG, MAXLONG, 0, 0);
								}
							}
						}

						// Go to next row (remember, the bitmap is inverted vertically)

						p32 -= bm32.bmWidthBytes;
					}

					// Create or extend the region with the remaining rectangles

					HRGN h = ExtCreateRegion(NULL, sizeof(RGNDATAHEADER) + (sizeof(RECT) * maxRects), pData);
					if (hRgn)
					{
						CombineRgn(hRgn, hRgn, h, RGN_OR);
						DeleteObject(h);
					}
					else
						hRgn = h;

					// Clean up

					SelectObject(hDC, holdBmp);
					DeleteDC(hDC);
				}

				DeleteObject(SelectObject(hMemDC, holdBmp));
			}

			DeleteDC(hMemDC);
		}	
	}

	return hRgn;
}