Microsoft KB Archive/27225

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INFO: Determining Available RGB Values of an Output Device

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Q27225

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The information in this article applies to:


 * Microsoft Windows Software Development Kit (SDK) 3.1
 * Microsoft Win32 Application Programming Interface (API), used with:
 * Microsoft Windows NT Server versions 3.5, 3.51, 4.0
 * Microsoft Windows NT Workstation versions 3.5, 3.51, 4.0

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SUMMARY
The best way to determine the colors supported by a device is to enumerate the solid pens from a Device Context (DC) associated with that device. The EnumObjects function enumerates the pens supported by a specified device and calls a callback function for each pen. To do so, EnumObjects requires three parameters: a device context associated with the desired device as the hDC parameter, OBJ_PEN as the value of the nOjbectType parameter, and the procedure-instance address of a callback function as the lpObjectFunc parameter.

MORE INFORMATION
The first parameter for the callback function, lpLogObject, points to a LOGPEN data structure that describes each enumerated pen. When the lopnStyle field of the LOGPEN structure contains the PS_SOLID value, the application can retrieve and store the value of the lopnColor field.

NOTE: For true color devices (devices that support more than 8 bits- per-pixel of color resolution), Windows enumerates only a subset of the supported pens. These devices support almost any color.

The following code demonstrates the process:

  int cMaxRGB, nCnt, nSolid; // Global variables for system RGB colors void GetColorList(HWND hWnd) {  HDC hdc; FARPROC lpProcCallback; HANDLE hmem; BYTE FAR *lpmem;

nCnt = nSolid = 0; hdc = GetDC(hWnd); cMaxRGB = GetDeviceCaps(hdc, NUMCOLORS); if (cMaxRGB >= 0x7FFF) return; // All colors available. Enumeration not required. lpProcCallback = MakeProcInstance(Callback, hInst); // Allocate space for color table hmem = GlobalAlloc(GHND, sizeof(COLORREF) * cMaxRGB); lpmem = GlobalLock(hmem);

EnumObjects(hdc, OBJ_PEN, lpProcCallback, lpmem);

FreeProcInstance(lpProcCallback);

// Use the color table stored in the first nSolid entries of a  // COLORREF array stored in lpmem.

GlobalUnlock(hmem); GlobalFree(hmem); ReleaseDC(hWnd, hdc); return; } The source for the enumeration callback function is below. The callback function must be listed in the EXPORTS section of the module definition (DEF) file.

  int FAR PASCAL Callback(LPLOGPEN lpLogPen, LPSTR lpData) {  nCnt++; if (lpLogPen->lopnStyle == PS_SOLID) // solid pen {     COLORREF FAR *lpDest = (COLORREF FAR *)lpData + nSolid++; *lpDest = lpLogPen->lopnColor; // remember the solid color }

if (nCnt >= cMaxRGB) return 0; return 1;

} Additional query words: 3.00 3.10 3.50 4.00

Keywords : kbOSWinNT350 kbOSWinNT351 kbOSWinNT400 kbOSWin95 kbSDKWin16

Issue type : kbinfo

Technology : kbAudDeveloper kbSDKSearch kbWin32sSearch kbWin32API kbWinSDKSearch