INFO: Determining Available RGB Values of an Output Device

• Microsoft Windows Software Development Kit (SDK) 3.1
• Microsoft Win32 Application Programming Interface (API), used with:
  • Microsoft Windows NT, versions 3.5, 3.51, 4.0
  • Microsoft Windows 95

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;
   } 

   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 : kbNTOS350 kbNTOS351 kbNTOS400 kbWinOS95 kbSDKWin16
Version : WINDOWS:3.1,95; winnt:3.5,3.51,4.0
Platform : WINDOWS winnt
Issue type : kbinfo